When cooling a part, the mold surface temperature should be uniform on both sides of the part. When the temperatures are not uniform, the molecules on the hot side have a longer time to cool so they shrink more. This makes them shorter, so the parts will bow towards the hot side of the part as shown in Figure 6: Cooling uniformity. The left image shows the temperature distribution and the right shows the warpage.
Figure 6: Cooling uniformity
Figure 7 shows a typical “Box” type structure that many injection molded parts have. In the box structure, there is an inside corner (the core) that is normally difficult to cool and where heat tends to concentrate. On the other hand, the cavity side is easy to cool, and there is a larger volume of mold to absorb the heat from the plastic. As a result, the inside of the corner runs hot, allowing more time for the molecules to cool down and shrink therefore collapsing the corner a bit. This will pull the sides of the box towards the core.
Jay Shoemaker has held several positions within Moldflow, including Design Engineer, Customer Services Engineer, and Senior Technical Instructor and Designer, his current role. Jay's primary responsibility is to develop technical training material and deliver training classes for Moldflow Plastics Insight (MPI) products. Learn more.
