Compression molding is one of the oldest and most dynamic methods of composite material production. It involves placing a certain amount of premix or prepreg into a metal mold and curing it under heat and pressure.

 

Main advantages of compression molding:

①High production efficiency, easy to achieve specialized and automated production;

②High product dimensional accuracy and good repeatability;

③Smooth surface, no secondary modification required;

④Can produce complex structures in one molding;

⑤Relatively low price due to mass production.
The disadvantages of compression molding lie in the complex mold manufacturing and large investment, coupled with the limitations of the press, making it most suitable for mass production of small and medium-sized composite products. With the continuous improvement and development of metal processing technology, press manufacturing level, and synthetic resin process performance, the tonnage and table size of the press are continuously increasing, and the molding temperature and pressure of the compression molding materials are also relatively reduced, enabling the size of compression molded products to gradually develop towards large-scale production. Currently, large automotive parts, bathtubs, and integrated bathroom components can be produced.
Compression molding processes can be classified into the following types according to the physical state of the reinforcing material and the type of molding material:

①Fiber material compression molding

A method of forming composite material products by placing premixed or prepreg fibrous molding materials into a metal mold under a certain temperature and pressure. This method is simple, easy to operate, and widely used. According to the specific operational differences, there are premix molding and prepreg molding methods.

②Shredded fabric compression molding

Cut the scraps of glass fiber cloth or other fabrics (such as linen, organic fiber cloth, asbestos cloth, or cotton cloth) that have been impregnated with resin solution into small pieces, and then form composite material products in a metal mold under heating and pressure.

③Fabric compression molding

Two-dimensional or three-dimensional fabrics woven into the required shape are impregnated with resin solution and then placed in a metal mold for heating and pressing to form composite material products.

④Laminate compression molding

Glass fiber cloth or other fabrics pre-impregnated with resin solution are cut into the required shape and then formed into composite material products in a metal mold under heating or pressure.

⑤Winding compression molding

Continuous fibers or fabrics (tapes) pre-impregnated with resin solution are provided with a certain tension and temperature through a special winding machine, wound onto a core mold, and then placed in a mold for heating and pressing to form composite material products.

⑥Sheet molding compound (SMC) compression molding

SMC sheets are cut according to the product size, shape, and thickness requirements, and then multiple layers of sheets are stacked and placed in a metal mold for heating and pressing to form products.

⑦Preform compression molding

First, preforms with shapes and sizes similar to the finished product are made from short-cut fibers. These are placed in a metal mold, and then a prepared binder (resin mixture) is injected into the mold. The product is then formed under a certain temperature and pressure.
There are many types of molding materials, which can be prepreg materials, premix materials, or preforms. Currently used molding materials mainly include: prepreg cloth, fiber premix, BMC, DMC, HMC, SMC, XMC, TMC, and ZMC.
 

I. Raw materials
(1) Synthetic resin

The molding materials used for composite material compression molding products require the synthetic resin to have:

①Good wetting properties for reinforcing materials to form good bonding at the interface between the synthetic resin and the reinforcing material;

②Appropriate viscosity and good fluidity, which can uniformly fill the entire mold cavity together with the reinforcing material under pressing conditions;

③Suitable curing speed under pressing conditions, and little or no by-products are produced during curing, with low volume shrinkage;

④Able to meet the specific performance requirements of the molded product. According to the above material selection requirements, commonly used synthetic resins include: unsaturated polyester resin, epoxy resin, phenolic resin, vinyl resin, furan resin, silicone resin, polybutadiene resin, allyl ester, melamine resin, polyimide resin, etc. To achieve specific performance indicators for molded products, after selecting the resin type and grade, appropriate auxiliary materials, fillers, and pigments should also be selected.
(2) Reinforcing materials

Commonly used reinforcing materials in molding materials mainly include glass fiber chopped strands, roving, twisted roving, continuous glass fiber bundles, glass fiber cloth, glass fiber felt, etc. A small number of special products also use asbestos felt, asbestos fabrics (cloth) and asbestos paper, as well as high-silica fiber, carbon fiber, organic fiber (such as aramid fiber, nylon fiber, etc.) and natural fiber (such as linen, cotton, boiled cloth, unboiled cloth, etc.). Sometimes, two or more types of fibers are mixed as reinforcing materials.
(3) Auxiliary materials

Generally include curing agents (initiators), accelerators, diluents, surface treatment agents, low shrinkage additives, release agents, colorants (pigments), and fillers.
 

II. Preparation of molding materials
Taking glass fiber (or glass cloth) impregnated resin as an example, the production process can be divided into two methods: premixing and prepreg.
(1) Premixing method First, cut the glass fiber into short cut fibers of 30-50mm. After loosening, fully knead it with the resin glue liquid in a kneading machine until the resin completely wets the glass fiber. Then, dry it until it reaches the appropriate viscosity. Its characteristic is that the fibers are loose and non-directional, with high production volume. The molding material produced by this method has a large specific volume and good fluidity, but the fiber strength loss is relatively large during the preparation process.
(2) Prepreg method The fiber prepreg method is to immerse a whole bundle of continuous glass fiber (or cloth) in glue, dry it, and then cut it into short pieces. Its characteristic is that the fibers are bundled and relatively dense, and the fiber strength loss is relatively small during the preparation of the molding material. However, the fluidity of the molding material and the compatibility between the material bundles are slightly worse.

 

SMC, BMC, HMC, XMC, TMC and ZMC production technology
 

Sheet Molding Compound (SMC) is a type of sheet molding material made by impregnating fibers or chopped fiber mats with resin paste and covering both sides with polyethylene film. It belongs to the prepreg mat range and is currently one of the most widely used molding materials internationally.
 

SMC is made by impregnating chopped glass fiber rovings or glass fiber mats with a resin paste mixed with unsaturated polyester resin, thickener, initiator, crosslinking agent, low shrinkage additive, filler, internal release agent, and colorant, and then covering both sides with polyethylene or polypropylene film to form a sheet molding material. As a rapidly developing new type of molding material, SMC has many characteristics:

① Good reproducibility, not affected by operators and external conditions;

② Convenient operation and handling;

③ Clean and hygienic operating environment, improving working conditions;

④ Good fluidity, can be molded into irregularly shaped products;

⑤ The molding process does not require high temperature and pressure, with a large variable range, which can significantly reduce equipment and mold costs;

⑥ Fiber length 40-50mm, good uniformity, suitable for pressing large thin-walled products with little cross-sectional change;

⑦ The resulting products have a high surface finish. After using low shrinkage additives, the surface quality is even better;

⑧ High production efficiency, short molding cycle, easy to achieve fully automated mechanized operation, and relatively low production cost.
 

As a new type of material, SMC has developed a series of new varieties according to different specific uses and requirements, such as BMC, TMC, HNC, XMC, etc.

① Bulk Molding Compound (BMC) Its composition is very similar to SMC. It is an improved premixed bulk molding compound that can be used for molding and extrusion. The only difference between the two lies in the material form and manufacturing process. BMC has a lower fiber content and shorter fiber length, about 6-18mm, and a larger filler content. Therefore, the strength of BMC products is lower than that of SMC products. BMC is more suitable for pressing small products, while SMC is suitable for large thin-walled products.

② Thick Molding Compound (TMC) Its composition and manufacturing are similar to SMC, with a thickness of up to 50mm. Because of the large thickness of TMC, the glass fibers can be randomly distributed, improving the wettability of the resin to the glass fibers. In addition, this material can also be used for injection and transfer molding.

③ High Molding Compound (HMC) and high-strength sheet molding compound XMC are mainly used for manufacturing automotive parts. HMC contains no or little filler, uses chopped glass fiber, with a fiber content of about 65%, and the glass fiber is directionally distributed. It has excellent fluidity and molding surface, and the strength of its products is about 3 times that of SMC products. XMC uses directional continuous fibers, with a fiber content of 70%-80%, and contains no filler.

④ ZMC ZMC is a molding technology. The three letters ZMC have no actual meaning, but they contain three meanings: molding material, injection molding machine, and mold. ZMC products maintain high strength indicators, excellent appearance, and high production efficiency. They combine the advantages of SMC and BMC and have achieved rapid development.

I. Raw materials of SMC
The raw materials of SMC are composed of three categories: synthetic resin, reinforcing material, and auxiliary material.
(1) Synthetic resin The synthetic resin is unsaturated polyester resin. Different unsaturated resins have a direct impact on the thickening effect of the resin paste, process characteristics, product performance, shrinkage rate, and surface state. SMC has the following requirements for unsaturated polyester resin:

① Low viscosity, good wetting performance for glass fiber;

② Sufficient reactivity with thickener to meet thickening requirements;

③ Fast curing, short production cycle, high efficiency;

④ The cured product has sufficient hot strength for easy hot demolding of the product;

⑤ The cured product has sufficient toughness and does not crack when the product undergoes certain deformations;

⑥ Low shrinkage rate.
(2) Reinforcing materials The reinforcing material is chopped glass fiber rovings or filaments. In unsaturated polyester resin molding compounds, the reinforcing material used for SMC is currently only chopped glass fiber mats, while the reinforcing materials used for premixes are more diverse, including chopped glass fiber, asbestos fiber, hemp, and various other organic fibers. In SMC, the glass fiber content can be adjusted between 5% and 50%.
(3) Auxiliary materials Auxiliary materials include curing agents (initiators), surface treatment agents, thickeners, low shrinkage additives, release agents, colorants, fillers, and crosslinking agents.
 

II SMC preparation process
The SMC production process mainly includes resin paste preparation, pasting operation, fiber cutting sedimentation and impregnation, resin thickening, etc. The process flow chart is as follows:
(1) Preparation and application of resin paste

There are two methods for preparing resin paste: batch method and continuous method. The batch method procedure is as follows:

① Pour unsaturated polyester resin and styrene into the mixing tank and stir evenly;

② Pour the initiator into the mixing tank and mix it with the resin and styrene;

③ Add thickener and release agent under stirring;

④ Add filler and low shrinkage additive under low-speed stirring;

⑤ Stop stirring when the components listed in the formula are dispersed, and let it stand for later use. In the continuous method, the resin paste in the SMC formula is divided into two parts: thickener, release agent, part of the filler and styrene as one part, and the rest of the components as another part. After separate metering and mixing, they are sent to the corresponding storage containers set on the SMC unit. When needed, they are metered by pipeline metering pumps and then enter the static mixer. After mixing evenly, they are transported to the paste application area of the SMC unit and then applied to the polyethylene film.
(2) Impregnation and compaction The lower carrier film coated with resin paste enters the short-cut glass fiber sedimentation chamber under the traction of the unit. The cut short-cut glass fibers are evenly sedimented on the resin paste. After reaching the required sedimentation amount, it leaves the sedimentation chamber with the transmission device and overlaps with the upper carrier film coated with resin paste. Then it enters a series of staggered roller arrays. Under the action of tension and rollers, the lower and upper carrier films tightly press the resin paste and short-cut glass fibers together. After repeated multiple times, the short-cut glass fibers are impregnated with resin and the air bubbles are expelled, forming a dense and uniform continuous SMC sheet.
According to the length of the molding cycle, composite material molding materials are generally divided into two types: rapid molding process and slow molding process. The rapid molding process is suitable for pressing small and thin-walled composite products, while the slow molding process is suitable for pressing large and thick-walled composite products.

 

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