What is the process of SMC compression moulding?

SMC compression moulding is to cut the plastic into the desired shape, determine the number of layers to be added, remove the two sides of the film, laminate and place it on the SMC mould.

The press moves down quickly when the SMC is added to the mould cavity. When the upper and lower moulds match, slowly apply the required moulding pressure. After a certain time of curing reaction, the SMC compression moulding process of the product is finished. In the moulding process, various moulding parameters and press operating conditions should be selected reasonably.

Molding temperature

The moulding temperature depends on the curing system of resin, the thickness of the product, the production efficiency and the complexity of the SMC part structure.

Generally speaking, the moulding temperature chosen for parts with large thickness is lower than that for thin-walled products to prevent excessive heat accumulation inside the thick products. For example, the moulding temperature is 135-145℃ when the thickness of the product is 25~32mm, while thinner products can be moulded at 171℃.

For the moulding process, a lower moulding temperature allows for a longer gel time and avoids pre-curing. In some cases, such as the moulding of deep-drawn products, etc., a lower moulding temperature should generally be selected. In summary, the moulding temperature should be selected as a trade-off between the highest curing speed and the best moulding conditions.

The moulding temperature of the sheet moulding compound is 120~155°C, and should be avoided above 170°C, otherwise bulging will occur on the product. Curing time will be extended at temperatures below 140°. If the temperature is below 120°, the basic curing reaction will not take place smoothly. When controlling the moulding temperature, pay attention to the temperature difference between the negative and positive mould must be maintained, generally speaking, the negative mould should be higher than the positive mould temperature of about 5.5 ℃.

Molding pressure

The moulding pressure of sheet moulding compound is slightly higher than that of polyester mass. The higher the degree of thickening of the sheet moulding compound, the higher the pressure required for moulding; the poorer the liquidity, the smaller the area for adding material, and the higher the moulding pressure required.

The moulding pressure also varies with the structure, shape and size of the product. For products with a simple shape, only 2.5~3MPa moulding pressure is required; for products with complex shapes, such as products with reinforcement, wings, and deep drawing structure, the moulding pressure can be 14-21MPa.

The size of moulding pressure is also related to the structure of the mould, the moulding pressure required for vertically parted moulds is lower than that for horizontally parted moulds; moulds with smaller gaps need higher moulding pressure than those with larger gaps.

Generally speaking, the molding pressure of SMC is 3.5~7.0MPa.

Curing time

Sheet moulding compound curing time at moulding temperature and its nature and curing system composition (curing agent type, dosage), moulding temperature, product thickness and other factors. Curing time is generally calculated by 40s/mm. For thick products (more than 3mm), for every 4mm increase, curing time increased by 1min.

Press operation

If the press is too slow to close after the material is added, the surface of the product will have a pre-cured patch or a defect with too large a missing material size. The above-mentioned defects are more likely to occur when the forming temperature is high. However, while achieving fast closing, the closing speed should be reduced at the end of the press stroke to slow down the closing process so that the enclosed air can escape smoothly. For the last 25mm stroke, the closing time should be 5~20s, if it is faster than 4s, it may damage the shear edge.

The compression moulding process of SMC is similar to thermoset moulding, the moulding pressure of SMC is slightly higher than that of BMC, the production process of SMC is simple and convenient, the production efficiency is high, easy to automate, and the products have good dimensional stability and smooth surface. MDC Mould has provided solutions for our customers in the automotive industry, bathroom field and daily necessities field.