Properties of molded products

Molded products have excellent electrical properties (especially anti-leakage properties), mechanical properties, heat resistance, fire resistance, chemical corrosion resistance and dimensional stability, and can be adjusted according to the needs of each component type and dosage to obtain products with special performance requirements. The main performance is as follows：

1.Electrical property

Molded plastics are widely used in high voltage electrical applications due to their arc resistance, acer performance, flame resistance, dimensional stability, moulding and low cost. The ability of arc suppression is mainly due to the presence of inert inorganic fillers such as hydrated alumina, silica and ceramic. Add a small amount of polyethylene powder (5% by weight) and use nylon fiber to improve arc resistance. In order to obtain better arc resistance, the resin and glass fiber content should be reduced to the minimum, however, such results reduce the mechanical properties.

2. Mechanical property

The mechanical properties of molding materials vary with the types and proportions of reinforced fibers and resin substrates used. In DMC, when the fiber length exceeds 6.35mm, the modification of product properties is very small.

Most SMC products are made from short-cut felt, and there is no published data on the effect of fiber length changes. Although the length of the general fiber is 50mm, this length is not necessarily suitable for all applications.

Increasing the content of reinforcing materials can improve the mechanical properties, but too much fiber content will bring inconvenience to molding. For example, when the fiber content in DMC exceeds 20%, it has little influence on its mechanical properties.

3. Heat resistance or fire resistance

Heat resistance refers to the ability of products to withstand thermal decomposition for long periods of time below the flammable temperature. "Short-term thermal strength" or "thermal strength" is related to the thermal deformation temperature of the resin. Although some fillers can improve the heat resistance of products, the heat resistance and thermal strength mainly depend on the properties of the resin. Flammability is a measure of apparent combustion, divided into "non-ignition", "self-extinguishing" and "flame-retardant" according to the ability and speed with which a material is able to extinguish easily or when the ignition source is removed. Polyester resins achieve flame resistance by adding halogen and phosphorous compounds and by using alumina hydrate as the main filler in the components.

Phenolic resins are inherently fire-resistant. Halogen compounds and phosphorous compounds, as well as HET anhydride used as hardener, can make epoxy resins flame resistant.

4.Dimensional stability

General molding materials have good dimensional stability, water absorption rate: small, thermal expansion coefficient and aluminum is very similar, when continuous exposure to high temperature, size is almost no change.

5. Corrosion resistance

The chemical resistance or corrosion resistance of molding material mainly depends on the selected resin matrix. The suitable resin and filler can be used to prepare the moulded material which can meet the requirement of resisting special bristle dew

DMC can be made of acid-resistant and alkali-resistant ligulates and acid-resistant epoxy resins. The choice of a resin for SMC is currently limited because the resin must also have a chemical structure that thickens easily. In common fillers, clay and silica have better corrosion resistance and other properties.

6.Contractility

The shrinkage of moulded material is very low after release, the typical maximum shrinkage rate is 0.004, and the shrinkage rate of many moulded materials is close to 0.001, which is mainly due to the small thermal shrinkage of glass fiber and inorganic filler. However, the combination of low-shrinkage, high-strength fibers with the resin systems commonly used for rapid curing and high thermal shrinkage results in greater stress on the resin matrix between the fibers. The collateral effects of this stress cause surface ripples, cracks, warping, and internal voids. These defects can be reduced by the use of organic fiber reinforced materials compatible with resin shrinkage or by the use of short or filamentous glass fibers.

Raw material for molding

The commonly used molding materials are mainly composed of resin matrix, reinforcing materials, fillers, pigments, etc., and according to the process and performance requirements, the resin matrix is also added with curing agent, thickener, internal release agent, solvent and other additives. Reinforcement material is the skeleton material of die pressing material, which mainly endows die pressing material with excellent mechanical properties and prevents the propagation of micro-cracks. The main varieties of reinforcement materials are various glass fiber felt, no twist roving, no twist coarse gauze, chopped fiber, ground fiber, glass yarn and fabric and other varieties of fiber. The main function of resin matrix in molding material is to bond the reinforcing material and filler together, which not only protects the reinforcing material, but also makes it uniform force under external load. Theoretically, most types of thermoplastic and thermosetting resins can be used as resin matrix materials. There are polyester resin, epoxy resin, amino resin and phenolic resin. A large number of various types of fillers are used in the molding material, the main function of which is to reduce the material cost, improve the process, improve the molding material some physical properties, appearance and give some characteristics. Fillers are mostly in powder form, such as clay, calcium carbonate, talcum powder and some fillers with special performance requirements.

The basic requirements of resin in molding are as follows: good infiltration performance of reinforcing materials and fillers to improve the bonding strength between resin and glass fiber; Resin to have the appropriate: viscosity, good fluidity, so that in the molding process of resin and glass fiber at the same time full of all corners of the cavity, to obtain a balanced strength of the molding products; Resin curing temperature is low, less volatile in the curing process, good technology (such as viscosity easy to adjust, good solubility with various solvents, easy to release mold, etc.), and can meet the mold products specific performance requirements. In addition, from the point of view of application or other point of view, the resin should also meet some other special performance requirements, such as corrosion resistance, heat resistance and so on. From the perspective of production efficiency, fast curing speed of resin is required, but for some large products with complex structure and higher requirements, the curing speed should be properly controlled. Resin curing after high mechanical strength, good toughness, avoid from the mold when taken out, in the case of bending cracking or breaking. Therefore, the selection of resin, will become a very important factor affecting the whole process.

Molding products are commonly used in thermosetting resin, thermoplastic resin in the molding process of the application of less. The main thermosetting resins are phenolic resin, epoxy resin, epoxy phenolic resin, polyvinyl butanal resin, unsaturated polyester resin, etc.