Bonding TPE to PC

[Jason S.]

I have a small two shot part where we are overmolding TPE over a PC core and the part requires a liquid tight seal. In the prototype phase we didn't have any issues with it, but now we have built production tooling and the parts are dimensionally equal but the surface finish on the parts from the new tool is a bit rougher. I'm not super experienced in overmolding so I am just trying to find the obvious and this is all I can see that has changed from the prototype tooling. Question is, could it be possible that I just need to get the production tooling polished to fix the sealing issues?

[Suhas]

Jason,
There could be other issues also other than the surface finish of the parts. Make sure the molding process is the same - match the actuals from one machine to another. Another factor is venting. I worked on one project where the customer had issues with binding in area. A short shot revealed that that was closer to the end of fill. Venting the mold correctly, helped the binding.
Suhas

[Shearstressin me out]

I have a small two shot part where we are overmolding TPE over a PC core and the part requires a liquid tight seal. In the prototype phase we didn't have any issues with it, but now we have built production tooling and the parts are dimensionally equal but the surface finish on the parts from the new tool is a bit rougher. I'm not super experienced in overmolding so I am just trying to find the obvious and this is all I can see that has changed from the prototype tooling. Question is, could it be possible that I just need to get the production tooling polished to fix the sealing issues?

How were the prototype parts made?

Through 3D-printing or through prototype injection molding tooling?

[iautry1973]

They didn't use mold release on the PC cores did they?

[rickbatey]

Are you over-molding in the same press or another press? The time between first component molding and the soft resin injection is CRITICAL. Too long poor bond; too soon you warp or damage the substrate. You know that differential shrinkage could also be an issue between the resins. What's the mold water temperature? Critical as well. Maybe you should give total process data for better root causes.
Rick.

[Jason S.]

Suhas, The process is exactly the same right down to the machine. Nothing has changed other than the tool that makes the core part. Shearstressin the prototype parts were molded on the exact same machine. Rick you may be on to something the two parts are run in the same machine but in deferent molds. There could be weeks or even months in-between when we run the core part and when we overmold. The water temp is consistent with the original prototype parts. I haven't changed any process parameters from the prototype tool.

[Jason S.]

It turns out the material supplier for the TPE suggests as high of a polish as possible on the core material. The theory is that the material doesn't always flow into the small pockets left from the EDM finish. This apparently reduces the bond strength due to a lack of surface contact.

[rickbatey]

Not sure how soft the TPE is but be careful not to polish that mold too much as the soft resins love to stick to polished surfaces. Seems they create a vacuum to the slick steel surface. I'd leave a small rib for the TPE to melt to increase the bond strength of the resins. BTW how hot is the PC mold water? Too cold and the PC won't pick up the mold steel finish.
Rick.

[chrisprocess]

Not sure how soft the TPE is but be careful not to polish that mold too much as the soft resins love to stick to polished surfaces. Seems they create a vacuum to the slick steel surface.

Second this. Had an experience with A2 surface finish where parts created a low pressure "vacuum" effect compressing the parts and overall dimensions.

We solved it by using a B1 draw polish in the same direction as ejection. A draw polish might also help with bonding too!

[Jason S.]

Rick, we will be polishing the tool that makes the core so the TPE will not come in contact with the polished tool steel at all. However it will come into contact with the PC core (first shot).
Not sure how soft the TPE is but be careful not to polish that mold too much as the soft resins love to stick to polished surfaces. Seems they create a vacuum to the slick steel surface. I'd leave a small rib for the TPE to melt to increase the bond strength of the resins. BTW how hot is the PC mold water? Too cold and the PC won't pick up the mold steel finish.
Rick.