BACK INJECTION MOLDING WITH ADDITIVE MANUFACTURED MOLD INSERTS USING ALL-INKJET PRINTED SUBSTRATES

Thomas Mitterlehner1, Christoph Beisteiner2, Hannes Rieger1, Peter Dunzendorfer3,

Georg Steinbichler1 Johannes Kepler University Linz, Austria

1. Institute of Polymer Injection Molding and Process Automation 2. Institute of Measurement Technology

3. Institute of Polymer Product Engineering

Abstract

Back injection molding is a well-known process in

the plastics industry. For example, the fabrication of decorated interior parts in the automotive industry is one of many fields of application. In the last few years, the additive manufacturing process rapidly expanded in injection molding. For instance, it is used for producing injection mold inserts made of metal or plastic. This technique is called Rapid Tooling. The aim of this paper is to investigate the back injection molding process with additive manufactured mold inserts made of Fullcure RGD 720 and to verify if this technique is feasible. Therefore, a substrate with all-inkjet printed conductor traces was placed in the mold insert. The injection molding was done with a standard injection molding machine. The conductive traces were printed with a standard inkjet printer manufactured by Epson. To generate additional information about the additive manufacturing material Fullcure RGD 720 a differential scanning calorimetry (DSC) was done.

Introduction

Back injection molding is a common used technique

to fabricate customized plastic parts. Generally, a substrate is placed in a mold which becomes overmolded by plastic injection afterwards. For example, labelling of packaging products is a typical field of application. Moreover, it is possible to use complex substrates which for example can be equipped with conductive traces. The costs for back injection molds are mostly higher than standard injection molds. Because of this it can be advantageous to fabricate prototypes in advance to test whether or not the designed part fulfils the properties.

There are several methods to fabricate prototypes. A

common used technology is the manufacturing of a mold made of aluminum. Another possibility is to make a mold out of silicone. With the growing additive manufacturing industry, there is another method coming up. If additive manufacturing is used to build injection molds or mold inserts it is typically called Rapid Tooling. This can be

done using metal powder or plastic resin. In this paper, a plastic resin called Fullcure RGD 720 was used to manufacture mold inserts. There are some techniques in the literature which explain this method. Kovács [1] did some work on the simulation and measurement of rapid tooling mold inserts made of Fullcure RGD 720. Noble [2] applied this technique to fabricate optical components. However, there is little research on the back injection molding process using additive manufactured injection mold inserts. To the best of our knowledge there is no information about the back injection molding with additive manufactured injection mold inserts and which parameters to be used.

Because the inserts are made of plastic resin the mechanical and thermal conditions are different in comparison to steel molds. Therefore, the major challenges are the durability of the printed inserts and the adhesion between the substrate and the injected plastic. For the experimental verification, a substrate will be placed in the additive manufactured mold inserts which then becomes overmolded. This was done with two different substrates. The substrates were equipped with all-inkjet printed conductive traces and light emitting diodes to show that back injection molding with additive manufactured mold inserts is not limited to simple injection molding parts. Nevertheless, the focus is on the back injection molding process itself. Therefore, no extensive explanation of the electrical behavior will be mentioned. Detailed information about the electrical behavior of the printed traces are described in [3-4]. In the first investigations, a polyethylene terephthalate substrate was used. In the second attempt a special paper similar to photographic paper was used.

With the additive manufactured inserts, nearly 50

prototypes were fabricated. A good adhesion between the inserted substrates and the injected plastic was established. However, there was a large influence on the deflection of the injection molding parts. The printed traces were still conductive after the injection molding procedure. This method enables the possibility of fabricating back injection molding parts within a few days and thus significantly reduces the development time.

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Materials

For injection molding a high density polyethylene was used, which was manufactured by Borealis. The material has a typical processing temperature between 180 and 240°C and a solid density of 960 kg/m³. The melt flow rate was 31 g/10min (190°C, 2.16 kg). Depending on the wall thickness and the molding parameters the shrinkage is in the range of 1 to 2% according to the material supplier.

For the additive manufacturing of the mold inserts the material Fullcure RGD 720 was used. This is an acrylic-based photopolymer which is initially liquid. A thermal analysis of the material is shown in the results. As support material SUP 705 was used. Both are manufactured by Stratasys.

Two different substrates were used in the experiment. The first one was a Pelikan inkjet overhead transparency (PIOT) which is made of polyethylene terephthalate. The foil was transparent and had a thickness of 0.14 mm. The second substrate was a photographic paper manufactured by Mitsubishi. This one also had a thickness of 0.14 mm. For the printing of the conductive traces the method described in [3] was applied.

CAD Design

For the experimental evaluation, a test-geometry was

designed. Therefore, a bar-shaped geometry was chosen. The bar had a length of 120 mm, a width of 40 mm and a thickness of 3 mm. To test the flexibility of the printed circuits, three radii were designed. Furthermore, a fingerprint-like texture was drawn. The aim of this was to show the molding behavior of additive manufactured mold inserts with fine textures. All drawings were made with PTC Creo Parametric 3.0, using the injection molding extension. The geometrical drawing of the bar-shaped test-geometry is shown in Figure 1.

Figure 1. Geometrical drawing of the bar-shaped test geometry, without substrate

After designing the injection molding part, the inserts was then construed from these drawings. Both halves had the dimensions 190 x 116 mm. The fixed half had a thickness of 22 mm and the moveable had a thickness of 17 mm. In Figure 2 a 3D view of the designed inserts and the injection molding part are presented.

Figure 2. Additive manufactured injection molding inserts (fixed half at the left and movable half in the middle) and the injection molding part (right).

The substrate only becomes overmolded partially, thus it reaches out of the cavity and the inserts. Therefore, an outlet at the end of the bar was designed. With this in mind, the conductive traces can easily be connected i.e. with a circuit board afterwards. The outlet had a width of 20 mm, a length of 10 mm and a depth of 1.5 mm. There were no additional designs for the tightening and no considerations relating to the substrate thickness.

Insert Manufacturing

The manufacturing of the inserts was done with an

Objet Connex 350 machine and the printing material Fullcure RGD 720. During the manufacturing process the resin gets jetted onto a building platform. This is managed by a printing head which is equipped with an ultra violet light tube. Due to the radiation of the Ultraviolet (UV) light the resin gets cross linked and hardens. The drawings of the inserts were transferred to the printer as a step file. Glossy mode was activated for the printing. This avoids printing support material on the last view layers, thus a smoother and more plane surface can be achieved. The printing of the inserts took 8 hours and 30 minutes.

After the additive manufacturing process the inserts

needed to be post processed, including the removal of the support material and the drilling of the ejector holes. After this steps the inserts were ready to use. To mount the inserts inside the injection molding machine a metal carrier mold was used. The fixation of the additive manufactured inserts was done with lateral holes by using screws. This ensures that the inserts stay in place. To center the inserts a conical sprue bush was used on the fixed half. The moveable half was centered by the ejector pin. The dimensions of the carrier mold were 245 x 195 mm. The total mold height was 201 mm. A sectional view of the total assembly is shown in Figure 3.

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Figure 3. Additive manufactured inserts (plastic) mounted in the carrier mold (steel), (a) clamping plate moveable half, (b) ejector spacer, (c) ejector pin and mount, (d) retainer plate moveable half, (e) additive manufactured insert moveable half, (f) additive manufactured insert fixed half, (g) retainer plate fixed half, (h) sprue bush, (i) clamping plate fixed half, (j) lateral holes for fixing the additive manufactured inserts

Substrates

Two different substrates were used in the

experiments. Both were equipped with a thermocouple, three surface mounted light emitting diodes, a strain gauge, a resistance for temperature compensation and a flat ribbon connector. The method described in [3] was used to print the conductive traces. For the printing of the thermocouples a second material was used to generate a thermoelectric voltage in response to a temperature difference. An inkjet printer manufactured by Epson was used to print conductive traces. As conductive material, a silver nanoparticle-based (Nag) dispersion from Mitsubishi was used. This material had conductivities up to 5 ⋅ 106 S/m [4] and was manufactured by Mitsubishi. That is to say, the standard ink was replaced by a conductive dispersion. The printing substrates had the format A4 (297 x 210 mm).

After the printing procedure, the cutting was managed

by a cutting plotter. Afterwards the light emitting diodes were soldered onto the intended locations. The total length of the substrate was 230 mm, the width 19 mm. The width at the top of the substrate, were the thermocouple is placed, was 5 mm. The ready-to-use substrates are shown in Figure 4.

Figure 4. Ready-to-use substrates equipped with a thermocouple, three surface mounted light emitting diodes, a strain gauge, a resistance for temperature compensation and a flat ribbon connector, photographic paper (top) Pelikan inkjet overhead transparency (bottom)

Injection molding

At the beginning of the injection molding cycle the

substrate was placed in the cavity of the movable half. To fix the foil in the insert a double sided adhesive tape was used. Therefore, only one fixing point was necessary, which was located near the gate. The tape was placed on the non-printed side and melt was back injected on the printed side. The mold closed with a clamping force of 100 kN. The screw diameter was 30 mm. The metering stroke was 22 cm³ and the injection speed 10 cm³/s. The plastic was metered with a screw speed of 0.25 m/s. The temperature of the melt was 200°C. The injection pressure limit was 120·105 Pa. The switch over type was set to pressure-dependent and had a value of 90·105 Pa. Moreover, the switch over was limited to the last 13 cm³ of the metering stroke. The holding pressure was 70·105 Pa and acted 5 seconds. The cooling time was set to 120 seconds.

Geometrical measurement

For the geometrical measurement five parts for each

substrate type and five parts without a substrate were made with the same process parameters. The fabricated parts were stored at a constant temperature of 23°C for one week and were than measured. For the measurement two points where defined. The first point was located near the gate and the second on away from the gate. Figure 5 shows the defined measurement points. At these points both the thickness and the width were measured. Moreover, the length and the deflection were measured.

Figure 5. Defined points of measurement

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For the measurement of the deflection the bar was placed on a plain surface. Afterwards the highest point of the deflection was measured with a dial gauge. For every measurement, a minimum of five test specimens were used.

Results and Discussion

The three key results of this empirical study are:

1. It is possible to fabricate back injection molding parts with additive manufactured inserts made of plastic 2. There is a large influence of the substrates on the deflection of the parts 3. The glass transition temperature of RGD 720 is at 55°C

Figure 6 shows the fabricated back injection molding

part. During the investigations 50 prototypes were fabricated within two days and the inserts are still functioning. Because of the poor heat conductivity, the cycle time was in the range of five minutes. There was an additional cooling time of about three minutes. This was necessary to cool down the inserts until the mold insert temperature reaches the glass transition temperature. Otherwise it is more likely to damage the inserts. If it is required, the cycle time can be decreased by applying air cooling.

Figure 6. Moveable half of the injection mold with a fabricated back injection molding part

Another important aspect was the clamping force. Because of the elastic behavior of the additive manufactured inserts there is a high potential for air traps. In other words, if the clamping force is too high the inserts hermetically seal. Because of that, the venting of the cavity is inhibited. Thus air is trapped in the molded part. In our experiments first air traps occurred at a clamping force of 180 kN. There are two possibilities to solve this issue. The first one is to reduce the clamping force and the second one is to place ejector pins at the critical locations.

The geometrical measurement reveals that the injection molding parts have an average thickness of 2 mm. In comparison to the nominal value of the designed part there is a shrinkage of 33% in thickness. The detailed results of the measurement are shown in Table 1, where firstly the nominal value of the designed part, secondly the dimensions of the injection molding part without a substrate, thirdly the dimensions of the injection molding part with the photographic paper and fourthly the dimensions of the injection molding part with the overhead transparency foil are provided. The measured properties of each measurement series were consistent without any change.

Table 1. Geometrical measurement of the thickness at two different measurement points

Thickness

Point 1 [mm] Point 2 [mm]

nominal value 3 3 no substrate 2.15 ± 0.14 1.95 ± 0.20

paper substrate 2.01 ± 0.04 1.87 ± 0.06 foil substrate 2.04 ± 0.08 1.90 ± 0.07

The values provided in Table 1 might suggest, that

the injection molding parts have a greater thickness at measurement point 1 compared to measurement point 2 but it has to be considered that all values are within the range of fluctuation. Moreover there is no change in thickness whether or not there is a substrate in use. The reason for this is that the substrate gets overmolded and thus is integrated in the injection molding part. Therefore the thickness of the injection molding part is independent of the substrate.

Table 2 shows the measurement of the width. The

designed part geometry had a constant width of 20 mm. The fabricated injection molding parts had an average width of 19 mm resulting in an average shrinkage of 5 %. Table 2. Geometrical measurement of the width at two different measurement points

Width

Point 1 [mm] Point 2 [mm]

nominal value 20 20 no substrate 18.99 ± 0.11 18.92 ± 0.18

paper substrate 19.25 ± 0.06 19.05 ± 0.08 foil substrate 19.13 ± 0.03 19.04 ± 0.08

The measurement of the length and the deflection

reveals that there is an average shrinkage of 2.5 % in length and a maximum deflection of 13.60 mm. The deflection of the injection molding parts only appears when substrates are in use. The characteristic deflection is represented in Figure 7. The detailed results of the

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geometrical measurement of the length and the deflection are provided in Table 3.

Figure 7. Fabricated back injection molding part with an overhead transparency foil showing the characteristic deflection Table 3. Geometrical measurement of the length (left) and the deflection (right)

Length and deflection [mm]

nominal value 120 0 no substrate 117.05 ± 0.11 0

paper substrate 117.43 ± 0.13 13.18 ± 0.97 foil substrate 117.33 ± 0.12 13.59 ± 1.82 The shrinkage of polyethylene MG9601 is normally

in range of 1 to 2 %. All things considered the measured shrinkages are all above the values provided from the manufacturer Borealis. Moreover, there is large deflection when substrates are in use. The reason for this might be the different thermal expansion coefficients of polyethylene and polyethylene terephthalate. This issue may also appear at conventional molds, but in this case, it can be easier handled because of the possibility to vary the mold temperatures.

The specific heat of Fullcure RGD 720 is shown in Figure 8. It was found that the glass transition temperature (Tg) is at 55°C. With this in mind the processing temperature of the injected plastic (200°C) is higher than the measured glass transition temperature. After 50 injection molding cycles the cavities of the inserts were measured. There was no change in height, depth or width, so the molding conditions can be assumed as consistent. This also suggests that molding inserts made of Fullcure RGD 720 are able to withstand abrupt temperature exposures up to 200°C or even higher. To avoid damages on the inserts, they must be cooled down under 55°C before the next injection molding cycle is started. Depending on the temperature the specific heat varies between 1165 and 2060 J/kgK. Furthermore, there is almost a linear increase of the specific heat under and above the glass transition temperature. This correlates with the data measured by Kovács [1].

Figure 8. Specific heat shown as a function of temperature, measured with differential scanning calorimetry (DSC)

Figure 9 shows the measured heat flux in the range from 30 to 450°C. In this graph the glass transition temperature also occurs at 55°C. The lowest peak indicates the degradation temperature which is located at 311°C. This means that under no circumstances the melt temperature of the injected plastic should rise above 311°C, otherwise there is a high risk to damage the inserts.

Figure 9. Heat flow shown as a function of temperature, measured with differential scanning calorimetry (DSC)

Conclusion

We have described a method to fabricate back injection molding parts with an additive manufactured mold inserts. Therefore, we used two different substrates with all-inkjet printed conductive traces. For the printing of the inserts we used an Objet Connex 350 machine. For the printing of the conductive traces we used a standard inkjet printer manufactured by Epson.

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The injection molding was done with a standard injection molding machine, which had a maximum clamping force of 500 kN. All things considered about 50 back injection molding parts were fabricated with this method.

We found there is a big deflection through the use of

the substrates. Moreover, the geometrical measurements of the fabricated parts revealed large shrinkages. The thermal analysis of the mold printing material indicates that the glass transition temperature is located at 55°C. Above all we show the possibility to fabricate back injection molding parts within a few days.

References 1. J. G. Kovács, F. Szabó, N. K. Kovács, A. Suplicz, B.

Zink, T. Tábi, H. Hargitai, Thermal simulations and measurements for rapid tool inserts in injection molding applications, Applied Thermal Engineering, Papers, 85, 44-51 (2015)

2. Noble, J., Walczak, K., Dornfeld, D. Rapid tooling injection molded prototypes: a case study in artificial photosynthesis technology, Procedia CIRP, 14, 251-256 (2014)

3. Beisteiner C., Gstöttenmayr R., Altmanninger T., Zagar B., Charakterisierung von mechanischen und elektrischen Eigenschaften funktionaler Inkjet-gedruckter Strukturen, tm - Technisches Messen, 83, 11, 647-659, (2016)

4. Beisteiner C., Mitterlehner T., Thurner T., Zagar B., An All-Inkjet Printed Strain Gauge and ThermocoupleEmbedded in Injection Molded Plastics, in: Association for Sensor Technology (Eds.): Proceedings Sensor 2017, AMA Service GmbH, (2017) [Submitted]

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