Some molds need to be textured. For example, if you’re making a mold of a medical part, like an endoscope, which a surgeon needs to hold with gloves, you need to develop a uniform texture for gripping.
Medical utensils often need to have a highly tactile capability. The grip is part of that, but something as basic as how the tool feels in your hand is important. When a surgeon is doing incredibly delicate work, they need to be able to not only hold their tools properly, but the tool should also feel comfortable in their hand.
The feel that a part has can also play an important role in the marketing of a part. Within the automotive industry, a lot of importance is placed on the feel of a vehicle’s interface, which goes back to how the surface finish was created in the mold for those parts.
Certain materials won’t release with inconsistent or flawed surface finishes - silicone, nylon, thermoplastics, and metal injection molding commonly require high-quality surface finishes for proper mold release. If you want to have a well-made, molded part, it needs to get out of the mold properly.
Some materials will have this issue intermittently if mold has a cruddy surface finish, which can cause even more headaches with not only your final parts but also with the longevity of the mold. Silicone and a lot of nylon materials, both of which are often used in the medical field, tend to have sticking issues, even if the surface finish is perfect.
In this situation, hedging your bets in favor of surface finish in your mold is a good idea. In other words, to give yourself the best chance of getting a quality part, significant care should be taken with surface finish, even if you have to sacrifice time in the process.
Surface finish can also affect the flow of your mold. As you inject a material, the material must be able to get to every crook and crevice within the mold. Surface finish can change how a material flows through a mold, possibly preventing material from reaching all parts or even (depending on the material) premature curing, leading to incomplete or inconsistent parts.
When the material is shot into the mold, more often than not, there is a certain window of time before the part begins to cure and solidify. If the material doesn’t cure evenly, the part can have unexpected weak spots or break apart when the mold releases. This is especially apparent if you’re dealing with thin channels in your mold.
Oftentimes when mold making, there will be callouts for the quality of surface finish in a specific area. If the callout isn’t directly related to a final part’s texture (or lack thereof), this is due to how the engineers and/or operators have projected the material flow. If an area has thin channels, specifically in the runners and gates of mold, there will often be a call for a high surface finish, just to be sure that material fills the mold.
A sub-par surface finish can also cause issues with splay, sometimes called “silver streaks.” This issue has the appearance of silver scratches on the finished part. Splay is caused by minuscule gas bubbles being dragged across the surface of the part when the mold cavity is filled.
When a material is heated before being injected into a mold (more often than not, some form of plastic), there is often moisture in that material. As it’s heated, this moisture turns to steam as the material is heated, and the result is tiny gas bubbles in your injection material.
As your part is curing in the mold, these gas bubbles work their way to the surface and cause splay. While this issue is a cosmetic problem, it can also weaken a molded part.
Splay can be prevented by using hygroscopic materials and checking that material is dried completely, but sometimes splay can still present a challenge. This is where quality surface finish comes into play. Vents are designed to allow the air in a mold to escape as it is filled, but with the right surface finish, those gas bubbles can be ushered to vents as well - reducing splay.