Die Preassure Losses During Injection Molding of Rubber Mixes

DIE PRESSURE LOSSES DURING INJECTION

MOLDING OF RUBBER MIXES

A. ARRILLAGA,* A. M. ZALDUA, R. M. ATXURRA, A. S. FARID

MATERIALS DEPARTMENT, LEA-ARTIBAI IKASTETXEA S.COOP., XEMEIN ETORBIDEA 19

MARKINA-XEMEIN 48270,VIZCAYA, SPAIN

A. S. FARID

LONDON METROPOLITAN POLYMER CENTRE, LONDON METROPOLITAN UNIVERSITY

166-220 HOLLOWAY ROAD, LONDON N7 8DC, UK

ABSTRACT

In order to fill the mold in a rubber injection molding process, it is necessary to inject the material into the closedmold. Rubber is usually injected under ram speed control, but it can be also injected under pressure control. In the pres-ent study, we have recorded the signals of pressure at three points during the filling of a spiral shape part. The behaviorsof two rubber compounds have been studied using a variety of combinations of process conditions (including mold tem-perature, mass temperature, ram speed and injection molding with and without pressure holding stage). In all conditions,the transducer located in proximity to the gate exhibits pressure decay at the last stage of mold filling. Initial CAE sim-ulations have been carried out using Moldflow software to check the capability of this sort of software to calculate pres-sure decay during the filling stage.

INTRODUCTION

Injection molding of rubber began in the early 1940’s, and the screw injection molding wasfirst reported by Cousino and Chrysler Corporation,1 being used today for the manufacturing ofa wide range of industrial products. These machines use to have two main parts:

a) The clamping unit, which opens and closes the mold, and keeps the mold closed duringthe injection and cure steps.

b) The injection unit, which contains two parts: the extruder and the chamber. First theextruder plasticizes and heats the material, so that later on this is stored in the injectionchamber prior to its injection into the mold.

When using injection molding, both processing variables and rubber compound variableshave a marked effect on rates of cure, rates of scorch and general injection performance asdefined by Wheelans.2 This author summarized the effect of process parameters such as screwspeed, barrel temperature, back pressure, injection pressure, nozzle diameter, ram speed andmold temperature. Tsai3 studied the effect of varying these parameters on degree of cure andstress-strain properties of the part. Kramer4 in 1994, made injection trials on a spiral shape moldchanging injection and mold temperature and injection pressure, and measuring the flow lengthresulting from such changes.

When injecting the material from the chamber into the mold, the injection shoot is usuallyintroduced under control of ram speed, but instead of filling the complete mold under ram speedcontrol, part of the shoot can be introduced under ram speed control and the remaining part ofthe shoot under control of pressure, that is, a pressure holding stage is applied.

In the present study, we have recorded the pressure signal values during the injection fillingof a spiral shape mold under various process conditions. We have included the filling under ramspeed control only and both ram speed and pressure control; the effect of process variables suchas mold temperature and mass temperature have also been considered. Measurements of pressure

62

* Corresponding Author. Ph: 34 94 616 9002; Fax: 34 94 616 9160; email : [email protected]

DIE PRESSURE LOSSES DURING INJECTION MOLDING OF RUBBER MIXES 63

were carried out along three points along the spiral.Similar trials where measurements of pressure were taken inside the cavity can be found in

literature. For instance, Bowers5 in 1987, injected black SBR into a two-flap-shape parts mold,equipped with pressure transducers; the assembly allowed for the possibility of varying the thick-ness of runners and cavities between 3 and 6 mm. Bowers also developed a mathematical codeto simulate temperature changes, pressure loses, and scorch levels during filling stage and degreeof cures during cure step, comparing calculated pressure values with measured ones, founding agood agreement. Graf and co-authors6 in 1989, in order to study rubber’s processability, modi-fied an injection machine by introducing pressure and temperature transducers in the injectionnozzle; here a “spider mold” was used which was equipped also with these sort of transducers.Tests were made with a black SBR, but no comparison to simulation results was made.

Isayev and his co-authors presented several studies using SBR formulations; in 1989, they7

developed a mathematical code for simulating the filling and cure stages of the injection mold-ing of thin rubber parts. It was applied for simulating the filling of two molds using a SBR com-pound. Parameters recorded were hydraulic pressure, ram speed and three pressure signals intothe cavity. The comparison between measured pressure values and simulated ones showed goodagreement. Later on, in 1991, the same authors8 performed additional studies comparing pre-dicted degree of cures and pressure drops with experimental values. In 1994, they9 presented anew approach to simulate cavity filling, packaging and flash formation, finding again goodagreement between experimental and predicted pressure traces. In 1996, they10,11 developed amathematical code taking into account the effect of vulcanization of the material as a salient rhe-ological feature, in order to enhance the quality of simulation. Finally, in 1998, they12 presenteda similar study to a previous study,8 but using a NR compound instead of SBR. Measurementsshowed a considerable error on the prediction of pressure traces, higher than in the studies donewith SBR, although prediction of cure states was satisfactory.

Leblanc and co-authors13 in 1991 sensorized a center-gated disk with three pressure trans-ducers, and made injection trials with compounds of NR, EPDM and Fluoroelastomers.Pressures into the injection nozzle and mold temperature signals were also recorded and injec-tion trials were made using different combinations of flow rate and injection temperature. Nocomparison to simulation results was made, but they concluded that although a constant value offlow rate was programmed, the real flow rate fluctuated differing from that which was pro-grammed.

Karam,14 in 1995, made injection trials on a spiral shape mold equipped with two pressuretransducers for a wide range of different process conditions using two compounds based on SBRand EPDM. He also recorded pressure and temperature signals inside the injection nozzle.Experimental pressure traces were compared to simulation results using “Fill” software. It wasconcluded that in order to define the temperature at which the rubber was introduced into themold, the best approach was to make a shoot into air at the same process conditions; make a com-pact ball with the shoot and measure its internal temperature with the aid of a temperature trans-ducer. Later on, in 1998, he15 again used the same mold for making injection trials with twocompounds of SBR and EPDM in order to study the influence of injection variables.Experimental pressure traces were compared to simulation results, and it was observed that sim-ulations which consider the phenomena of material’s compressibility, wall slippage and the effectof vulcanization rheology produced accurate results.

The present study is part of a bigger research project focused on assessing the capabilitiesof Moldflow simulation software for simulating rubber injection molding process, where the res-olution of the appropriate governing equations (Momentum, Continuity and Energy) in conjunc-tion with relevant rheological and cure kinetic models (see our previous paper)16 provides amethod to determine such parameters as flow patterns, pressure drops as well as temperature andcure distributions.

64 RUBBER CHEMISTRY AND TECHNOLOGY VOL. 82

In the present study simulations were launched using “Reactive Molding” module of theMoldflow package using the same process variables as used in the machine, in order to comparethe calculated pressure traces with the experimental ones. However, prior to launching the sim-ulations it is necessary at first to define the thermal and rheological properties of the compounds;and, to define the geometry of the cavity with a suitable mesh model.

EXPERIMENTAL

MATERIALS

Two rubber compounds were studied. One based on NBR with peroxide curing system ofknown formulation and another based on an industrial EPDM compound based on a sulfur cur-ing system. Table I summarizes the recipe of the NBR compound. Two batches were preparedfor this material in Elastorsa S.A. Company:

- Batch of approximately 1,7 kg, prepared in an small internal mixer of 2 liter: This mate-rial was used to perform the necessary tests to characterize the material for simulationspurposes.

- Batch of approximately 57 kg, prepared in an internal mixer of 70 liter. This materialwas used to make the injection trials in both machines.

TABLE IRECIPE FOR NBR FORMULATION

NBR

Ingredient phr

Europrene N3345 100

N-330 60

DOP 10

Stearic acid 1

ZnO 5

Percadox BC40MB 65

(Total) 1825

TEST EQUIPMENT

Two rubber injection machines were used:- Rep V37 vertical rubber injection molding machine, with LIFO injection unit, having

the following characteristics:- Maximum hydraulic pressure: 250 bar- Maximum injection pressure over the material: 1700 bar- Clamping force: 100 ton- Ram Diameter: 48 mm- Maximum injection capacity: 0,5 liter

- RUTIL T1A vertical rubber injection molding machine, with FIFO injection unit, hav-ing the following characteristics:- Maximum hydraulic pressure: 230 bar- Maximum injection pressure over the material: 1700 bar- Clamping force: 500 ton- Ram Diameter: 90 mm- Maximum injection capacity: 2,5 liter

Note that there exists a relation in between the hydraulic pressure and the pressure over the

material into the injection unit, that is, they are proportional. For instance, a hydraulic pressure of125 bar means a pressure of 850 bar over the material in the injection unit in the LIFO machine.Usually, what we control and measure in the injection machines is the hydraulic pressures.

MOLD

A spiral - shape geometry was designed (see Figure 1). It has a constant thickness of 6 mmand 20 mm width, and although it may be longer, we have limited its length to 1150 mm to suitour purposes. The vertical cone represents the sprue, with a initial diameter of 4 mm, a final oneof 11,5 mm and a length of 113 mm. The total theoretical volume to be injected represents 161,8 cm3.

A mold of dimensions 400 x 360 x 418 mm was machined for the production of the com-ponent illustrated in Figure 1; the Figure 2 shows a scheme of the mold, including the marks ofthe spiral. Pressure measurements were done in 3 positions (Ps1, Ps2 and Ps3), which were donein the upper surface of the spiral.


FIG. 1. — 3D representation of the rubber part.


FIG. 2. — 2D representation of the spiral mold, defining the dimension marks ofthe spiral and the position of the 3 pressure transducers (Ps1, Ps2 and Ps3).

DATA ACQUISITION SYSTEM

As mentioned previously, pressure signals were recorded along three locations in the cavity.Two Kistler 6157BB Type transducers were utilized for measurements of pressure in the range0-2000 bar, so when signals for the three points are required, it was necessary to make at leasttwo measurements. The pressure acts over the entire front of the sensor and is transmitted to thequartz measuring element, which produces a proportional electric charge in PC (Pico coulomb);with the help of an amplifier (Kistler Type 5039A322), this signal is converted into a voltage inthe -10 V to + 10 V range. This last signal is introduced in the data acquisition system (OROSOR35), where later on, it is converted into a pressure vs. time signal with the aid of appropriatesoftware (OROS NVGate 2.2).

In addition to the pressure signals in the mold interior, hydraulic pressure and injection ramspeed signals were recorded from the injection machine’s main display using a video recorder at15 Hz (15 data per second), so that information pertaining to the real flow rate could be defined.In order to overlap the different result traces, as time scales differ for each recording, time datawere adjusted considering as reference the end of filling time.

PROCESS PARAMETERS USED IN EXPERIMENTAL TRIALS

Tables II and III represent the combination of process parameters used for the injectionmolding of the two rubber formulations referred to earlier. Combinations A, E and I include thevariation of temperatures and ram-speed, where the cavity-filling was completed in one singlestep, just under ram speed control. Combination B is similar to A, but the cavity is not completelyfilled, whereas in combination C the filling is finished under pressure control stage.

TABLE IIPROCESS PARAMETERS USED FOR NBR IN EACH COMBINATION

Parameter\combination A a) B b) C c) E a) I a)

Extruder speed, rpm 80 80 80 80 80

Extruder temperature, °C 75 75 75 60 90

Chamber temperature, °C 85 85 85 70 100

Mold temperature up, °C 180 180 180 160 200

Mold temperature down, °C 180 180 180 160 200

Lineal ram speed, 01 mm/s d) 190 190 190 190 190

Shooting stroke, 01mm e) 1100-50 1020-35 1020-35 1020-25 1150-50

Pressure (holding), bar 0 0 125 0 0

Time (holding), s 0 0 8 0 0

Screw delay, s 40 40 40 90 40

Curing time, min 2 2 2 2,5 2a) The cavity is filled completely (100 %) under ram speed control.b) The cavity is not filled completely (about 98,5 % of the total cavity volume is filled).c) The cavity is filled about 98,5% under ram speed control, introducing the remaining volume under pressure control,

that is, a pressure holding stage is applied after the 1st filling stage.d) Ram speed units are defined in 0,1 mm/s for the REP machine. If a value of 190 is set, means a ram speed value of

190x0’1 mm/s = 19 mm/s. Considering the ram has a diameter of 48 mm, we could calculate the equivalent flow rate of 34’4 cm3/s.

e) Shooting stroke, defined in 0,1 mm. The amount of material introduced under ram speed control stage is definedfixing the initial and final position of the ram during the injection shoot. A value of 1100 – 50 means that a total of 1100-50=1050*0’1= 105 mm of material are injected. Considering the ram has a diameter of 48 mm, we couldcalculate in this way the volume of material introduced into the mold (190 cm3).


TABLE IIIPROCESS PARAMETERS USED FOR EPDM IN EACH COMBINATION

Parameter/combination A a) E I

Extruder speed, rpm 80 80 80

Extruder temperature, °C 75 60 90

Chamber temperature, °C 85 70 100

Mold temperature up, °C 180 160 200

Mold temperature down, °C 180 160 200

Lineal ram speed, 01 mm/s d) 120 120 120

Shooting stroke, mm e) 1075-75 1075-90 1075-75

Pressure (holding), bar 0 0 0

Time (holding), s 0 0 0

Screw delay, s 40 100 40

Curing time, min 2 3 2a) The cavity is filled completely (100 %) under ram speed control.b) The cavity is not filled completely (about 98,5 %).c) The cavity is filled about 98,5% under ram speed control, introducing the remaining volume under pressure control

stage.d) Ram speed units are defined in 0,1 mm/s.e) Indicates the initial and final position of the ram during the injection shoot, in “0’1 mm” units.

For the trials about to be discussed a Rep V37 machine was used. First the parameters foreach combination were programmed. After having programmed the machine at least three shootswere conducted initially in order to acquire measurements necessary for process stabilization.Thereafter, three to four shoots were recorded; however, only results for one record per combi-nation are shown because of their repetitive nature.

INJECTION MOLDING SIMULATIONS

Injection molding simulations were carried out for the NBR compound according to theprocess parameters defined as combination “A” in Table II (that is, completely filling the moldunder ram speed control), for modeling the injection molding process in the LIFO (REP)machine. Simulations were also done to model the process in the FIFO (RUTIL) machine,according to the process parameters described in Table VIII, for the NBR. The “ReactiveMolding” module of Moldflow 6.1 CAE software was used. It is noteworthy that prior to launch-ing the simulations it was necessary to represent the cavity to be filled using a mid-plane meshin order to define the properties of the material and to fix the process parameters. Together withthe cavity, part of the injection unit was also modeled as a cold-runner assembly at a temperatureequal to that of the injection temperature, as represented in Figure 3. This cold-runner assemblyhas the same size as the injection unit of the REP V37 machine, with a nozzle diameter of 5,3mm and a ram diameter of 48 mm. The cold-runner assembly was not modeled for the RUTILmachine. Altogether, 1470 nodes, 2279 triangle elements and 41 1D elements were used.


TABLE IVCONSTANT VALUES FOR THE KAMAL-SOUROUR a) AND INDUTION MODELS FOR

NBR COMPOUND (CHARACTERIZED BY MDR)

NBR- Cure kinetics NBR Induction

Parameter Value Parameter Value

A1, s-1 0* A,s 3,6979E-03

E1/R, °K 0* B, °K 3591

A2, s-1 exp(32,506)

E2/R, °K 16305

m 0,33163

n 1,5309a) Although the Kamal-Sourour model includes the K1 as a parameter, data fitting concluded that the best fits is obtained

when the K1 is considered as zero; that’s why A1 and E1 becomes zero, too.

The NBR compound was characterized in terms of thermal, rheological and cure behavioras summarized below:

° Specific heat: DSC was used for these measurements (ASTM E1952-05); a TAInstrument Q100 device was used. Values were measured in the temperature range 65to 215 °C, obtaining values between 1,53 and 1,83 J/g. °C (data were registered every5 °C). Measurements were made in the cured material.

° Thermal conductivity: A modified PVT measuring device form the Basque CountryUniversity was used. Values were measured in the temperature range 80 to 180 °C,obtaining values between 0,19 and 0,24 W/m.K (data were registered every 10 °C).Both the Specific Heat and the Thermal Conductivity are parameters that vary with tem-perature and cure degree of the material. The dependence with cure degree is usuallyvery low, and that’s why it is neglected commonly, as do Moldflow. However, the vari-ation with temperature is important.

° Cure characterization: Moldflow utilizes the Kamal-Sourour Model17 to define cure


FIG. 3. — Mesh model used in Moldflow for simulations. Cavity is described using shell typetriangular elements, whereas sprue and injection unit are described using 1D (Beam) elements.

kinetics as a function of temperature and time. As reported in our previous study,16 curekinetics can be defined using various methodologies. In the present study we used anMDR instrument from Alpha Technologies; the method used is described fully inASTM D5289; the relevant equations for the analysis are:

(1)

where and (2)

MDR tests were made a 150, 160, 170 and 180 °C. First, K1, K2, m and n parameters werecalculated at each temperature. Then, from the values of K1 and K2 at each temperature, the A1,A2, E1 and E2 parameters were calculated. “Grace” software package18 was used for this purpose.“m” and “n” values were fixed as an average value of those measured at each temperature.

The overall kinetic scheme included the characterization of the induction period definedunder the concept of “Anisothermal induction time” as proposed by Isayev et al.:19

where (3)

Considering the induction times (ti) at 150,160,170 and 180 °C, the A and B fitting param-eters where calculated using “Grace” software package.

Table IV summarizes the cure kinetics parameters for our material.

° Rheology: Material’s rheology was characterized using a Rosand RH 7.2 ExtrusionCapillary Rheometer (ASTM D5099). Measurements were made at 80, 90 and 100 °C,using capillaries of 2 mm diameter and 8, 12, 16 and 25 mm length. Apparent viscosi-ty data were measured, so Rabinowitch20 and Bagley21-22 correction were applied inorder to calculate the corrected or real viscosity data. Wall slip and yield stress phe-nomena were neglected. Then, viscosity were fitted to the reactive viscosity model; themathematical statement of which is:

(4)

where (5)

Using “Grace” software package, first the η0(T), τ* and n parameters were calculated. Then,considering the η0(T) values at 3 temperatures, B and Tb parameters were calculated. Final “τ*”and “n” parameters are fixed as an average value of the value measured at each temperature.

Table V summarizes the rheological parameters for our material.

η0 T B T Tb( ) = ( ). exp

η α γη

η γ

τ

α

α α

α

, ,.

TT

Tn

g

g

C C

( ) =( )

+( )⎛

⎝⎜

⎞

⎠⎟

⋅−

⎛

⎝⎜

⎞

⎠⎟

∗

−

+ ⋅

0

01

1

1 2

ti A B T= ( ). exp /tdt

ti t

l=

( )∫0

K A E T2 2 2= ⋅ −( )exp /K A E T1 1 1= ⋅ −( )exp /

dadt K K m n= + ⋅( ) ⋅ −( )1 2 1α α


TABLE VCONSTANT VALUES FOR THE REACTIVE VISCOSITY MODEL NBR COMPOUND (CHARACTERIZED BY MDR)

NBR Value

Tao*, Pa 24700

N 0,2750

B, Pa.s 6,0733

Tb, °K 4888,9

c1 a) 1

c2 a) 0

agb (gelation conversion) 0,1a) We consider that material doesn’t start to cure before the cavity is completely filled, so c1 and c2 parameters were set

to 1 and 0, respectively.b) Gelation conversion, that is, cure degree at gel point, was set to 0,1 as default.

The main process parameters include:- Mold temperature: We have set temperatures of 180 °C for the cavity surface in the sim-

ulation (as fixed for the experimental trials), both for the FIFO machine and the LIFOmachine. In the real process, what is controlled is the mold temperature at the positionat which the temperature transducers are located in the mold, so the real temperature atthe cavity surface is lower, and doesn’t keep constant during all the molding cycle.

- Material temperature: Temperatures of 75/85 °C and 80/85 °C for the extruder/chamberwere set for injection trials in the LIFO and FIFO machines. So, injection temperaturecould be defined as 85 °C for making the simulation, but it cannot be assumed that thematerial will enter the mold at the temperatures indicated; in addition, the cycle timeand the design of the injection unit will also affect the temperature at which the mate-rial enters the mold. In the present investigation - as proposed by Karam14 - we haveperformed injection shoots into air using our stipulated process conditions. Consideringthis, we have set a temperature of 97 °C for the injection molding process on the LIFOmachine and 95 °C in the FIFO machines.

- Injection time: The Moldflow package needs to specify an injection time or a ram-speedprofile. Usually a constant vale of ram speed, that is, a constant flow rate is defined, butthe LIFO machine simulations were conducted using the same ram speed profile andinjection time that was used (measured) in the REP machine. However, for the FIFO(RUTIL) machine, realizing that ram-speed-profile definition was poor, only the injec-tion time data was used as reference to do the simulation, that is, a constant ram speed(flow rate) was set.

- Mold-melt heat transfer coefficient was set to a standard value of 25000 W/m2 °C.- A default cure time of 300 s was set for both cases.

RESULTS AND DISCUSSION

MEASUREMENTS OF PRESSURE INTO CAVITY, HYDRAULIC PRESSUREAND RAM SPEED, IN LIFO (REP) MACHINE

Figures 4 shows the traces for hydraulic pressure, ram speed and pressure into the cavity(Ps1, Ps2 and Ps3) for NBR, when injecting all the cavity volume under ram speed control, thatis, under process variable combinations “A” of Table II.


“Phydra” represents the evolution of hydraulic pressure done by the machine to inject thematerial into the mold, in bars; instead of working with the hydraulic pressure, we could plot alsothe injection pressure value, which in the case of the LIFO (Rep V37) machine will be 6’8 timesthe hydraulic pressure. “Ram speed” indicates the ram speed at which the ram pushes the mate-rial into the cavity, the values are shown in 0,1 mm/sg units (so a value of 100 wants to mean alineal speed of 10 mm/s). “Ps1”, “Ps2” and “Ps3” are the pressure values, in bars, recorded atthree points into the cavity as indicated in Figure 2.

This NBR formulation is certainly a viscous compound. Maximum hydraulic pressure (250bar, the one of the machine) is achieved in the early stages of mold-filling. More pressure is nec-essary to continue pushing the material at 19 mm/s, so the machines pushes the material at thatlimit hydraulic pressure of 250 bar (that is, 1700 bar inside the injection unit) until the mold-fill-ing sequence is completed. Considering the ram speed and hydraulic pressure profiles, weobserve that there is an initial acceleration of the ram speed, but, once maximum hydraulic pres-sure is achieved thereafter the ram speed decreases up to a specific value (time = 6 s and Ramspeed = 6,8 mm/s) at which point it again starts to rise achieving a maximum of 8,3 mm/s. Thisbehavior is unexpected from a theoretical stand point as one would expect the ram speed to con-tinue decreasing.

Three pressure traces were recorded in the mold interior. Concerning Ps1, which is the trans-ducer located near the gate, there is a time lapse (of about 1,2 s) between the start of the injec-tion sequence and the point at which pressure starts to increase in Ps1; this is clearly related tothe time required by the material to arrive at the position where the transducer measuring the Ps1signal is located. It is found that pressure continues increasing at this point, and before the mold-filling sequence is completed, the pressure reaches a maximum (1060 bar). After having reachedthe maximum point, the pressure decreases (up to a value of 903 bar at end of injection). Thisbehavior is again contrary to expectation, as it is natural to expect the pressure to continueincreasing during the injection sequence or at least reach limiting or plateau value (especially inthis case where maximum hydraulic pressure is achieved during the injection stage and filling is


FIG. 4. — Results of hydraulic pressures, Ram speed and Pressures inside the mold variation,when injecting the mold with Combination “A” process parameters, in NBR..

completed under this maximum pressure, which is our case). In view of the aforementioned pro-cessing anomalies it was thought expedient to investigate this phenomenon. At first thought it istempting to suggest that this peculiar behavior might be related to the increase on ram speed dur-ing the final stage of mold-filling. It is noteworthy that when the injection sequence ends (11 s),the pressure declines and stabilizes during the curing stage.

As for the position designated Ps2, which is the transducer located in an intermediate posi-tion of the spiral cavity, pressure increases continuously throughout the various processingstages. Moreover, there is no evidence to suggest any inclination of pressure decay as was foundfor Ps1. Nevertheless, in the initial stage of curing sequence there appears to be a sudden increaseof pressure in this position (from 303 to 389 bar); but, later it declines and eventually stabilizesduring the rest of curing sequence.

Regarding PS3, which is the transducer located at the end of the spiral cavity, there is hard-ly any increase of pressure for NBR during the filling stage, but pressure starts to increase dur-ing the cure stage. Note that Ps3 scale is different from Ps1 and Ps2, so that, although it seemsPs3 pressure data become much higher than the two others, really it is lower.

EFFECT OF CHANGING THE MATERIAL

In order to ascertain whether or not the phenomenon described earlier in terms of ram speedand Ps1 results are a direct consequence the salient properties of the rubber compound, a test-runwas repeated using a commercial rubber compound based on EPDM. The process variables usedfor the run are designated “Combination A” in Table III. The traces obtained are shown in Figure5. For the sake of expediency, measurements at location Ps3 were also carried out.

It was noticed at the onset that the EPDM compound had a lower viscosity than that of NBRas it was found that hydraulic pressure drops were lower for similar process conditions. In caseof EPDM, we observed that pressure reaches a maximum of 154 bar, instead of 250 bar foundfor the NBR compound, so maximum pressure of the machine is no achieved. Regarding the real


FIG. 5. — Results of hydraulic pressures, Ram speed and pressures inside the mold variation,when injecting the mold with Combination “A” process parameters, in EPDM.

ram speed: for EPDM, at the initial stage of filling, there is a definite acceleration followed by aperiod over which the speed stabilizes at the programmed value of about 12 mm/s. It was noticedthat during the injection phase the hydraulic pressure achieves a maximum of pressure (154 bar)and thereafter starts to decline. This behavior in the pressure suggests that the material enteringthe mold, resulting in a concomitant pressure drop, has a lower viscosity than the previous inject-ed material; thus, less pressure is necessary to inject the material at the programmed ram speedleading to a decay in pressure. Moreover, at the instance at which the hydraulic pressure beginsto decrease the ram speed also starts increases, but eventually the machine controls it and dropsto the programmed value. For NBR no decay in hydraulic pressure was observed; however, theincrease of ram speed witnessed in the last stage of mold filling leads us to similar conclusion.The behavior just described alludes to the possibility that it may be due to viscosity differencesin the material stored in the injection chamber, and this undoubtedly will be related to tempera-ture and as consequence viscosity differences in the material entering the mold.

Concerning location Ps1: the cavity is filled under ram speed control (not achieving the max-imum hydraulic pressure); at this location a pressure decay is observed in the last stages of fill-ing (from 925 to 763 bar). For position Ps2, as with NBR, pressure continues increasing con-stantly during all the filling sequence. In the initial steps of cure (very close to the end of fillingsequence), there is a sudden increase of pressure in the Ps2 position (from 350 to 414 bar); butsoon after it falls and stabilizes during the curing sequence. As for position Ps3, a small declineof about 54 bar is detected in the latter stages of the filling sequence; but, later the pressure con-tinues increasing and stabilizes, once again during the cure stage.

It is important to realize that in relation to the traces for hydraulic pressure and ram speed;and, consideration of position Ps1, the same phenomenon as mentioned earlier is observed irre-spective of the type of material used.

EFFECT OF TEMPERATURE

Similar measurements to those mentioned previously were made using both low and hightemperatures for the injection unit and mold. Figures 6 and 7 represent the measurements forNBR and EPDM conducted at the low temperature range; in contrast, Figures 8 and 9 correspondto higher temperatures. Process conditions used were those that are described as combinations Eand I in Tables II and III.



FIG. 6. — Results of hydraulic pressures, Ram speed and pressures inside the mold variation,when injecting the mold with Combination “E” process parameters, in NBR.

FIG. 7. — Results of hydraulic pressures, Ram speed and pressures inside the mold variation,when injecting the mold with Combination “E” process parameters, in EPDM.

When using lower temperatures, EPDM exhibits similar traces to those obtained previously,but requires more hydraulic pressure (170 as opposed to 154 bar) to force the material into themold owing to the higher viscosity of the material. Maximum values for mold pressures are near-ly equal (so it’s not shown the difference in viscosities), but the shapes of the curves are similar.When injecting NBR, again maximum hydraulic pressure is achieved; but the material’s higher


FIG. 8. — Results of hydraulic pressures, Ram speed and pressures inside the mold variation,when injecting the mold with Combination “I” process parameters, in NBR.

FIG. 9. — Results of hydraulic pressures, Ram speed and pressures inside the mold variation, when injecting the mold with Combination “I” process parameters, in EPDM.

viscosity is seen because the material arrives slightly faster to the point at which maximumhydraulic pressure is attained. The material continues injecting under pressure control; ramspeeds are lower than those found at higher temperatures (5,1 mm/s in contrast to 8,3 mm/s atend of injection).

On the other hand, when using higher temperatures, NBR is less viscous. Again, maximumhydraulic pressure is achieved; however, the significance of reduced viscosity is again apparentin the sense that greater time is required to attain this maximum. Furthermore, ram speed valuesare higher during the pressure controlled stage (9,8 mm/s as opposed to 8,3 mm/s at end of theinjection sequence). As for EPDM processed at a relatively temperature: Figure 9 shows thatlower hydraulic pressure (147 versus 154 bar) is necessary to inject the part.

Although measurements were made at different mold and injection temperatures, the pres-sure decay phenomenon was observed in the vicinity of position Ps1 for both materials. Relatedto this, NBR exhibits also an increase in ram speed at the latter stages of the injection sequence.

EFFECT OF INCOMPLETE FILLING OF MOLD

Figures 10 represents the traces obtained for NBR for the process in which there is a delib-erate incomplete filling of the mold. We want to check if the previous phenomenon appears, notcompleting mold filling. The process conditions used for this aspect of the investigation are des-ignated as “Combination B” in Table II.

The general shapes of the hydraulic pressure and ram speed traces are similar to the shapesillustrated in Figure 4. For positions PS1 and PS2 we observe the same behavior as detailed ear-lier (including the pressure decrease at last stage of curing). Therefore, we conclude that thereare no differences as compared to earlier behavior regarding ram speed, hydraulic pressure, Ps1,and Ps2 in terms of complete (100%) or incomplete (98%) filling of the mold cavity. As expect-ed, there are substantial differences in the pressure trace for location Ps3; no increase of pressurewas detected during the filling sequence because cavity filling was no completed, so no materi-


FIG. 11. — Results of hydraulic pressures, Ram speed and pressures inside the mold variation, when injecting the mold with Combination “C” process parameters, in NBR.”B”.

al arrives to the position of Ps3; as the material begins to cure in the incompletely filled mold, itis accompanied by dilation and material arrives to the position of the transducer, generating asmall pressure of about 8 bar.

EFFECT OF USING PRESSURE CONTROL TO COMPLETE MOLD FILLING

We want to analyze the behavior when apart form a 1st stage of mold filling under ram speedcontrol, an additional stage of pressure holding is applied. Figure 11 represents the tracesobtained for NBR when injecting the material under the process conditions designated as“Combination C” in Table II.

All traces obtained during the ram speed controlled stage are almost identical to those shownin Figure 10. During the pressure holding stage, hydraulic pressure is kept constant at the pro-grammed value; as a result, ram speed is also maintained at an approximate constant value.

In terms of the pressures developed, it was found that at Ps1 the pressure is constant at about386 bar for a duration corresponding to half the cycle and finally reducing to approximately 232bar just prior to finishing the sequence; then during subsequent cure, the pressure decreases andstabilizes. As for Ps2, the pressure increases slightly from 301 to 364 bar on commencement ofthe holding sequence, thereafter it maintains more or less a constant value for the remainingphases comprising a complete molding cycle. As regards Ps3, there is no increase of pressureduring the filling stage (pressure starts to increase just before finishing the holding stage).

Hence, although rubber compounds were introduced into the mold cavity using variouscombinations of processing parameters (injection and mold temperature, complete and incom-plete filling of mold) it was found that, the pressure at Ps1, in all cases, showed a rise to maxi-mum pressure followed by a decline in the pressure in latter stages of the mold filling sequence.This behavior is contrary to expectation. In addition recall the behavior exhibited by the pro-cessing parameters when processing NBR, showing an increase of ram speed under pressure con-trolled injection (because machine’s maximum pressure was achieved). In an attempt to explain


FIG. 11. — Results of Hydraulic pressures, Ram speed and pressures inside the mold variation,when injecting the mold with Combination “C” process parameters, in NBR. “B”.

the observed overall behavior it was though expedient to carry out further trials. The nature andsignificance of these trials are discussed in the next Section.

SPECIAL TRIALS

The trials discussed here were based solely on NBR.Measurements at Low Mold and Material Temperature .— It was thought that a possible rea-

son for the pressure decay reported on numerous occasions in the present paper might be due therapid formation of an external layer of rubber (layer of rubber in direct contact with the mold sur-face) while the rubber is resident in the mold; and, that this cured layer perhaps acts as a bufferleading to the attenuation of pressure at the core. This could well be a reason for the decay shownin Ps1, but not for the decay in hydraulic pressure. It should be borne in mind that the leadingedge of the material injected into the mold is the initial portion of the material that travels to theend of the mold cavity. Therefore, the pressure that is monitored at Ps1 is the pressure of thematerial that continues to enter the mold behind the leading edge. Having made this importantpoint, we merely state that tests were repeated according to process parameters demarcated undercombination “A” in Table II; but, using lower mold and material temperatures as well as a lowerram speed (see Table VI). The selected temperature was not considered excessive so as to engen-der premature curing during the injection sequence, that is, the temperature used was well belowthat required for substantial decomposition of the peroxide curing agent. Care was also taken toensure that the ram speed used was relatively benign in order avoid excessive heating of thematerial due to shearing.

TABLE VIMOLD AND INJECTION TEMPERATURES USED AT TESTS DONE AT LOW TEMPERATURE

NBR

Initial Low temperature test

Extruder temperature, °C 75 60

Chamber temperature, °C 85 70

Mold temperature up, °C 180 104

Mold temperature down, °C 180 106

Ram speed, 01 mm/sa 190 75 a Ram speed units are defined in 0,1 mm/s, so that a value of 190 means a ram speed of 19 mm/s.

Figure 12 shows the relevant traces for the purposes of comparison; the traces are for theoriginal test conditions for NBR and for those obtained at low mold and material temperatures.The traces clearly indicate that even at low mold and material temperatures, the pressure decayat Ps1 is still apparent (from 739 to 411 bar). Thus, in terms of our initial trials it may be educedthat the external layer of material does not cure, or if it does it does not do so to an extent thatcauses substantial pressure alleviation at the material’s core.


Measurement of Overshooting. — Rheology measurements were made again at differentshear rates using a Rosand RH 7.2 assembled with a capillary of 2 mm diameter and a length of25 mm in order to ascertain whether or not our material exhibits overshooting phenomena in thepressure traces.

Figures 13 and 14 represent the test evolution for pressure recordings during the extrusioncapillary test at different shear rates. They represent the rheology tests done with NBR, withoutpassing it previously through the injection unit and after passing it. Vertical axis indicates the


FIG. 12. — Comparison of result traces obtained at different mold and material temperaturecombinations for NBR: (a) Initial tests (High temperature); (b) Tests done at low temperatures.

FIG. 13. — Extrusion capillary rheometer tests for raw NBR. The ram speed sequence appliedwas 0’5, 0’25, 0’5, 1’25, 2’5, 5, 12’5, 25, 50, 125, 250 and 500 mm/s. For each ram speed,

pressure goes to a plateau value, showing no overshooting.

pressure measurement value, whereas the horizontal one indicates “data reading”, which isequivalent to test time. First a ram speed of 0,5 mm/s is set (equivalent to a shear rate of 1,88 s-1);pressure starts to increase and it stabilizes at a certain pressure value, that is, a plateau isachieved. If instead of a plateau, a “jump” appears in this region, this means material has over-shooting. Pressure value is recorded and used for viscosity calculation. Then, the system jumpsto the next ram speed value. Overshooting was not observed in both cases.

Effect of the Content of Carbon Black .— A possible reason for the pressure decay phe-nomenon may be partially or solely due the presence of carbon black in the rubber compound.To investigate this matter an NBR mix was prepared without carbon black. Here, instead of mix-ing oil and NBR separately, an oil extended NBR masterbatch was utilized.

The reader should recall that Figure 4 shows the pertinent results using process parametersdesignated as “Combination A” for the original recipe of NBR. In contrast, Figure 15 representsthe pressure traces for a similar NBR compound, but without carbon black; clearly the pressuredecay for the Ps1 transducer is again noticeable. Therefore, we conclude that pressure decay phe-nomenon is not a consequence of the presence of carbon black in the rubber compound. Noticetoo, that pressure drops for the unfilled compound are lower than that for the initial filled com-pound, so its viscosity is lower than the original one.


FIG. 14. — Extrusion capillary rheometer tests for processed NBR. The ram speed sequence appliedwas 0’5, 0’25, 0’5, 1’25, 2’5, 5, 12’5, 25, 50, 125, 250 and 500 mm/s. For each ram speed,

pressure goes to a plateau value, showing no overshooting.

Measurement of Temperature for the Injected Material .— It is clear that each volume frac-tion increment of rubber stored in the chamber along the injection unit is subjected to a complexand varying time-temperature history just prior to injection into the mold cavity. It was thoughtthat the pressure decay reported earlier during the filling stage could be due to temperature dif-ferences between the material stored in the chamber. The portion which is injected in the lastfinal moments of the injection stage (that is, the portion stored in the upper most part of thechamber) will be subjected to a relatively extensive residence time and therefore will obviouslybe at a higher temperature as compared to the remainder of the compound. As a consequence ofthe long residence time it is reasonable to assume that the last portion of material in the cham-ber will enter the mold cavity at a much reduced viscosity. But not only the residence time couldaffect on the temperature distribution, the design and operating way of the injection unit couldalso lead to temperature differences. In the light of the preceding discussion it was thought desir-able to investigate the effects of viscosity and its possible connection in engendering the pressuredecay phenomena. As a precursor to this investigation, using NBR compound as the base rubber,shoots into air were carried out adopting the same process conditions as those used in Comb A,Table II. Slices were cut from the material that was injected into air at different positions andcompact balls were made from each slice with considerable celerity. The internal temperature ofeach ball was measured using a temperature transducer as proposed by Karam.14 Three to fourtemperature measurements were conducted for each shoot slice; measurements were made atfour positions (see Figure 16). Table VII gives a summary of the measured temperatures.


FIG. 15. — Results of hydraulic pressures, Ram speed and pressures inside the mold variation, when injecting themould with Combination “A” process parameters, in non-filled NBR.

TABLE VIITEMPERATURE MEASUREMENTS FOR DIFFERENT SLICES OF NBR INJECTION SHOOTS

Trial 1 Trial 2 Trial 3 Trial 4 AverageA - Initial part, °C 104,5 102,4 103,5 101,5 103,0B - Intermediate part, °C 109,0 111,5 108,0 109,2 109,4C - Intermediate part, °C 111,4 113,1 109,9 109,4 111,0D - End part, °C 101,1 101,5 101,2 Not measured 101,3

On analysis, the results indicate that the initial part or leading edge of the shoot is at a lowertemperature than the remainder of the shoot. Somewhat surprisingly the temperature at the endof the shoot showed a much lower temperature than expected. In passing, the present authorswish to state that the experiments could be refined by fitting a temperature transducer in the noz-zle; however, it is believed that this arrangement would not necessarily lead to greater accuracyin our measurements.14

It is certainly possible that the temperature variations discussed above will cause commen-surate changes in compound viscosity resulting in the decay of hydraulic pressure; and, the pres-sure at the Ps1 transducer. Higher temperature means lower viscosity.

Injection Trials in a FIFO System. — The results obtained thus far indicate unequivocallythat there are differences along the shoot and consequently may be associated with a variabletime-temperature history. In order to ascertain whether or not the differences to which we referare caused by the type of injection unit it was thought expedient to repeat the measurements ona FIFO injection-unit system instead of the LIFO injection unit. Figure 17 and 18 show dia-grammatic views of the injection-unit systems of the LIFO and FIFO machines respectively.


FIG. 16. — Determination of the slices of the shoot where temperature has been measured.

Tests performed on the FIFO machine were carried out using similar process conditions (seeTable VIII) to those used for “Combination A” given in Table II and III. On this occasion the cav-ity was filled to 100% under ram speed control. Figures 19 and 20 give the dose, hydraulic pres-sure, and traces for locations PS1 and Ps2 for both NBR and EPDM compounds. Evidently,recording of hydraulic pressure and dose is very poor; nevertheless, it can be clearly seen thatpressure continues to increases without any semblance of decay for the duration of the injectionsequence. The pressures at Ps1 and Ps2 also increase during the injection cycle. Notice too thata velocity trace is presented; this was constructed from the knowledge of the dose and thereforeits reliability is debatable. Moreover, it is felt that the data represented by the trace is not rec-ommended as a point of reference for performing simulations.


FIG. 17. — Scheme of a LIFO system, as used in REP machines: (a) Represents the position at whichthe material is stored; (b) Represents the position at which the chamber has been emptied.

FIG. 18. — Scheme of a FIFO system, as used in Rutil machines.

TABLE VIIITEST CONDITIONS FOR MEASUREMENTS DONE IN FIFO MACHINE

Parameter/combination NBR EPDM

Extruder temperature, °C 80 80

Chamber temperature, °C 85 85

Mold temperature up, °C 180 180

Mold temperature down, °C 180 180 (190)

Ram speed, % of the maximum of the machine a) 36 20

Shooting stroke, mm b) 34—1 34—1

Holding pressure, bar 0 0

Holding time, s 0 0

Cure time, min 2 2a) Ram speed is set as a percentage of the maximum available ram speed of the machine.b) Shooting stroke, defined in mm. A value of 34 – 1 means that a total of 34-1=33 mm of material are injected.

Considering the ram has a diameter of 90 mm, we could calculate in this way the volume of material introduced into the mold (210 cm3).


FIG. 19. — Results of hydraulic pressures, ram speed and pressures inside the mold variation,when injecting the mold with NBR, in FIFO (Rutil) machine.

We conclude that FIFO machines give the expected conventional pressure traces becausesuch machines are capable of storing the material in the chamber where a more homogeneoustemperature distribution is prevalent. This feature leads to consistent material temperature, andas consequence viscosity, on entering the mold.

INJECTION MOLDING SIMULATIONS

Figure 21 presents the experimental data for the mold-filling sequence for the LIFO machinein terms of: the evolution of the three pressure recordings; the hydraulic pressure; and, ramspeed. Similar to Figure 21, Figure 22 represents the relevant data for the FIFO machine. Asmentioned, we use the information of injection time and ram speed profile to launch the simula-tions for LIFO machine, whereas just only the injection time is used for FIFO machine. Only theresults for injection stage are represented.


FIG. 20. — Results of hydraulic pressures, ram speed and pressures inside the mold variation,when injecting the mold with EPDM, in FIFO (Rutil) machine.

Figures 23 and 24 represent simulations constructed by the Moldflow computer package.Figures 25 and 26 superimpose the computer simulation data and results of experimental trialsfor both conditions described above.


FIG. 21. — Representation of the pressure traces recorded into the mold, hydraulic pressureand ram speed profile, for injection of NBR compound under process parameters

Combination “A” in Table II, during the filling stage in LIFO machine.

FIG. 22. — Representation of the pressure traces recorded into the mold and the valuesfor hydraulic pressure, for injection of NBR compound under process parameters

defined in Table VIII, during the filling stage in FIFO (Rutil) machine.


FIG. 23. — Representation of the simulation results for NBR, injected in LIFO machine.

FIG. 24. — Representation of the simulation results for NBR, injected in FIFO machine.

In the case of the LIFO machine, despite the fact that the simulation was constructed usingthe real ram speed profile, the shapes of the experimental and simulated traces for hydraulic pres-sure show some level of incongruity. This lack of agreement is attributed to the experimentaltrace reaching the maximum value (250 bar) that can be attained by the machine; whereas the


FIG. 25. — Comparison of measured pressures and simulated ones for NBR, in LIFO machine.

FIG. 26. — Comparison of measured pressure and simulated ones for NBR, in FIFO machine.

simulation indicates that hydraulic pressure continues increasing throughout the duration of theinjection phase, reaching a maximum of 184 bar. Notice also, as expected, Moldflow softwaredoes not detect the pressure decay shown in Ps1, although the value at end of the injectionsequence is similar (903 bar in the injection trials as opposed to 851 bar in the simulation).Regarding position Ps2, similar traces were obtained, with a deviation from 435 bar (simulated)to 363 bar (experimental) at the end of injection. The aforementioned observations may be attrib-uted to:

a) Poor characterization of the material’s rheological behavior or even perhaps a salientinadequacy of the Moldflow simulation package.

b) Inaccurate description of the actual mold and material temperatures profiles: The moldtemperature used was an approximation; in hindsight it may have been expedient tomake some measurements of the cavity surface. Nevertheless, we are of opinion that themain cause for the deviation in the results is related to temperate variations of the mate-rial resident in the injection chamber leading to concomitant temperature and viscosityfluctuations of each increment of material entering the mold cavity. Such variations arenot taken into account in the Moldflow simulation.

Owing to differences observed in the LIFO machine, it was thought desirable to repeat theexperiments utilizing the NBR formulation on a FIFO machine thus providing a new set of sim-ulations. Here an estimate of the ram speed profile was used in the calculations. The Results arepresented in Figure 26 and clearly show that there is no decay of pressure in the latter stages ofinjection for location Ps1 thus suggesting that temperature variations in FIFO machines can beneglected. Note that the shapes of the pressure traces (Hydraulic, Ps1 and Ps2) are remarkablyidentical for both experimental and simulation results despite the discrepancy in absolute values.Clearly therefore there is a need to improve the characterization of real process variables (injec-tion temperature, mold temperature and ram speed) as well as material rheological behavior.

CONCLUSIONS

This paper presents and discusses pressure profiles recorded during the filling and initialstages of curing for a spiral-shape cavity mold during rubber injection-molding. Pressures atthree locations along the spiral were measured in conjunction with the evolution of machine vari-ables such as ram speed and hydraulic pressure. These measurements were made using a diverserange of different test conditions: variation of mold and material temperature, ram speed; and,filling the mold under ram speed control only or under both ram speed and holding pressure con-trol. Tests were performed two rubber compounds: peroxide-cured NBR and sulfur-curedEPDM.

On comparison of the two compounds, EPDM exhibited a lower viscosity due to lower pres-sure profiles obtained for both hydraulic and mold pressures. Decay in pressure was observed atposition Ps1 during the last stages of mold filling contrary to expectation. Moreover, in the finalstages of mold filling hydraulic pressure decreased for EPDM; whereas an increase in ram speedwas observed for NBR. These variations in pressure are mainly due to reductions in the viscosi-ty. Additional investigations were carried out to obtain a consummate understanding of the phe-nomenon. Positions PS2 and PS3 (located in the intermediate and final positions of the spiral) donot give adequate information concerning the phenomena.

The decay in pressure at Ps1 was thought to be due to the formation of an initial cured layerleading to attenuation of pressure in the bulk of the material. This belief was readily negatedbased on experiments conducted at low and high temperatures using NBR and EPDM.Rheological measurements made with NBR (both processing it previously in the injection unitand not doing it) showed that pressure before die entrance reaches a plateau without displayingovershooting. Peak pressure measurements at position Ps1 using gum NBR (NBR without car-


bon black) suggested that the pressure decaying phenomena was not associated with the presenceof carbon black.

Investigation into the affect of temperature changes upon viscosity indicated that decayinghydraulic pressure during mold filling is attributed changes in viscosity caused by temperaturevariations (up to 8 °C) of the material resident in the chamber. The temperature distribution ofmaterial in the chamber was found to be dependent upon position within the chamber. This tem-perature variation is related to machine type. A system capable of engendering a uniform tem-perature history exhibits a conventional pressure profile without any tendency to decline; abehavior that is typical of most injection-molding processes. On the other hand, systems in whichmaterial temperature gradients persist produce pressure profiles that decrease markedly at adefined period during injection.

Pressure measurements at a given location (Ps2) in the LIFO agree quite well with simulat-ed values. The Moldflow software is incapable of predicting actual hydraulic pressure and pres-sure profiles at a given defined position (Ps1) because among other reasons, the software doesnot have the utility to compute variations in material temperature during the injection phase.Nevertheless, the FIFO machines showed good agreement between actual and simulated pres-sures profiles, although absolute values differed. Consequently, further work is needed inattempting to obtain an accurate description of material rheological behavior in relation to typi-cal injection and mold temperatures.

The primary objective of the present study was to investigate and monitor cavity pressureprofiles during rubber injection-molding; and, propose possible reasons for the pressure-decay-ing phenomenon observed near the gate. An important secondary objective was concerned withassessing the capabilities of the Moldflow software to predict process behavior and thereby con-struct a methodology that can provide an accurate correlation between experimental and simu-lated results.


TABLE IXNOMENCLATURE

Abbreviation NameCAE Computer Aided EngineeringSBR Styrene-butadiene rubberNR Natural rubberEPDM Ethylene-propylene-diene rubberNBR Acrylonitrile-butadiene rubberkg Kilogram (Mass unit)LIFO Last in First Out Injection UnitFIFO First in First Out Injection Unit1D Mesh element of one single dimension (Beam ele-

ments), useful for modeling sprues, runners,DSC Differential Scanning CalorimeterPVT Pressure-Volume-TemperatureMDR Moving Die Rheometerdα/dt Rate of cureα Alpha, cure degree. Its value goes between 0 (0%

cure) and 1 (100% cure).T Temperature (Kelvin)K1, K2 (Kamals model) Fitting constants, variable with temperatureA1, E1, A2, E2 ,m, n (Kamals model) Fitting constantst—: Non-isothermal induction timeti(T) Induction time at temperature TA,B Fitting constants in the non-isothermal induction

time modelη Viscosityγ Shear rateη0 Zero Viscosity; its a fitting constant, variable with

temperatureαg Alpha gelτ*, n,B, Tb, c1,c2 Fitting constantsPs1,Ps2, Ps3 Pressure transducers at position 1,2, and 3 into the

mold

ACKNOWLEDGEMENTS

London Metropolitan University, for the supervision from Dr. A.S. Farid. HidrorubberIbérica SA Company, for using their FIFO injection machine. CTR SA Company, for using theirAlpha MDR Rheometer. Elastorsa SA Company, for preparing the rubber mixes. Lea ArtibaiIkastetxea S.Coop., AZARO Fundazioa, FEDER and Basque Country Government, for givingfounding for the development of this study.

REFERENCES

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[ Received October 2008, revised March 2009 ]


Die Preassure Losses During Injection Molding of Rubber Mixes

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