【Abstract】Introduced characteristics of polyphenylene sulfide material, combined with a certain type of fixed plate plastic parts, injection molding process of slender, cross-arranged and thin-walled microporous plastic parts is analyzed, injection molding mold structure and mold parts processing technology are explored, analyzed mold base selection, parting surface selection principle, demoulding structure design of mold; focused on introducing cross grid groove mold forming structure scheme and its structure. Manufacturing method and manufacturing cost are compared, finally a mold that meets processing and use requirements is designed.

Full name of polyphenylene sulfide is polyphenyl sulfide. It is a thermoplastic resin with a phenylsulfide group in main chain of molecule. It is a crystalline polymer and is abbreviated as PPS in English. It has advantages of high mechanical strength, high temperature resistance, corrosion resistance, radiation resistance, chemical resistance, flame resistance, good thermal stability, and excellent electrical properties. It can be used in temperature range of 180℃ to 220℃.
Plastic part material is PPS A7-04, which is a special engineering plastic reinforced with imported PPS resin, glass fiber, inorganic fillers and various lubricants. It has excellent mechanical and electrical properties. Main disadvantages are poor toughness, low impact strength, and insufficient melt viscosity. Melting point of PPS A7-04 is as high as 285℃~300℃, molding temperature is 300℃~330℃, molding shrinkage rate is very small, only 2.5‰, flow performance is good, and it is easy to injection molding. However, during pouring, solidification is fast, shrinkage is small, and it is easy to decompose. Therefore, a higher injection pressure and injection speed should be selected, which requires relatively high injection conditions for injection machine.

Figure 1 shows plastic parts of a certain type of connector fixing plate, its material is PPS A7-04. Base is a small plastic part, thin-walled slender, cross-shaped narrow grooves and fine holes. Length is 60mm and width is only 4mm, thickness is 1.5mm, and the thinnest point is only 0.5mm. In vertical direction, there are 29 fine grooves with a width of 0.3mm and 30 rows of fine holes with a diameter of ϕ 0.66mm. In addition, there are two horizontal grooves with a width of 1mm, which intersect vertical grooves to form a cross grid-like groove.
Cross grid groove divides plastic part into many tiny squares, and blocks are connected by only 0.5mm thick wall. This structure not only brings great challenges to selection of mold structure, mold processing and manufacturing, but also easily causes quality problems of warpage and deformation of plastic parts. During injection molding process, due to cutting of cross grid grooves, material flow is blocked and molding is difficult. To ensure quality of injection molded parts, only increase mold temperature and injection pressure, but in this way, molding quality and appearance of plastic parts will be affected to a certain extent, warpage deformation due to cooling shrinkage will also be aggravated. Through inquiring about assembly and use, I learned that cross grid groove of fixed plate is used for piling up glue and exhausting during work. Size requirements are not high, so mold is designed according to maximum size to reduce influence of groove on fluidity of plastic, thereby improving molding quality and appearance of plastic part.

 

Figure 1 Plastic parts of fixed plate

According to actual situation of base, plastic part has 60mm long cross grid grooves and small insert pins. In order to facilitate demolding and molding of mold, secondary ejector mold base is selected. The working process is: fixed mold plate and movable mold plate are divided into molds, plastic parts are left in push plate with inserts, then pull rod (or pull plate) is used to drive push plate to move, movement distance is limited by limit nails to separate plastic part from insert, so as to realize the first demoulding; then plastic part is ejected from push plate by movement of ejector rod to realize second demolding, and plastic part is completely separated from mold to complete demolding.

Whether parting surface is reasonable or not has a great influence on quality of plastic parts, mold design and manufacturing and performance. Design should be based on various factors such as structural shape, dimensional accuracy, pouring system, overflow system, demoulding method and manufacturing process of plastic part, and a reasonable selection should be made. General principle is to ensure molding quality of plastic parts, facilitate demolding of plastic parts, simplify mold structure, and reduce cost of mold manufacturing.
When selecting parting surface, following should generally be followed: ①Parting surface should be selected at the largest contour of plastic shape; ②To facilitate smooth demolding of plastic, try to keep plastic part on the side of movable mold plate when mold is opened; ③Ensure precision requirements of plastic part; ④Meet appearance quality requirements of plastic parts; ⑤It is convenient for mold processing and manufacturing; ⑥Position of plastic parts in cavity should be arranged reasonably; ⑦It is conducive to exhaust.
Analyzing shape of plastic part and processability of mold, there are two options to choose from. One is to place plastic part cavity on push plate, that is, to separate from upper surface of plastic part, as shown in Figure 2; the other is to place all plastic part cavity on fixed mold plate, that is, to separate from lower surface of plastic part, as shown in Figure 3. Both solutions have little effect on molding quality and molding conditions of plastic parts, and both can be produced. The biggest difference lies in design and manufacture of push plate insert and way of demoulding.

 

Figure 2 Option One

 

Figure 3 Scheme two

Push plate is an important component of plastic mold ejection mechanism. Its function is to eject plastic part in a large area to achieve purpose of separating insert and plastic part. For molds with complex structure and shape, it is often necessary to process a cavity or a forming structure on push plate. In order to facilitate processing, it is often designed as a mosaic structure. For the first parting scheme, structure of push plate insert is shown in Figure 4. According to its shape, it can be called vertical groove insert type, which is to make groove in vertical direction of molding and fix it on movable mold plate, use same fixing method and demolding method as round hole inserts; horizontal groove is cut by vertical groove and is formed by CNC slow-moving wire cutting. When mold is closed, round hole insert is inserted into fixed mold plate to achieve purpose of forming through hole. Vertical groove insert is inserted into push plate insert and spliced with horizontal groove divided into small pieces on it to form a cross groove. When demolding, push plate moves upward with one ejection mechanism, movable mold plate does not move, so that round hole inserts and vertical groove inserts are separated from push plate, plastic parts remain in push plate cavity. As mold continues to move, ejector pin ejects plastic part from cavity to complete demolding.

 

Figure 4 Vertical slot inlay
Second parting scheme is to design cavity and cross grid grooves on push plate, use CNC EDM to process them as a whole. Cross grid groove forming insert is shown in Figure 5. Molding insert is fixed on push plate and moves with push plate during demolding. Demolding process is: when mold is opened, with separation of fixed mold and movable mold, plastic part is separated from fixed mold under tightness of insert pin and crisscross groove, stays on push plate. Subsequently, when ejection mechanism moves to a certain position, insert is separated from plastic part. At this time, plastic part is wrapped on cross grid groove forming insert, as ejection mechanism continues to move, ejector rod pushes plastic part away from insert from mold, thereby completing demolding.

 

Figure 5 Integral insert

In the first solution, cross vertical groove adopts mosaic method, push plate insert can be directly cut and formed by CNC slow-moving wire. Cavity surface is accurate, easy to process, and there are few processing steps. After damage, it is easy to replace. However, vertical groove inserts are too thin and reciprocating demolding needs to be placed on movable mold plate. Its length needs to be 39.46mm, and its thickness is only 0.4mm, so it is extremely easy to deform during processing, making it difficult to assemble mold. Moreover, due to splicing of vertical grooves, under high injection pressure, burrs are easily formed at butt joint position, which affects molding quality and appearance.
In second solution, integral insert is fixed on push plate, horizontal and vertical grooves do not have inlays, so problems in the first solution can be effectively avoided. However, due to existence of cross groove, forming part of head has many small sinking cavities, which can only be processed by EDM, discharge electrode can be processed by CNC slow wire cutting to solve processing problem. This will increase manufacturing difficulty and cost of insert to a certain extent, after insert is broken, it can only be reprocessed. But as far as entire mold is concerned, processing cost will be greatly reduced, processing steps of vertical groove inserts are reduced, and processing difficulty is avoided; fitter is easy to assemble, there is no need to correct size of each groove, and there is no need for a series of problems such as inability to assemble due to processing errors or processing burrs. Based on above analysis, considering advantages and disadvantages of two schemes, combined with production conditions of workshop, mold finally chose second design scheme.

Since base is relatively small and slender, it does not require high requirements for maximum injection volume and maximum clamping force of injection molding machine, so main factor in determining cavity is accuracy requirements of plastic part. According to production experience, number of cavities is inversely proportional to dimensional accuracy. Each additional cavity will reduce dimensional accuracy of plastic part. In order to fully ensure accuracy of plastic parts and make full use of mold plate, cavity is finally determined to be one mold and two cavities, symmetrically arranged on both sides of main runner. Considering fluidity of plastics, it is guaranteed that plastics flow smoothly, quickly and without disorder. It can be seen from cavity layout that mold runner can be processed directly to cavity through cold slug hole, that is, straight gate feed is processed; air groove can be ground at the other end, thereby reducing air pressure in cavity, improving molding quality and accuracy of small sinking cavity divided by cross groove, reducing warping deformation of plastic part.

Mold cooling is to ensure stability of plastic molding temperature and improve quality of plastic parts. Melting point of PPS exceeds 280℃, heat distortion temperature exceeds 260℃, and it can be used for a long time at 200°C. During injection molding, mold temperature is 60℃ to 80℃, injection nozzle temperature is 290℃ to 300℃, and injection pressure is 60 to 70 MPa. After mold is processed, plastic part is trial-processed, and plastic part is well formed. After actual test, warpage deformation of plastic part is only 0.05mm, quality is high. Actual picture is shown in Figure 6.

 

Figure 6 Physical image of plastic part molding

Plastic part is a new type of connector. Plastic part has a slender structure, a cross grid groove, and a thin-walled pore structure. Molding structure itself is not complicated, but because of distribution of cross grid fine grooves and ϕ 0.67mm fine holes on slender plastic part, processing difficulty of mold is increased, molding difficulty of plastic part and risk of warping deformation quality are also increased.
Through analysis of plastic parts and selection of structural schemes, this paper successfully developed a mold that meets requirements of use, accumulated certain experience, which can be used as a reference for mold design and manufacture of this type of structural plastic parts.