In the November/December 2017 issue of Composites Manufacturing, we focused on choices and considerations in building glass fiber reinforced molds. In this follow-up column, we will discuss actual mold building, best practices for mold maintenance, and materials and technological innovations.
Composite Mold Making
GFRP molds generally are the least expensive option and normally the simplest to construct. They often can be built in days rather than months required for alternatives, such as machined metal molds. Further, GFRP molds can easily be modified or simply rebuilt if design or process changes are required.
A traditional GFRP mold will likely be made in the following manner:
- Tooling gel coat (18-22 mils thick)
- Vinyl ester barrier coat (18-22 mils thick) (optional)
- Skin coat (0.045-0.090 mils thick) (optional)
- Bulking materials (0.200” + thick)
- Core materials ( optional, but if used for stiffness, minimum 3/8”)
- Bulking materials (0.200”+ thick)
- Bracing materials (amount and thickness depends on size and frame stiffness required)
Gel coat should always be fresh material and designed for mold making. Look for ones marketed as “tooling gel coat.” These will be harder and more temperature-resistant than conventional production gel coats (i.e. marine, sanitary or sanding). Tooling gel coats are also designed to be more corrosion- and abrasion-resistant and able to hold a gloss far better than conventional gel coats. Previously, a second layer of tooling gel coat was used if heavy sanding/polishing of the mold was expected. More recently, especially as shrink-controlled bulking materials have evolved, less sanding has become the norm. So the second gel coat layer has evolved into using a vinyl ester barrier coat to “push” the glass fibers further away from the mold surface, while providing a layer of toughness over the more brittle second layer of tooling gel coat.
Skin coats are often applied to pick up complex mold details, especially where glass fiber bridging can occur in tight radii. These skin coats should be made with hybrid vinyl ester resins, specifically designed for thin skin cures. Any air voids under or within the skin coat must be ground out and repaired before proceeding.
Bulking layers have evolved over the last 25 years. The method employed by most companies today involves shrink-controlled bulking technologies, where thicker layers can be used per application (0.120” to 0.200”) multiple times per day. Mold builds of one to three days are common with fast cure, shrink-controlled technologies. Core materials, such as balsa or synthetic materials, can be included in a mold build where lighter weight or stiffness is required.