The FRP mold-making process is a molding method in which liquid epoxy resin is mixed with organic or inorganic materials to serve as the matrix material, and-using a prototype as a reference-the mold is manually fabricated layer by layer. The specific process for hand lay-up FRP mold fabrication is as follows:
Design of the Parting Line
Whether the parting line is designed rationally has a significant impact on the ease of processing operations, the fabrication of the mold itself, and the quality of the final molded part. Generally, based on the characteristics of the prototype-and provided that smooth demolding of the prototype and precise alignment between the upper and lower mold halves are ensured-the position and shape of the parting line should be kept as simple as possible. Therefore, the positions of the parting line and the gates must be selected correctly and rationally; undercuts (reverse draft angles) are strictly prohibited to prevent demolding failures. The prototype should be secured along the parting line using smooth wooden boards to facilitate the separate fabrication of the upper and lower mold halves. When applying the release agent to the prototype and the parting line, it is essential to apply it evenly and thoroughly. It must be applied in 2 to 3 coats, with each subsequent coat applied only after the previous coat of release agent has completely dried.
Application of the Gelcoat Layer
Once the release agent has completely dried, a specialized mold gelcoat should be applied using a brush in two separate coats. The application must be even, and the second coat should be applied only after the first coat has reached its initial set (gelation) stage. The gelcoat used is black, and the total thickness of the gelcoat layer should be controlled to approximately 0.6 mm. It is important to note here that the gelcoat should not be applied too thickly, as this can lead to surface cracking and wrinkling.
Preparation of the Resin Mixture
Depending on the viscosity of the room-temperature-curing resin, it may be preheated to an appropriate temperature. Next, mix 100 parts (by weight) of WSP6101-type epoxy resin with 8 to 10 parts (by weight) of acetone (or propylene oxide butyl ether) in a clean container. After stirring until uniform, add 20 to 25 parts of curing agent (the quantity of curing agent added should be adjusted appropriately based on the ambient temperature at the worksite). Stir rapidly, then subject the mixture to vacuum degassing for 1 to 3 minutes to remove any air bubbles from the resin liquid; the mixture is then ready for use.
Layer-by-Layer Fiberglass Layup
Once the gelcoat has reached its initial set-feeling soft to the touch but no longer tacky-apply the prepared epoxy resin mixture over the gelled gelcoat using a brush. Immediately lay down a layer of chopped strand mat, then use a brush to consolidate the mat layer, ensuring uniform resin distribution and expelling any trapped air bubbles. In some instances, a pointed tool may be required to puncture and release stubborn bubbles. The second layer of chopped strand mat must be applied only after the resin mixture applied to the first layer has gelled. Subsequently, the layup process may proceed in an alternating pattern of one layer of fiberglass cloth followed by one layer of chopped strand mat. After laying up every 2 to 3 layers, one must wait until the exothermic peak of the resin curing reaction has passed-specifically when the resin mixture becomes noticeably more viscous (typically after about 60 minutes at 20°C)-before proceeding with the next layer, continuing this process until the desired mold thickness is achieved. During the layup, the fiberglass cloth must be laid down smoothly and flat; the seams between adjacent pieces of cloth should be staggered relative to one another, and overlaps should be avoided as much as possible at sharp corners or edges. The quantity of resin applied to each layer must be strictly controlled: it should be sufficient to fully saturate the fibers but not excessive. High resin content makes it difficult to expel air bubbles and results in greater exothermic heat generation during curing, leading to higher shrinkage rates; conversely, low resin content increases the risk of delamination. Once the first half of the mold has cured, trim away any excess flash (overhang), clean any debris from the mold surface and the exposed surface of the original prototype, then apply a mold release agent, create the gelcoat layer, position the resin injection ports and air vents, and proceed with the layup of the second half of the mold. After the second half of the mold has cured, trim away its excess flash. To ensure the mold possesses sufficient structural integrity and to prevent deformation, appropriate support ribs, fasteners, alignment pins, and other structural components may be bonded in place to complete the mold assembly.
Demolding and Finishing
For molds fabricated at ambient temperature (approximately 20°C), a basic cure and structural set are typically achieved after 48 hours, at which point the mold is ready for demolding. During the demolding process, it is strictly forbidden to strike the mold with hard objects; instead, use compressed air-applied intermittently-to gradually separate the finished mold from the original prototype (master model). After demolding, and depending on the specific requirements for the mold's application, mechanical processing-such as drilling-may be performed on the mold. This is particularly critical in "dead zones" (hard-to-reach corners) where the material struggles to flow and fill during casting or injection molding; if no air vents were pre-planned, it is absolutely essential to drill some vents in these areas. Next, the mold undergoes post-processing: typically, the surface is progressively sanded using wet-and-dry sandpaper ranging from 400-grit to 1200-grit, followed by surface polishing using a polishing machine. Once all these procedures are complete, the mold is ready for use. Several key points require special attention: first, the master mold (pattern) must be smooth; second, the mold release agent must be applied evenly; third, no air bubbles should be present when applying the initial layer; and fourth, the sanding process must be executed meticulously. Of course, there are numerous other intricate details involved in the intermediate stages, which cannot all be listed here individually.
