Today we will take a closer look at the production process of SMC products.
1、 Preparation before suppression
(1) Quality inspection of SMC
The quality of SMC sheets has a significant impact on the forming process and product quality. Therefore, before pressing, it is necessary to understand the quality of the material, such as the resin paste formula, the thickening curve of the resin paste, the glass fiber content, and the type of glass fiber sizing agent. Single weight, film peel resistance, hardness, and quality uniformity.
(2) Cropping
According to the structural shape of the product, the feeding position, and the process, the shape and size of the sheet cutting are determined. A sample is made, and then the material is cut according to the sample. The shape of the cut is mostly square or circular, and the size is usually 40% to 80% of the projected area of the product surface. To prevent contamination from external impurities, the upper and lower films are only removed before loading.
(3) Preparation of equipment
① Familiar with the various operating parameters of the press, especially adjusting the working pressure, press running speed, and table parallelism.
② The installation of the mold must be horizontal and ensure that the installation position is in the center of the press table. Before pressing, the mold must be thoroughly cleaned and coated with release agent. Before adding materials, clean gauze should be used to evenly wipe the release agent to avoid affecting the appearance of the product. For new molds, oil must be removed before use.
2、 Feeding
(1) Determination of feeding amount
The feeding amount of each product can be calculated according to the following formula during the first pressing: the feeding amount (g)=the volume of the product (cm? 0? 6) X 1.8 (g) (2) The determination of the feeding area The size of the feeding area directly affects the compactness of the 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. The general feeding area is 40% -80%, and if it is too small, it will lead to the orientation of the glass fiber due to the long process. Reduce strength, increase waviness, and even fail to fill the mold cavity. Excessive size is not conducive to exhaust and can easily cause internal cracks in the product.
(3) Feeding location and method
The location and method of feeding directly affect the appearance, strength, and directionality of the product. Normally, 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 all ends of the mold cavity during molding. The feeding method must be conducive to exhaust. When laminating multiple layers of sheets, it is best to stack the blocks in a pagoda shape according to their size. In addition, try not to add the blocks separately, as it may cause air entrapment and fusion zone, leading to a decrease in product strength.
3、 Forming
When the material block enters the mold cavity, the press quickly descends. When the upper and lower molds match, slowly apply the required molding pressure, and after a certain curing time, the product molding is completed. During the molding process, it is necessary to reasonably select various molding process parameters and press operating conditions.
(1) Forming temperature
The molding temperature depends on the curing system of the resin paste, the thickness of the product, production efficiency, and the complexity of the product structure. The molding temperature must ensure the smooth initiation and crosslinking reaction of the curing system, and achieve complete curing.
Generally speaking, the molding temperature selected for thick products should be lower than that for thin-walled products, which can prevent excessive heat accumulation inside the 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 ℃.
The increase in molding temperature can shorten the corresponding curing time; On the contrary, when the molding temperature decreases, the corresponding curing time needs to be extended. The molding temperature should be selected by balancing between the highest curing speed and the best molding conditions. It is generally believed that the molding temperature of SMC is between 120-155 ℃.
(2) Forming pressure
The molding pressure of SMC varies with the product structure, shape, size, and SMC thickening degree. Simple shaped products only require a molding pressure of 5-7MPa; For products with complex shapes, the forming pressure can reach 7-15MPa. The higher the thickening degree of SMC, the greater the required molding pressure. The size of the forming pressure is also related to the mold structure. The molding pressure required for vertical parting structure molds is lower than that for horizontal parting structure molds. A mold with a smaller fit gap requires higher pressure than a mold with a larger fit gap.
In summary, the determination of forming pressure should consider multiple factors. Generally speaking, the molding pressure of SMC is between 3-7MPa.
(3) Curing time
The curing time (also known as insulation time) of SMC at the molding temperature is related to its properties, curing system, molding temperature, product thickness, and color. The curing time is generally calculated as 40s/mm. For products with a thickness of 3mm or more, for every 4mm increase, the curing time increases by 1min.
4、 Press operation
Due to SMC being a rapid curing system, rapid closure of the press is crucial. If the press closes too slowly after feeding, it is prone to pre curing and patching on the surface of the product, resulting in material shortage or excessive size. While achieving rapid closure, the mold closing speed should be carefully adjusted at the end of the press stroke to slow down the closing process and facilitate exhaust.