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Guide to compression molding powder

Join Date: 2022-07-28

In compression molding, high quality molding powder is indispensable. High quality compression molding raw materials have uniform and consistent heating and plasticizing speed in compression molds, good molding processability and product quality. This article introduces what is compression molding powder is and the process of dry and wet preparation of molding powder.

What is molding powder?

Compression molding powder is a powder material used for molding plastic products. The preparation process of compression molding powder mainly includes the preparation of raw materials and the mixing of raw materials. The preparation of raw materials is based on the selected formula of the product to carry out the necessary raw material pretreatment, metering and conveying processes. Raw material mixing is the use of machinery to disperse each component of the raw material to obtain a uniform component of the material process.

The preparation of compression molding powder includes dry and wet methods. Dry method is a solid resin as the substrate, adding powder filler in the dry state for the mixing and plasticizing process; wet method is a soluble liquid resin as the substrate, adding powder filler in the wet state for the mixing and plasticizing process. The important ones in industry are phenolic molding powder, urea-formaldehyde molding powder, melamine-formaldehyde molding powder, etc.

compression-molding-powder

Dry process of molding powder production

The dry process is widely used in production due to its high production efficiency and consistent quality of the powder. The dry process is also divided into roller press and screw extrusion method, and China mostly uses the roller press method which is dusty but less energy consuming and low cost.

  1. The first step is the preparation of raw materials, fillers should be selected to meet the fineness requirements of the species, resin and other solid materials for crushing and sieving.
  2. The second step is mixing. Powder in the ball mill, Z-type kneader, high-speed mixer and other mixing equipment for mixing, the order of adding material to the uniformity of material mixing has a great impact, should first be mixed with resin and other groups in addition to filler, and then add filler for mixing, because the amount of filler is very large, if mixed together will affect the mixing efficiency and uniformity.
  3. The third step is the hot roller press. Hot rolling is a key process in the production of compression molding powder, and the quality of the molding powder depends on the temperature and time of hot rolling after the formula is determined. The temperature of the two rollers is also different, the temperature of the low temperature roller should be higher than the softening point of the resin, and the high temperature roller is 20-30℃ higher than that of the low temperature roller. 2 roller gap can be adjusted, and its size determines the thickness of the material sheet, usually the thickness of the material sheet is controlled as 1.5~3mm. When the roller is pressed, the material is heated and melted on the roller, and is squeezed, sheared and kneaded through the roller gap to make the components better impregnated and more uniformly dispersed. In order to mix more uniformly available manually or multiple scrapers to work on the rollers to cut off the pieces of material, turned over and then rolled and repeated several times.
  4. The fourth step is crushing. After cooling, it can be crushed and sieved. The fineness of the crushing is to ensure that the molded powder has good looseness and certain specific volume (usually <2), and the more uniform the granularity, the better. The crushed molded plastic powder also has to be magnetically selected to remove metal impurities and can be packaged after the batch.

wet manufacturing process of molding powder

The wet manufacturing process of compressed molding powder is to mix the thermosetting resin solution with various ingredients (such as fillers, pigments, etc.), and the wet mixture is fully mixed in the Z-shaped mixer at the specified temperature, then sent to the vacuum oven for drying, removing the solvent, and then finely ground to obtain the molding powder. The fluidity of the finished product is controlled by the drying time and temperature. Wet production can be divided into solution method, emulsion method and resin material impregnation method according to the form of resin.

Solution method

The phenolic resin is mixed with the corresponding solvent (solvent can be a mixture of alcohol and various aromatic solvents) according to the ratio, and then injected into the Z-shaped mixer, and then add oleic acid, hexamethylene tetramine and pigment, stirring 20min, then add wood powder in 4 times, each time, stirring 5min, after adding, continue to stir for 1h. The temperature of the whole process should not exceed 45°C, vacuum degree is Finally, the mixture will be dried to be the finished product.

Emulsion method

Put the phenolic resin emulsion with oleic acid and hexamethylenetetramine aqueous solution into Z-shaped mixer, stir for 10min, then add iron red powder and wood powder in four times, stirring for 5min for each addition. finally, stir for several minutes at 40~45°, and send to vacuum drying oven for drying. Drying should be constantly turned, control the temperature at 70 ~ 80 °, the vacuum degree of 0.087 ~ 0.091MPa, the time depends on the requirements of the finished product, if the liquidity is large, it can shorten the time; such as the requirements of high electrical insulation performance, it should be longer.

Resin material impregnation method

Phenol, formaldehyde, catalyst and filler are added into the vacuum double-lobe mixer for stirring, and heated to 65~85° at the same time, and the resin polycondensation reaction is completed after 1.5~2.5h of heat preservation. Then the mixture is vacuum dried in the mixer to make the water content at 7%~8%, and then the mixture is further dried in the vacuum drying oven to 3% water content. This method has a good diffusion effect, so that the surface of the raw material can be fully infiltrated, but because the reaction can not be carried out completely, so the raw material loss is large.

Compression molding powders are used for compression molding, high strength composites, powder coatings, 3D printing and other demanding applications. For compression molding, the powder can be used alone or mixed with lubricants, modifiers, additives and/or reinforcements prior to molding to a basic shape without internal stress. In addition, our deep knowledge of compression molding powders allows us to perfectly control the compression molding process and increase production efficiency.

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