Compression molding (also known as press molding or compression molding) is a process in which powdered, granular or fibrous plastics are placed into a mold cavity at molding temperature, and then the mold is closed and pressurized to form and solidify it. Compression molding can be used for thermosetting plastics, thermoplastics and rubber materials.

Introduction

The compression molding process uses the characteristics of each stage in the resin curing reaction to achieve product molding, that is, the molding material plasticizes, flows and fills the mold cavity, and the resin solidifies. In the process of the molding material filling the mold cavity, not only the resin flows, but also the reinforcing material flows with it. Therefore, the molding pressure of the compression molding process is higher than other process methods and belongs to high-pressure molding. Therefore, it requires both a hydraulic press that can control the pressure and a high-strength, high-precision, and high-temperature resistant metal mold.

Applications

Mainly used as structural parts, connectors, protective parts and electrical insulation parts. Widely used in industry, agriculture, transportation, electrical, chemical, construction, machinery and other fields. Due to the reliable quality of molded products, they are also used in weapons, aircraft, missiles and satellites.

Advantages

(1) The loss of raw materials is small and will not cause excessive losses (usually 2% to 5% of the product quality).

(2) The internal stress of the product is very low, the warping deformation is also very small, and the mechanical properties are relatively stable.

(3) The wear of the mold cavity is very small and the maintenance cost of the mold is low.

(4) The cost of molding equipment is low, the mold structure is simpler, and the manufacturing cost is usually lower than that of injection molds or transfer molding molds.

(5) It can form relatively large flat products. The size of the products that can be formed by molding is only determined by the clamping force of the existing molding machine and the size of the mold plate.

(6) The shrinkage rate of the product is small and the repeatability is good.

(7) A mold with a large number of cavities can be placed on a given template, which increases productivity.

(9) It can adapt to automatic feeding and automatic removal of products.

(10) High production efficiency, facilitating specialized and automated production.

(11) The product has high dimensional accuracy and good repeatability.

(12) The surface is smooth and no secondary modification is required.

(13) Capable of producing products with complex structures in one go.

(14) Mass production, relatively low price.

Shortcoming

(1) The molding cycle in the entire production process is long, the efficiency is low, and it consumes a lot of physical energy for the workers.

(2) Compression molding is not suitable for complex products with depressions, side slopes or small holes.

(3) In the manufacturing process, it is difficult to completely fill the mold and certain technical requirements are required.

(4) After the curing stage, different products have different stiffness, which affects the product performance.

(5) This process has some shortcomings for products with high dimensional accuracy requirements (especially for multi-cavity molds).

(6) The flash of the final product is thicker, and the workload of removing the flash is large.

(7) The disadvantages of compression molding are that the mold manufacturing is complex and the investment is large. In addition, it is limited by the press and is most suitable for mass production of small and medium-sized composite products.

Molding process

The compression molding process can be divided into the following types according to the physical state of the reinforcing material and the type of molding material:

(1) Fiber material molding method: The premixed or pre-impregnated fiber molding material is put into a metal mold to form a composite material product under a certain temperature and pressure.

(2) Fabric scrap molding method: Cut the scraps of glass fiber cloth or other fabrics such as linen, organic fiber cloth, asbestos cloth or cotton cloth soaked in resin glue into pieces, and then heat and pressurize them in a mold to form composite products. This method is suitable for molding products with simple shapes and general performance requirements.

(3) Fabric molding method: The two-dimensional or three-dimensional fabric pre-woven into the desired shape is impregnated with resin glue, then placed in a metal mold and heated and pressurized to form a composite product.

(4) Lamination molding method: The glass fiber cloth or other fabric pre-impregnated with resin glue is cut into the required shape, and then heated or pressurized in a metal mold to form a composite product.

(5) Winding molding method: The continuous fiber or cloth (tape) pre-impregnated with resin glue is wound around the core mold with a certain tension and temperature provided by a special winding machine, and then placed in the mold for heating and pressure to form a composite material product.

(6) Sheet plastic (SMC) molding method: The SMC sheet is cut according to the product size, shape, thickness and other requirements, and then the multiple layers of sheets are stacked and placed in a metal mold for heating and pressurization to form the product.

(7) Preform molding method: First, place the preformed blank with a shape and size similar to the chopped fiber finished product into a metal mold, then inject the prepared binder (resin mixture) into the mold and mold it under a certain temperature and pressure.

(8) Directional auxiliary molding: The unidirectional prepreg product is laid in the direction of the main stress, and then molded. The fiber content in the product can reach 70%, which is suitable for molding products with high unidirectional strength requirements.

(9) Molding powder compression molding method: Molding powder is mainly composed of resin, filler, curing agent, colorant and release agent. The resin is mainly thermosetting resin (such as phenolic resin, epoxy resin, amino resin, etc.). Thermoplastic resin with high molecular weight, poor fluidity and high melting temperature that is difficult to inject and extrude can also be made into molding powder. The molding process of molding powder and other molding materials is basically the same. The main difference between the two is that the former does not contain reinforcing materials, so the strength of its products is lower and it is mainly used for secondary load-bearing parts.

(10) Adsorption preform molding method: The glass fiber is made into a preform with a structure similar to that of the molded product by the adsorption method (air adsorption or wet slurry adsorption), and then placed in a mold, and resin paste is poured on it, and molded under a certain temperature and pressure. This method uses low-cost materials and can use longer chopped fibers. It is suitable for molding products with more complex shapes and can be automated, but the equipment cost is relatively high.

(11) Bulk molding compound molding method: Bulk molding compound (BMC) is a fiber-reinforced thermosetting plastic, and is usually a fully mixed bulk prepreg composed of unsaturated polyester resin, chopped fibers, fillers and various additives. Low shrinkage additives are added to BMC, which greatly improves the appearance performance of the product.

(12) Felt molding method: This method uses resin (mostly phenolic resin) to impregnate glass fiber felt, then dries it into pre-impregnated felt, cuts it into the required shape, places it in a mold, and heats and presses it into a product. This method is suitable for molding large thin-walled products with simple shapes and little thickness variation.

Molded varieties

There are many types of molding materials, which can be prepreg materials, premixed materials, or blanks. The types of molding materials currently used mainly include: prepreg tape, fiber premix, BMC, DMC, HMC, SMC, XMC, TMC and ZMC.

Raw materials

Synthetic resin

The molding materials used in composite molded products require synthetic resins to have the following characteristics: ① good wettability to the reinforcing material so as to form a good bond at the interface between the synthetic resin and the reinforcing material; ② appropriate viscosity and good fluidity so that they can evenly fill the entire mold cavity together with the reinforcing material under pressing conditions; ③ suitable curing speed under pressing conditions, and no or few by-products are produced during the curing process, and the volume shrinkage is small; ④ able to meet the specific performance requirements of the molded products. 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. In order to achieve specific performance indicators for molded products, after selecting the resin type and brand, the corresponding auxiliary materials, fillers and pigments should also be selected.

Reinforcement Materials

Commonly used reinforcing materials in molding materials include glass fiber slit wire, untwisted roving, twisted roving, continuous glass fiber bundle, glass fiber cloth, glass fiber mat, etc. There are also a small number of special products using asbestos felt, asbestos fabric (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, scoured cloth, unscoured cloth, etc.). Sometimes two or more fiber mixtures are used as reinforcing materials.

Supplementary Materials

Generally, auxiliary materials include curing agent (initiator), accelerator, diluent, surface treatment agent, low shrinkage additive, release agent, colorant (pigment) and filler.

Molding material preparation

Taking the molding material made of glass fiber (or glass cloth) impregnated with resin as an example, its production process can be divided into two types: premixing method and prepreg method.

(1) Premixing method: First, cut the glass fiber into 30-50 mm short fibers, fluff them up, and then fully knead them with the resin glue in a kneader until the resin completely soaks the glass fiber, and then dry (air dry) to an appropriate viscosity. Its characteristics are loose and non-directional fibers, large production volume, and 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 dip, dry and cut a whole bundle of continuous glass fiber (or cloth) into glue. Its characteristics are that the fibers are bundled and relatively compact. The fiber strength loss is small during the preparation of the molding material, but the fluidity of the molding material and the compatibility between the bundles are slightly poor.

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

Sheet Molding Compound (SMC) is a type of sheet molding compound made of resin paste impregnated fiber or chopped fiber mat, covered with polyethylene film on both sides. It belongs to the range of prepreg mats. It is one of the most widely used molding materials in the world.

SMC is a sheet molding material made of unsaturated polyester resin, thickener, initiator, cross-linking agent, low shrinkage additive, filler, internal release agent and colorant, which is impregnated with resin paste to form short-cut fiber roving or glass fiber mat, and then covered with polyethylene or polypropylene film on both sides.

As a new type of molding material with rapid development, SMC has many characteristics:

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

② convenient operation and processing;

③ clean and hygienic operating environment, improved working conditions;

④ good fluidity, can be formed into special-shaped products;

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

⑥ fiber length 40 ~ 50mm, good quality uniformity, suitable for pressing large thin-walled products with little cross-section change;

⑦ the surface finish of the obtained product is high, and the surface quality is more ideal after using low shrinkage additives;

⑧ high production efficiency, short molding cycle, easy to realize full-automatic 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 difference between the two lies only in the material form and production process. BMC has a lower fiber content and a shorter fiber length, about 6 to 18 mm, 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 production are similar to SMC, with a thickness of up to 50 mm. Due to the large thickness of TMC, the glass fibers can be randomly distributed, which improves the wettability of the resin to the glass fibers. In addition, this material can also be used for injection and transfer molding.

③ High-strength molding compound (HMC) and high-strength sheet molding compound XMC are mainly used to manufacture automotive parts. HMC does not add or adds little filler, uses short-cut glass fiber, the fiber content is about 65%, the glass fiber is directional, has excellent fluidity and molding surface, and its product strength is about 3 times that of SMC products. XMC uses directional continuous fiber, the fiber content is 70% to 80%, and does not contain filler.

④ ZMC ZMC is a molding technology. The three letters ZMC have no actual meaning, but include three meanings: molding compound, injection molding machinery and mold. ZMC products not only maintain high strength indicators, but also have excellent appearance and high production efficiency. They combine the advantages of SMC and BMC and have achieved rapid development.

SMC raw materials

Synthetic resin

The synthetic resin is an unsaturated polyester resin. Different unsaturated resins have a direct impact on the thickening effect of the resin paste, process characteristics, and product performance, shrinkage rate, and surface state. SMC has the following requirements for unsaturated polyester resin: ① low viscosity and good glass fiber impregnation performance; ② sufficient reactivity with the thickener to meet the thickening requirements; ③ rapid curing, short production cycle, and high efficiency; ④ the cured product has sufficient hot strength to facilitate thermal demoulding of the product; ⑤ the cured product has sufficient toughness and does not crack when the product undergoes certain deformation; ⑥ low shrinkage rate.

Reinforcement Materials

The reinforcement material is chopped glass fiber roving or raw yarn. In unsaturated polyester resin molding compounds, the only reinforcement material used for SMC is chopped glass fiber mat, while there are many reinforcement materials used for premixes, including chopped glass fiber, asbestos fiber, hemp and other various organic fibers. In SMC, the glass fiber content can be adjusted between 5% and 50%.

Supplementary Materials

Auxiliary materials include curing agents ( initiators ), surface treatment agents, thickeners, low shrinkage additives, release agents, colorants, fillers and cross-linking agents.

Preparation process

The process flow of SMC production mainly includes resin paste preparation, paste operation, fiber cutting, sedimentation and impregnation, resin thickening and other processes. The process flow is as follows:

(1) Preparation of resin paste and paste operation: There are two methods for preparing resin paste – intermittent method and continuous method. The intermittent method procedure is as follows: ① Pour unsaturated polyester resin and styrene into the batching kettle and stir evenly; ② Pour the initiator into the batching kettle 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 until all the components listed in the formula are dispersed, and let it stand for use. The continuous method is to divide the resin paste in the SMC formula into two parts, namely, thickener, release agent, part of filler and styrene as one part, and the remaining components as another part. After measuring and mixing them respectively, they are sent to the corresponding storage container set on the SMC unit. When needed, they are measured by the pipeline metering pump and enter the static mixer. After mixing evenly, they are transported to the paste area of ​​the SMC unit and then coated on the polyethylene film.

(2) Impregnation and compaction: The lower supporting film coated with resin paste enters the chopped glass fiber sedimentation chamber under the traction of the unit. The cut chopped glass fibers are evenly settled on the resin paste. After reaching the required sedimentation amount, they leave the sedimentation chamber with the transmission device and overlap with the upper supporting film coated with resin paste. Then they enter a series of staggered roller arrays. Under the action of tension and rollers, the lower and upper supporting films press the resin paste and chopped glass fibers tightly together. After repeated cycles, the chopped glass fibers are impregnated with resin and the bubbles in them are driven out, forming a dense and uniform continuous SMC sheet.

Process

(1) Adding materials: Add a specified amount of material into the mold as needed, and the amount of material added directly affects the density and size of the product. If too much material is added, the product will have thick burrs, poor dimensional accuracy, and difficulty in demolding, and may damage the mold; if too little material is added, the product will be loose, have poor gloss, and even cause material shortage and produce waste.

(2) Mold Closing: After the material is added, the male and female molds are closed. When closing the mold, use fast speed first, and then slow speed when the male and female molds are in contact. The fast-first-then-slow operation method is conducive to shortening non-production time, preventing mold scratches, and preventing the raw materials in the mold groove from being carried out by air due to too fast mold closing, and even causing the insert to move and the molding rod to be damaged. After the mold is closed, the pressure can be increased to heat and pressurize the raw materials.

(3) Exhaust: When molding thermosetting plastics, moisture and low molecular weight substances are often released. In order to remove these low molecular weight substances, volatile substances and air in the mold, after the plastic reaction in the mold cavity of the plastic mold has been carried out for an appropriate time, the pressure can be released and the mold can be loosened to exhaust for a very short time. Exhaust operation can shorten the curing time and improve the physical and mechanical properties of the product, and avoid stratification and bubbles inside the product; but exhausting too early or too late is not good. Exhaust purpose cannot be achieved too early; if it is too late, the gas cannot be discharged because the surface of the material has solidified.

(4) Curing: The curing of thermosetting plastics is to maintain the molding temperature for a period of time so that the condensation reaction of the resin reaches the required cross-linking degree and the product has the required physical and mechanical properties. Plastics with low curing rates can also be temporarily cured when the product can be completely demolded, and then post-processing can be used to complete the entire curing process to improve the utilization rate of the equipment. The molding curing time is usually the pressure holding time, which generally ranges from 30 seconds to several minutes, and most do not exceed 30 minutes. Too long or too short curing time will affect the performance of the product.

(5) Demolding: Demolding is usually done by ejecting the rod. Products with molding rods or certain inserts should first be removed with special tools to remove the molding rods, etc., and then demolded.

(6) Mold cleaning: After demolding, the mold cavity and mold surface are usually cleaned with compressed air. If the fixatives on the mold are tight, they can be cleaned with a copper knife or copper brush, or even with a polishing agent.

(7) Post-treatment: In order to further improve the quality of the product, thermosetting plastic products are often post-treated at a higher temperature after demolding. Post-treatment can make the plastic cure more completely; at the same time, it can reduce or eliminate the internal stress of the product, reduce the moisture and volatiles in the product, etc., which is beneficial to improve the electrical properties and strength of the product.