SMC mold
SMC is sheet molding compound.
The main raw materials of SMC are composed of GF (special yarn), UP (unsaturated resin), low shrinkage additives, MD (filler) and various auxiliaries.
SMC has the advantages of superior corrosion resistance, softness, easy engineering design, and flexibility. Its mechanical properties are comparable to some metal materials. The products it manufactures have the advantages of good rigidity, deformation resistance, and wide operating temperature range.
At the same time, the size of SMC products is not easily deformed and has excellent heat resistance; it can maintain its performance well in cold and hot environments, and is suitable for outdoor UV resistance and waterproof functions.
Widely used, such as car front and rear bumpers, seats, door panels, electrical appliances, bathtubs, etc.
BMC mold
BMC is the abbreviation of (Bulk Molding Compounds), which is bulk molding compound.
BMC is a thermosetting plastic that is mixed with various inert fillers, fiber reinforcements, catalysts, stabilizers and pigments to form an adhesive "putty-like" composite material for compression molding or injection molding. It is often made by extrusion. Shape into granules, logs or strips to facilitate subsequent processing and shaping.
BMC has many unique properties, such as high hardness, light weight, corrosion resistance, UV resistance, good insulation, and excellent thermal properties, which make BMC more desirable than thermoplastics. At the same time, since many components can be molded at the same time as these parts, there is no need for post-processing, which is more economical from a production perspective.
At present, BMC molds have been used in automobiles, energy, electrical appliances, catering services, household appliances, optical instrument components, industrial and construction supplies and other areas. Such as car tail light covers, electrical boxes, meter boxes, etc.
1. Preparation before suppression
(1) Quality inspection of SMC/BMC: The quality of SMC sheets has a great impact on the molding process and product quality. Therefore, it is necessary to understand the quality of the materials before pressing, 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. Unit weight, film peelability, hardness and quality uniformity, etc.
(2) Cutting: Determine the shape and size of the sheet according to the structural shape of the product, the feeding position, and the process, make a sample, and then cut the material according to the sample. The cutting shape is mostly square or circular, and the size is usually 40%-80% of the projected surface area of the product. In order to prevent contamination from external impurities, the upper and lower films are removed before loading.
Molding process flow chart
2. Preparation of equipment
(1) Be familiar with the various operating parameters of the press, especially adjust the working pressure, press operating speed and table parallelism.
(2) The mold must be installed horizontally and ensure that the installation position is in the center of the press table. Before pressing, the mold must be thoroughly cleaned and a release agent applied. Before adding materials, wipe the release agent evenly with clean gauze to avoid affecting the appearance of the product. For new molds, oil must be removed before use.
3. Adding ingredients
(1) Determination of the feeding amount: The feeding amount of each product can be calculated according to the following formula during the first pressing:
Adding amount = product volume × 1.8g/cm³
(2) Determination of the feeding area: The size of the feeding area directly affects the density of the product, the flow distance of the material and the surface quality of the product. It is related to the flow and solidification characteristics of SMC, product performance requirements, mold structure, etc. Generally, the feeding area is 40% to 80%. If it is too small, the process will be too long, which will lead to glass fiber orientation, reduce strength, increase waviness, and even fail to fill the mold cavity. If it is too large, it is not conducive to exhaust and may easily cause cracks in the product.
(3) Feeding position and method: The feeding position and method 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 asymmetrical and complex products, the feeding position must ensure that the material flow reaches all ends of the mold forming cavity at the same time during molding. The feeding method must be conducive to exhaust. When stacking multiple layers of sheets, it is best to stack the pieces of material in a pagoda shape with small top and large bottom. In addition, try not to add the material blocks separately, otherwise air entrapment and welding areas will occur, resulting in a decrease in the strength of the product.
(4) Others: Before adding materials, in order to increase the fluidity of the sheet, preheating at 100°C or 120°C can be used. This is especially beneficial for forming deep drawn products.
4. Forming
When the material block enters the mold cavity, the press moves down quickly. When the upper and lower molds are matched, the required molding pressure is slowly applied. After a certain curing system, the molding of the product is completed. During the molding process, various molding process parameters and press operating conditions must 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 is initiated, the cross-linking reaction proceeds smoothly, and complete curing is achieved. Generally speaking, the molding temperature selected for thicker products should be lower than that of thin-walled products. This 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℃; while thinner products can be molded at 171℃. As the molding temperature increases, the corresponding curing time can be shortened; conversely, when the molding temperature decreases, the corresponding curing time needs to be extended. The molding temperature should be chosen as a trade-off between maximum curing speed and optimal molding conditions. It is generally believed that the SMC molding temperature is between 120 and 155°C.
(2) Molding pressure: SMC/BMC molding pressure varies with the product structure, shape, size and SMC thickening degree. Products with simple shapes only require a molding pressure of 5-7MPa; products with complex shapes require a molding pressure of up to 7-15MPa. The higher the thickening degree of SMC, the greater the required molding pressure. The size of the molding pressure is also related to the mold structure. The molding pressure required for vertical parting structure molds is lower than that of horizontal parting structure molds. Molds with smaller clearances require higher pressure than molds with larger clearances. Products with high requirements on appearance performance and smoothness require higher molding pressure during molding. In short, many factors should be considered when determining the molding pressure. Generally speaking, the SMC molding pressure is between 3-7MPa.
(3) Curing time: The curing time of SMC/BMC at the molding temperature (also called the holding time) is related to its properties, curing system, molding temperature, product thickness and color and other factors. The curing time is generally calculated as 40s/mm. For products thicker than 3mm, some people think that for every 4mm increase, the curing time will increase by 1 minute.
5. Molding production process control
(1) Process control
The viscosity (consistency) of SMC should always remain consistent during pressing; after removing the carrier film of SMC, it cannot be left for a long time. It should be pressed immediately after removing the film and should not be exposed to the air to prevent excessive volatilization of styrene; keep SMC The feeding shape and feeding position of the sheet in the mold should be consistent; keep the temperature of the mold at different positions uniform and constant, and should be checked regularly. Keep the molding temperature and molding pressure constant during the molding process and check them regularly.
(2) Product testing
Products should be tested for the following aspects:
Appearance inspection: such as glossiness, flatness, spots, color, flow lines, cracks, etc.;
Mechanical property testing: bending strength, tensile strength, elastic modulus, etc., whole product performance testing; other properties: electrical resistance, media corrosion resistance.