Arc-shaped base assembly is a 1/3 ring structure, diameter of outer ring is φ125mm, diameter of inner ring is φ112mm, maximum wall thickness is 6.5mm, minimum wall thickness is 2.5mm, thickness of the entire base is 5mm, there are 2 metal shrapnel embedded in the middle, 3 screw holes are evenly distributed in thickness direction for later installation and fixation, material is PA66, including 33 %glass fiber.
From shape and structure of plastic part, it can be seen that due to change of wall thickness and open 1/3 circle structure, the biggest difficulty in injection molding lies in control of its warpage deformation, followed by positioning of metal shrapnel in-mold injection molding.

 

Results of melt flow analysis are shown in Figure 2. Point gate is used to feed from middle, the overall flow is relatively normal, and there is no phenomenon of insufficient filling.

 

Warpage analysis results are shown in Figure 3. Maximum comprehensive deformation of plastic part reaches 1.074mm, and deformation is concentrated at both ends of base joint. From perspective of deformation in Y direction, deformation is mainly caused by shrinkage deformation in Y direction, as shown in Figure 4.

 

 

Based on results of structural analysis and mold flow analysis, it is necessary to deal with deformation to prevent warpage deformation during shrinkage process from being too large to cause size of molded plastic part to not meet requirements of drawing. When it is impacted and deformed by molten plastic, size of molded plastic part does not meet requirements.

According to results of mold flow analysis, deformation is mainly concentrated at both ends of arc, and the overall deformation of plastic part can be solved by solving deformation problem at the end. Combined with mold flow analysis of maximum deformation in Y direction, size of arc part at tail end is correspondingly increased. Mold forming segmented mosaic structure is shown in Figure 5, tail end is divided into deformation area 1 and deformation area 2, different deformation amounts are processed according to results and trends of mold flow analysis. Insert processing is performed for area 2 with large deformation. If set deformation amount is not as expected and size is out of tolerance, it is only necessary to repair or re-process insert to reduce the overall rework amount to shorten mold manufacturing time and save cost.

 

Since shrapnel of in-mold injection molding is relatively thin, if an effective fixing method is not adopted, molten plastic will form a huge impact force on shrapnel during injection molding process, resulting in failure of in-mold injection molding of shrapnel. From structure and molding position of shrapnel, shrapnel is completely suspended in the middle of cavity and must be fixed.

(1). Fixation of in-mold injection molding part and non-molding part can be solved by setting of parting surface. As shown in Figure 6, parting surfaces at different positions are set on inner and outer circles of plastic part, outer surface sinks to movable mold as a whole, inner parting surface of plastic parts are contact surfaces between shrapnel and mold parts to avoid shrapnel hanging in air. Fixed mold and movable mold insert form a concave-convex fit on inner circular parting surface. In addition, using non-in-mold injection molding part of shrapnel, inserts are used to position shrapnel, and at the same time, connected wire parts are clamped with inserts. Distribution of card wire inserts is shown in Figure 7.

 

 

(2) Reduce injection impact force of in-mold injection molding part. Impact force is reduced by method of material reduction, that is, steel processing is performed in cavity to reduce deformation space of shrapnel in mold cavity, so as to avoid excessive deformation of shrapnel caused by impact force of molten material, resulting in failure of the shrapnel in-mold injection molding. Structure of plastic part processed by material reduction is shown in Figure 8. Depth of material reduction is flush with placement surface of shrapnel, that is, flush with parting surface.

 

The overall structure of mold is shown in Figure 9. Standard mold base, model CT-3340-A70-B100-C80, is used. Fixed and movable mold inserts are sunk into fixed and movable mold plates respectively. Method is shown in Figure 6. Layout is carried out in the way of intermediate feeding and 2 cavities. As shown in Figure 10, after mold is opened, molding shrapnel is placed in corresponding positioning insert. After injection molding, plastic parts are pushed out and demoulded by a push rod.

 

1. Fixed die base plate 2, fixed die plate 3, guide post 4, movable die plate 5, support block 6, reset rod 7, movable die base plate 8, gate sleeve 9, fixed die insert 10, movable die insert 11 , Pull rod 12, Push rod 13, Push rod fixing plate 14, Screw 15, Push plate

 

After optimization, injection-molded arc-shaped base assembly is shown in Figure 11. Size of arc-shaped base assembly after forming meets assembly requirements, and deformation is within expected range, indicating that anti-deformation design has achieved expected effect; shrapnel has no deformation and displacement during forming process, size meets requirements, positioning design achieves expected effect.