We are developing a new product. It's molded by BI-injection machine. The base material is Acetal(POM), the top material has to be something soft. We have tried Santoprene, TPR, and a lot of other soft materials. But all failed. Can any one give me some advice?

Thanks a lot.

by fail .... do you mean that it is not adhering to the acetal

Hi Kevin, thanks. Right, it's not adhering to the acetal tightly. The soft parts are tiny (4~5mm long and 2~3mm wide) scattered on the acetal part.

is there a way that you can put a light tapered hole into the acetal part so that when you overmold it, the soft material will grab on, or do you need total adhering throughout the entire surface area of the part

You mean, for the soft part, make the bottom wider and top narrower? How to eject the part?

put a tapered hole on the acetal part. When you put the acetal part in the tool to overmold the soft material the tpe will go into the hole that you created on the acetal part and the taper hole with act as an undercut to the tpe. You will not have to worry about ejection because you are not creating an undercut on the steel itself. Does this make sense?

Are the soft parts on both sides? if so, the hole could go through the acetal part and join both sides.
You could also leave a raised boss in the acetal part that the soft material can shrink around and grip tighter.

Hi Yez - you need to give us more information on the Hard part (POM). The geometry of the POM is important to obtain the right adhesion. You also need to check if the POM grade you use is suitable for overmolding (some materials contain release agents, that can make adhesion of the second material hard).

Your going to have trouble getting an amorphous material to stick to a crystalline material like POM, they are not miscible and some remelting occurs when you over shoot to form adhesion. Acetal has a high lubricity to it and is often used for moving parts. Could you pick another material and live with the difference in shrink? I'm thinking ABS. The only other choice is as suggested here by mechanically locking the elastomer to the part. The problem I see with this is non uniform adhesion, IE. Puckering, or a release of the elastomer on the product over time.

Also you may want to try an elastomer with a higher Tm / Tg than the POM substrate so that some "remelting" occurs when you shoot over the Acetal, its a long shot but they may have just enough partial miscibility to stick.

Just an after thought did you try a rough or knurled finish on the bond side of the substrate.

Thanks a lot guys for your suggestion. I will explain each of your suggestions to our engineers and see what we will try next. I will keep you informed of our progress.

Thanks again for your great help.

Have a good day!