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Elimination of Process Elimination of Process Constraints Constraints

in Plastics Injection Molding in Plastics Injection Molding Using Isothermal MoldingUsing Isothermal Molding

ThermoCeramiX, Inc. Shirley, MAThermoCeramiX, Inc. Shirley, MAandand

David Kazmer, Department of Plastics Engineering, University of Massachusetts LowellDavid Kazmer, Department of Plastics Engineering, University of Massachusetts Lowell

Theoretical Limits of Injection Theoretical Limits of Injection MoldingMolding

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IntroductionIntroduction Nearly all injection molding processes can be continuously Nearly all injection molding processes can be continuously

improved with respect to performance and/or cost. improved with respect to performance and/or cost. Continuous improvement in molding technologies are Continuous improvement in molding technologies are

providing molders with increases in productivity and providing molders with increases in productivity and reductions in materials and energy usage. reductions in materials and energy usage.

With competition, the processes are commoditized and With competition, the processes are commoditized and differentiated along a performance: cost curve in which differentiated along a performance: cost curve in which nearly all producers maintain similar profit margins nearly all producers maintain similar profit margins determined by market forces plus or minus some variation determined by market forces plus or minus some variation associated with the efficiency of their internal processes. associated with the efficiency of their internal processes.

As time progresses, however, the magnitude of potential As time progresses, however, the magnitude of potential improvements are reduced as the process performance improvements are reduced as the process performance approaches unknown but real constraints. approaches unknown but real constraints.

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CAEanalysis

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Gasassist

Co-injection

RapidProto’ing

Hotrunners

Two-shotmolding

AllElectric

DynamicFeed

Stackmolds

Scorim

SPCSQC

Auto-mation

PulsedHeating

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The maturation of several molding The maturation of several molding technologies is evidenced by the S-shaped technologies is evidenced by the S-shaped curves in Figure 1.curves in Figure 1.

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Breakthroughs in process, mold, material, Breakthroughs in process, mold, material, and/or machine designs are required to relax and/or machine designs are required to relax the existing set of process constraints, and the existing set of process constraints, and thereby enable higher levels of performance thereby enable higher levels of performance at lower costs. at lower costs.

A stream of innovation has sustained the A stream of innovation has sustained the plastics industry by providing new process plastics industry by providing new process capabilities to design and manufacture more capabilities to design and manufacture more complex products at reasonable costs. complex products at reasonable costs.

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Process DesignProcess Design The performance of conventional molding processes The performance of conventional molding processes

are governed by the physics of pressure, flow, and are governed by the physics of pressure, flow, and thermal dynamics, with significant trade-offs thermal dynamics, with significant trade-offs required in the design of the part geometry, molding required in the design of the part geometry, molding process, and polymeric materials. process, and polymeric materials.

For instance, a thin-walled product may require very For instance, a thin-walled product may require very high injection pressures and a lower viscosity resin. high injection pressures and a lower viscosity resin. High injection pressure drives the need for a high High injection pressure drives the need for a high clamp tonnage, and may also result in reduced part clamp tonnage, and may also result in reduced part properties and high scrap rates. Lower viscosity properties and high scrap rates. Lower viscosity resins may also tend to reduce the structural resins may also tend to reduce the structural properties of the thin walled, molded product. properties of the thin walled, molded product.

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For these reasons, it is desirable to consider the For these reasons, it is desirable to consider the development of new molding processes that development of new molding processes that decouple filling, packing, and cooling. decouple filling, packing, and cooling.

Specifically, it is desirable to maintain the Specifically, it is desirable to maintain the temperature of the mold surface above the glass temperature of the mold surface above the glass transition temperature of the polymer during the transition temperature of the polymer during the filling. Such isothermal mold filling would provide filling. Such isothermal mold filling would provide two benefits:two benefits:

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First, isothermal filling would prevent the cooling First, isothermal filling would prevent the cooling of the polymer melt and development of the of the polymer melt and development of the solidified layer, thereby enabling longer fill times solidified layer, thereby enabling longer fill times to be used and decreasing the injection pressure to be used and decreasing the injection pressure required to fill the mold. required to fill the mold.

Second, isothermal filling would allow for the Second, isothermal filling would allow for the equilibration of pressure throughout the cavity equilibration of pressure throughout the cavity after mold filling. The packing stage could then after mold filling. The packing stage could then proceed from a uniform state.proceed from a uniform state.

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Isothermal MoldingIsothermal Molding Isothermal filling of Isothermal filling of

mold cavities has been mold cavities has been an active area of an active area of academic and industrial academic and industrial research for some time. research for some time.

ThermoCeramiX, Inc. ThermoCeramiX, Inc. has developed has developed electroresistive heating electroresistive heating technology based on the technology based on the deposition of insulative deposition of insulative and conductive layers and conductive layers that can be deposited to that can be deposited to conform to the shape of conform to the shape of the molded part.the molded part.

Figure 4: TCX Mold Cavity Heating System

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A resistance heater deposited on the mold surface A resistance heater deposited on the mold surface provides direct control of the temperature at the provides direct control of the temperature at the polymer interface. polymer interface.

To reduce the power requirements and also To reduce the power requirements and also provide excellent abrasion resistance, the heater is provide excellent abrasion resistance, the heater is deposited between two asymmetric ceramic layers. deposited between two asymmetric ceramic layers.

The design variables include the material The design variables include the material properties and thickness of the insulative layers, properties and thickness of the insulative layers, the energy density of the deposited heater, and the energy density of the deposited heater, and the energy density of the cooling system the energy density of the cooling system determined by design of the mold and selection of determined by design of the mold and selection of the mold coolant temperature. the mold coolant temperature.

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TCX Heating AdvantagesTCX Heating Advantages Improved plastic flowImproved plastic flow

Conformal – TCX heating systems can follow the Conformal – TCX heating systems can follow the part shape for more uniform heatingpart shape for more uniform heating

Conserves energy by heating the part not the Conserves energy by heating the part not the mold mold

Reduced cycle time in some applicationsReduced cycle time in some applications

Can be used for single or multiple zone Can be used for single or multiple zone applicationsapplications

Very compact, usually less than .75 mm thickVery compact, usually less than .75 mm thick

Operates with Existing Machines and Control Operates with Existing Machines and Control SystemsSystems

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To demonstrate the capability of the developed To demonstrate the capability of the developed isothermal molding technology, a spiral mold was isothermal molding technology, a spiral mold was built. In the mold, two symmetric spiral cavities built. In the mold, two symmetric spiral cavities are provided. One cavity has a conventional are provided. One cavity has a conventional design with a wall thickness of 1 mm and a flow design with a wall thickness of 1 mm and a flow length of 1000 mm…a flow length to wall length of 1000 mm…a flow length to wall thickness beyond conventional injection molding. thickness beyond conventional injection molding. The other cavity is geometrically identical but has The other cavity is geometrically identical but has a spiral heater deposited below the mold walls. a spiral heater deposited below the mold walls. This mold design allows direct comparison This mold design allows direct comparison between conventional molding and isothermal between conventional molding and isothermal molding at the same process conditions molding at the same process conditions

DemonstrationDemonstration

3232Figure 5a: Spiral Mold Plan View

3333Figure 5b: Spiral Mold Side View

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Molding trials were conducted Molding trials were conducted on a 1988 HPM molding machine on a 1988 HPM molding machine with an 80 ton hydraulic clamp. with an 80 ton hydraulic clamp. A polypropylene resin with a A polypropylene resin with a melt flow of 20 was utilized with melt flow of 20 was utilized with a melt temperature and mold a melt temperature and mold temperature of 450F and 120F, temperature of 450F and 120F, respectively. A maximum respectively. A maximum hydraulic pressure of 30% (600 hydraulic pressure of 30% (600 psi) was specified with a 20 psi) was specified with a 20 second injection forward time. second injection forward time. This set of conditions was This set of conditions was chosen to guarantee a pressure chosen to guarantee a pressure limited flow situation with limited flow situation with sufficient time for melt flow. sufficient time for melt flow. Figure 6 provides a photograph Figure 6 provides a photograph of the two molded spirals of the two molded spirals without heat being provided. without heat being provided.

This figure demonstrates that the spiral This figure demonstrates that the spiral cavities are mostly balanced in a cold cavities are mostly balanced in a cold condition. The ceramic layers on the condition. The ceramic layers on the isothermal side do slightly reduce the isothermal side do slightly reduce the heat transfer rate as previously heat transfer rate as previously predicted, and slightly increase (14%) predicted, and slightly increase (14%) the flow to the isothermal side of the the flow to the isothermal side of the mold when they are not activated as mold when they are not activated as compared to the conventional molding.compared to the conventional molding.

Figure 6: Short Shot without heatFigure 6: Short Shot without heat

3535B side of Spiral Mold in Injection Molding MachineB side of Spiral Mold in Injection Molding Machine

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When heat is applied to provide When heat is applied to provide isothermal filling at the same isothermal filling at the same machine conditions, however, machine conditions, however, the plastic melt flows the plastic melt flows substantially farther as shown in substantially farther as shown in Figure 7. In this molding, 220V Figure 7. In this molding, 220V was provided to the deposited was provided to the deposited heaters, providing heaters, providing approximately 600W along the approximately 600W along the length of the spiral. As this length of the spiral. As this heating level, the isothermal heating level, the isothermal mold cavity was maintained at mold cavity was maintained at 512F, above the melt 512F, above the melt temperature! The flow length temperature! The flow length was no longer dominated by the was no longer dominated by the development of a solidified layer development of a solidified layer in the mold cavity and, in in the mold cavity and, in theory; any flow length could be theory; any flow length could be achieved by extending the filling achieved by extending the filling time. time.

In this experiment, however, the lack of heat provided In this experiment, however, the lack of heat provided to the sprue bushing and gate allowed the to the sprue bushing and gate allowed the development of a solidified layer in the feed system, development of a solidified layer in the feed system, which reduced the rate of flow to the isothermal cavity. which reduced the rate of flow to the isothermal cavity.

Figure 7: conventional vs. isomolded Figure 7: conventional vs. isomolded

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To investigate the relationship between heating To investigate the relationship between heating power and flow length in the conventional and power and flow length in the conventional and isothermal molding processes, a series of isothermal molding processes, a series of experiments were conducted at increasing supply experiments were conducted at increasing supply voltages. voltages.

Figure 8 provides the estimated mold surface Figure 8 provides the estimated mold surface temperature for the isothermal and conventional temperature for the isothermal and conventional mold cavity while the coolant was being mold cavity while the coolant was being convected at a constant 120F. It is observed that convected at a constant 120F. It is observed that the mold wall temperature is concave up, which the mold wall temperature is concave up, which could be expected since the heater power varies could be expected since the heater power varies with the square of the supplied voltage.with the square of the supplied voltage.

3838FigureFigure 8: Spiral Mold Temperature vs. Heater 8: Spiral Mold Temperature vs. Heater

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For each molding cycle, the same injection pressure For each molding cycle, the same injection pressure and flow rate were utilized. and flow rate were utilized.

As can be observed from Figure 9, the increasing As can be observed from Figure 9, the increasing heater power and mold wall temperature enable heater power and mold wall temperature enable greatly improved flow conditions – a 63% increase in greatly improved flow conditions – a 63% increase in flow compared to conventional injection molding. flow compared to conventional injection molding.

With isothermal molding, the advancing plastic melt With isothermal molding, the advancing plastic melt will never solidify in the cavity, and the flow length will never solidify in the cavity, and the flow length should continue to increase with increasing injection should continue to increase with increasing injection forward times. forward times.

The results would have been further improved if larger The results would have been further improved if larger and/or heated sprue bushings and gates were also and/or heated sprue bushings and gates were also utilized. utilized.

4040Figure 9: Spiral Flow Length vs. Heater VoltageFigure 9: Spiral Flow Length vs. Heater Voltage

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Cost SavingsCost Savings

The areas of cost savings are: The areas of cost savings are:

Reduction in wall thicknessReduction in wall thickness

Reduction in cycle timeReduction in cycle time

Reduction in clamp tonnageReduction in clamp tonnage

Reduction in associated hourly ratesReduction in associated hourly rates

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For discussion purposes, For discussion purposes, consider the top cover of a consider the top cover of a laptop or rear housing of an laptop or rear housing of an LCD display shown in Figure LCD display shown in Figure 10. This part is approximately 10. This part is approximately 300 mm by 200 mm, center 300 mm by 200 mm, center gated with a wall thickness of gated with a wall thickness of 1.8 mm, which corresponds 1.8 mm, which corresponds to a flow length: wall to a flow length: wall thickness ratio of 100:1. thickness ratio of 100:1. Molded of a high flow ABS/PC Molded of a high flow ABS/PC blend with an apparent blend with an apparent viscosity of 300 PaSec, and a viscosity of 300 PaSec, and a 1 sec injection time, this part 1 sec injection time, this part requires an injection pressure requires an injection pressure of 166 MPa (24,200 psi) of 166 MPa (24,200 psi)

Figure 10: Example ApplicationFigure 10: Example Application

and clamp tonnage of 560 and clamp tonnage of 560 mTons. The melt and mTons. The melt and mold coolant temperature mold coolant temperature are 280C and 90C, are 280C and 90C, respectively with a cycle respectively with a cycle time of 13.96 seconds. time of 13.96 seconds.

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The marginal cost of the molded product is driven The marginal cost of the molded product is driven by material and processing costs. Given a by material and processing costs. Given a material cost of $2/kg, the material costs would material cost of $2/kg, the material costs would be approximately $0.216 per part. Given an be approximately $0.216 per part. Given an hourly rate of $95/hour for a 560 mTon machine, hourly rate of $95/hour for a 560 mTon machine, the processing cost per part is approximately the processing cost per part is approximately $0.369. $0.369.

Consider a reduction in wall thickness from 1.8 to Consider a reduction in wall thickness from 1.8 to 1.4 mm, which would result in a 22% material 1.4 mm, which would result in a 22% material savings. Such a reduction would normally be savings. Such a reduction would normally be impossible per conventional injection molding impossible per conventional injection molding without adding gates or other major process without adding gates or other major process changes. changes.

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With isothermal molding, an injection time of 4 With isothermal molding, an injection time of 4 seconds is chosen, which allows a significant seconds is chosen, which allows a significant decrease in the injection pressure from 167 MPa decrease in the injection pressure from 167 MPa to 69 MPa (ref. eq. (2) and the isothermal curve of to 69 MPa (ref. eq. (2) and the isothermal curve of Figure 2). There is a net reduction in required Figure 2). There is a net reduction in required clamp tonnage from 560 to 232 mTons. clamp tonnage from 560 to 232 mTons.

Even with the extended injection time, the net Even with the extended injection time, the net cycle time is reduced due to the significantly cycle time is reduced due to the significantly reduced cooling time associated with the reduced cooling time associated with the reduction in wall thickness. As such, the reduction in wall thickness. As such, the processing cost is reduced by 52% due to the use processing cost is reduced by 52% due to the use of a less expensive molding machine with of a less expensive molding machine with increased production rates. increased production rates.

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Isothermal molding does, however, require the Isothermal molding does, however, require the additional costs of adding and removing heat each additional costs of adding and removing heat each cycle. For this application, a 4.65kW heater is utilized, cycle. For this application, a 4.65kW heater is utilized, being pulsed for 4 seconds each cycle. Given an energy being pulsed for 4 seconds each cycle. Given an energy cost of $0.12/KwHr and cooling system efficiency of cost of $0.12/KwHr and cooling system efficiency of 25%, the additional costs associated with adding and 25%, the additional costs associated with adding and removing heat corresponds to $0.003 per part.removing heat corresponds to $0.003 per part.

The cost analysis is provided in Table 1. It is impressive The cost analysis is provided in Table 1. It is impressive to note the potential cost savings (40%) that are to note the potential cost savings (40%) that are possible by eliminating the process constraints in possible by eliminating the process constraints in plastics injection molding. Again, this discussion has plastics injection molding. Again, this discussion has not considered the additional benefits that may be not considered the additional benefits that may be associated with enabling higher levels of performance associated with enabling higher levels of performance or quality in the molded productsor quality in the molded products

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Table 1: Cost ImpactTable 1: Cost ImpactGeometry Conventional Isothermal

Width (mm) 300 300Length (mm) 200 200

Thickness (mm) 1.8 1.4

ProcessingViscosity (Pa Sec) 300 300

Melt Temperature (C) 280 280Mold Temperature (C) 90 53

Injection Time (sec) 1 4Injection Pressure (Mpa) 167 69Clamp Tonnage (mTons) 562 232

Cooling Time (sec) 12.96 7.84Cycle Time (sec) 13.96 11.84

Heater Power (kW) N/A 4.65

CostsEnergy Cost ($/KwHr) N/A $0.12

Material Cost ($/kg) $2.00 $2.00Machine Rate ($/Hour) $95.23 $54.03Heater Energy Cost ($) $0.000 $0.003

Material Cost ($) $0.216 $0.168Processing Cost ($) $0.369 $0.178

ComparisonTotal Marginal Cost ($) $0.585 $0.349

Net Difference ($) ($0.237)Net Difference (%) (40.41)

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ConclusionsConclusions Plastics injection molding is perceived by many as a Plastics injection molding is perceived by many as a

mature technology. However, many performance mature technology. However, many performance constraints in plastics injection molding still exist that constraints in plastics injection molding still exist that prevent the development and manufacture of higher prevent the development and manufacture of higher performance products at lower cost. A primary issue performance products at lower cost. A primary issue is not whether these performance constraints can be is not whether these performance constraints can be overcome, but rather which performance constraints overcome, but rather which performance constraints should be overcome. With respect to control of the should be overcome. With respect to control of the melt temperature in plastics injection molding, this melt temperature in plastics injection molding, this paper has provided analytical, experimental, and paper has provided analytical, experimental, and economic proof of feasibility. This analysis provides economic proof of feasibility. This analysis provides convincing argument that control of melt convincing argument that control of melt temperature should be overcome and beneficially temperature should be overcome and beneficially utilized in many commercial applications. utilized in many commercial applications.

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TCX heater deposition technology is being TCX heater deposition technology is being developed for many industrial applications developed for many industrial applications including the plastics industry. including the plastics industry.

The initial proof of principle molding experiments, The initial proof of principle molding experiments, while limited in scope, and the analysis above, while limited in scope, and the analysis above, demonstrate significant promise for isothermal demonstrate significant promise for isothermal molding using TCX’s heating technology. molding using TCX’s heating technology.

Further long term cycle testing, evaluation with Further long term cycle testing, evaluation with differing polymers and mold and part geometries differing polymers and mold and part geometries as well as strategic partnerships are required to as well as strategic partnerships are required to move the technology into molding practice. move the technology into molding practice.

Development projects with customers molding Development projects with customers molding decorative automotive parts, consumer optics, decorative automotive parts, consumer optics, and tires are now in progress.and tires are now in progress.