Abstract: For molding of large plastic parts supporting industrial control computer circuit boards, a glass fiber modified reinforced polypropylene (PP) composite material was selected for injection molding. Mold structure is a three-plate mold, opened in three stages. Two point gates were designed for gating mold cavity to ensure even and full filling. Considering numerous secondary features of plastic part, especially many deep ribs of irregularly shaped studs, multiple local inserts were designed for molding these local features, improving strength of local molding inserts while reducing processing difficulty of these molded parts. To address difficulty of venting in certain areas of mold cavity, multiple venting inserts were installed to enhance venting. A locking mechanism and a fixed-distance tie rod mechanism were implemented for mold opening and closing control; a spring mechanism was designed for reliable demolding of runner waste; a positioning mechanism ensures molding accuracy of plastic part and service life of mold; and three slider mechanisms were used for molding, demolding of straight side walls and side holes on straight walls of plastic part. Mold structure is reasonable, molded parts are easy to process, practical and reliable, and can provide a useful reference for design of similar molds.
Polypropylene (PP) has good high-temperature resistance, and it will not deform significantly within the range of 100~150 ℃; Secondly, it has good mechanical properties. Modified PP has good mechanical load-bearing capacity, and it is not easy to react chemically with other chemicals. It is non-toxic, non-absorbent, and has good stability in use. When used in electronic products, it has good electrical insulation properties. Modified PP can be used to make insulating shells for household appliances and washing machine inner tubs. It is widely used as an insulating material for wires, cables, and other electrical appliances. In particular, modified PP composite materials play a very important role as a load-bearing material in production of electronic components. Based on usage requirements and injection molding requirements of a large industrial control computer circuit board support frame, this paper designs an injection molding mold for injection molding of this plastic part. Given complex structure of plastic part and its difficulty in casting, designed mold is a three-plate mold with one cavity, two-point gates, and a single cavity. Design of molded parts and demolding mechanism in mold structure has good design reference value.

Structure of industrial control computer circuit board support frame plastic part is shown in Figure 1. Base of plastic part is a rectangular plate, with numerous additional features on the top, bottom, and four sides. These features include: multiple rectangular, waist-shaped, and circular through holes arranged on the front view of plate; multiple Ø7 mm studs b1, countersunk holes b2, tower-shaped studs b3, countersunk columns b4, recessed bases b5, and hooks b6 arranged on the back view of plate; four sides c1, c2, c3, and c4, where c1, c3, and c4 are straight walls, and c4 also has a grid hole s1. Plastic part has external packaging dimensions of 292.2 mm * 47.8 mm * 58.7 mm, an average wall thickness of 2.7 mm, and a reinforcing rib thickness of 1.5 mm. In wall thickness design of plastic part, maximum wall thickness is 3.2 mm and minimum is 1.5 mm, and wall thickness is relatively uniform, making it suitable for injection molding production. In demolding design of plastic part, demolding angle varies from 50′ to 1°20′ depending on feature height. In design of various holes in plastic parts, some holes have a depth-to-wall-thickness ratio greater than 4, classifying them as deep holes. Holes b1, b3, and b4 fall into this category, and therefore are designed as stepped holes. The overall molding accuracy of plastic part is MT6 grade, while dimensional control between local hole positions is MT4 grade (GB/T 14234-2008).

 

Fig. 1 Plastic parts of industrial computer circuit board support frame
b1—Ø7 mm stud; b2—Sink hole; b3—Tower shaped stud; b4—Sink stud; b5—Sunken abutment; b6—Buckle hook; c1～c4—Side wall number; s1—Grid hole number

Plastic parts are made of a modified PP material. This modified PP material consists of 60-80 parts by weight of homopolymer PP, 20-40 parts of ethylene-vinyl alcohol copolymer, and 1-10 parts of compatibilizer (a reaction product of polypropylene maleic anhydride graft and ethylene-vinyl alcohol copolymer), mixed at 170-190℃. This PP composite material has high toughness, with an impact strength as high as 210 J/m, and high gas barrier properties, with a water vapor permeability rate close to 2000 g·μm/(m2·24 h), making it suitable for manufacturing carrier substrate in electronic control equipment. Adding 15% glass fiber and other additives to PP composite materials reduces shrinkage rate to 0.8%~0.92%.

From perspective of mold design required for injection molding, considering shape and structural characteristics of this plastic part, following challenges exist when using injection molding: First, plastic part is relatively large, and flow distance of molten material in mold cavity is long. A suitable gating system must be selected to ensure sufficient filling of mold cavity and excellent molding quality. Second, plastic part has many structural holes, which easily generate more weld lines, reducing the overall strength of plastic part. Third, plastic part has many deep walls and deep ribs. Numerous deep ribs and deep piers make venting during filling process difficult, easily leading to incomplete filling. Fourth, molded parts require numerous small inserts to ensure strength of molded parts in specific areas and reduce the overall processing difficulty. Fifth, demolding molded parts is difficult, mainly due to strong clamping force exerted by deep ribs and deep piers, which is unevenly distributed, resulting in high demolding resistance in mold opening direction and making demolding difficult. Additionally, three sides (c1, c2, c3) of molded parts are all straight walls, and multiple grid holes are added to these three side walls, requiring lateral core pulling.

To address difficulties in injection molding of plastic parts, following mold design scheme is proposed. As shown in Figure 2, to reduce difficulty of mold manufacturing, a planar parting surface (PL) should be used to part main body of mold cavity, thus reducing the overall processing difficulty of molded parts. Secondly, a multi-point gating scheme is adopted for mold cavity to shorten flow length of molten material, ensure filling effect of molten material in mold cavity, and ensure effective filling control. Therefore, mold cavity will be gated using two gates, G1 and G2. Using two-point gating combined with mold temperature control can effectively eliminate weld lines generated when molten material flows through individual hole features. To solve venting problem in mold cavity, small inserts 3 to 9 are set for various types of features, and venting grooves are set on inserts to allow high-pressure air in mold cavity to be quickly expelled, avoiding dead zones in molten material filling and preventing localized material shortages in plastic part. Venting inserts 15 are also required for venting in special locations. Demolding of plastic part is designed in two parts: First, for demolding deep ribs, deep pillar holes, etc., flat ejector pins 14 and push tubes 13 must be used for ejection. For other locations, ejector pins 11, 12, etc., of varying diameters are used as needed. Second, for three sides c1, c3, and c4, one side core-pulling slider 10, side core-pulling slider 16, and side core-pulling slider 17 are each used for side core-pulling demolding in directions F1, F2, and F3 respectively.

 

Figure 2 Parting and molded part design
1–Cavity insert; 2–Core insert; 3-9–Sub-inserts; 10, 16, 17–Sliders; 11, 12–Ejector pins; 13–Ejector sleeve; 14–Blade ejector; 15–Ventilate inserts; G1, G2–Point gate; F1, F2, F3–Core pulling direction

5.1 Overall Structure Design: Mold cavity layout is shown in Figure 3. Considering large size of plastic part and need for three-sided side core pulling, a one-cavity mold layout is used. A three-plate mold base (LKM standard mold base) is used, and a two-point gating system (point gate G1, point gate G2) is used. As shown in the bottom view of fixed mold in Figure 3b, in mold cavity cooling system design, cavity insert 1 is cooled by one Ø8 mm cooling pipe; core insert 2 is cooled by one Ø8 mm cooling pipe; and sliders 16, 17, and 10 are each cooled by one Ø8 mm cooling pipe. Final demolding of plastic part is achieved by ejector pins and push plates. Mold opening and closing control mechanism includes a set of two symmetrically angled locking mechanisms 19, mainly used to control opening and closing of fixed and moving mold plates; a combined fixed-distance tie rod 21, mainly used to control opening distance of fixed mold plate and stripper plate; and a resin buckle 18, mainly used for frictional linkage movement of fixed and moving mold plates. Four long guide pillars (guide pillars 20) are used to guide motion during mold plate opening and closing action.

 

Figure 3. Mold Cavity Layout Design
(a) Moving mold vertical view (b) Fixed mold bottom view
1~17—Same as Fig.2; 18—Resin locking; 19—Locking mechanism; 20—Guide pillar; 21—Combined fixed distance pull rod
5.2 Gating System Design: Design parameters for horizontal runner, pull rod, and gate of gating system are shown in Figure 4. Horizontal runner of point gate gating system uses trapezoidal cross-section runner shown in Figure 4a, with a width of 7.5 mm and a depth of 6 mm, and an included angle of 20° on trapezoidal sides. Horizontal runner uses a mushroom-shaped pull rod with a diameter of 6 mm, as shown in Figure 4b. Injection depth of G1 and G2 point gates is 0.5 mm, and gate diameter is 0.8 mm, as shown in Figure 4c.

 

Figure 4. Gating system design
(a) Horizontal runner (b) Sprue puller (c) Point gate
5.3 Locking Mechanism: Mold structure design is shown in Figure 5. As can be seen from Figure 5a, locking mechanism 19 consists of components 22 to 26. Insert rod 23 controls movement of locking core 22 through its lower flange, thereby controlling locking engagement between locking core 22 and locking hook 24, achieving purpose of controlling closing and opening/closing of fixed mold plate 43 and moving mold plate 44.

 

Fig. 5 Mold structure design
(a) Design of mold opening and closing control mechanisms, et al. (b) Design of mold ejection and demoulding mechanism, et al.; 22—Lock cylinder; 23—Insert rod; 24—Lock hook; 25, 35, 51—Springs; 26—Lock base; 14, 27, 37—Ventilate inserts; 28—Small round hole inserts; 29—Sprue bushing; 30—Sprue puller; 31—Grub screw; 32—Pressure pad; 33—Flat head screw; 34—Scrap ejection sleeve; 36, 52, 60—Screw bolts; 38—Short pull rod; 39—Long pull rod; 40, 49—ABS insulation board; 41—Fixed clamp plate; 42—Runner stripper plate; 43—Fixed mold plate; 44—Moving mold plate; 45—Spacer parallel; 46—Ejector retainer plate; 47—Ejector plate; 48—Moving clamp plate; 50—Transverse pin; 53—Angle pin; 54—Pressure resistant and wear-resistant plate; 55—Slider body; 56—Bottom wear-resistant plate; 57—Release link; 58—Stop pin; 59—Ejector guide pillar; 61—Precision positioning block; 62—Returning spring; 1-21 are same as Fig. 2, Fig. 3. P1, P2, P3—Mould opening surface
5.4 Fixed-distance tie rod mechanism: As shown in Figure 5a, combined fixed-distance tie rod 21 is composed of a short tie rod 38 and a long tie rod 39. Short tie rod 38 controls opening distance of stripper plate 42 (mold opening surface P2), long tie rod 39 controls opening distance between fixed mold plate 43 and stripper plate 42 (mold opening surface P1). Mold opening distance is 120 mm for surface P1, 10 mm for surface P2, the total length of runner waste is 112 mm, opening distance for surface P3 is 60 mm.
5.5 Springing Mechanism: As shown in Figure 5a, to ensure smooth demolding of horizontal runner waste from stripper plate 42, two springing mechanisms are provided. Each springing mechanism includes parts 32 to 35. When surface P2 is fully opened, spring 35 pushes ejector sleeve 34 to eject horizontal runner waste from stripper plate 42.
5.6 Positioning Mechanism: As shown in Figure 5b, to ensure accurate repositioning of cavity insert 1 and core insert 2 when closed on surface P3, four precision positioning block assemblies 61 are added to mold to ensure precise positioning in the X+, X-, Y+, and Y- directions on mold opening plane.
5.7 Ejector Plate Mechanism: Ejector plate mechanism is a conventional mechanism. Difference is that return spring 62 and return rod 57 are separately configured independently. Six ejector plate guide pillars 59 are used, and they must be positioned using locating pins to ensure reliability of ejector plate repositioning accuracy.
5.8 Venting Enhancement Measures: To achieve sufficient venting within mold cavity, venting inserts 14, 27, 36, and 37 are installed to enhance venting within mold cavity during injection filling, preventing air entrapment and ensuring molding quality of plastic part.
5.9 Slider Mechanism: In mold, slider side core-pulling mechanism designed to meet demolding requirements of plastic part's side and side holes is as follows.
(1) For slider 10, designed inclined guide pillar slider mechanism assembly includes parts 10 and 51~56, as shown in Figure 5b. Inclined guide pillar 53 of this slider mechanism drives tail of slider, uses spring 51 for auxiliary driving and positioning.
(2) Slider mechanism structure designed for sliders 16 and 17 is similar to that of slider 10, as shown in Figure 6.

 

Fig. 6 Design of slider mechanism
16, 17—Slide; 20—Guide pillarr; 25—Spring Note: E-E correspond to cross-sectional positions in Fig. 3

As can be seen from Figures 5 and 6, after mold is installed on injection molding machine, its working principle is as follows:
(1) Mold Closure Injection. Mold closes, and injection molding machine nozzle injects molten plastic into mold cavity through sprue sleeve 29. After filling, holding pressure, and cooling, mold is ready to open.
(2) P1 surface opens. Driven by moving platen of injection molding machine and guided by guide pillar 20, moving mold first opens at P1 surface. During opening, runner waste and plastic part separate at point gates G1 and G2.
(3) P2 surface opens. As moving mold continues to descend, mold opens at P2 surface. When opening is complete, stripper plate 42 and ejector sleeve 34 push runner waste out of mold.
(4) P3 surface opens. Moving mold continues to descend. Under limit of fixed-distance tie rod 21, fixed platen 43 cannot continue to descend. Locking mechanism 19 unlocks, and P3 surface is opened. After moving mold descends to a certain position, it stops descending and waits for ejection. During opening of P3 surface, sliders 10, 16, and 17 complete side core pulling.
(5) Ejection. Ejector pin of injection molding machine ejects, pushing ejector plate 47 and its ejector elements to eject plastic part from core insert 2, achieving complete demolding.
(6) Reset. During reset, ejector plate 47 and its ejector elements are pushed by reset spring 62 to reset first, then mold resets and closes in the order of P2-P1-P3. After mold is fully closed, it waits for next injection cycle.

In conjunction with molding of industrial control computer circuit board support frame, a modified PP composite material was selected for injection molding. Designed injection mold is a two-point gate three-plate mold with a cavity layout of one cavity per mold, opened in three stages. In design of molded parts, different structures of local inserts were used for molding different studs, holes, etc., to ensure strength of inserts, reduce processing difficulty of molded parts, and facilitate venting of mold cavity. To address problem of difficult venting in local areas, multiple venting inserts were used to enhance venting, solving problem of trapped air during molding of plastic part. For demolding of deep-walled sections and deep studs in plastic parts, reinforced flat ejector pins, push tubes, and ejector rods were designed to ensure deformation-free ejection. To address requirements for straight-wall molding of three sidewalls and molding of side holes on sidewalls, three side-slider mechanisms were designed for molding and demolding of three sidewalls. Mold structure is simple and practical, and molded part structure is rationally designed, facilitating machining. It can provide a useful reference for mold design of similar plastic parts.