What Is Compression Molding?

Compression molding, also known as compression molding, press molding, etc., is to add powdered or loose granular solid plastics directly to the mold, and gradually soften and melt them by heating and pressure. Cured into plastic parts, mainly used to form thermosetting plastics, but also for thermoplastics. Compared with injection molding, its advantages are that ordinary hydraulic presses can be used and the structure of the compression mold is simple (no pouring system). In addition, the compression plastic parts have less internal orientation structure, the plastic part has a smaller shrinkage rate and uniform performance. The disadvantages are long forming cycle, low production efficiency, high labor intensity, difficult to control the precision of plastic parts, short mold life, and difficult to realize automated production.

The principle of compression molding is shown in Figure Xin.

Compression molding is mainly used for the molding of thermosetting plastics. Compared with injection molding, the advantages of compression molding are: it can be produced using ordinary presses; because the compression mold has no pouring system, the mold structure is relatively simple; there is less oriented organization in the plastic part. Low degree of orientation, relatively uniform performance, small molding shrinkage, can produce some fillers with chip shape, sheet shape or long fiber shape; poor fluidity, plastic parts that are difficult to be formed by injection molding, large area and thickness Small large flat plastic parts. ^[1]

(1) Preparation before molding

Thermoset plastics are relatively easy to absorb moisture and are susceptible to moisture during storage. Therefore, plastics should be preheated and dried before processing. At the same time, because the specific volume of the thermosetting plastic is relatively large, in order to make the molding process smoothly, sometimes the plastic is pre-pressed first:

Preheat and dry. Before molding, the thermosetting plastic should be heated. The purpose of heating is twofold: one is to preheat the plastic to provide a hot material with a certain temperature to the compression mold, so that the plastic is uniformly heated in the mold, and shorten the compression molding cycle; the second is to dry the plastic to prevent Excessive moisture and low molecular volatiles are contained in the plastic to ensure the molding quality of the plastic part.

Preload. Pre-compression refers to compacting loose powdery, granular, chip-like, sheet-like or long-fibrous molding materials into plastics with a uniform weight and uniform shape at room temperature or slightly above room temperature before compression molding. The shape of the parison so that it can be easily put into the compression mold to feed the pre-compacted blank is generally disc-shaped or disc-shaped, and it can also be pressed into a shape similar to that of plastic parts. The pre-pressing pressure can usually be selected within 40 ~ 200MPa, and the density of the blank after pre-pressing should preferably reach about 800k of the density of the plastic part to ensure that the blank has a certain strength.

(2) Compression molding process

The mold should be preheated after being installed on the press. If the plastic parts have inserts, the preheated inserts should be placed in the mold cavity before feeding. The molding process of thermosetting plastics can generally be divided into feeding, mold closing,

The quality of plastic parts is not only affected by the performance of the plastic raw materials and the size of the mold structure, but also related to the formulation and control of the molding process parameters. This is the three elements of molding often mentioned in production: temperature, pressure, and time.

1) molding temperature

Molding temperature refers to the mold temperature required during pressing. At this temperature, the plastic melt flows in the mold cavity, fills the cavity, and solidifies. The mold temperature is not equal to the temperature of the plastic melt in the cavity. For thermosetting plastics, the maximum temperature of the plastic melt is higher than the temperature of the mold due to the heat generated during the crosslinking reaction of the plastic. When thermoplastics are compression-molded, the temperature of the plastic melt in the cavity is limited to the mold temperature. If the molding temperature is too high, the curing time will be short, but it will cause mold filling difficulties, make the surface of the plastic part dull, dull, and even swell, deform and crack. If the molding temperature is too low, the curing time is slow and the molding time is long. Therefore, the determination of the mold temperature should comprehensively consider all factors, it is also the key to ensure the molding quality of plastic parts.

2) forming pressure

The molding pressure refers to the unit pressure on the projected area of the plastic part by the press during compression molding. Its role is to force the plastic melt to flow and fill the mold cavity, avoid defects such as bubbles and loose structures due to low-molecular volatiles inside the plastic part, ensure that the plastic part has a fixed shape and size, prevent deformation, and improve its internal quality.

3) time

When thermosetting plastics are compression-molded, they need to be held at a certain temperature and pressure for a certain period of time to fully cross-link and solidify to become an excellent plastic part. This period of time is called compression time. The compression time is related to the type of plastic (resin type, volatile content, etc.), the shape of the plastic part, the process conditions (temperature, pressure) of the compression molding, and the operating steps (whether exhaust, pre-press, pre-heat) and so on. As the compression molding temperature increases, the plastic solidifies faster and the required compression time decreases: an increase in compression pressure will also reduce the compression time, but it is not as obvious as increasing the temperature. In addition, the compression time will increase as the wall thickness of the plastic part increases. The length of compression time has a great impact on the performance of plastic parts. The compression time is too short, the plastic is not hardened (under-ripe), the appearance quality of the plastic part is poor, the mechanical properties are reduced, and it is easy to deform. Properly increasing the compression time can reduce the shrinkage of plastic parts and improve its heat resistance and other physical and chemical properties. However, if the compression time is too long, not only the productivity will be reduced, but also the plastic parts will shrink too much, the internal stress will increase, and the plastic parts will easily break. Generally, the compression time of phenolic plastic is 1 ~ 2min, and the silicone plastic is 2 ~ 7min. ^[2]