Quick link of the Troubleshooting Guide:
There is a bit you can do to help improve adhesion between PC and TPE; two materials that don't typically love each other. I am working on a fix right now on one of our products having just this problem.
- Make sure the material has been dried properly to obtain the best adhesion possible (which may not be great).
- I believe you should expect better results in a two shot process as the initial shot is still warm when it receives the TPE attaining a better bond. In an insert mold operation, the part is rarely warm and sometimes has had the opportunity to take on moisture if it has been sitting around.
We had a clear nylon vessel that exhibited a "splay like" condition that emanated from the gate region. In our case, the mold had valve gates and what was happening is that some of the melt was being left on the face a sides of the valve pin. The next shot these now solidified pieces of nylon would break loose as the incoming melt stream re-melted the prior shot but because of the viscosity differences would result in a splay like appearance to the naked eye. Only under 30X microscopic exam could you actually see the residual debris and the flaring trails behind them like comets.
Slow cooling relieves internal stresses of metals. Plastics, especially ones with more crystaline (less amorphous) structure shrinks more as they cool slowly. Slower cooling promotes more internal crystaline structure, which is more brittle and shrinks more. If you want a more dimensional stable part, when the injection mold opens, drop the part into cold water (it will have less internal stress. Think of the melted plastic having no internal stresses. Then as it cools in the mold with varying rates, stresses are formed. Plastic parts shrink more when then they cool slowly.
Slower cooling does usually relieve some of the molded in stress. Many products are annealed after injection molding just for this purpose. The other factor that affects the linier stress is the velocity. At slower velocities a greater amount of stress in the direction of flow occurs. Of course faster velocities used to achieve a consistent viscosity will tend to entrap more air. Venting is very important. There must be vents along the entire flow path, not just at the end of fill. Weak weld lines may also become more evident after annealing, shrinkage will increase.
There are many reasons for Quick Mold Change. We have customers who have multiple tools and also run them all in multiple colors. Prior to installing quick mold change equipment they would run a mold, change the color multiple times, then change the mold and run the next mold through various colors... This required huge inventories and all the problems that come with large batch sizes.
With our quick mold change systems installed on all of their presses they now run every mold prior to making a color change. Change time is less than 10 minutes. Lot sizes are much smaller. Many problems solved. The flexibility and efficiency now allow them to change scheduling on a dime when needed. And the cost per quick mold change system installed on a press is a fraction.
There are thousands reason to use hot runner system in injection molding but seems lacking of info not prefer the hot runner. As I know the hot runner can be considered as the extension of the barrel. In some cases it combines with the injection gate that it can inject to the part directly.