Design of Injection Mold for Eccentric Alignment

Time:2021-02-17 13:52:00 / Popularity: / Source:

Abstract: According to demand plan of window products, matching injection molding technology was selected, focusing on demolding system, support structure, etc., to cleverly solve problem of deviation between injection center and geometric center of mold, realize smooth implementation of injection mold design.

1 Product process analysis

Design of Injection Mold 
Window products need to be completely transparent, have certain strength and toughness requirements, have no obvious molding traces on the surface, especially front panel must not have any defects, product needs 10,000 pieces, and batch is not too large. Based on above considerations, injection molding process is adopted, molding efficiency is high, production cost is low, and surface quality of product is good; PC (polycarbonate) plastic is used, with a transparency of 90%, rigid and tough, has high impact resistance, high dimensional stability and a wide range of melting temperatures, which can fully guarantee mass production of product.

2 Mold design

Successful completion of injection molding process requires matching molds, plastics, processes and molding equipment. Among them, mold design and manufacturing are very important factors. Mold design process and main points of window products are as follows:

2.1 Mold layout

Design of Injection Mold 
Window product is a flat shallow shell part with a wall thickness of 2 mm, which is basically uniform. Overall size is 186 mm * 186 mm * 7.5 mm. There are 6 cylindrical holes with an outer diameter of 7 mm and an inner diameter of 3.5 mm on the back of product. Engineering drawing is shown in Figure 1. According to characteristics of low batch demand of product and high production efficiency of injection molding, manufacturing cost of mold is integrated, mold cavity is laid out as one cavity, as shown in Figure 2. Single-cavity injection molds generally use point gates or direct gates for gating, but they are not suitable for products with strict surface quality and transparent windows. Based on above two main problems, form of gating is set as a flat side gate, which will cause injection center an geometric center of mold base to not overlap. Problems caused by this situation can be dealt with and overcome through structural improvement and optimization to ensure smooth implementation of design scheme.

2.2 Gating system

According to mold cavity layout of one cavity, as well as requirements of product transparency and high surface quality, window injection mold uses side gate method, main runner bushing uses standard parts with an outer diameter of 10 mm and a total length of 50 mm. Sub-runner adopts a semicircular cross-section, a diameter of 8 mm, and a length of 14 mm. Gate adopts a flat rectangular structure with a size of 1.2 mm*2.4 mm*0.8 mm (length*width*height).

2.3 Demoulding system

Window product is a fully transparent product. According to design experience, ejection structure of push plate structure should be used, which will cause mold structure to be complicated and cost will increase; in order to reduce cost of mold, this solution adopts traditional ejection form of ejector. Position is set at the edge of product to reduce traces left by thimble on the surface of product. Specific position is shown in Figure 2. Due to one-cavity mold cavity layout, coupled with side gate gating, geometric center of mold does not coincide with injection center, which will cause overall layout of ejector pin to deviate from geometric center of ejection mechanism. For this reason, following two requirements are proposed: (1) Ejection mode of injection molding machine adopts double ejector pins to ensure overall force balance of push plate; (2) Ejection mechanism and movable mold part must have guide posts and guide sleeves to ensure smooth and balanced movement of ejection mechanism, as shown in Figure 2.

2.4 Supporting structure

Regarding problem of offset between geometric center of mold and center of injection molding in this design, during injection process, injection pressure is unevenly distributed and mainly concentrated on the side of product. Therefore, it is necessary to design a support structure to ensure that movable mold plate has sufficient strength to resist deformation. Generally, there are two methods: (1) Add an appropriate number and diameter of support columns between movable mold plate and movable mold seat plate; (2) Add a pad of appropriate thickness between movable mold plate and support block. In order to facilitate manufacturing and assembly, this scheme chooses second way to increase backing plate, as shown in Figure 2. According to calculation of following empirical formula, material of backing plate is 45# steel, and thickness is 35 mm. At the same time, molded parts of molded column holes are made into inserts, which is convenient for processing and mold repair, also saves materials. Backing plate structure also ensures smoothness of inserts.
Where: P is required maximum injection pressure (in MPa); S is projected area of product in mold opening direction (in mm2); [τ] is allowable shear stress of 45# steel (in MPa); A It is shear area of backing plate in mold opening direction (unit: mm2).

2.5 Mold assembly drawing

According to product's dimensions and characteristics of cavity layout, LKM standard mold base was selected with a specification of 3045, but positions of movable and fixed mold plates were changed. Specific situation is shown in Figure 2. Movable and fixed mold plate forming part, under condition of ensuring  shape accuracy, mirror polishing processing method can ensure surface quality of front and back of window product and avoid forming defects of processing knife. Cooling water channel adopts 8 mm water holes and center distance is 30 mm. Based on design of above mechanism and structural parts, mold can be opened and closed smoothly, with cooperation of guide post and guide sleeve of demolding mechanism, ejector mechanism can eject product smoothly, it can move freely under action of reset rod, and there is no uneven force. After practical application, current mold structure is stable, window products produced are flat and not deformed, surface quality is smooth, transparency is high, and there are no obvious surface defects, which ensures normal production of product.

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