Is it possible to mold a tpe over a rigid plastic structure and completely cover it in a single shot/ mold? If so, how do you register the rigid structure inside the mold?
The only way I can think of is to over-mold part of the structure, leaving some of it exposed to register it in the mold, then coming back with a second over-molding process and mold to cover the rest. If you were to do it this way, using the same color tpe, can you hide the seam between the two shots? Would the tpe fuse together along this line?
Not knowing what the finish requirements are, have you looked at dip molding or spray application?
Your seam will be dependent on the capability of your mold vendor and your design. There are techniques to hide seams and part lines with cosmetic “features”, but it tough to know what’s acceptable. What geometry are you making that requires 100% coverage of TPE? I’m wondering how you’ve managed to design your way into this corner.
If you try to do a triple shot you’d need to account for how you’re going to hold it the second time around and create the shutoffs needed between the second and third shot. Also plan for some potential damage to the part caused by clamping it into the third tool.
No, we think it would have to be 3 shots in a shuttle mold where the part is removed and put into another die. And the final product will end up with a seam.
1st shot would be the center.
2nd shot would be the first over mold
3rd shot, the part would be removed from the mold, fixtures onto the second shot and the third shot would cover the areas you couldn’t get with the second shot.
Thanks for the replies everyone. It sounds like the best option is to use the two shots to improve the aesthetics and make it two different colors.
So how about this, I watched a “How It’s Made” video this afternoon were they shot a hard casing over a rubber core to make a golf ball. They were able to fully encapsulate the rubber core with a single shot. You can see it here at min 2:00 - YouTube
It looks to me like it is some very precise machine settings (mold temp, injection force, etc…) to get the plastic to flow around the core and leave it perfectly in the center. Can someone verify this?
The spherical shape probably opens some possibilities with the flow. The ring of material and multiple injection points keep the ball centered, two pins that retract vertically could keep the ball in place top and bottom. Or, a slight variation in the flow of material directed under the ball could lift it in the molding. The cross section shows three layers, the process does not.
I visited a golf factory in Nevada and saw the rubber wound core process. For the final shell, two hemispheres of plastic were compression molded together around the rubber band wound core, then the joint trimmed as in the video.
You still have to hold the core/first shot in place with pins, but the pins can retract as the plastic flows. You will still get a mark on the surface where the pins retracted, but it may be hard to see. Also, that can occasionally lead to an uneven wall thickness (the substrate moves) but it may not matter.