Ask any group of composites fabricators, and it is quickly apparent that everyone has encountered a problem with sticking a part in a new mold. It is just a fact of life in the open molding business, that on occasion a new mold will stick the initial part, regardless of the fabricators perception of the thoroughness of mold preparation.
When dealing with polyester gel coat surfaced molds, understanding the dynamics of mold sticking is essential knowledge.
The scenario usually goes something like this: We used the same mold release, the same method of application, and in fact the same people did the mold preparation. The mold before this one released with no problem, however this time the part stuck on the first pull. Why?
There are four interacting factors that have a direct bearing on the possibility of sticking a part in a new mold:
- The reactivity of the tooling gel coat surface
- The preparation of the tooling surface
- The type of mold release and the method of application
- The initial application of the production gel coat on the new mold surface
To examine the mechanism by which production gel coat sticks to a new mold surface, it is necessary to understand how adhesives work. Once you know what makes things stick, the secret of making them not stick can be unlocked.
Understanding Adhesives and Anti-Adhesives: The bond strength between two materials is determined by the surface chemistry of the individual materials, or more specifically the surface energy available. This can also be referred to as surface reactivity in the case of cross linked polymeric materials. The work of adhesion is the force required to separate two particles on the surface of individual substrates.
Adhesives exhibit high surface energy and are reactive, whereas non-stick surfaces are not reactive, and exhibit low surface energy. For example, polyester resin has a rather high affinity for bonding to a cured polyester laminate, but it will not bond with Teflon or polyethylene. The cured laminate has high surface energy, while Teflon is very inert and therefore presents a low energy surface.
This brings us to a discussion on the function of mold release agents. The primary purpose of a mold release is to form a low energy barrier between the surface of the mold, (i.e. the tooling gel coat surface) and the molded part. In the formation of this inert barrier, the mold release must be capable of overcoming the inherent surface energy of the tooling gel coat.
If the reactivity of the mold surface is low enough to be overwhelmed by the non-reactive barrier of the mold release, the initial parts out of a new mold will release. However, if the tooling surface is highly reactive, the mold release may not be able to overcome the available surface energy, and sticking will occur.
More about Reactive and Non-Reactive Surfaces in my next post.
Take care and good glassing to you,
Steve
