Negative pressure switch panel product is shown in Figure 1. Maximum size of product is 182.00 mm * 100.00 mm * 25.50 mm, average thickness of plastic part is 2.00 mm, material of plastic part is ABS+PC, shrinkage rate is 1.005, and weight of plastic part is 51.20 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, etc.

 

Figure 1 Product Picture of Negative Pressure Switch Panel
It can be seen from Figure 1 that plastic part has a flat and complex shell structure, fixed mold part has protrusions and four openings of different sizes. There are 3 raised cylinders on the top surface of plastic part, movable mold part is hollowed out for connecting self-tapping screws. Plastic part material is ABS+PC, which is commonly known as plastic alloy, which has good rigidity and strength. Plastic alloy is especially suitable for manufacturing precision plastic parts. There are four kinds of materials that are commonly used raw materials for precision plastic parts of chassis, namely ABS, PC, PA and POM. Now with widespread use of new engineering materials such as PPO and PPS, they are used to make more precise movement-type plastic parts. For example, sensor moving piston in an electronic sphygmomanometer is made of PPO. For precision plastic parts, product tolerances must be accurately marked on 2D product drawing, geometric tolerances must be marked when necessary. For size without tolerance requirements, so-called free tolerance size, its tolerance also needs to have an acceptance standard. Figure 3 is an example of free dimension tolerances of plastic parts for reference during design.
4 sides of plastic part are 90° vertical, and each side has a small rectangular hole. Therefore, four sides need to be designed with large slider cores. There is a slot with a width of 1.5mm on both sides of each rectangular small square hole. When assembling, this part can be partially elastically deformed, so that buckles of matching parts can be smoothly installed.

 

According to size of plastic part and four-side core-pulling mold structure, cavity layout of mold design is 1 cavity. Gating system design uses a three-plate mold for point gate pouring, number of gates is 4 points. See mold design drawing 2.
Shape of plastic part is relatively regular, and parting surface is flat. Reasonable selection of parting surface is an important part of designing injection molds. It is not only conducive to layout of gating system, but also improves processability, avoids critical load of injection molding machine, and ensures that plastic parts remain on the side of ejection system after mold is opened. Moreover, it is beneficial to simplify mold structure, improve dimensional accuracy and surface quality of product. Design of gating system, design of ejection system and design of parting surface are the three elements that determine basic structure of a set of molds.
All plastic injection mold design books will write relevant chapters on parting surface design. Parting surface refers to the largest projection surface on plastic part that can make plastic part separate from mold. Mold can be divided into two parts, movable mold and fixed mold, by parting surface, plastic parts can be demolded by opening these two parts. Parting surface is a separable surface that exists on mold. Generally, a set of molds has only one parting surface. When there is no push plate, two plate molds usually have only one parting surface, which coincides with parting surface. Difference between parting surface and parting surface is shown in Figure 4. Therefore, when reviewing mold structure, parting surface will be drawn on plastic part, direction of movable and fixed mold will be marked with an arrow. Generally, there is only one parting surface on plastic part, the rest are partial parting surfaces, such as parting surface when slider is core-pulled.
Parting surface and parting surface are easy to confuse. When mold is opened, condensate of gating system of two-plate mold needs to be taken out from parting surface, condensate of pouring system of three-plate mold needs to be taken out from parting surface. Plastic parts need to be taken out from parting surface, and metal inserts need to be placed in mold from parting surface.
Negative pressure switch panel products need to be designed with large slider cores on all four sides. Division of four large sliders requires attention. Slider cannot be designed to be in line contact at junction. Two large sliders are widened to collide with two sliders on the side of up and down. Four sliders are of insert structure, which is easy to adjust. All sliders are driven by oblique guide posts. Oblique wedge is an insert structure.
There are many kinds of sequential mold opening mechanisms for three-plate molds. Most of them use stroke limit screws and resin shutters to control opening and closing of mold. Advantages of resin shutters are low cost and easy replacement. In injection molding of some high-precision transparent products, resin shutters are not suitable, because in long-term opening and closing of mold, there is friction between resin shutter and metal template, so tiny powders will fall down on the product. Due to four-sided large slider core-pulling of this set of molds, corners of mold plate are limited in space. Therefore, pull plate structure is used instead of resin shutter.

 

Ejection of plastic parts adopts a thimble and a cylinder to eject. Designed center toss to guide thimble plate. Cooling circuits of front and rear molds are all straight-through water transport.

 

 

 

 

Figure 2 Die diagram of negative pressure switch panel

 

Figure 3 Free tolerance of plastic parts

 

Figure 4 Difference between parting surface and parting surface