Molding process of two inner side inclined slider core-pulling of plastic part and analysis of influence of mold forming structure on quality of plastic part are introduced, correct selection and design of gating system, design of mold forming part and assembly structure.

Inner sides of two long sides of plastic part have slide core-pulling molding with symmetrical requirements. In structure design of injection molding mold, mold structure in which inclined slider is guided upward and downward is generally used in integral movable mold module to ensure working strength of mold, reliability and safety of core pulling of inclined slider during moving process.
Plastic part is used under environmental conditions with a temperature of about 25℃. Material used is ABS engineering plastic material commonly used in general structural parts as raw material of plastic part. Because ABS is an engineering plastic widely used in home appliance housings, various structural parts, connectors, decorative parts, etc., it has a moderate melt index, good melt fluidity and moldability during injection molding. Wall thickness of plastic parts is uniform (both are about 2.5mm), which meets process requirements of injection molding.

 

Figure 1 Parts diagram of double inner side with guide chute housing

This plastic part is a rectangular rectangular shell with an external dimension of 270*170*40mm and a wall thickness of about 2.5mm. Thickness of each wall is relatively uniform, structure and manufacturability are good. Bottom surface of shell is the best parting surface between movable and fixed molds. Structural graphics and related dimensions of plastic part are shown in Figure 1. Inside and shape of plastic part are all rectangular structures. Main forming parts: two inner sides should have completely symmetrical guide chutes to ensure that corresponding assembled parts move flexibly and reliably in guide chutes without blocking.

ABS is a terpolymer of acrylonitrile, butadiene and styrene. Due to respective characteristics of these three components, ABS has good comprehensive mechanical properties. Its melting temperature is 210℃, decomposition temperature is above 250℃, heat distortion temperature is generally around 93℃, and continuous working temperature is around 70℃, which can fully satisfy long-term use of plastic part under environmental conditions of around 35℃. Mold shrinkage rate of ABS plastic varies from 0.3% to 0.8%.

a. Dimensional accuracy analysis of plastic parts. Inside and outside of plastic part are rectangular thin-walled hollow parts, two inner sides have symmetrical guide chute forming requirements. In addition, inner dimension of 120 +0.25+0.10 *220+0.25+0.10 mm has corresponding matching tolerance requirements. However, dimensional accuracy of inner wall thickness is not high, and allowable tolerance size variation range is relatively large, which is easy to control in actual production process. Tolerance value of free size can be checked according to MT7.
b. Surface quality analysis of plastic parts. Inner surface and bottom surface of guide chute on both inner sides of plastic part should be smooth and free of burrs. Parts drawing of plastic part is shown in Figure 1. It should be ensured that molded surfaces of inner and bottom surfaces of guide chute after molding can reach surface roughness value Ra1.6μm, while the rest of surface of plastic part does not have a higher roughness requirement.
c. Structural process analysis of plastic parts. One end of guide chute on inner side of plastic part is deflected 15° downward and the other end is deflected 20° upward, which requires precision for symmetry of guide chute on both sides after forming. Control process accuracy in mold structure design and processing of main mold working parts.
In order to strengthen repeated sliding strength of relevant guide ram for assembly in guide chute on both sides, a number of 1.5*1.5mm reinforcing ribs are designed. Arrangement of reinforcing ribs also increases filling flow channel of plastic melt in mold cavity, which is conducive to molding process requirements of plastic part. Wall thickness of plastic parts is relatively uniform (about 2.5mm), which meets process requirements of injection molding.

(1) Selection of parting surface. When selecting parting surface, according to selection principle of parting surface, maximum contour of plastic part should be used as parting interface of moving and fixed molds. At the same time, it facilitates exhaust of mold cavity and keeps plastic parts in movable mold part as much as possible. In design of this injection mold, considering that plastic part is a hollow rectangular shell, its bottom surface is the largest contour surface, selection of parting surface is very intuitive, that is, large bottom surface of plastic part is used as horizontal parting surface between moving and fixed molds.
(2) Design of gating system. Because plastic part has a thin-walled hollow rectangular structure and has a large volume, guide chutes on two inner sides of plastic part need to be core-pulled to form. Design of gating system and positioning form of setting will directly affect molding quality of plastic parts. This is mainly due to large volume of plastic part, longer flow of plastic melt in mold, and lateral molding of inner side of plastic part. Such as adopting form of setting point gates on upper end faces of plastic parts on both sides of frame, it is very easy to produce melting marks or insufficient filling of melt. After comparison and analysis, a feed system with two latent gates on inner sides of both ends of plastic part is adopted, that is, a four-point latent gate feed system. Gate position of feed point is at 4 small points on inner side of two ends of shell. After injection molding is finished, while plastic part is ejected and demolded, runner push rod 10 arranged under latent runner simultaneously pushes out gate. After plastic part is separated from gate, it will not affect appearance quality of plastic part (see Figure 2 of schematic diagram of gating system).

 

Figure 2 Schematic diagram of gating system (four-point latent gate)
1. Latent gate 2. Split runner 3. Main runner 4. Gate push rod

Guide chute on two symmetrical inner sides of plastic part is formed with core pulling structure of inclined guide slider, as shown in Figure 3. Angle of guide slope is mainly to ensure that core pulling distance of inclined slider after final guide is in place can meet smooth demoulding of plastic part without interference. If angle of inclined surface is selected too large, although moving distance required for demoulding can be increased, friction resistance between slider and inclined surface is increased; if angle of inclined surface is selected too small, although friction resistance between slider and inclined surface is reduced. However, space required to easily form moving distance required for demolding is insufficient, and plastic part cannot be demolded. After calculation, guide slope adopts 9° inclination, as shown in Figure 3. Depth dimension of guide chute is 1.5mm, mold opening distance after inclined guide slide is finally pushed into place is 3mm, which fully meets working requirements of mold. In design of push-out structure of plastic part, in order to make plastic part smoothly demold after being formed, a form in which push rods 17 are symmetrically arranged at lower part of inner side of each circumference is adopted, as shown in FIG 4. Ejection mechanism is guided by a push plate guide post 20 fixed on movable mold backing plate 25.

 

Figure 3 Core-pulling structure diagram of double inner side inclined slider

1. Fixed mold cavity block 2. Inner inclined slider 3. Moving mold cavity 4. Moving mold guide sliding insert

 

Figure 4 Layout of push rods in movable mold

Assembly structure of mold is shown in Figure 5.

 

Picture Figure 5 Mold assembly drawing
Due to hollowness, thin wall, and large volume of plastic part, cavity of fixed mold part of shape of molded plastic part is designed in the form of an integral cavity block 3 inserted into fixed mold plate 2. Movable mold part is inserted into movable mold plate 11 in a structure composed of three insert sets (pieces 4, 5, and 9 respectively). Relative position of large core in mold is ensured, displacement of core when subjected to injection pressure is avoided, strength of mold is increased, and assembly of mold is also facilitated. Structure of assembled oblique core-pulling moving slider is shown in Figure 6. Separation of movable and fixed molds after injection molding is shown in Figure 7. After movable mold and fixed mold are separated, latent gate push rods are pushed simultaneously with each push rod. Eject runner and plastic parts respectively, as shown in Figure 8.

 

Picture Figure 6 Push rod layout in movable mold

 

Figure 7 Injection molding is completed, moving and fixed molds are open

 

Figure 8 Moving and fixed molds are fully opened, slider is pushed out, and plastic part is ejected