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preheating of compression molding

Join Date: 2022-08-02

Compression molding is a common method of high-volume manufacturing, and it is common practice to preheat the material before placing it in the compression mold. This is because preheating softens the polymer and shortens the production cycle time. So what is compression molding preheating? What does preheating do? How many methods of preheating are there? Please read on.

What is preheating?

Preheating is sometimes performed before compression molding of composite materials. The role played is firstly to increase the material temperature, and the heated material will be suitable for compression molding. The second is drying, i.e., removing moisture and other volatile materials from the material to avoid degradation of product quality caused by improper storage of raw materials.

The purpose of preheating

  • Shorten the molding cycle. Plastic is a poor conductor of heat, heat transfer from the surface of the cavity to the inside of the part, takes a long time to complete the curing reaction, preheating can speed up the curing speed to reduce the hot compression molding time.
  • Reduce the molding pressure. After preheating, the molded plastic becomes soft, has good liquidity when molding, and molding pressure can be reduced by 30% to 60%, so you can use a smaller tonnage of hydraulic press compression molding area of larger products, or in the fixed tonnage of the hydraulic press to increase the number of a compression mold.
  • Improve the quality of molded parts. Preheating can improve the fluidity of the molded plastic, improve the homogeneity of the molded plastic curing, reduce the shrinkage rate and internal stress of the products, improve the surface finish of the products, and thus improve the physical and mechanical properties of the products.
  • The preheating method also helps to extend the service life of compression molds. This is because the molten plastic avoids excessive friction in the mold cavity.


Preheating Methods

There are various methods of preheating molded plastics, commonly used are hot plate heating, oven heating, infrared heating, and radio frequency heating, etc.

Hot plate heating

The hot plate is a metal plate that can be turned horizontally, it is often placed next to the hydraulic press and heated by electricity or steam to the plate. When preheating, the molded plastic is divided into several parts according to a certain quality and placed on the hot plate in successive batches and covered with a layer of cloth. The pre-press must be turned regularly to ensure even heating on both sides.

Oven heating

The oven is equipped with forced air circulation and a temperature control device, the temperature can be adjusted in the range of 40~230°. The preheated material is placed in the oven with a tray, and the preheating time and temperature are controlled as required. The thickness of the material layer is usually 2.5cm, and if the material layer is thicker, it should be continuously turned to ensure uniform preheating.

Infrared heating

The equipment used is a box equipped with an infrared lamp, and the inner wall of the box is coated with white paint or chrome plating. The heat is generated by infrared light irradiation. When the infrared light irradiates the material, first of all, the surface of the radiation temperature rises, and then the heat is gradually transferred to the interior of the material by means of heat conduction. Infrared heating efficiency is high, but one should pay attention to controlling the power of infrared lamps, the number of lamps, irradiation distance height, and time, in order to prevent the material surface overheating caused by scorching.

Radiofrequency heating

Under the action of radio frequency electric field, the polar material molecules undergo orientation polarization (i.e. molecular movement with the external electric field direction change and constantly change), so that the strong friction between molecules, so that the material temperature rises. Since this heat is generated from the material itself, the molded plastic is heated more uniformly.

Compared with other heating methods, the efficiency of radio frequency heating is high, which can greatly shorten the preheating time, especially when molding thick products, the use of radio frequency heating is more favorable. However, due to the rapid heating, the moisture in the molded plastic is not easy to remove, and the drying effect is not good, so the electrical properties of the products are not as good as the oven preheating.

Because of the existence of reactive points on the molecules of thermosetting plastics, it is particularly sensitive to temperature. The time and temperature of preheating should depend on the specific species of plastic. At a given preheating temperature, the preheating time should be limited to obtain the maximum flow, too much time will reduce the flow.

MDC Mould always aims to produce compression molded products better and faster, and we will choose whether to use preheating as needed. In addition, any performance in our past compression molding solutions shows that MDC is capable of selecting the best preheat parameters for different types of molded plastics.

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