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 when designing a mold. The fluidity is easy to cause excessive flashing, the filling cavity is not dense, the plastic structure is loose, the resin and the filler are accumulated in the head, the mold is easy to stick, the mold is released and the cleaning is difficult, and the hardening is too early. However, if the fluidity is small, the filling is insufficient, the forming is difficult, and the forming pressure is large. Therefore, the fluidity of the selected plastic must be compatible with the requirements of the plastic parts, the forming process and the forming conditions. The mold design should consider the gating system, the parting surface and the feeding direction according to the flow performance.
The fluidity of thermoset plastics is usually expressed in terms of Laceg mobility (in millimeters). The value is large and the fluidity is good. The plastics of each variety are usually divided into three different grades of fluidity for different plastic parts and forming processes. Generally, the plastic parts are large in area, the inserts are large, the core and the insert are weak, and the complex shapes with narrow deep grooves and thin walls are not suitable for filling, and plastics with better fluidity should be used. Plastics with a fluidity of 150 mm or more should be selected for extrusion molding, and plastics with a flow of 200 mm or more of Rasig should be applied during injection molding. In order to ensure that each batch of plastics has the same fluidity, in practice, the batch method is used to adjust, that is, the plastics with different fluidity of the same variety are used together, so that the fluidity of each batch of plastics is compensated to each other to ensure the quality of the plastic parts. . The Lacyg fluidity value of commonly used plastics cannot be displayed in tabular form, but it must be pointed out that the plasticity of the plastic is not only determined by the plastic type, but also is often affected by various factors when filling the cavity to make the plastic actually filled. The ability of the cavity changes. Such as fine particle size (especially round pellets), high humidity, water content and volatiles, proper preheating and forming conditions, good mold surface finish, proper mold structure, etc. are all beneficial to improve fluidity. Conversely, poor preheating or forming conditions, poor mold flow resistance, long plastic storage period, overdue, high storage temperature (especially for aminoplasts) can cause the actual flow properties of the plastic to fill the cavity to decrease and cause filling. bad.
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