See Figure 1 for product of storage box for leg guards. Maximum dimensions of product are 110.80 mm * 74.55 mm * 15.68 mm, average thickness of plastic part is 2.54 mm, material of plastic part is PP, shrinkage rate is 1.016, and weight of plastic part is 20.00 grams. Technical requirements for plastic parts are that there should be no defects such as peaking, underfilling, flow lines, pores, warping deformation, silver streaks, cold material, and spray lines.

 

Figure 1 Product picture of storage box for leg guards
Leg guard storage box is a storage box for a certain medical and health care equipment, which has high requirements. It can be seen from Figure 1 that plastic part is a flat pear-shaped shell, which is divided into two parts, left and right, connected by a plastic hinge in the middle. Plastic hinge is a commonly used structural form in packaging products. It is simple in structure, easy to use, low in product and injection molding processing costs. Plastic hinges are mostly made of PP or PE materials. Proper plastic hinge design is important, but so is processing and precision of hinge. When injecting plastic hinge products, it is necessary to understand principle the orientation of polymer melt during injection molding.

 

Figure 2 3D mold design drawing

 

Fig. 3 Back mold core map
Main points of plastic hinge design and processing are as follows:
(1) When plastic melt passes through narrow forming hinge channel, shear rate increases so that polymer has a clear flow orientation arrangement. Thickness of middle part of hinge is 0.25~0.38mm, and its length is about 0.5mm. Overlap length of hinge is about 1.5mm. Too long and thin hinges will cause too much pressure loss when melt flows through, cavity on the other side of hinge will not have enough pressure maintaining and feeding.
(2) Gate of small plastic part must be arranged on same side of hinge, so that melt flows along longitudinal direction of hinge, and polymer molecular chains are aligned in bending direction. There must be no flow defects and welded seams at hinges. For large plastic parts, glue must be injected at two places on both sides of hinge, two gates on the left and right should not make molten plastic converge at hinge.
(3) Since thickness of plastic hinge is very thin, it is necessary to prevent degradation and breakage of molecular chain caused by excessive shear stress and shear rate when melt flows through hinge gap. In order to avoid excessive residual stress at hinge, necessary rounded corners or transition sections should be provided at the entrance and exit of hinge. There should be a corresponding reasonable injection process during injection to avoid excessive cavity pressure caused by melt being blocked at hinge.
(4) After hinged plastic part is taken out of mold, hinge should be bent several times immediately while it is hot, that is, box is opened and closed, repeated several times. Bending angle is 180゜, which will make hinge extend 200%~300% in length direction, and make hinge thickness thin from 0.25~0.38 mm to about 0.13 mm. Polymer molecules at hinge are further stretched and oriented, effectively improving fatigue strength and tensile strength.
(5) Plastic hinge products with large output have realized automatic flipping, opening and closing robots in the mold. An example is flip top of a mass-produced shampoo bottle.
(6) Processing accuracy of multi-cavity mold is very important. Electrodes should be made of better red copper, pay attention to precision of electrodes and EDM, and keep plastic hinges of each hole to have same thickness.
Cavity layout of mold design is 1 out of 2, and 3D drawing of mold design is shown in Figure 2. Mold base is standard mold base CT 3035 A70 B90 C90; arrangement direction of mold is that slider is arranged along horizontal direction. Four corners of front and rear mold cores are positioned by positioning corner, mold core steel is made of Japanese Daido PAK90 high-polishing performance steel. Due to thinner plastic parts, mold is a small mold, front and rear mold cooling systems adopt a straight-through water delivery circuit. As mentioned above, because plastic part is small, only one gate can be opened, and latent gate can feed glue, as shown in Figure 4. Molten plastic enters the other side through a very thin plastic hinge, so that under action of shear stress, melt passes through hinge to quickly form crystallization and orientation. Therefore, plastic hinge here has good strength and will not break after tens of thousands of uses.

 

Figure 4 2D ranking map
Ejection of plastic parts adopts thimble ejection and inclined ejection.
Part inside plastic part corresponding to slider has a bevel, so a lifter needs to be designed here. Both sliders are T-slot driven sliders. T-slot driving slider has a simple structure and is suitable for core-pulling of sliders with small spatial positions.