Cycle time of a part in injection molding process is very important as the rate of production and the quality of the parts produced depend on it, whereas the cycle time of a part can be reduced by reducing the cooling time which can only be achieved by the uniform temperature distribution in the molded part which helps in quick dissipation of heat. Conformal cooling channel design is the solution to the problem which basically “conforms” to the shape of cavity in the molds. This paper describes the analytical study of cooling analysis of different types of cooling channel designs. The best cooling channel design is also selected on the basis of minimum time to reach ejection temperature, uniform temperature distribution, and minimum warpage of part. “Creo Elements/Pro 5.0” is used to model the case study, its molds, and the cooling circuit whereas analytical study is done using “Autodesk Moldflow Advisor 2013 (AMFA).”
Injection molding is a major part of the plastic industry and is a huge business worldwide, consuming approximately 32 wt% of all plastic. It is in the second place to extrusion, which consumes approximately 36 wt% [
In the cooling phase, heat transfers between the molten material inside the cavity and the cooling fluid (generally water) flowing through the cooling channels inside the mold, until ejection temperature is achieved and part is stable enough for demolding. Thus this rate of heat exchange is very important and directly related to the time taken by the cooling phase. So it is important to understand and optimize the cooling channel design to optimize the rate of heat transfer in an injection molding process. Proper design of the cooling channel is required for a faster cooling phase. Historically, the cooling channels have been created by drilling several straight holes (cooling channels) inside the mold core and cavity. Such type of cooling channels is called as “Conventional Cooling Channels (CCC).” However the cooling process in CCC is too long because of nonuniform cooling of part. If the part’s temperature can be reduced more quickly and uniformly, it will shorten the cooling time without compromising on part quality because nearly uniform temperatures can be held in part by using conformal cooling [
(a) Straight drilled cooling channel. (b) Conformal cooling channel.
Research in conformal cooling system has mainly focused on simulation studies and testing of prototype conformal cooling molds using various techniques [
In this study the time to reach ejection temperature, time to reach part ejection temperature (freezing time), shrinkage, and temperature variance have been studied for four different cooling channels.
In this case study, a plastic food container has been modeled using Creo Elements/Pro 5.0 (Figure
CAD model of food container.
The appropriate location for cooling channels is inside the mold cavity or core with proper distance apart from the mold surface and between the successive cooling channels. Figure
Layout of cooling channels in injection mold.
As the proposed mold material is Tool Steel
(a) Conventional cooling channel design. (b) Series conformal cooling design. (c) Parallel conformal cooling channel. (d) Conformal cooling channel with additive cooling lines.
In conventional cooling channel (CCC) straight drilled cooling lines are used (Figure
The mold flow analysis is performed after assigning Polypropylene (PP) Purell HM671T material to the plastic part. The part weight for this material is 43.17 grams, excluding sprue and gating system. The thermal and mechanical properties of PP are shown in Table
Thermal and mechanical properties.
Number | Property | Value |
---|---|---|
1 | Density (g/cm3) | 0.9 |
2 | Melt temperature (°C) | 168 |
3 | Thermal conductivity (10−4 cal/sec cm °C) | 2.8 |
4 | Heat capacity (cal/g °C) | 0.9 |
On AMFA first of all “gate location analysis” is performed and results show that the best location is on the base of the part (Figure
Best gate location.
In the second step “molding windows” analysis is performed to evaluate the optimum material conditions required for part production. The purpose of molding windows is to improve manufacturability of the part. Molding windows of the case study for Polypropylene Purell HM671T material is shown in Figure
Molding windows.
Using these optimal conditions Fill + Pack + Warp + Cooling quality analysis is performed. Dual domain meshing is used and the number of nodes and triangles in the mesh is 9225 and 18446, respectively. Figure
Confidence of fill.
Then at the end “cool” analysis is performed for each cooling channel design. The cooling fluid is pure water with inlet temperature 25°C and at 4 lit/min volume flow rate. The coolant Reynolds number is 9401.5 which indicates that flow of water is fully turbulent because the flow in a round pipe is turbulent if the Reynolds number is greater than approximately 4000 [
The simulation results in terms of time to reach ejection temperature (time required to reach the ejection temperature, which is measured from the start of fill), time to reach part ejection temperature (time required by the part to freeze), volumetric shrinkage at ejection, and the temperature variance of the part are discussed here. These results show that conventional cooling channels take 15.63 seconds to reach ejection temperature for the case study part as shown by red color in Figure
Time to reach ejection temperature with (a) conventional cooling channel and (b) series conformal cooling channels.
However, time required to reach ejection temperature decreased to 14.63 seconds by use of conformal cooling channel connected in series. This shows that using same pitch and the cooling channel diameter conformal cooling channels connected in series provides 6.39% faster cooling as compared to the conventional cooling channels (Figure
The simulation results show that both parallel conformal cooling channel and conformal cooling channel with additive cooling lines take 14.13 seconds to reach ejection temperature. Thus these combinations are 9.5% faster as compared to the conventional cooling channels (Figure
Time to reach ejection temperature with (a) parallel conformal cooling channels and (b) conformal cooling channels with additive lines.
Similarly, when the results of time to reach part ejection temperature are studied, the CCC takes 5.664 seconds, SCC takes 5.205 seconds, PCC takes 5.113 seconds, and the CCAL takes 5.024 seconds. This shows that the CCC consumes maximum time and CCAL takes minimum time to reach part ejection temperature. The results also show that SCC, PCC, and CCAL are 8.1%, 9.7%, and 11.29% faster, respectively, as compared to the CCC. Thus CCAL is the fastest cooling channel combination as compared to others (Figure
Time to reach part ejection temperature with (a) conventional cooling channels, (b) series conformal cooling channels, (c) parallel conformal cooling channels, and (d) conformal cooling channels with additive lines.
The next parameter of comparing these cooling channels is percentage of “volumetric shrinkage at ejection.” This parameter gives information about the percentage reduction in volume of the part with respect to the original part at the time of ejection. The lesser the value of the volumetric shrinkage is, the higher the part accuracy will be. The simulations results show that CCC shows 11.39%, SCC shows 9.89%, PCC shows 8.498, and CCAL shows 8.477% shrinkage (Figure
Percentage volumetric shrinkage at ejection with (a) conventional cooling channels, (b) series conformal cooling channels, (c) parallel conformal cooling channels, and (d) conformal cooling channels with additive lines.
The results show that CCC shows maximum shrinkage whereas SCC, PCC, and CCAL show 13.16%, 25.39%, and 25.57% less shrinkage as compared to the CCC, respectively. Thus CCAL shows minimum shrinkage among other cooling channel combinations.
The last parameter of comparison in this study is “temperature variance” among different regions of the part with respect to the average ejection temperature of the part. This temperature is responsible for the warpage of the part. If the mold temperature is not equal on two mold walls, this leads to thermokinetic asymmetry of melt flow. This, in turn, causes the asymmetrical structure development in the part cross-section. As a result, different stresses in part’s cross-section occur, which result in part warpage [
Temperature variation in part with (a) conventional cooling channel, (b) series conformal cooling channels, (c) parallel conformal cooling channels, and (d) conformal cooling channels with additive lines.
Figure
Comparison of time to reach ejection temperature.
Comparison of time to part ejection temperature.
Comparison of volumetric shrinkage at part ejection.
Comparison of part temperature variance.
Table
Summary of Fill + Pack + Cool analysis results.
Number | Cooling channel | Time to reach ejection temperature |
Time to reach part ejection temperature |
Volumetric shrinkage |
Temperature variance |
---|---|---|---|---|---|
1 | CCC | 15.63 | 5.664 | 11.39 | 9.723 |
2 | SCC | 14.63 | 5.205 | 9.89 | 6.436 |
3 | PCC | 14.13 | 5.113 | 8.498 | 5.693 |
4 | CCAL | 14.13 | 5.024 | 8.477 | 5.297 |
On the basis of comparison of the results shown in Table
In this case study, four different types of cooling channel layouts are studied named as “conventional cooling channels” (CCC), “series conformal cooling channels” (SCC), “parallel conformal cooling channels” (PCC), and “conformal cooling channels with additive cooling lines” (CCAL) for cooling of a food container. Simulations for filling, packing, and cooling of molded part are performed on Autodesk Mold Flow Advisor. From simulation results it is being concluded that although the conformal cooling lines provide better cooling as compared to conventional cooling lines, yet using the additive cooling lines with conformal cooling channels provides even more uniform cooling and takes less cooling time because not only these conform to the geometry of the part but also the additive lines improve the cooling.
It is also very important to ensure uniform cooling of molded part in the injection mold (especially in case of complex parts) by proper cooling system design. It can be done with the help of CAE simulation software like Autodesk Mold Flow Advisor.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research work is a part of “Design and Fabrication of Energy Efficient Injection Molding Machine” sponsored by Pakistan Science Foundation (letter no.: PSF/Res/Invenandinnov/P-HITEC/2013). The authors of this research work are thankful to Dr. Syed Touseef Mohi-ud-Din Mr. Nizar Ullah Khan, Mr. Adeel Akhtar Shah, and Mr. Luqman Ahmad Nizam for their technical support and guidance.