An Old/New Process for Manufacturing Composite Parts
Resin vacuum infusion is a cost effective process for making high quality composite parts. For many applications, parts manufactured using the resin vacuum infusion process have higher quality, better consistency, higher glass content, higher specific strength, higher specific stiffness, better interior finish, faster cycle time and lower cost compared to the open mold process.
Background
Resin infusion and the many variations of the process have been around for over 50 years. In the past few years, the materials, supplies, and processes used for the resin vacuum infusion process have improved drastically thus making it commercially feasible for a wide range of products. It is a viable alternative to the open mold process for many applications.
Vacuum Infusion Process
Any composite or fiberglass part that is made using the open mold process can be vacuum infused
(refer to the Fiberglass Design Guide for more information on the open mold process). Standard fiberglass molds will work for the process if the molds have vacuum integrity (no cracks) and have enough flange width to seal the vacuum bag. The part is typically gel coated in the mold similar to the open mold process. Then the dry reinforcements (fiberglass, carbon, Kevlar etc), core (balsa, or foam), and ribs are precut and placed on to the mold dry. After the materials have been loaded, a vacuum bag or an inner mold is placed over the material and sealed against the mold. High vacuum (25 in Hg or more) is used to evacuate the air from the part and resin (polyester, vinyl ester or epoxy) is pulled into the part by the vacuum. The process is very simple in concept; however, it requires detailed planning and design so the parts can be infused in a reasonable amount of time without any dry spots. The rate of infusion depends on the viscosity of the resin, the distance the resin has to flow, the permeability of the media, and the amount of vacuum. Therefore, the choice of materials, flow media, resin flow layout, and location of vacuum ports are critical in making good parts.
Advantages of Vacuum Infused Parts:
- The quality of the process is not dependent on the skill of the operator. Once the process has been designed, part quality will be consistent.
- The laminate quality will be higher than parts made with the open mold process. There will not be any entrapped air; the laminate will be perfectly consolidated and the wall thickness will be consistent.
- Minimal part shrinkage and distortion.
- Infused laminates will have glass content by weight ranging from 35% to 50% compared with 25% to 35% for chopper gun laminated parts. The parts will have higher specific strength and specific stiffness.
- Complex parts with ribs, core, and brackets can be made all together in one step eliminating secondary bonding operations.
- The part can be made net size thus reducing post-finishing time.
- The non-molded side of the part can have a good finish if an inner mold is used.
- Faster cycle time. Cycle times of 2-3 hours are typical so a mold can be turned 3-4 times per shift.
- No air pollution from volatile organic compounds.
Disadvantages of Resin Vacuum Infusion:
- Material and supply costs are higher.
- Cosmetic finish on the gel coat is not as good as open mold process due to fabric print through; however, a barrier coat can be used to improve the finish.
- Tooling cost is higher.
- Each product will require some process development.
Ideal Applications for Resin Vacuum Infusion:
- Parts complex in shape and difficult to laminate.
- Parts requiring high quality laminates (no air voids, high glass content, perfect laminate consolidation).
- Parts that need low shrinkage and low distortion.
- Parts that have core materials.
- Parts that require multiple turns per day (can make 4-5 parts per mold per day).
- Parts that require ribs, and internal structure.
- Parts that need good finish on the non-molded side.
- Parts that need extensive post finishing.
Example of a project Performance Composites is working on:
A high quality professional speaker cabinet used in the outdoor environment was fabricated by Performance Composites using vacuum resin infusion. Compared to the open mold hand laminated part, the resin infused parts was lighter by 10%, had much better laminate consolidation and higher glass content which provided better acoustic performance. Also the infused cabinet reduced the labor time by 25%, resulting in a net cost savings of 15%. The quality of the cabinet was consistent and did not depend on the skill of the technician fabricating the part. Also the composite sandwich was void free and perfectly consolidated.
The cabinet consists of 2 outer layers of 1.5 oz fiberglass mat, ½” thick balsa core with resin flow channels, and an inner layer of 1.5 fiberglass oz mat. The outer 2 glass layers are precut and fitted in the mold. Then the balsa core is cut and fitted into the mold. The gaps between the balsa panels are filled with syntactic foam to eliminate voids. The inner layer of glass is then placed over the balsa core. Tacking adhesive is used to hold the glass and balsa in place during the material loading process. A vacuum bag is placed over the mold and the air is evacuated from the part using a vacuum pump (approximately 27 in Hg). A low viscosity polyester resin is then infused into the part, which took about 4 minutes. The part was kept under vacuum until the resin cured (about 30 minutes).
