Fig。1 plastic shell model Fig。2  Fill time

Moldflow analysis

In the paper MPI software is used to analysis the fill time and filling pressure, weld lines, air traps, maximum clamping force and warpage。 Through the analysis of these parameters, the best injection programs are selected。

Fill time。 Gating system design directly affects the melt filling behavior, while the filling analysis of their ultimate goal is to get the best gating system design。 Fill analysis is from the beginning to the cavity that is filled with injection of the entire process of analysis of the flow front location。 The analysis used to predict the filling behavior of workpiece is reasonable, whether the balance of filling, whether to complete the whole workpiece fill。 Filling the purpose of the analysis is to obtain the best gate location, gate number and the best casting system layout。 Fig。2 shows the results of the analysis of filling time。 Through the different shades of color, the change of state as well as the melt flow during filling is reflected, from which can see whether the cavity is filled, and whether the process of filling is balance。 As can be see from Fig。2, Filling time is 2。825s, and the color shade is softer transition。 The indicates that the filling process is relatively flat, and that the entire injection molded parts can be filled within a short period of time, liquidity balance of good and filling and reasonable situation。 Thus the analysis can be proved that gating system design shown in Figure 2 is feasible。

Air traps。 The circle shown in Fig。3 is air trap。 Air trap location map is shown in the figure。 Melt front bubbles gathered in injection parts within or cavity surface are basically around the lateral side of the injection parts, and concentrated in the parting surface。 In the case air traps mainly in the inner surface of plastic shell, and most appear in the parting surface, as shown in Fig。3。 So air in the melt filling will automatically discharge, the air traps does not affect the appearance of the quality of the product。 Therefore, the case of gas very easily through the mold parting surface gap discharge, there is no trapped air phenomenon, which can effectively prevent the formation of air traps injection molded parts, and scorch the surface defects such as defective。

Fig。3 Air traps Fig。4 Weld lines

Weld lines。 When two or more of the flow front integration, it will form a weld line。 In the weld-line position the molecules are tend to change strongly, so that the mechanical strength significantly weakened in the position, and weld line is not visually obvious。 In this case, increasing the mold temperature and melt temperature make two melt encounters merge better。 It is advisable to increase the screw speed or improve the design of gating system to reduce the friction generated heat, while maintaining melt flow rate to reduce flow channel dimensions。 If weld lines can not eliminated, then they should be located the place that it is less sensitive on the workpiece area to prevent the impact of the mechanical properties and appearance of workpiece quality。 By changing the gate location, or to change the workpiece thickness can change the location of weld lines。

The weld lines position is shown on the plastic pieces in Fig。4。 After a Moldflow optimization analysis, improving the die temperature, increasing adequate the gate and reducing appropriately clamping force, opening of the exhaust system in the welding seam, the product weld lines will become Obvious, reduced scrap rates, to meet actual production needs。 The results from Fig。4 can be seen that there is no large area and continuous weld lines, so workpiece surface quality will not be affected。

Sink index。 Shrinkage lines refer to the phenomenon of surface depressions in the moldings。 Although these depressions are small, but as long as viewing from different angles, but it is visible。 Shrinkage lines visibility and its surface texture is the role of the color components, so deep is the distinguishing criteria。 The main reason of lines appearance is that in the cooling process heat is contracted。 Figure 5 shows the depth of shrinkage lines。 In the case lines depth is very small, shrink lines very clearly does not affect the product's appearance。