As design software becomes increasingly powerful, plastic parts are becoming more complex, and demolding them is becoming more difficult. Many plastic parts require not only core pulling during demolding but sometimes also secondary core pulling. Due to diversity of product structures, secondary core pulling mechanisms also vary. Secondary core pulling mechanisms are typically designed in two situations: (1) Plastic part has a structure that hinders core pulling, making it impossible to pull out with a single core pull. The first core pull removes structure hindering core pulling, and secondary core pull removes structure hindering demolding. (2) Core pulling area of plastic part has a large clamping force, and a single core pull will damage plastic part. Purpose of secondary core pulling is to reduce core pulling force to avoid damaging plastic part during core pulling.

As shown in Figure 1, intake manifold of a Kawasaki motorcycle engine is an irregularly shaped plastic part. Plastic part features four curved pipes (with an included angle of 63.53° between two ends), one straight pipe on the side, and four stepped holes at one end communicating with it. Part's dimensions are 424 mm * 256 mm * 111 mm (irregular shape, referring to space occupied by part in mold). Material is 30% glass fiber reinforced polyamide 6 (PA6-GF30), with a shrinkage rate of 0.5%. Surface defects are not permitted.

 

Figure 1: Part Structure
As shown in Figure 2, parting surface is selected at maximum contour of part. Due to irregular shape, parting surface is curved. Since two ends of curved pipe are not perpendicular, choosing orientation shown in Figure 2 reduces number of core-pulling areas, simplifies core-pulling structure. As shown in Figure 2(a), right side of curved pipe end and concave end face require core-pulling. Because there is also an oblique hole inside curved pipe hindering core-pulling, secondary core-pulling is required here. Straight tube on the side is a deep hole, side core is covered inside and out, resulting in a large core-pulling force. To prevent damage to plastic part during core pulling, a secondary core-pulling is required here. As shown in Figure 2(b), right side of plastic part has a concave-convex structure, requiring core pulling here; in addition, there is a pillar on left side of plastic part that is inclined to demolding direction, also requiring core pulling.

 

Figure 2 Parting surface

As shown in Figure 2(a), right side of plastic part requires secondary core pulling. First core pulling removes inclined hole inside bend (which is horizontal in mold), and second core pulling removes core inside bend. Force of first core pulling is not large, so it can be driven by an inclined guide pillar. Force of second core pulling is larger, slider is at 26.47° to horizontal direction, so a hydraulic cylinder is used for driving. Because inclined hole is a stepped hole, larger end is at other end, other end of inclined hole also needs core pulling, which can be done using an lifter on moving mold side. Straight tubes on the side require secondary core pulling. A small core can be pulled first using a slanted guide post, followed by a second core pulling using a larger slider. As shown in Figure 2(b), right side of plastic part requires core pulling; due to large slider, a hydraulic cylinder is used. Left side of plastic part can be core pulled using a lifter on fixed mold side.
Large size of plastic part requires multiple injection points. Therefore, gating system can use point gates to inject material onto product surface, selecting inside of curved tubes and exposed concealed parts as gates to meet appearance requirements. Considering large size of plastic part, a one-cavity mold structure is adopted.
Ejection mechanism can use ejector pins as ejection elements. Since ejector pins act on non-appearance surfaces, they will not affect appearance quality of plastic part, structure is simple and low-cost.
Plastic part is large and thick. To improve cooling, cooling system uses multiple cooling channels around cavity, and all sliders are designed with cooling channels. Mold appearance is shown in Figure 3.

 

Figure 3 Mold Appearance

As shown in Figure 4, due to complex structure of plastic part, for ease of processing and maintenance, moving mold core and fixed mold core adopt an integral structure for molding of outer surface, and an insert structure for molding of non-outer surface. This ensures both quality of outer surface and convenience of processing and maintenance. To expel gas from cavity, an venting groove is designed on one side of parting surface of fixed mold core. Additionally, venting can be achieved through ejector pins, inserts, and side cores. To ensure quality of outer surface of plastic part, outer surface is molded using an integral side core structure. Since parting surface is curved, a large lateral force is generated during injection molding. To improve positioning accuracy during mold closing and to withstand lateral force generated during injection, locking jaws are designed at four corners of moving mold core and fixed mold core for positioning inner mold body. Locking jaws are also designed at four corners of plates A and B for positioning.

 

Figure 4 Fixed mold core and moving mold core

Figure 5 shows gating system. Due to large size and wall thickness of product, mold adopts a point gate and multiple injection points. Gate locations are selected in non-visible and concealed areas to meet customer's appearance requirements. Analysis using Moldflow software confirms that molding requirements are met.

 

Figure 5 Gating System

Figure 6 shows mold core pulling mechanism. This mold has 5 sets of core pulling mechanisms: a core pulling mechanism for front of plastic part, a slanted secondary core pulling mechanism for back of plastic part, a secondary core pulling mechanism for side of plastic part, a lifter core pulling mechanism on fixed mold side, and a lifter core pulling mechanism on moving mold side.

 

Figure 6 Core Pulling Mechanism
As shown in Figure 3, mold requires 4 parting operations. Mold opening process is as follows: Clamping mechanism 1 and clamping mechanism 2 lock parting surface PL3 and parting surface PL4 respectively. During mold opening, under action of compression spring, parting surface PL1 opens first, breaking sprue and separating sprue runner from plastic part. Simultaneously, as PL1 opens, sprue runner runner is pulled out of runner, and PL1 is used to remove runner runner runner. After PL1 opens a certain distance, parting surface PL2 opens, separating sprue runner runner from sprue hook and sprue sleeve. After PL2 opens a certain distance, ejector 1 unlocks, PL3 opens, and lifter on fixed mold side completes core pulling. After PL3 opens a certain distance, ejector 2 unlocks, PL4 opens, and plastic part is removed. Because mold parting surface has requirements for mold opening sequence and distance, this mold has a set of sequential fixed-distance parting mechanisms. Parts controlling mold opening sequence are compression spring, sprue hook, ejector 1, and ejector 2, while opening distance of parting surface is limited by fixed-distance tie rod.

Side of plastic part uses an inclined guide pillar secondary core pulling mechanism (Figure 6), structure of which is shown in Figure 7. There is a long through hole and a stud at this location. Core-pulling position of slider is at through hole and stud. During core-pulling, due to strong clamping force of hole and stud, molded part is difficult to demold, which can easily cause deformation or damage to plastic part. A secondary core-pulling mechanism usually involves a small slider within a large slider. During core-pulling, small slider moves first, and after moving a certain distance, large slider moves, thus completing secondary core-pulling. To address this, mold is designed with a small slider in the middle of large slider to fix core of through hole onto small slider. Oblique guide post hole of large slider is machined into an elongated hole to allow for clearance. During mold opening, due to elongated hole, large slider will move with a delay. Oblique guide post first drives small slider to pull out core of through hole, eliminating clamping force of through hole on core. When oblique guide post simultaneously contacts both sliders, both sliders move simultaneously, completing the entire core-pulling process. When large slider begins core-pulling, clamping force of plastic part on it has become very small, preventing damage to plastic part during core-pulling.

 

Figure 7 Secondary core-pulling mechanism with inclined guide post
Following issues need to be considered when designing this mechanism: (1) Driving force of inclined guide post must be sufficient, otherwise a hydraulic cylinder must be used; (2) When small slider inside is pulling core backward, there must be a safe positioning mechanism, such as limit screw shown in Figure 8; (3) Delayed action of large slider must be safe and reliable.

 

Figure 8 Secondary core-pulling mechanism with inclined guide post + hydraulic cylinder

Back of plastic part is a secondary core-pulling mechanism with inclined guide post + hydraulic cylinder (Figure 6), and its structure is shown in Figure 8. This is because there is an inclined hole in bend, and core cannot be pulled out in one pull. Therefore, core in inclined hole is pulled out first, then the entire core is pulled out together. Secondary core-pulling mechanism is also designed to make a small slider inside large slider. Small slider is used for pulling core of inclined hole in bend, and large slider is used for pulling core of bend. In terms of core-pulling power, small slider is driven by mold opening power and inclined guide post. Due to its large mass and 26.47° angle with horizontal, large slider is not sufficiently driven by inclined guide post, so a hydraulic cylinder is used. During mold opening, inclined guide post drives small slider to extract core from inclined hole. After mold opening, hydraulic cylinder drives large slider to extract core from curved hole. When product is ejected, lifter core-pulling mechanism on moving mold side moves, extracting core from the other end of inclined hole (inclined hole is a stepped hole, with larger end at the other end).

Difficulty in designing fixed mold side core-pulling mechanism is lack of core-pulling power. Solutions are: (1) For fixed mold core pulling with a small stroke, a spring is used as power source; (2) For molds with a large core-pulling stroke and large core-pulling force, springs cannot meet requirements, and mechanical clamping mechanisms or hydraulic cylinders can be used. Since external springs are unreliable, external hydraulic cylinders are bulky and costly, this mold utilizes mold opening power and employs an lifter mechanism to achieve inclined core pulling on fixed mold side. Three key points need to be mastered when designing a fixed mold side lifter mechanism: (1) guiding mechanism of lifter; (2) ejection power source of lifter; and (3) reset mechanism of lifter. Currently, most fixed mold side lifter mechanisms use an ejector plate on fixed mold side to fix lifter seat, with a spring as power source.
As shown in Figure 9, this mold is a point gate mold, and mold frame is a simplified fine sprue mold frame. If lifter seat is directly fixed to sprue ejector plate, since gap between sprue ejector plate and plate A is used to remove solidified material from sprue, lifter will detach from guide block and cannot reset. To achieve inclined core pulling on fixed mold side, a plate is added between sprue ejector plate and plate A to fix lifter seat. This plate is separable from plate A. Using mold opening power, when this plate separates from plate A, lifter (core) and plastic part generate an inclined relative movement, pulling out from inclined hole to complete core pulling. When mold closes, lifter (core) resets under push of lifter seat. This structure is simple, practical, and has ideal results.

 

Figure 9. Fixed Mold Side Angled Core Pulling Mechanism

As shown in Figure 10, this mold uses an ejector pin ejection mechanism. Action surface is non-appearance surface of plastic part, resulting in a smaller demolding force and meeting surface quality requirements of product.

 

Figure 10. Ejection Mechanism

Cooling system is shown in Figure 11. Plastic part has an irregular shape, large volume and wall thickness, and releases a lot of heat during cooling, requiring good cooling performance. Cooling channels were designed on fixed mold core, moving mold core, lifter plate, sprue ejector plate, and core pulling slider. To improve cooling efficiency, in addition to arranging as many cooling channels as possible around cavity, dense water-blocking plate-type cooling channels were designed on fixed mold core and moving mold core.

 

Figure 11. Cooling System

Because this mold uses a point gate, a simplified fine sprue mold base was selected. To achieve core-pulling function of fixed mold side lifter, an additional lifter mounting plate was added. Plates A and B have a length and width of 750 mm and 700 mm respectively, and thicknesses of 210 mm and 250 mm respectively. Mold thickness is 816 mm.

Injection mold designed and manufactured according to above description has undergone trial molding, is now in mass production. Manufactured products have been inspected, meet design requirements, with stable plastic part quality. This indicates that injection mold structure designed in this paper is reasonable, its operation is reliable, it can meet production requirements.

(1) Secondary core-pulling mechanism can usually be designed with a small slider inside a large slider. It utilizes mold opening power and is driven by a slanted guide post. Slanted guide post hole for small slider is designed as a round hole, and slanted guide post hole for large slider is designed as an oblong hole. During mold opening, slanted guide post first moves small slider to perform a first core-pulling operation. After small slider moves a certain distance, it moves large slider together to perform a second core-pulling operation. If slanted guide post is not powerful enough, second core-pulling operation can be driven by a hydraulic cylinder.
(2) Lifter core-pulling on fixed mold side can be achieved by adding a plate to fix lifter seat. Utilizing mold opening power, motion generated by separating lifter seat fixing plate and plate A is converted into slanted motion of lifter, thus achieving core-pulling. Mold must be designed with a sequential fixed-distance parting mechanism to control mold opening sequence and mold opening distance.