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thermoset compression moulding

Join Date: 2022-07-27

Are you looking for a custom thermoset molding solution? Read this article to learn everything about thermoset plastic compression molding.

What are thermosetting plastics?

Thermosetting plastics are plastics that can cure when subjected to heat or other conditions. Thermosetting plastics are also divided into two types: formaldehyde cross-linked and other cross-linked types.

Thermosetting plastics can be softened and flow when heated for the first time, and become hard when heated to a certain temperature, resulting in a chemical reaction of cross-linking and curing, and this change is irreversible. Thereafter, when heated again, it can no longer become soft and flowing. Compression molding of thermoset plastics uses the plasticized flow during the first heating, fills the cavity under pressure, and then cures into products of definite shape and size.

Thermoset plastics are mainly used for heat insulation, wear resistance, insulation, high voltage electricity and other harsh environments, the most commonly used should be frying pan handles and high-voltage electrical shells. Commonly used thermosetting plastic varieties are phenolic resin, urea-formaldehyde resin, melamine resin, unsaturated polyester resin, epoxy resin, silicone resin, polyurethane, etc.

thermosetting plastics

Compression molding of thermosetting plastics

The compression molding process of thermosetting plastics is a physicochemical change process. The resin of thermoset plastic is linear or branched before curing, and after curing, the molecular chains form a chemical bond between them and become a three-dimensional mesh structure, which not only cannot be molten again, but also cannot be dissolved in many chemical solvents.

Before compression molding, we also need to pay attention to the following factors of thermosetting plastics.

1)Flowability

The fluidity of a thermoset plastic is its ability to fill the mold cavity when subjected to heat and pressure. The fluidity is firstly related to the nature of the molded plastic itself, including the nature of the thermosetting resin and the composition of the molded plastic. Low relative molecular mass of resin, low reaction degree, small and spherical filler particles, low molecular content or high water content is good fluidity.

Secondly, the mold and molding process conditions are related to the mold cavity surface is smooth and streamlined, then the material flow is good. In addition, preheating or increasing the molding temperature of the molded plastic before molding can also improve the fluidity.

Different molded products require different fluidity, and molding complex shapes or thin-walled products require a large fluidity of the molded plastic. If the fluidity is too small, it is difficult to fill the mold cavity, resulting in lack of material. But the fluidity can not be too big, too big will make the mold plastic melt and overflow out of the thermosetting mold caused by the waste of raw materials. At the same time in the cavity can not be compacted, resulting in product quality decline.

(2) Curing rate

Curing rate is unique to thermosetting plastic molding is also the most important process performance, can measure the speed of chemical reaction when molding thermosetting plastics. It is the ratio of the time required for the physical and mechanical properties of the product to reach the best value under a certain temperature and pressure of thermosetting plastic to the thickness of the specimen (s/ mm), the smaller the value, the greater the curing rate.

Curing rate is mainly determined by the nature of the cross-linking reaction of thermosetting plastics. Curing rate should be moderate, too small, the production cycle is long, low productivity. Curing rate is too large will lead to a decline in fluidity and may be cured in advance, which is not suitable for molding thin-walled and complex-shaped products.

(3) Molding shrinkage rate

After the thermosetting plastics are compressed and molded at high temperature and then cooled to room temperature, their dimensions will shrink in all directions. Molding shrinkage rate is the difference between the one-way size of the mold cavity and the corresponding one-way size of the product and the one-way size of the mold cavity at room temperature and pressure, and the products with large molding shrinkage rate are prone to warpage and even cracking.

The shrinkage of thermosetting plastic products caused by many factors: first of all, thermosetting plastics in the molding process of chemical cross-linking, its molecular structure changes, the density becomes larger, resulting in shrinkage; secondly, because of the plastic and metal thermal expansion coefficient difference is very large, after cooling plastic shrinkage than metal mold is much larger; again is the product is released from the mold due to pressure drop there is an elastic response and plastic deformation, so that the volume of products change.

4) Compression rate

Thermoset plastic is generally powder or granular material, its relative density, and the relative density of the product is very different. The volume of the molded plastic changes greatly before and after molding, and the compression rate can be used to express. Compression rate of large materials need to be compressed mold loading room is also large, consuming mold materials, is not conducive to heat transfer, low production efficiency, and easy to mix into the air when loading. The usual way to reduce the compression rate is to pre-press the material before compression molding.

thermoset-compression-molding

Notes on compression molding of thermosetting plastics

For general thermoset plastic compression molding, usually choose prepreg dough or bulk material first. In the case of molding flat structures, sheet molding compounds should be prepared.

Filling the cavity with the "lowest viscosity" state. Due to the special rheological behavior of thermoset plastics, the "lowest viscosity" state is short-lived and must be used to fill the cavity at this time. The selection of MDC's advanced molding equipment and experienced operators is a key factor in ensuring high quality molded products.

Pay attention to the control of temperature, pressure and time. If the temperature is not well controlled, the material will not be able to fill the cavity nor will the product be cured completely. It is well known that thermosetting plastics contain a large amount of fillers or reinforcing materials. Therefore, its viscosity is larger than that of thermoplastic, and its molding pressure is naturally much higher than that of thermoplastic. In addition, during the curing reaction, water vapor and low-molecular volatile gases are generated, leaving bubbles on the surface of the product. In view of these circumstances, thermosetting plastics are often molded at high temperature and pressure.

After the thermosetting plastic products are released from the mold, they usually have to be post-treated. The post-treatment is usually carried out in an oven at a temperature of 120-160° and a time of 5-20h. After post-treatment, the heat deflection temperature of the products can be increased, and their electrical properties and dimensional stability can be improved to varying degrees; otherwise, the products are prone to deformation, warping or cracking during use.

MDC provides OEM thermosetting molds. We will choose the right thermoset plastic formulation according to your product characteristics. In addition we have years of experience in molding thermoset composites to ensure that your products have the best performance. We successfully navigate the ever-changing material chemistry to ensure quality while minimizing scrap. Contact us now for thermoset molding solutions.

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