PART 2: The Importance of Mold Temperature When Processing Polycarbonate
Don’t be afraid to increase mold temperature to improve part quality when making PC parts. Take a look at a few examples here.
Several years ago, one of my clients chose to injection mold a simple 6 x 6 in. (152 x 152 mm) plaque from polycarbonate. The wall thickness of the plaque was 0.5 in. (12.7 mm), which enabled my client to machine a variety of small parts from the part. The first parts produced from the mold were clear, since the primary concern with a wall this thick was voids. While there were no voids, the part was so full of surface flow lines that you could not see through it. When we visited the molder to work on improving the part appearance, we found the mold temperature set at 75 F (24 C). When asked how I wanted to approach the problem, my first response was that I wanted to increase the mold temperature.
When asked how high I wanted to go, I replied that I was not sure yet but that I wanted to have the opportunity to go as high as 220 F (105 C). The response I received was one that I hear a lot: “The material will never set up.” The accompanying graph shows a plot of the modulus of PC as a function of temperature. There is a lot of discussion in our industry about how to determine cycle time, but I maintain that it is fundamentally about how rapidly the material being molded can achieve an ejectable modulus. This graph shows that at 220 F the modulus of PC is only about 15% lower than it is at room temperature. If you follow the red line from right to left, it is quite apparent that most of the stiffness that PC will eventually have when cooled to room temperature will have developed by the time the material has cooled to 265 F (130 C).
At the same time, I noticed that the melt temperature was 610 F (321 C). We agreed that we would lower the melt temperature to 500 F (260 C) to reduce the amount of heat we would have to remove from the part. The parts came out completely free of flow lines and with the desired transparency and no change in cycle time. What we did not know on that day was that two years later the parts that had been run in the cold mold would begin to crack while sitting on the shelf. The cracks appeared first on the part perimeter a little less than 1/8 in. (3.2 mm) from both part surfaces and grew steadily over time.
This depth gave an excellent illustration of the transition between the material close to the mold surface that cooled rapidly and the material farther into the wall that cooled much more slowly. It was a demonstration of the manner in which high levels of molded-in stress influence the long-term behavior of a material. Some of the parts molded at the higher mold temperature are almost eight years old now and have yet to show any of these defects.
Recently, two other clients faced with periodic brittle performance of polycarbonate parts solved the problem with an increase in mold temperature. In one case the problem was brittle failure at the gate when the part was subjected to a standardized flexural test. In the other case the problem manifested as stress cracking when the molded parts were exposed to a fluid used in a test designed to measure the internal stress in the part. In this instance, the problem was being remedied by annealing the parts after they were molded. But even with the annealing step, there was still some fallout from stress cracking. In both cases the failures were eliminated by increasing the mold temperature. In the case of the parts that had required annealing, this secondary operation was no longer required.
These two cases had something in common. Both processors were already using mold temperatures that fell within the range recommended by the material supplier. In the case of the part that failed in flexure at the gate, the mold temperature was specified to be 170-190 F (77-88 C) and a qualified process had been constructed around these settings. But changes in the composition of a colorant being added to the material created the performance problem, and a brief design of experiments (DOE) showed that increasing the mold temperature to 210-230 F (99-110 C) stopped the failures. In addition, the DOE showed that a small increase in melt temperature would also help with stress relief; it is likely that with an increase in melt temperature, an increase in the mold surface temperature also occurs. However, the increased melt temperature resulted in a measurable though acceptable decline in the average molecular weight of the polymer, while the increased mold temperature involved no such side effect.
In the case of the part that required annealing, the mold temperature initially was 160 F (71 C). I recommended that the mold temperature be increased to 210-230 F (99-110 C). Initially, there was concern because the nominal wall thickness of the part was 0.200 in. (5 mm). But the parts were moldable at the higher mold temperatures. In this case the processor had the option of changing the grade of material and I recommended as a follow-up that they switch from a nominal 13 melt-flow-rate PC to a nominal 6 MFR, since this also improves stress-crack resistance and decreases the tendency for sink marks to develop in thick walls.
These examples point out the value of pushing the envelope when using mold temperature to improve part quality. It is interesting that when looking at a data sheet for recommendations on mold temperatures for PC, there can be differences for the same grade of material, depending upon where the document was published.
The other day I found three different data sheets for the same grade of material. Two of these, published for Asia and the Americas, gave a recommendation of 158-203 F (70-95 C) while the data sheet published for the European market gave a range of 176-230 F (80-110 C). It seems that material suppliers, at least in some parts of the world, are reluctant to suggest mold temperatures that require equipment that goes beyond the standard water heating unit. Yet experience has repeatedly demonstrated the benefits of going beyond the standard practice. Most often, cycle time can be kept constant by reducing the melt temperature, which is commonly set higher than it needs to be. Frequently, the melt temperature is elevated because the mold temperature is too low. The best way to prevent spending extra time removing thermal energy from a material is to not put this energy into the material in the first place.
This same principle works well for a wide variety of amorphous polymers, including acrylic, ABS, and the polysulfone family. As the graph here shows, amorphous materials set up a lot faster than most people realize.
About the Author
Mike Sepe is an independent, global materials and processing consultant whose company, Michael P. Sepe, LLC, is based in Sedona, Ariz. He has more than 40 years of experience in the plastics industry and assists clients with material selection, designing for manufacturability, process optimization, troubleshooting, and failure analysis. Contact: (928) 203-0408 • firstname.lastname@example.org.
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