What do you mean by ‘co-molding’? Do you mean compression moulding? Or the moulding of composites?


Ok, I’ve never heard of co-molding before. As far as I’m aware it should still just be referred to as compression moulding (or 'press moulding) even though it is using composites.

I’ve never had experience in compression moulding composites, but it is possible because you are essentially moulding either powdered or dough material, which is mixed before entering the mould - the moulds are heated (rather then heating en route to the gate in injection moulding) which causes the polymerisation, i.e. creates the hardened plastic/composite etc.

That’s my (limited) knowledge, unless others know different.


In the aircraft industry we used the term co-molding, but in the racing industry we just did it, without really calling it anything. In both cases, we were making honeycomb cored carbon fiber structures. The “co-molding” we were doing was putting in aluminum, steel or phenolic inserts, usually where something needed to be bolted up to the structure.

But is that not just a lay-up process (GRP)? You aren’t acutally compression moulding anything, are you?

It would probably help if you could be a little more specific – what are the two materials you’re trying to co-mold? Are they both thermosets? And when you use the term “composite” are you referring to something like carbon fiber, that’s laid-up by hand, or a filled polymer (like glass-fill nylon, for example), or simply two polymers that must mold together?

My experience with co-molding is limited to injection molding, and I have to say I’ve never heard of a two-material compression mold. My guess is that if it were possible, you’d have to take a molded base part and place it manually into a compression mold, which would probably wreck the base part. It works for things like metal inserts, but they’re quite a bit stronger.

Anyhow, more info, and let’s see what else we can come up with.

Also, here’s an older discussion thread that gives a little more specific information on the co-molding process:



Are you talking about co-molding or overmolding? What type of Thermosets are you interested in?

Overmolding is a general term that also includes insert molding; whereas co-molding is a little more specific. Overmolding just means applying one material over another.

Co-Molding usually refers to molding material onto a part that is already in a tool… as in double-shot, or triple-shot injection molding. Co-Molding usually doesn’t allow significant undercuts though.

Insert Molding usually involves molding material around a part that has been previously molded and then inserted into a new tool. This allows for completely covering the first material in the second, but presents new concerns regarding material bonding, handling, etc.

To understand the compression co-molding of thermosets, you might look at silicone… like keypads, etc. Keypads can use different pieces of silicone that may be colors or durometers. These pieces are all inserted into the same mold, like puzzle pieces, and then the mold is closed and compresses the pieces into one part.

GLS has some good info on molding: http://www.glscorporation.com/resources.php


not exactly…

co-molding is a form of overmolding. insert-molding is a form of overmolding.

in co-molding, both materials are formed in the same tool.

in insert-molding, both materials are formed in separate tools.

the difference is that insert-molding doesn’t restrict the shape of second material to the molding conditions of the first material.

There are tons of online resources about these subjects that can give you a better idea - just check out the website of some plastics manufacturers, or even the links through core77.

Good Luck