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.
Bracing materials range from wood to metal tubes/squares and should be designed to provide added mold stiffness and strength. A word of caution: Keep tab thicknesses to a minimum so as not to transfer any shock (thermal or mechanical) onto the mold surface. Cracked/broken tabs are easily repaired while in production. Gel coat repairs on molds, caused from external damage, will require the molds to be pulled from production.
Composite Mold Maintenance
Mold maintenance typically consists of:
- Training employees on demolding and mold maintenance.
- Cleaning off excess material and debris, blowing off and/or wiping down dust.
- Inspecting for mold damage or defects. (Small damage areas may be clayed to maintain schedules.)
- Performing preventative maintenance. Paste wax or reapply mold release as needed.
- Recording and tracking mold use and maintenance data, along with any needed repairs.
- Covering mold for storage, preferably not outside.
Mold sealers and releases take many forms and are often neglected. Today, most FRP composite manufacturers use “semi-permanent” type mold releases. In higher volume production operations, most manufacturers have standard maintenance that includes an accurate accounting of the number of cycles that a mold has experienced, especially in “low draft angle” molds. You should discuss best practices with your mold release manufacturer, but one best practice to live by is the 80 percent rule: For instance, if production discovers that 30 parts can be pulled from a mold before sticking, then reapply mold release after the 24th pull.
Running the mold until failure or repairing every part becomes expensive and limits product quality, so it should be avoided! Having a preventive maintenance system in place and training personnel is key. Proper demolding procedures and training will significantly improve FRP mold longevity, lower repair time and ensure employee safety.
Mold Making Innovations
Controlled shrink tooling materials include older, but still popular ATH filled systems (generally limited to 18 to 25 percent glass fiber by weight). Newer unfilled systems allow a much higher glass fiber content (40 percent glass fiber by weight), and the latest, infused versions allow up to 70 percent glass or carbon fiber content, along with the inclusion of well-consolidated core materials.
Many of these higher fiber content materials are finding a home in both traditional composite markets, as well as in the prepreg, 350 F cure temp markets and the thermoforming area (where temperatures in excess of 400 F are common). Thermal shock-resistant tooling gel coats are highly suggested for these applications.
Finally, a quick discussion on additive manufactured plugs and molds. Several manufacturers – such as Cincinnati Incorporated (CI), Ingersoll Machine Tools, Thermwood Corporation and 3D Platform – now offer very large formatted additive printers for rapid plug making and limited production runs with direct-to-mold 3-D printed molds. The deposition rate is no longer grams of materials per minute, it’s pounds per minute. It is common to see a “roughed in” mold (or plug) printed in a couple of hours and converted into a gel coated, high gloss surface within one to three days, depending on size.
In conclusion, GFRP molds provide some of the least costly ways to manufacture FRP composite parts, ranging from decorative figurines to 80-plus meter wind energy molds. How that will change as 3-D and subtractive mold making evolves remains to be seen. But for now, GFRP molds provide a very attractive means to quickly manufacture high-quality, non-reinforced or reinforced composite parts.
Disclaimer: Opinions, statements and technical information within the Best Practices column are that of the authors. ACMA makes no warranty of any kind, expressed or implied, with respect to information in the column, including fitness for a particular purpose. Persons using the information within the column assume all risk and liability for any losses, damages, claims or expenses resulting from such use.
source: Composites Manufacturing