SMC compress molding, is to place the molding compound in the cavity of the female mold, close the male mold, and use the pressure and heat to melt the material into the cavity to form the same product as the cavity. A molded article which is formed by heating, solidifying, and demolding after cooling.
The main advantages of the compression molding process:
1 high production efficiency, easy to achieve specialization and automated production;
2 product size accuracy and repeatability;
3 smooth surface, no need for secondary modification;
4 can form a complex product at a time;
5 Because of mass production, the price is relatively low.
In the MC/BMC molding process, it is important to control the “3 points”, that is, 3 main process parameters: molding temperature, molding pressure and molding time.
First, the molding temperature
The molding temperature is the mold temperature specified during the molding process. This process parameter determines the heat transfer conditions of the mold to the material in the cavity, and has a decisive influence on the melting, flow and solidification process of the material.
The temperature change of SMC/BMC molding compound during molding is more complicated. Since plastic is a poor conductor of heat, the temperature difference between the center and the edge of the material at the beginning of molding is large, which will cause the curing cross-linking reaction not in the inner and outer layers of the material. Start at the same time.
The surface layer is hardened by heat to form a hard shell layer, and the curing shrinkage of the inner layer material is limited by the outer hard shell layer, so that residual compressive stress is often present in the surface layer of the molded product, while the inner layer is carried. There is residual tensile stress, and the presence of residual stress causes warpage, cracking, and strength degradation of the product.
Therefore, measures are taken to minimize the temperature difference between the inside and outside of the material in the cavity, and eliminating uneven curing is one of the important conditions for obtaining high quality products.
The molding temperature of SMC/BMC molding compound depends on the exothermic peak temperature and curing rate of the curing system. Usually, the temperature range of the curing peak temperature is slightly lower than its curing temperature range, generally about 135-170 ° C and determined by experiment. The system with a fast curing rate takes the temperature at a low point, and the system with a slow curing rate takes a higher temperature.
When forming a thin-walled product, the upper limit of the temperature range is taken. The thick-walled product can be taken as the lower limit of the temperature range. However, when the thin-walled product with a large molding depth is used, the lower limit of the temperature range should be taken because the process is long to prevent the material from solidifying during the flow. .
Appropriately increasing the molding temperature without impairing the strength of the product and other performance indicators is advantageous for shortening the molding cycle and improving the quality of the product. The molding temperature is too low, not only the viscosity of the material after melting is high, the fluidity is poor, and the crosslinking reaction is difficult to proceed sufficiently, so that the strength of the product is not high, the appearance is dull, and the mold sticking and the ejection deformation occur during demolding.
Second, the molding pressure
The molding pressure is usually expressed by the molding pressure (MPa), which is the ratio of the total force exerted by the FRP hydraulic machine on the mold to the projected area of the mold cavity in the pressing direction.
The role of the molding pressure in the compression molding process is to tightly close the mold and to densify the material, as well as to promote the flow of the melt and to balance the pressure generated by the volatilization of low molecular weight in the cavity.
A molding material having a large compression ratio consumes a large amount of energy when it is densified, so that a high molding pressure is required for molding, so that the molded bulk material has a higher pressure than the molded blank, and the SMC/BMC molding material is high. It is also higher in pressure than the molded powder.
When molding materials with high melt viscosity and fast cross-linking rate, as well as products with complex shapes, thin walls, deep or large areas, it is necessary to overcome the large flow resistance to fill the cavity, thus requiring higher molding. pressure. The high molding temperature accelerates the crosslinking reaction, resulting in a rapid increase in the viscosity of the melt, so it is required to use a high molding pressure.
The high molding pressure has a series of advantages such as increasing the density of the product, lowering the molding shrinkage rate, prompting rapid flow filling, overcoming swelling and preventing the occurrence of pores. However, excessive molding pressure reduces the service life of the mold, increases the power consumption of the hydraulic machine, and increases the residual stress in the product.
Therefore, when processing thermosetting plastic molded products, pre-pressing, preheating, and appropriately increasing the molding temperature are used to avoid high molding pressure. If the preheating temperature is not properly increased or the preheating time is extended, the partial solidification fluidity is lowered during the preheating process, and not only the molding pressure but also the higher molding pressure is used to ensure that the material fills the cavity.
Third, molding time
The molding time is also called compression molding insulation/holding time. It refers to the time when the material is heated and solidified in the mold after the mold is completely closed or after the last venting and closing of the mold.
The role of the molding time in the molding process is mainly to allow the molded body having the cavity shape to have sufficient time to complete the curing. Curing refers to the formation process of a bulk structure during the formation of a thermosetting plastic. From the nature of the chemical reaction, the curing process is a process in which the crosslinking reaction proceeds.
However, the "complete curing" of the process does not mean that the crosslinking reaction has proceeded to the end, that is, all the reactive groups that can participate in the crosslinking have all participated in the reaction. This term is technically meant that the crosslinking reaction has been carried out to the appropriate extent and that the overall physical and mechanical properties of the article or other specifically specified properties have met the desired specifications.
Obviously, the degree of cross-linking of the product is unlikely to reach 100%, and the degree of cure can exceed 100%. The phenomenon that cross-linking is more than required for complete curing is generally referred to as "over-ripening", and vice versa.
The molding time is determined by the curing rate of SMC/BMC molding compound, the shape and wall thickness of the product, the structure of the mold, the molding temperature and the molding pressure, as well as the factors such as pre-compression, preheating and whether the exhaust is formed during molding. Among all these factors, the influence of molding temperature, product wall thickness and preheating conditions on the molding time is most significant.
Suitable preheating conditions are beneficial to shorten the molding time by speeding up the temperature rising process of the material in the cavity and filling the cavity. When the molding temperature is increased, the molding time is shortened and the thickness of the product wall is increased. Extend the molding time.
When the molding temperature and the molding pressure are constant, the molding time becomes a key factor determining the performance of the product. If the molding time is too short, the resin cannot be completely cured, the product is under-cooked, and the mechanical properties are poor, the appearance lacks luster, and warpage and deformation are likely to occur after demolding. .
Appropriate extension of the molding time not only overcomes the above disadvantages, but also reduces the molding shrinkage of the product and improves its heat resistance, strength properties and electrical insulation properties. However, excessively prolonging the molding time will over-cook the product, which not only reduces the production efficiency, increases the energy consumption, but also increases the shrinkage rate due to excessive cross-linking, resulting in a large internal stress between the resin and the filler; Dark blistering, severe rupture of the product.