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Evolution of Additively Manufactured Injection Molding Inserts Investigated by ThermalSimulations

Hofstätter, Thomas; Pedersen, David B.; Tosello, Guido; Hansen, Hans N

Published in:A I P Conference Proceedings Series

Link to article, DOI:10.1063/1.5088311

Publication date:2018

Document VersionPublisher's PDF, also known as Version of record

Link back to DTU Orbit

Citation (APA):Hofstätter, T., Pedersen, D. B., Tosello, G., & Hansen, H. N. (2018). Evolution of Additively ManufacturedInjection Molding Inserts Investigated by Thermal Simulations. A I P Conference Proceedings Series, 2065,[030053]. https://doi.org/10.1063/1.5088311

AIP Conference Proceedings 2065, 030053 (2019); https://doi.org/10.1063/1.5088311 2065, 030053

© 2019 Author(s).

Evolution of additively manufacturedinjection molding inserts investigated bythermal simulationsCite as: AIP Conference Proceedings 2065, 030053 (2019); https://doi.org/10.1063/1.5088311Published Online: 06 February 2019

Thomas Hofstätter, David B. Pedersen, Guido Tosello, and Hans N. Hansen

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Evolution of Additively Manufactured Injection Molding Inserts Investigated by Thermal Simulations

Thomas Hofstättera*, David B Pedersena, Guido Toselloa, and Hans N Hansena

aTechnical University of Denmark, Department of Mechanical Engineering, Kongens Lyngby, Denmark

*thohofs@mek.dtu.dk

Abstract. Injection molding using inserts from vat polymerization, an additive manufacturing technology, has been investigated for pilot production and rapid prototyping purposes throughout the past years. A standard mold is equipped with additively manufactured inserts in a rectangular shape of (20 x 20 x 2.7) mm3 and (60 x 80 x 10) mm3 produced with vat photo polymerization. This contribution discusses the heat transportation within the inserts made from a thermoset material, brass, steel, and ceramic material. It therefore elaborates on the possibilities of injection molding as well as thethermal challenges connected with the use of polymer inserts. They are an essential part for further calibrations of the injection molding process, which suffers from reduced lifetime due to the poor thermal conductivity of polymer inserts as compared to metal inserts. Multiscale inserts combining micro features at larger inserts in the cm-range.

Keywords: Additive Manufacturing, Injection Molding, Inserts, Simulation, Fiber-Reinforcement

INTRODUCTION

Injection molding (IM) inserts made by additive manufacturing (AM) rapid prototyping (RP) such as vat photo polymerization (VP) are already state of the art as discussed in [1-4] combining advantages in product design cycle time, manufacturing complexity, environmental impacts, as well as a certain freedom of shape which comes with the AM process. IM inserts are produced from thermoset photopolymers (PhPs) and then mounted into a standardized mold, which is usually manufactured from steel in a traditional way or by novel metal AM processes allowing for a more complex cooling system.

IM inserts from AM face challenges in connection with heat transfer coefficients and therefore cooling time in the IM machine, which can result in an increase of the molding cycle time by an average of 100 % as shown in Figure 1 (right). Insert cooling is therefore of importance. It has already been shown that IM shapes manufactured by the use of metal AM improve the efficiency of cooling by the means of cooling channel geometry and therefore possible cooling liquid throughput in a para-conformal manner adapted to the use of IM inserts, but standardized for multiple insert geometries.

In order to understand the effects of thermal material properties, an evolution process as shown in Figure 1 (left) has been defined to consist of three steps: 1) small AM insert without reinforcement by fibers, composites or similar material compositions; 2) small AM inserts with composite reinforcement consisting of a classical and para-conformal cooling scheme; 3) multiscale AM inserts with composite reinforcement consisting of a classical and para-conformal cooling scheme. [5] The inserts are anticipated in a rectangular shape of (20 x 20 x 2.7) mm3 and (60 x 80 x 10) mm3. Multiscale is anticipated as a combination of an insert with outer diameters as well as cavity dimensions in the cm-range and in the VP process generated micro structures, which come without any additional manufacturing effort when using a VP process.

Reinforcement by the use of short fibers or similar composite materials plays an important role in the lifetime of IM inserts and reduces the crack propagation velocity. It is therefore an incremental part for the use of higher volume inserts in order to increase the lifetime to a reasonable value. [4]

Proceedings of PPS-34AIP Conf. Proc. 2065, 030053-1–030053-5; https://doi.org/10.1063/1.5088311

Published by AIP Publishing. 978-0-7354-1793-9/$30.00

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FIGURE 1. Development steps of IM inserts. a) and b) represent sub-steps and refinements of the development step. (left) Molding cycle with significantly higher natural cooling time as compared to metal IM inserts. (right)

METHODS

Numerical simulations were conducted using the Comsol Multiphysics® version 5.3a with modules for laminar fluid flow and heat transfer as well as the multiphysics module for non-isothermal flow. The interface between the flowing and cooling injected material was approximated by applying effusivity between the materials. Comparison materials were chosen according to commonly used insert materials presented in Table 1: PhP, brass, steel, silicon carbide (SiC).

Effusivity was considered according to the following equations approximating the interface between the liquid injected acrylonitrile butadiene styrene (ABS) material and the cavity material as listed above. = ++ =

Whereas … … ℎ … …

Table 1 Part materials, development steps and interface temperature.

Part Material Steps Ti with ABS Conventional Mold Steel 1-3b 32.2 C Insert PhP 1-3b 121.0 C

Brass 1-3b 30.0 C Steel 1-2b 32.2 C SiC 1-2b 27.4 C

Injected Material ABS 1-3b n/a Cooling Fluid Water 2a-3b n/a

Para-conformal cooling channels were simulated using water as cooling fluid. It was assumed that the cooling flow

is provided at a constant rate at the inlet of 5 l/min or 10 l/min for a high throughput (HT) cooling whereas it is assumed that the fluid flow stays stationary over the entire cycle. This assumption is reasonable as the effect of the temperature difference can be neglected as no phase changes of the liquid appear due to low temperature and high pressure.

The molding cycle was assumed according to prior experiments presented in [4] with 1.5 s injection, 7 s packing, 11.5 s mold opening and natural convection cooling. This cycle was simulated for 3 times after which a stationary cycle was reached. Boundary conditions were changed according to the physical conditions during the molding cycle (closed or open mold).

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RESULTS

Simulations and experiments show a high interface temperature between the injected polymer and the PhP insert. This fact stands in relation with the effusivity parameter as well as the weak heat transfer within the insert. The energy input into the insert is sufficient enough to heat the entire insert. Heat is later removed from the system through (1) conduction into the standardized mold, (2) free convection at the outer boundaries (3) heat transport through the cooling water.

The heat transportation through the steel is sufficient enough to equalize the temperature in between the inserts if not all inserts are made from the same material. Figure 2 shows the influence of a single insert made from PhP (upper left and upper insert) on other inserts made from inserts with higher conductivity.

FIGURE 2. Temperature of one mold side with PhP inserts in the upper left cavity and upper cavity. The right figure shows

transparent features in order to see the cooling system. The effect of para-conformal cooling on the temperature development of the IM inserts is shown in Figure 3

(step 2a) and Figure 4 (step 2b) whereas the para-conformal cooling approach will allow for a shorter cooling time as the cooling temperature is already lower at the time of ejection. This conforms with research like [6] where the authors claim to reach an improvement of cycle time due to conformal cooling by 53 %.

FIGURE 3. Cooling of the surface of the insert with standard cooling. Ejection of the molded part appeared after 8.5 s.

FIGURE 4. Cooling of the surface of the insert with para-

conformal cooling. Ejection of the molded part appeared after 8.5 s. The improved cooling allows for a shorter molding cycle as the temperature of the PhP insert reaches lower values quicker.

The development steps 3a and 3b elaborate on the fact that fiber-reinforced PhP IM inserts provide a longer lifetime. That makes them suitable for larger geometries, cavities as well as larger surface areas around the injected

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polymer. All factors result in a higher thermal energy introduced into the insert which requires either higher cycle time or effective cooling mechanisms.

An evaluation plotted in Figure 5 and Figure 6 shows the temperature difference as compared to standard cooling. It can be shown that the enhanced para-conformal cooling mechanisms allow for a reduced temperature as compared to standard cooling for both PhP and brass, which served as example material in this case. It can be shown that multiscale PhP inserts have significant thermal disadvantages which can be compensated for by HT cooling, which again was made possible due to para-conformal cooling.

It is moreover shown that an increased absolute heat capacity consequently increases the insert temperature where-as the temperature of the cooled inserts is significantly lower than the temperature for simple para-conformal cooling. The higher absolute heat capacity can therefore be compensated.

FIGURE 5. Difference to standard cooling of PhP inserts. A significant influence of the multiscale geometry

(step 3) can be seen in the cooling behavior.

FIGURE 6. Difference to standard cooling of brass

inserts. The influence of the multiscale geometry (step 3) on the cooling behavior is significantly reduced as compared to Figure 5.

Investigations on the temperature of the water cooling channels are presented in Figure 7 whereas it can be seen the velocity distribution generates more smoothly for the para-conformal cooling channels due to the optimized geometry and rounded edges. The cooling power equals a 2.9 kW cooling power for standard cooling and 5.7 kW for para-conformal cooling at the same throughput rates. The temperature variations in the round parts of the cooling channels show furthermore that it might not be necessary to cool the entire volume of the standardized mold, but rather a smaller part as natural convection on the outside removes energy from the standardized mold and therefore stabilizes temperature in the outer region.

FIGURE 7. Temperature of cooling channels for conventional cooling (left) and para-conformal cooling (right). Note the higher temperature at the front side where the inserts are located.

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ELABORATIONS FOR FUTURE WORK

Past elaborations on the mechanical stability of fiber-reinforced PhP IM inserts [4] have shown an increased lifetime due to a significantly reduced crack propagation velocity. This allows for the further development in step 3 where (1) geometrical scales are significantly larger, (2) cavities and therefore the volume of injected polymer are bigger, (3) the contact surface between the injected polymer and the insert is increased, and (4) warping due to the VP process is (a) increased in the process and (b) more significant due to the larger geometries. All factors result in higher stresses due to thermally introduced internal stresses, and stresses due to limitations of space in the standardized mold, which need to be accounted for by the manufacturing and post-processing of the insert. Experiments performed e.g. by [4] used a polishing post-processing to remove the warpage on the back of the insert, which increased the lifetime significantly.

Another strong influencing factor is given by effusivity resulting in significant disadvantages of polymer inserts as compared to metal inserts with higher conductivity. Future scientific investigations need to account for this and provide solutions to improve conductivity of the PhP insert. Suggestions made by multiple sources in literature (e.g. [7]) indicate that the choice of fiber material significantly influences the thermal conductivity of the final material. Challenges in the fiber-matrix interface as mentioned in [2] as well as fiber orientation [8, 9], which also influences the crack propagation, will need to be accounted for.

CONCLUSION

Fiber-reinforced IM inserts made from PhP provide significant advantages in terms of lifetime of the insert during the molding cycle and account for the disadvantages of the low thermal conductivity. Moreover, para-conformal cooling allows for a higher throughput of cooling water and therefore reduced cycle time of the IM process.

The combination of both soft tooling as well as metal AM need to be minded in order to improve the IM process in its heat transfer considerations when implementing an adapted cooling geometry as well as higher throughput of cooling water. Furthermore, the process provides the flexibility of digital processes and conventional injection molding.

ACKNOWLEDGEMENTS

The Technical University of Denmark, Department of Mechanical Engineering is gratefully acknowledged for funding the PhD project “Additive Manufacturing of Fiber-Reinforced Polymers”. The research presented in this paper has been conducted in the framework of the activities of the Manufacturing Academy of Denmark (MADE, http://en.made.dk/), Work Package 3 “3D Print and New Production Processes”, funded by Innovation Fund Denmark (https://innovationsfonden.dk/en).

REFERENCES

1. Kovács, József Gábor, et al. "Thermal simulations and measurements for rapid tool inserts in injection molding applications." Applied Thermal Engineering 85 (2015): 44-51.

2. Hofstätter, Thomas, et al. "State-of-the-art of fiber-reinforced polymers in additive manufacturing technologies." Journal of Reinforced Plastics and Composites 36.15 (2017): 1061-1073.

3. Hofstätter, Thomas, et al. "Applications of fiber-reinforced polymers in additive manufacturing." Procedia CIRP 66 (2017): 312-316.

4. Hofstätter, Thomas, et al.. (2016). Evolution of surface texture and cracks during injection molding of fiber-reinforced, additively-manufactured, injection molding inserts. In ASPE Summer Topical Meeting 2016. ASPE–The American Society for Precision Engineering.

5. Hofstätter, Thomas, et al. "Thermal behaviour of additively manufactured injection moulding inserts." euspen’s 18th International Conference & Exhibition. 2018.

6. Kitayama, Satoshi, et al. "Multi-objective optimization of injection molding process parameters for short cycle time and warpage reduction using conformal cooling channel." The International Journal of Advanced Manufacturing Technology88.5-8 (2017): 1735-1744.

7. Barbero, Ever J. Introduction to composite materials design. CRC press, 2017. 8. Hofstätter, Thomas, et al. "Flow Characteristics of a Thermoset Fiber Composite Photopolymer Resin in a Vat Polymerization

Additive Manufacturing Process." 34th Annual Meeting of the Polymer Processing Society (PPS34). 2018. 9. Hofstätter, Thomas, et al. "Internal Fiber Structure of a High-Performing, Additively Manufactured Injection Molding

Insert." 34th Annual Meeting of the Polymer Processing Society (PPS34). 2018.

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