Plastic molding process|Flowability of plastic molding

Plastic molding process

The ability of a plastic to fill a cavity at a certain temperature and pressure is called fluidity. This is an important process parameter that must be considered in mold design. High fluidity is easy to cause excessive overflow, uncompact filling cavity, loose plastic parts, separate accumulation of resin and filler, easy mold sticking, difficulty in demoulding and cleaning, and premature hardening. However, if the fluidity is small, the filling is insufficient, it is not easy to form, and the forming pressure is large. Therefore, the fluidity of the selected plastic must be adapted to the requirements of the plastic part, the forming process and the forming conditions. When designing the mold, the gating system, parting surface and feeding direction should be considered according to the flow performance. Thermoset flow is usually expressed in Lassig flow (in millimeters). The larger the value, the better the fluidity. Each type of plastic is usually divided into three different grades of fluidity for different plastic parts and forming processes. Generally, when the area of ​​plastic parts is large, there are many inserts, the cores and inserts are weak, and the complex shapes with narrow deep grooves and thin walls are unfavorable for filling, plastics with better fluidity should be used. Plastics with a Lasig fluidity of more than 150mm should be used in extrusion molding, and plastics with a Lasig fluidity of more than 200mm should be used in injection molding. In order to ensure that each batch of plastics has the same fluidity, in practice, the batch method is often used to adjust, that is, the same variety of plastics with different fluidity are matched to make the fluidity of each batch of plastics compensate each other to ensure the quality of plastic parts. . See Table 1-1 for the Lasig fluidity values ​​of commonly used plastics, but it must be pointed out that in addition to the type of plastics, the fluidity of plastics is often affected by various factors when filling the cavity, which makes the plastic actually fill the cavity ability changes. Such as fine and uniform particle size (especially round pellets), high humidity, high water content and volatile matter, appropriate preheating and forming conditions, good mold surface finish, and appropriate mold structure are all conducive to improving fluidity. On the contrary, poor preheating or forming conditions, poor mold structure, large flow resistance, or long plastic storage period, overdue period, high storage temperature (especially for aminoplastics), etc., will lead to the actual flow performance of plastic filling the cavity. bad.