Low-pressure molding process for composite materials

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The characteristic of the low-pressure molding process for composite materials is that the reinforcing material is laid manually, impregnated with resin, or the laying of the reinforcing material and resin is assisted by simple tools. Another feature of the contact low-pressure molding process is that no molding pressure needs to be applied during the molding process (contact molding), or only a low molding pressure is required (0.01–0.7 MPa of pressure is applied after contact molding, with the maximum pressure not exceeding 2.0 MPa). In the low-pressure molding process, the material is first shaped into the desired design using a female mold, a male mold, or matching molds; it is then cured through heating or at room temperature, removed from the mold, and subjected to further processing to obtain the final product. Processes that fall into this category include hand lay-up molding, spray molding, bag molding, resin transfer molding, autoclave molding, and thermocompression molding (low-pressure molding), among others. The first two of these are contact molding. Among the low-pressure molding processes, the hand lay-up process is the first to be developed in the production of polymer-based composites, and it has the widest range of applications; all other methods are developments and improvements upon the hand lay-up process. The greatest advantage of the contact molding process is its simple equipment, wide applicability, low investment, and rapid results. According to statistics from recent years, the use of low-pressure molding processes still accounts for a large proportion in the production of composite materials across various countries around the world; for example, 35% in the United States, 25% in Western Europe, 42% in Japan, and 75% in China. This demonstrates the importance and irreplaceability of the low-pressure molding process in the industrial production of composite materials; it is a process method that will never become obsolete. However, its biggest drawbacks are low production efficiency, high labor intensity, and poor product consistency. I. Raw Materials The raw materials used in low-pressure molding include reinforcing materials, resins, and auxiliary materials. (1) Reinforcing materials – Requirements for reinforcing materials in contact molding: ① The reinforcing materials should be easily impregnated by the resin ； ②It has sufficient deformability to meet the forming requirements of complex product shapes ； ③Bubbles are easily deducted ； ④Meeting the physical and chemical property requirements necessary for the product’s intended use ； ⑤Reasonable price (as cheap as possible), with a wide supply. Reinforcement materials used for contact molding include glass fibers and their fabrics, carbon fibers and their fabrics, aramid fibers and their fabrics, etc. (2) Matrix material: Requirements of the matrix material for low-pressure molding processes: ① It should be easily permeable to fiber reinforcement materials under hand-laying conditions, allow for easy removal of bubbles, and have a strong bonding strength with the fibers ； ②It should be able to gel and cure at room temperature, with minimal shrinkage and low levels of volatiles ； ③Appropriate viscosity: generally 0.2–0.5 Pa·s, with no gelation occurring ； ④Non-toxic or low-toxicity ； ⑤Reasonable price and guaranteed source. Resins commonly used in production include unsaturated polyester resin, epoxy resin; phenolic resin and bismaleimide resin are also used, as well as polyimide resin, among others. Performance requirements of resins for several contact molding processes:
Molding method – Performance requirements for resins
Coating application: (1) Does not flow during molding, easy to defoam; (2) Uniform color without discoloration; (3) Fast curing, no wrinkles, good adhesion to the layup resin.
Hand layup molding: (1) Good impregnation properties, easy penetration into fibers, easy removal of bubbles; (2) Fast curing after laying up, low heat release, minimal shrinkage; (3) Low volatiles, non-sticky surface of the product; (4) Good interlayer adhesion.
Spray molding: (1) Meets all the requirements of hand layup molding; (2) Thixotropy is restored more quickly; (3) Temperature has little effect on resin viscosity; (4) The resin should have a long usable period, and its viscosity should not increase upon addition of accelerators.
Bag molding: (1) Good impregnation properties, easy penetration into fibers, easy removal of bubbles; (2) Fast curing, low heat release during curing; (3) Resin does not flow easily, strong interlayer adhesion.
Auxiliary materials: In contact molding processes, auxiliary materials mainly refer to fillers and pigments, while curing agents, diluents, toughening agents, etc., belong to the resin matrix system. II. Molds and Release Agents (1) Molds Molds are the primary equipment in various contact molding processes. The quality of the mold has a direct impact on the product’s quality and cost; it must be carefully designed and manufactured. When designing molds, the following requirements must be taken into account: ① Meeting the precision requirements of the product design, with accurate mold dimensions and smooth surfaces ； ②It must have sufficient strength and stiffness ； ③Easy to demold ； ④It has sufficient thermal stability ； ⑤Light weight, abundant material supply, and low cost. (2) Mold structure: Compression molding molds are divided into three types: female molds, male molds, and matching molds. Regardless of the type, they can be designed as integral molds or assembled molds depending on size and molding requirements. (3) Mold material: When selecting a mold material, the following requirements must be met: ① It should be capable of satisfying the requirements regarding the dimensional accuracy, appearance quality, and service life of the product ； ②The mold material must have sufficient strength and stiffness to ensure that the mold does not deform or get damaged during use ； ③Not affected by resin erosion, does not interfere with resin curing ； ④It has good heat resistance; the mold does not deform during the curing of the product or during heating-based curing ； ⑤Easy to manufacture, easy to demold ； ⑥Reduce the mold weight during the day to facilitate production ； ⑦It’s cheap and the materials are easily available. Materials that can be used as hand-laid molding molds include: wood, metal, gypsum, cement, low-melting-point metals, rigid foam plastics, and fiberglass-reinforced plastic. (4) Basic requirements for release agents: ① It shall not corrode the mold, nor affect the curing of the resin; its adhesion force to the resin should be less than 0.01 MPa ； ②Short film-forming time, uniform thickness, and smooth surface ； ③Safe to use, with no toxic effects ； ④Heat-resistant, capable of being cured at elevated temperatures ； ⑤Easy to use and inexpensive. The release agents used in the contact molding process mainly include film-type release agents, liquid release agents, as well as paste and wax-based release agents.