T-Minus 30 Minutes and Casting
“It ain’t rocket science” as they say, however, the resin-casting/liquid molding process does require “the right stuff” in order to ensure positive results. Whether you’re producing one or two prototypes, or manufacturing a low-volume production run of a few hundred, your goal should always be to cast perfect parts and eliminate rejects.
One way to achieve this is to document the equipment, supplies and procedures necessary for each project ahead of time; prior to hand-mixing and pouring or meter-mix dispensing your material. A pre-casting checklist will help eliminate the need to scramble at the last minute, while your resin gels, in order to incorporate something into the process that you overlooked. Every project is different; however, there are some basic essentials required for just about every application.
Many materials have a different mix ratio by weight and by volume, and one of the key factors to ensure success is ratio accuracy, especially when hand-mixing versus meter-mixing. In this case, an accurate gram scale is an essential tool. Try to avoid using measuring cups or beakers to portion out your components by volume. The volume ratio should only be considered when using accurate meter-mix dispensing equipment, and even with this type of equipment, ratio checks should always be calculated by weight.
In most cases, you will be pouring and mixing materials in containers other than those they were packaged in. Plastic and/or metal buckets and mixers are recommended over paper or wood products since these can retain moisture that will contaminate your mix.
It is good practice to calculate your formula and document it prior to actually weighing out the components, and be sure to tare your scale before pouring off the material (this involves “zeroing-out” the scale with the empty mixing container on it). You should try to achieve ratio accuracy within ±2% of the manufacturer’s specification.
Be sure to wear a protective smock or apron, eye wear and gloves, not only for safety, but to avoid the difficulties of cleaning resin and pigments off of skin and clothing. Also, make sure to keep plenty of lint-free towels at your disposal to clean off any excess or spilled material from your containers and scale.
If you are using vacuum degassing or pressure casting equipment, there are some general guidelines to follow. In order to effectively degas liquid materials, your vacuum chamber must be capable of pulling a minimum of 29.6 inHg. If curing your materials inside a pressure chamber is required, 60-80psi is recommended in order to compress air bubbles down to microscopic size. Periodic maintenance of pumps and seals is critical to keeping this equipment in good working order, and a “preventative maintenance log” should be formally documented and kept on file. Thermoset resins can be quite costly and it would be a shame to go through all the previous steps just to find you have a leak in your tank.
Here is a basic checklist to keep on hand before embarking on your next casting project:
- Temperature and humidity in your work environment is controlled and kept constant throughout the process; 20-25°C (68-77°F) with relative humidity of 43-47%.
- All equipment and tools are clean, calibrated and fully operational.
- Liquid materials, molds, inserts and encapsulates have been properly stored, pre-heated and coated with mold release if necessary.
- Plenty of containers, mixers, and towels are within easy access.
- Casting work benches are level and free of extraneous materials.
- Work instructions and formulations are close at hand for quick review and verification if needed (always double-check your formula/mix ratio).
- Clock or timer is within easy viewing distance. A general rule-of thumb is that your material should be mixed, degassed and poured within ½ – ¾ of the material’s gel time.
- Trays, flat-plates and carts are readily available for easy transport of poured molds around your work environment or into vacuum and pressure vessels.
- Protective eye wear, latex gloves are on and any other safety precautions in place.
- Good Luck and Have Fun!
