Instrument core bracket is key support part of PCB board of wave soldering equipment control system. Product diagram is shown in Figure 1. Maximum size of product is 206.85mm * 126.80 mm * 8.60 mm, average thickness of plastic part is 1.20 mm, material of plastic part is ABS+PC, shrinkage rate is 1.004, and weight of plastic part is 6.28 grams. Technical requirements for plastic parts are that there must be no defects such as peaks, underfilling of injection molding, flow lines, pores, warpage deformation, silver streaks, cold materials, jet lines, bubbles, etc.

 

Figure 1 Product diagram of instrument movement bracket
As can be seen from Figure 1, plastic part is shaped as a flat bracket. Main glue positions are all connected by narrow strips. Plastic parts are thin and plastic fluidity is poor. Plastic parts do not have various undercuts, there is no need to design slider cores and lifters. Difficulty of mold design is design of gating system and ejection system. There are also many difficulties in mold processing.
Before mold design, product analysis of plastic parts is required. According to product structure, determine type of mold design structure. Glue position of instrument core bracket is narrow and melting plastic process is longer. This kind of plastic part must be designed as a three-plate mold with a fine nozzle, with multiple points of glue. Hot runner can also be designed to feed glue. If a two-plate mold is designed and side gate is filled with glue, plastic part cannot be filled and mold design fails.

 

Plastic parts have poor rigidity and are easy to deform. Material selection ABS+PC, commonly known as plastic alloy, is to enhance rigidity and strength of plastic parts.
First of all, make sure that you must use a point gate to enter glue. Second is a few problems with glue. Development and application of injection mold CAD/CAE technology has greatly reduced cost of mold design and processing, has doubled efficiency. Among them, application of Moldflow software is the most representative. It can not only simulate and analyze flow process of thermoplastic melt entering mold, but also simulate and analyze gate position, pressure distribution, cooling process and injection molding process conditions of injection molded part, find out possible defects, improve success rate of a mold trial, reduce production costs, shorten production cycle. Gate is a small and short passage connecting runner and cavity. It is a key part of gating system, plays role of adjusting and controlling flow rate, feeding time and preventing backflow. Gate design has a great influence on molding quality of injection molded parts. It mainly includes design of shape, number, size and feeding position of gate. On the one hand, gate design must ensure a fast, uniform, and balanced single-directional flow filling mode; on the other hand, it must avoid occurrence of jetting, stagnation, and depression. At present, number and location of gates are generally selected based on experience, design is often unreasonable, and flow balance in cavity cannot be guaranteed. Using injection molding CAE software for flow analysis, it is easy to predict influence of different gate numbers and positions on flow balance, then make corresponding trade-offs and design modifications based on analysis results.
Through simulation analysis of entire injection molding, flow analysis of filling time, injection pressure and cooling analysis, the best gate plan for injection mold of instrument movement bracket is determined. Because structure of plastic part of instrument movement bracket is more complicated, in order to make fluid have better flow performance during injection molding, it is necessary to design 6 point gates, as shown in Figure 5.

 

Mold design cavity rank is 2 cavities. Mold base is standard mold base DCI3550. Production batch of plastic parts is large. Two sets of molds are designed and produced respectively, 1 cavity and 2 cavity respectively, and attached file 3D is 1 cavity, which is only for reference in analyzing structure of plastic parts. Plastic parts are large in size and thin in structural strength. Mold design needs to pay attention to ejection balance. Ejector components are thimble and cylinder. It is necessary to pay attention to length of thimble when making mold to ensure that each thimble moves at the same time to prevent plastic part from being ejected and deformed.
When making mold, it is necessary to pay attention to large fluctuations of parting surface, where there is a rubbing position, cut insert, minimize spark and clear corners. Electrode processing should use good copper materials to make corners of front and rear molds clear and good exhaust.
Mold is designed with a taper locator on each edge of mold base, four corners of front and rear mold cores are machined to facilitate accurate mold clamping.
Front and rear molds are designed for direct water transport to ensure normal injection molding.

 

 

Figure 2 Mould diagram of instrument movement bracket

 

Figure 3 3D diagram of back mold core

 

Figure 4 3D image of the front mold core

     

Figure 5 3D diagram of gating system