Product of external bracket of imager is shown in Figure 1. Maximum dimensions of product are 213.00 mm * 148.95 mm * 41.28 mm; average thickness of plastic part is 3.00 mm, material of plastic part is ABS, shrinkage rate is 1.005, and weight of plastic part is 122.76 grams. Technical requirements for plastic parts are that there should be no defects such as peaking, underfilled injection molding, flow lines, pores, warping deformation, silver streaks, cold material, and jetting lines, and meet ROSH environmental protection requirements.

 

Figure 1 Image instrument external bracket product map
Previous article introduced main points of mold design for inner bracket of imager. Inner and outer brackets can be assembled together to form a product. This article describes mold design of outer bracket. It can be seen from Figure 1 that plastic part is shaped as a flat shell with same dimensions as inner bracket. There are large rectangular openings on upper surface of inner bracket, which is convenient for designing gating system in inner hole, but multiple small holes on the surface will produce glue lines, hinder flow of molten plastic, and shrinkage rate of plastic parts is small. Surface of external bracket of imager is flat, partially sunken, and there are some pillars on the back. Both ends of plastic part need to be designed with large sliders for core pulling, as shown in Figure 3.

 

Figure 2 3D drawing of mold
Size of plastic part is large, mold design cavity arrangement is 1 cavity, mold base model is simplified fine nozzle FCI-2940-A80-B70-C90-L270, gating system is one-point gate, two-point glue injection. For high-precision molds, fine frames of front and rear molds need to be processed in mold base factory, and frame depth should not exceed 1/2 of thickness of mold plate. Generally speaking, a set of molds has three levels of positioning, and one, two or three of them can be selected according to specific conditions of mold. The first stage is positioning and guiding of guide post and guide sleeve, second stage is taper positioning block and zero-degree positioning block on mold base, and the third stage is positioning of front and rear mold cores. Tiger mouth design of front and rear mold cores is shown in Figure 4. Front and rear mold cores are positioned by tiger mouth, which complements shortcomings of only using guide posts and guide sleeves for positioning and guiding. In the design of positioning corner, in most cases, rear mold core is convex, and front mold core is recessed. Height and shape of mold core are determined and designed reasonably, so as not to cause waste of steel and processing man-hours. On the other hand, inclination angle of positioning corner generally does not exceed 10°. The smaller angle, the higher positioning accuracy, the better it is to protect wear-through surface and protect small inserts from breaking. Taper positioning block and zero positioning block on mold base must be designed once when mold is designed, and processed together in mold base factory when mold base is ordered, so as to ensure mold clamping accuracy. Therefore, it is difficult to achieve high precision in fine positioning added later.
Size of plastic part is large, and design of gating system is very critical. For flat plastic parts of this size, it is ideal to design gate at the top of plastic part. Therefore, two requirements must be ensured, one is to realize separation of gate aggregate from product when mold is opened, and the other is to ensure that gate aggregate is pulled out smoothly and does not break when mold is opened. Compared with two-plate mold, three-plate mold can realize direct separation of gating system aggregate and product through multiple openings of mold, avoiding subsequent man-hours of cutting off material, saving labor costs, at the same time protecting product and avoiding secondary damage to product when removing material head. It is more and more widely used in molds. When point gate is processed, it is necessary to pay attention to fact that gate mark on plastic part is less than 0.1mm after gate is broken. In recent years, in European and American countries, application of three-plate molds and point gates has become less and less, and they are basically replaced by hot runners or nozzles.

 

Figure 3 Die parting surface diagram

 

Figure 4 Front and rear mold core positioning diagram
Due to seam on the edge of plastic part, seam position on the edge of rear mold core is concave, as shown in Figure 4. This kind of seam is only good for cutting inserts. Mold materials with different characteristics can be selected for insert removal, so as to meet hardness, surface roughness, wear resistance and corrosion resistance requirements of different structural parts on mold, and high-quality mold materials can be saved on premise of meeting mold requirements. In addition, adding beryllium copper inserts locally in mold can solve cooling problem of mold. Method of removing inserts can meet requirements of mold forming process. Because mold is easy to produce bad plastic parts due to gas in cavity during injection molding process, in order to make the gas in cavity discharge out of mold smoothly, usually fitting gap of insert can be used to exhaust.
Mold is cooled by ponds and straight-through water cooling. Because slider is wide, it is driven by two inclined guide columns, and a straight-through cooling circuit is designed on slider.