compression mould

Compression molding process is based on the characteristics of each stage of the curing reaction of thermosetting plastics or other moulding materials.

Compression mold uses a hydraulic press in which a mold is fixed on the top and bottom platens of the press. The molds can be classified as hand molds, used for trial runs; semiautomatic, in which the press follows a programmed cycle but the operator manually loads and unloads the press, and automatic, which operate under a fully automatic press cycle (including automatic loading and unloading).

The process of compression molding is as follows:

• Step1 - Powdery, granular or fibrous plastic is put into the heated metal mold at the molding temperature.
• Step2 - Close the mold and apply pressure to force the material into contact with all mold areas.
• Step3 - Maintain heat and pressure until the molding material is cured.
• Step4 - The product is ejected by ejector, repairing the flash.
• Step5 - After demolding, clean the mold cavity to start the next production.

Compression mold structure

The difference with the injection mold is that the compression mold has no pouring system, and the mold is equipped with a feeding chamber. There is a heating device in the cavity or feeding chamber. The structure of the compression mold can be divided into the following major parts.

(1) Cavity part. The cavity is the location where the part is directly formed, where the mold cavity is composed of a punch, a core, and a cavity.

(2) The feeding chamber is different from the injection mold. There is a feeding chamber containing raw materials above the compression mold cavity.

(3) Heating and cooling system. Thermosetting plastic compression molding needs to be performed at a higher temperature, so the mold must be heated.

In addition, like the injection mold, the compression mold is usually also provided with a guide mechanism, a demoulding mechanism, a lateral split core pulling mechanism, etc., which will not be repeated here.

What are the different types of compression mould?

There are different types of compression molds in the market, they may have different names due to different compression molding materials, such as SMC mold, BMC mold, GMT mold, LFT mold, but more often they are classified according to the compression mold structure.

According to the connection with the press

1. Movable mold. The mold is not fixed on the press. In general, mold parting, loading, closing and taking out the product after molding are all doing outside the press. Mold itself doesn’t need the heating system. This kind of mold is suitable for mass produce medium and small products. The movable mold has simple structure, short manufacturing cycle and low cost. However, the process of materials putting, mold opening and parts taking are all manual operation with high labor intensity and low production efficiency.
2. Stationary mold. The mold is fixed on the press and has a heating system. The whole production process including mold parting, loading, closing and taking out the product after molding are all doing on the press. The working life is long, and it is suitable for high volume production of products with large size.
3. Semi-stationary mold. This mold is somewhere in between we talked above. Generally, the cavity is fixed on the press. The core can be moved along the guide rail and positioned with the positioning block.

According to the closed form of cavity and core

1. Open mold. The mold is characterized by no feeding chamber. This kind of mold structure is simple, low cost, durable, easy to demolding, easy to install and insert.
2. Closed mold. The feeding chamber of the mold is the continuation part of the upper part of the mold cavity, without extruding surface. The pressure applied by the press all act on the product. There is only little material overflow of the mold. The compactness of the product is good, the mechanical strength is high, and the flash is in the vertical direction, easy to remove. This kind of mold is suitable for the products with complicated shape, thin wall and long flow, and also for the mold with small fluidity, large unit pressure and high density. Its disadvantage is quantity of material must be controlled accurately. The side wall of the feeding chamber is easily damaged due to friction between the die punch and the side wall of the feeding chamber, and the wall of the feeding chamber with damage marks is easy to damage the surface of the products.
3. Semi-hermetic mold. There is a feeding chamber on the mold cavity, there is extruding ring in the mold cavity, the product's compactness is better than the open mold molding products, and easy to ensure the height direction dimension accuracy, when demolding can avoid abrading products.

compression mould design

When designing compression mould, it is necessary to design the cavity according to the size and moulding method of the product, besides, it is also required to select the appropriate steel according to the quality requirements and demand of the product surface. Compression mould should be made of mould steel with easy cutting, compact structure and good polishing properties. Such as: P20, 718, 1.2738 (adjust freely), etc. In addition, it is necessary to consider the related problems of machining accuracy and control the dimensional tolerance, shape tolerance and surface roughness

In order to improve the wear resistance and corrosion resistance of compression mould surface, surface treatment is usually carried out. Chromium plating is one of the most common methods. Chromium plating can keep metallic luster for a long time, and it is stable in various acidic media without chemical reaction. Chromium plating has excellent wear resistance and high heat resistance. When the temperature reach to 500℃, there is no obvious change of its appearance and hardness. In addition, nitriding treatment can greatly improve wear resistance.

The parting line design of compression mould should be easy to demoulding, and the way of simplifying the ejection mechanism can be adopted. When the radial dimension of the product requires high precision, the influence of flash thickness on the dimensional accuracy of compression products should be considered, and the vertical parting line can easily ensure the dimensional accuracy of the product

Compression mould temperature directly affects product quality and production efficiency, and the temperature design of compression mould is usually between 140℃ and 160℃. Heating system includes electric heating, steam heating and oil heating. When designing the temperature system of the mould, choosing the appropriate heating method and ensuring the uniform surface temperature of the mould can reduce the deformation and improve the dimensional stability.

In compression mould design stage, new mould software (CAD/CAM, etc.) is used to master the whole process of compression mould design, manufacturing, assembly, inspection, testing and production, including mould design and manufacturing cost estimation, mould assembly evaluation, mould parts manufacturing process evaluation, mould structure and formability evaluation.

What are the advantages and disadvantages of compression mold?

compression mold advantages

• Molds for compression molding are generally simpler than their injection mold counterparts. There is no sprue and runner system in a compression mold.
• Better quality and consistency, high precision, uniform wall thickness and good surface finish can be achieved with good mold design.
• Reduced production time, increased productivity and efficiency, the reason behind this is that there is no waiting period between each cycle.
• Reduced tooling costs, MDC uses high-quality steel to improve the mold life.

compression mold disadvantages

• The process itself is generally limited to simpler part geometries due to the lower flow capabilities of the starting thermosetting materials.
• Provision must be made to heat the mold, usually by resistance heating, steam or hot oil circulation.
• The half-mold needs to be perfectly aligned, otherwise the part will not be removed correctly and the surface quality will be affected.

What kind of mold steel is used for compression mold?

The steel used for compression molds is alloy steel, carbon steel. Alloy steel is a high alloy steel with very low carbon content and is therefore very tough. Carbon steel contains more carbon than alloy steel and is less hard, but is also cheaper than alloy steel. The most common steel used in compression molds is alloy steel. This is because the mold needs to be able to withstand the pressure of molten metal and low alloy steels can withstand higher temperatures than carbon steels.

MDC mold steel suppliers are ASSAB, BUDERUS, LKM, AUBERT&DUVAL, DAIDO, FINKL, HITACHI, ALCOA, BOHLER, SAARS TAHL, BAOSTEEL, etc. In addition to quality steel suppliers, we also use the best heat treatment process, and milling capability to meet the requirements of high precision compression mold. MDC controls the flash within an ideal range through CNC processing technology and the processing technology of mold steel to reduce the waste of materials as much as possible.

Is compression molding cheap?

Compression mold may not be cheap, but it's not as expensive as you might think. But when you need to make a large number of parts quickly and economically, compression mold can be cost effective. This is because it requires less material, less energy, less labor, less equipment, and less overall resources.

Since 1997, MDC Mould have been designed and manufactured for compression moulding of SMC, BMC and GMT materials. Since then, the great achievement can be used to produce automobile moulds (bumper mould, front panel mould, top panel mould, bottom cover mould, pickup box mould, foot plate mould), bathroom moulds (shower base mould, sink mould, washing tank) mould, bathtub mould), electronic mould (meter box mould, connecting box mould, enclouse mould). After effective optimization, the compression mould we produce has a longer mould life and higher precision of product.