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1. Preparation before pressing of SMC products
(1) Quality inspection of SMC
The quality of SMC sheet has a great influence on the molding process and product quality. Therefore, it is necessary to understand the quality of the material before pressing, such as resin paste formula, resin paste thickening curve, glass fiber content, and type of glass fiber sizing agent. Single weight, film peelability, hardness and mass uniformity, etc.
(2) Cut
According to the structure and shape of the SMC product, the feeding position and the process determine the shape and size of the sheet cutting, make a template, and then cut the material according to the template. The shape of the cut is mostly square or round, and the size is mostly 40%-80% of the projected area of the product surface. In order to prevent the contamination of external impurities, the upper and lower films are peeled off before loading.
(3) Preparation of equipment
①Familiar with the operating parameters of the press, especially to adjust the working pressure, the running speed of the press and the parallelism of the table.
②The mold installation must be level, and ensure that the installation position is in the center of the press table. Before pressing, thoroughly clean the mold and apply a mold release agent. Use clean gauze to wipe the release agent evenly before feeding, so as not to affect the appearance of the product. For new molds, oil must be removed before use.
2. Feeding of SMC products
(1) Determination of feeding amount
The feeding amount of each product can be calculated according to the following formula during the first pressing = product volume X 1.8.
(2) Determination of the feeding area Determine the size of the feeding area
It directly affects the compactness of the SMC product, the flow distance of the material and the surface quality of the product. It is related to the flow and curing characteristics of SMC, product performance requirements, mold structure, etc. Generally, the feeding area is 40%-80%. If it is too small, the glass fiber will be oriented due to the long process. Reduce the strength, increase the waviness, and even fail to fill the cavity. If it is too large, it is not conducive to exhaust, and it is easy to produce cracks in the product.
(3) Feeding location and method
The feeding position and method directly affect the appearance, strength and directionality of the product. Under normal circumstances, the feeding position of the material should be in the middle of the mold cavity. For asymmetric complex products, the feeding position must ensure that the material flow reaches each end of the mold cavity at the same time during molding. The feeding method must be favorable for venting. When the multi-layer sheets are stacked, it is best to stack the material blocks in a pagoda shape according to the upper and lower ones. In addition, try not to add the material blocks separately, otherwise, air accumulation and welding areas will occur, resulting in a decrease in the strength of the product.
3. SMC product molding
When the material block enters the cavity, the press moves down rapidly. When the upper and lower molds are matched, the required molding pressure is slowly applied, and after a certain curing time, the molding of the product is completed. During the molding process, various SMC moulding process parameters and press operating conditions should be reasonably selected.
(1) Molding temperature
The molding temperature depends on the curing system of the resin paste, the thickness of the product, the production efficiency and the complexity of the product structure. The molding temperature must ensure that the curing system initiates, the cross-linking reaction proceeds smoothly, and complete curing is achieved. Generally speaking, the molding temperature of SMC products with large thickness should be lower than that of thin-walled products, which can prevent excessive heat accumulation inside thick products due to excessive temperature. If the thickness of the product is 25-32mm, the molding temperature is 135-145℃. And thinner products can be formed at 171 ℃.
When the molding temperature is increased, the corresponding curing time can be shortened; on the contrary, when the molding temperature is lowered, the corresponding curing time needs to be extended. Molding temperature should be selected as a trade-off between maximum cure speed and optimum molding conditions. It is generally believed that the SMC molding temperature is between 120-155 °C.
(2) Forming pressure
The pressure of the SMC molding process varies with the product structure, shape, size and the degree of SMC thickening. Products with simple shapes only need a molding pressure of 5-7MPa; products with complex shapes, the molding pressure can reach 7-15MPa. The higher the degree of SMC thickening, the higher the molding pressure required. The size of the molding pressure is also related to the mold structure. The molding pressure required for the vertical parting structure mold is lower than that of the horizontal parting structure mold. A mold with a smaller fit requires higher pressure than a mold with a larger clearance. In short, the determination of forming pressure should consider many factors. Generally speaking, the SMC molding pressure is between 3-7MPa.
(3) Curing time
The curing time of SMC at the molding temperature is related to its properties and curing system, molding temperature, product thickness and color and other factors. The curing time is generally calculated at 40s/mm.
4. SMC product press operation
Since SMC is a fast curing system, fast closing of the press is important. If the press is closed too slowly after feeding, pre-cured patching spots may easily appear on the surface of the product, or there will be material shortages and excessive size. While achieving rapid closing, the closing speed of the mold should be carefully adjusted at the end of the press stroke to slow down the closing process and facilitate exhaust.
