What Is Compression mold?

Compression molds are abbreviated as compression molds. Plastic compression molding is a method of compression molding. Plastic raw materials are directly added to the open mold cavity, the mold is closed, and the plastic becomes a flowing state and fills the cavity under the action of heating and pressure. , And then through chemical cross-linking or physical cooling to make the product hardened and shaped. The mold used in this method is a compression mold, and compression molding can be used for thermosetting plastics, thermoplastics and rubber materials.

Compression molding can be divided into compression molding and lamination molding according to the nature of the material and the characteristics of the molding process. Compression molding is also called compression molding. This molding method is to put powdery, granular, crumb-like or fibrous plastic into a heated mold and heat it to melt, and fill the mold cavity with pressure to form Molded products with the same shape as the cavity.

For thermoplastics, because the mold is heated and cooled alternately, the production cycle is long, the productivity is low, and the mold is easily damaged, this method is rarely used in production, but it can be used for thermoset plastics with a larger molding area. The plastics processed by the molding method mainly include phenolic plastics, amino plastics, epoxy resins, silicones, rigid polyvinyl chloride, polyethylene, copolymers of ammonia and vinyl acetate.

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

Scope of application

Mainly used as structural parts, connectors, protective parts and electrical insulation parts. It is widely used in industry, agriculture, transportation, electrical, chemical, construction, machinery and other fields. Because of the reliable quality of molded products, they have also been used in weapons, aircraft, missiles, and satellites.

advantage

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

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

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

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

(5) Larger flat-shaped products can be formed, and the size of the products that can be formed by molding is only determined by the clamping force of the existing molding press and the size of the template.

(6) The shrinkage 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, and the productivity is high.

(9) It can be adapted to automatic feeding and automatic taking out of products.

(10) High production efficiency, easy to realize specialized and automated production.

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

(12) The surface is smooth and clean without secondary modification.

(13) Products with complex structures can be formed at one time.

(14) Mass production, relatively low price.

shortcoming

(1) The molding cycle in the entire production process is long, the efficiency is low, and the staff has a large physical consumption.

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

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

(4) After the curing stage is over, different products have different stiffness, which has an impact on product performance.

(5) For products with high dimensional accuracy requirements (especially for multi-cavity molds), the process is somewhat short.

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

(7) The shortcomings of compression molding are that the mold manufacturing is complicated, the investment is relatively large, and coupled with the limitation of the press, it is most suitable for mass production of small and medium composite materials.

Forming process

The compression molding process can be divided into the following types according to the physical state of reinforcement materials and the types of molded materials:

(1) Fiber material molding method:
Put the pre-mixed or pre-impregnated fibrous molding material into a metal mold, and form a composite product under a certain temperature and pressure.

(2) Moulding method of broken cloth:
Cut the scraps of glass fiber cloth or other fabrics such as linen cloth, organic fiber cloth, asbestos cloth or cotton cloth soaked in resin glue into pieces, and then heat and press in a mold to form a composite product. This method is suitable for forming 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, and then placed in a metal mold to heat and press to form a composite product.

(4) Laminated molding method:
The glass fiber cloth or other fabrics pre-soaked with resin glue are cut into the required shape, and then the composite product is formed by heating or pressing in a metal mold.

(5) Winding and molding method:
The continuous fiber or cloth (belt) pre-impregnated with resin glue is wound on the core mold through a special winding machine to provide a certain tension and temperature, and then put into the mold for heating and pressing to form a composite product.

(6) Molding method of sheet plastic (SMC):
The SMC sheet is cut according to the product size, shape, thickness, etc., and then the multi-layer sheets are stacked and placed in a metal mold to heat and press to form the product.

(7) Preformed blank molding method:
First, the preformed blanks with similar shapes and sizes of short-cut fiber products are put into a metal mold, and then the prepared binder (resin mixture) is injected into the mold, and then molded under a certain temperature and pressure.

(8) Moulding for directional auxiliary equipment:
Laying unidirectional prepreg products in the direction of main stress, and then compression molding, the fiber content of the products can reach 70%, which is suitable for forming products with high unidirectional strength requirements.

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

(10) Adsorption preform molding method:
Adopt the adsorption method (air adsorption or wet slurry adsorption) to preform the glass fiber into a preform with a structure similar to the molded product, then place it in the mold, and pour the resin paste on it, at a certain temperature and Molded under pressure. The material used in this method is low in cost, long chopped fibers can be used, and it is suitable for forming products with more complex shapes. It can be automated, but the equipment cost is relatively high.

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

(12) Felt molding method:
This method uses resin (mostly phenolic resin) to impregnate glass fiber mat, and then dry it into a prepreg mat, which is cut into the required shape and placed in a mold, heated and pressurized to form a product. This method is suitable for forming large-scale thin-walled products with simpler shapes and little change in single thickness.

Molded varieties

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

Raw materials

Synthetic resin

The molding compound used for composite molding products requires the synthetic resin to have:

① It has good wettability to the reinforced material, so as to form a good bond at the interface between the synthetic resin and the reinforced material;

②It has proper viscosity and good fluidity, and can evenly fill the entire cavity with the reinforcing material under pressing conditions;

③It has a suitable curing speed under the pressing conditions, and no by-products or few by-products are produced during the curing process, and the volume shrinkage rate is small;

④It can meet the specific performance requirements of molded products.

According to the above material selection requirements, the commonly used synthetic resins are: 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 make the molded products reach specific performance indicators, after selecting the resin variety and brand, the corresponding auxiliary materials, fillers and pigments should also be selected.

Reinforced material

The commonly used reinforcement materials in molded materials mainly include glass fiber open yarn, 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, high silica fiber, carbon fiber, organic fiber (such as aramid fiber, nylon fiber, etc.) and natural fiber (such as linen cloth, cotton cloth, scouring cloth, non-cooking cloth, etc.) and other varieties. Sometimes two or more fiber blends are used as reinforcing materials.

Supplementary materials

It generally includes auxiliary materials such as curing agent (initiator), accelerator, diluent, surface treatment agent, low shrinkage additive, release agent, colorant (pigment) and filler.

Molded material preparation

Taking the molded 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 30-50mm chopped fiber, after fluffing, kneading with resin glue in a kneader until the resin completely infiltrates the glass fiber, and then drying (drying) to proper viscosity. Its characteristic is that the fiber is loose and unoriented, and the production volume is large. The molded material produced by this method has a large specific volume and good fluidity, but the fiber strength loss is large during the preparation process.

(2) Pre-soaking method:

The fiber prepreg method is made by dipping, drying, and cutting the entire bundle of continuous glass fibers (or cloth). Its characteristic is that the fibers are formed into bundles and are relatively compact, and the fiber strength loss is small in the process of preparing the molded material, but the fluidity of the molded material and the compatibility between the material 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, belonging to the scope of prepreg material. It is one of the most widely used molding materials in the world.

SMC is a resin paste impregnated with chopped fiber roving or glass fiber mat with unsaturated polyester resin, thickener, initiator, crosslinking agent, low shrinkage additive, filler, internal release agent and coloring agent, etc. Sheet-shaped molded material formed by covering both sides with polyethylene or polypropylene film.

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

①Good reproducibility, not affected by the operator and external conditions;

②Convenient operation and processing;

③The operating environment is clean and hygienic, which improves working conditions;

④Good fluidity, can form special-shaped products;

⑤The molding process does not require high temperature and pressure, and the variable range is large, which can greatly reduce the cost of equipment and molds;

⑥The fiber length is 40-50mm, and the quality is uniform, suitable for pressing large thin-walled products with little change in cross section;

⑦The 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 fully automatic mechanized operation, and relatively low production cost.

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

①Bulk Molding Compound (BMC) Its composition is very similar to SMC. It is an improved pre-mixed bulk molding compound that can be used for molding and extrusion molding. The difference between the two is only in the material form and manufacturing process. The fiber content in BMC is low, the fiber length is short, about 6-18mm, and the filler content is large. 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. Products.

②Thick Molding Compound (TMC) Its composition and production are similar to SMC, with a thickness of 50mm. Due to the large thickness of the TMC, the glass fibers can be randomly distributed, which improves the wettability of the resin to the glass fibers. In addition, the material can also be injection and transfer molding.

③High-strength molding compound (Hight Molding Compound, HMC) and high-strength sheet molding compound XMC are mainly used to manufacture automobile parts. There is no or little filler in HMC, chopped glass fiber is used, the fiber content is about 65%, the glass fiber is oriented and distributed, and it has excellent fluidity and molding surface. Its product strength is about 3 times the strength of SMC products. XMC uses oriented continuous fiber with a fiber content of 70% to 80% without filler.

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

SMC raw materials

Synthetic resin

The synthetic resin is an unsaturated polyester resin, and different unsaturated resins have a direct influence on the thickening effect, process characteristics, product performance, shrinkage rate, and surface state of the resin paste.

SMC has the following requirements for unsaturated polyester resin:

①Low viscosity, good wettability to glass fiber;

② It has sufficient reactivity with the thickener to meet the thickening requirements;

③ Fast curing, short production cycle and high efficiency;

④The cured product has sufficient thermal strength to facilitate the thermal release of the product;

⑤The cured product has sufficient toughness, and the product will not crack when some deformation occurs;

⑥Lower shrinkage rate.

Reinforced material

The reinforcing material is chopped glass fiber roving or raw silk. Among the unsaturated polyester resin molding compounds, the reinforcing material used for SMC is only chopped glass fiber mat, while there are many reinforcing materials used for premix, including chopped glass fiber, asbestos fiber, hemp and other various organic fiber. 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, mold release agents, colorants, fillers, and crosslinking agents.

Preparation Process

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

(1) Resin paste preparation and paste operation: There are two methods for the preparation of resin paste-batch method and continuous method.

The batch method procedure is as follows:

①Pour the unsaturated polyester resin and styrene into the batching kettle and stir evenly;
②Pour the initiator into the batching kettle and mix it with the resin and styrene;
③Add thickener and release agent under stirring;
④ Add fillers and low shrinkage additives under low-speed stirring;
⑤ Stop stirring until the components listed in the formula are dispersed, and let it stand for use.

Continuous method

The resin paste in the SMC formula is divided into two parts, that is, thickener, release agent, part of filler and styrene as one part, and the remaining components are the other part. After measuring and mixing, they are sent to the SMC unit to set up. In the corresponding storage container, when needed, it is metered by the pipeline metering pump and then entered into the static mixer, mixed evenly and transported to the paste area of ​​the SMC unit, and then coated on the polyethylene film.

(2) Impregnation and compaction:

The lower load-bearing film coated with resin paste enters the chopped glass fiber settling chamber under the traction of the unit, and the cut chopped glass fibers uniformly settle on the resin paste. After reaching the required settling amount, it leaves the settling chamber with the transmission device. It is superimposed with the upper carrier film coated with resin paste, and then enters into a series of staggered arrays of rollers. Under the action of tension and rollers, the lower and upper carrier films tightly tighten the resin paste and chopped glass fibers. After pressing together, after many times of repetition, the chopped glass fiber is impregnated with resin and the bubbles in it are driven out to form a dense and uniform continuous SMC sheet.

Process flow

(1) Adding materials:
Add a specified amount of material to the mold as needed, and the amount of material directly affects the density and size of the product. If the amount of feed is large, the product will have thick burrs, poor dimensional accuracy, difficult to demold, and may damage the mold; if the amount of feed is small, the product will not be tight and the gloss will be poor, and even cause shortage of materials and waste products.

(2) Closed mold:
Even if the male mold and the female mold are closed after feeding. When closing the mold, use fast speed first, and change to slow speed when the male and female molds are in quick contact. The quick first and slow operation method is beneficial to shorten the non-production time, prevent the mold from scratching, and avoid the raw material in the mold cavity from being carried out by the air due to the rapid clamping, and even the displacement of the insert and the destruction of the molding rod. When 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 substances are often released. In order to eliminate these low-molecular substances, volatiles and air in the mold, after the plastic reaction in the mold cavity of the plastic mold has progressed to a suitable time, the pressure can be released to loosen the mold row. Anger for a short time. The 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; however, the exhaust is too early, sooner or later, and the exhaust cannot be achieved too early; if it is too late, the surface of the material has already been removed. The curing gas cannot be discharged.

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

(5) Demoulding:
Demoulding is usually done by ejector rods. Products with molding rods or some inserts should be removed first with special tools, and then demolded.

(6) Mold blowing:
After demolding, compressed air is usually used to blow the cavity and the mold surface of the mold. If the fixation on the mold is tight, it can also be cleaned with a copper knife or a copper brush, or even a polishing agent brush.

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