Base is similar to a semi-enclosed box-shaped piece. Coil is installed inside box surrounded by vertical ribs. Outer side where vertical ribs are located and B and C surfaces are exterior surfaces. No push rod marks or obvious parting lines are allowed. End surface where height of vertical rib is located is mating surface with upper shell. Try to avoid setting flat push rods. Side A is installation surface. Bottom surface must be horizontal, a push rod or parting surface can be set. In order to realize two functions of contactor rail installation and screw installation, this plastic part is designed with 2 dovetail grooves and 4*φ5.2mm through holes. In order to be consistent with installation dimensions of other models of contactors, distance and position of 4*φ5.2mm holes cannot be changed, resulting in 2*φ5.2mm holes penetrating dovetail groove that needs to be formed by side core pulling. This is a difficulty in mold design. Material of plastic part is reinforced flame-retardant nylon. Vertical ribs have a large area, a high height, and a thin wall thickness of only 1.2 mm. Allowable draft angle is small, plastic part is easy to stick to mold and trap air during molding. φ14mm hole and fan-shaped side hole are heat dissipation holes, which are located on appearance surface and need to be formed by side core pulling. How to ensure smooth forming and demoulding of vertical ribs and side holes, while ensuring that there are no push rod marks or obvious parting lines on appearance, is also a difficult point in mold design. 3.0, 9.3, and 12.8mm slots are clamp installation slots and require side core-pulling molding.

Considering size, shape, structure and output of base, mold is designed as 2 cavities. In order to ensure molding, demoulding, and push-out of plastic parts, plastic parts to be formed are usually placed and separated in direction shown in Figure 2(a). A point gate is set in the center of bottom surface of plastic part, and a three-plate mold structure is adopted. All side cores and components are installed on movable mold side. However, plastic part to be formed is placed as shown in Figure 2(a). No matter how it is parted, there will be parting lines and push rod marks on exterior surfaces such as B, C surfaces (see Figure 1) and outer surfaces where vertical ribs are located, and mold structure is complex. Because surface A is installation surface, push rod marks and parting lines are allowed. In order to meet requirements of plastic parts, mold can only be placed and separated to molded in direction shown in Figure 2(b) during mold design, which is opposite to conventional box-shaped parts placement and molding direction.

 

 

There are three places in plastic part that need to be formed by side core pulling. The first place is 3.0, 9.3, and 12.8mm clamping block installation grooves; second place is dovetail groove; third place is φ14mm step hole and fan-shaped side hole on vertical rib. Core pulling distance on 12.8mm long clamp block installation groove side is longer. Considering parting and pushing out of plastic parts to be formed, core pulling distance here can only be designed on movable mold side. Formation of second dovetail groove and third fan-shaped side hole must be combined with formation of 2×φ5.2mm through hole and vertical rib.
Plastic parts to be formed are placed as shown in Figure 2(b) to avoid push rod marks on appearance. However, considering large area, high height, and thin wall thickness of vertical ribs, in order to prevent molded plastic parts from sticking to mold, in addition to increasing draft angle of vertical ribs, most of ribs are usually formed on side core, and side core is designed on the side of movable mold. But this will produce parting lines on B and C surfaces or outer surfaces of vertical ribs, which will affect appearance of plastic parts. At the same time, due to strong adhesion force on inner side of ribs, plastic parts will deform greatly after side core is demoulded. Whether 2×φ5.2mm through hole is formed on one side or both sides of movable or fixed mold, dovetail groove is formed by an inclined push rod or side core pulling, mold structure is relatively complex, there are sequence requirements for opening and closing mold, mold manufacturing cost is high, later debugging and maintenance are difficult. Therefore, option of forming vertical bars and B and C surfaces through side core is eliminated.
Based on above analysis, vertical bars and surfaces B and C can only be set in fixed mold. In order to ensure smooth demoulding of vertical bars and avoid mold sticking, consider using two different cores to form inner and outer sides of vertical bars, and two cores can slide with each other, that is, when opening mold, first detach one side to reduce mold sticking force and obtain deformation space required for subsequent demolding of plastic part, and then detach the other side. This structure is fixed mold core pre-core structure, that is, molding on same side of movable mold or fixed mold needs to be demoulded separately twice instead of once. Fan-shaped side hole part can only be formed through side core, but side core must be designed on same side of forming vertical rib cavity, that is, on the side of fixed mold, and parted along φ14mm step hole. Fixed mold tunnel type side core pulling is used. This can avoid parting line on vertical rib surface and B and C surfaces, and meet appearance requirements of base. After above molding structure is determined, molding structure of dovetail groove and 2×φ5.2mm hole that penetrates it is also determined. Since the two penetrate each other, if round hole is formed on one side, it is necessary to delay side core pulling or have mold opening and closing sequence requirements. Due to space and position constraints, from perspective of continuous and stable molding in later stage, structure of forming dovetail groove through an inclined push rod was eliminated, and core-pulling molding on the side of slider was chosen, which has a simpler structure, is easier to maintain and adjust. Considering strength of 2*φ5.2mm through-hole core and side core-pulling structure of dovetail groove, part mold along through line with dovetail groove, half of movable and fixed molds are formed, so that opening and closing of circular core and side core will not interfere, there is no requirement for order of mold opening and closing. Main parting and molding structure is shown in Figure 3.

 

Mold structure is shown in Figure 4, which mainly explains fixed mold core pre-core pulling and fixed mold tunnel type side core pulling structures. Mold has a two-plate mold structure, with sliding blocks on three sides of movable mold and side core pulling, and tunnel-type side core pulling on both sides of fixed mold. Latent gate is used for pouring. Fixed mold core 12 forms inner side of vertical ribs of plastic part and is fixed on fixed mold base plate 8 through fixed mold core fixing plate 13. Fixed mold cavity plate 17 forms outer side of vertical rib of plastic part and is fixed on fixed mold plate 4. There is a sliding fit between fixed mold core and fixed mold cavity plate, a spring is installed between fixed plate of fixed mold and fixed mold plate, a die buckle is installed between fixed mold plate and movable mold plate. Round side core 18 is fixed on slider 16 and crosses fixed mold cavity plate. Boss shape (see Figure 5) with a width of 62mm and a height of 20mm on slider 16 is installed on fixed mold plate, can slide horizontally along groove formed by fixed mold plate guide rail 28 and fixed mold plate table. There is a "T"-shaped groove on the slope of slider, which cooperates with "T" shape on locking block 15 to function as a sliding guide and side core puller. Locking block 15 is fixed on fixed plate of fixed mold. When fixed mold plate is separated from fixed mold fixed plate, slider 16, guided and acted by "T" shaped groove of locking block, not only slides horizontally along groove formed by fixed mold guide rail 28 and fixed mold plate, but also moves downward with fixed mold plate, driving round side core to both move downward and slide horizontally with fixed mold cavity plate, completing side core pulling; movement direction is opposite when closing mold. Because core-pulling distance on this side is short, "T"-shaped groove of slide block and locking block are not separated.

 

1. Moving mold plate 2. Base 3. Die buckle 4. Fixed mold plate 5. Fixed distance screw 6. Fixed distance tie rod 7. Spring 8. Fixed mold base plate 9. Water inlet pipe 10. Seal ring 11. Water barrier 12 . Fixed mold core 13. Fixed mold core fixed plate 14. Outlet pipe 15. Locking block 16. Slider 17. Fixed mold cavity plate 18. Round side core 19. Moving mold guide rail 20. Moving mold cavity plate 21. Pad Block 22. Push rod fixed plate 23. Push plate 24. Moving mold base plate 25. Block groove side core 26. Gate plate 27. Gate sleeve 28. Fixed mold guide rail 29. Screws 30. Dovetail groove side core

Figure 4 Mould Structure

 

When opening mold, fixed mold plate and fixed mold fixing plate are separated first under action of spring. At this time, fixed mold core begins to be demoulded. At the same time, round side core begins to pull out core under action of slider 16 and locking block 15. After fixed mold core and side molding core are completely demoulded, fixed distance pull rod 6 starts to act. At this time, fixed mold plate does not move, movable mold plate continues to move downward. Under action of block groove and dovetail groove side molding core, outer side of vertical rib is separated from fixed mold cavity plate, then clamping groove and dovetail groove complete side core pulling, movable mold part is demoulded, and molded plastic part is pushed out. Side core-pulling structure of dovetail groove and clamping block installation groove is same as conventional side core-pulling structure of movable mold slider, and will not be described again.
Mold gate is set on fixed model cavity plate. Position of gate on mold cavity plate is determined for overlap compensation to ensure strength of gate, making the overall flow channel shorter, leaving less traces of condensate on gate, and having little impact on appearance of molded plastic parts, enabling automated production.

Structure of tunnel-type side core-pulling locking block and slider of fixed mold is shown in Figure 5. In addition to "T" shaped structure, 3° slope on locking block cooperates with fixed mold plate to play an anti-locking role. Round side cores 18 can be fixed on slider. When side cores are smaller in size and larger in number, they can be fixed on slider through a fixing plate.

(1) Wall thickness of plastic part is uneven, wall thickness of vertical ribs is thin, and wall thickness of bottom part is thick. In order to ensure smooth demoulding of molded plastic parts, shorten molding cycle and reduce warpage deformation, separate cooling water channels must be designed for pre-drawn fixed mold core, annular cooling water channels must be designed around moving and fixed mold cavity plates.
(2) Pre-drawn fixed mold core 12 and fixed mold cavity plate 17 are both molded parts. When mold is opened, distance between them is large. Considering that it is easy to guide and slide when closing mold, molded part is not damaged, mating surface needs to be set with a taper of 0.5°. Mating surface of sprue sleeve and the fixed mold cavity plate must also have a certain taper. Taper is determined based on diameter, length, etc. of sprue sleeve. It is recommended to be about 1.5°. Since side core pulling distance is small for round side core, mating surface with fixed mold cavity plate does not need to have a taper.
(3) Fixed mold core pre-pulled core structure realizes segmented demoulding. First, inner wall of vertical rib is detached, and then outer wall of vertical rib is detached, which decomposes demoulding force of vertical rib and avoids deformation of plastic part caused by poor demoulding due to excessive adhesion force of fixed model core. On the other hand, after inner wall of the vertical rib with the largest mold adhesion force is demoulded, mold adhesion force of outer wall of vertical rib is smaller. When mold is demoulded under action of dovetail groove and side core of clamp installation groove, plastic part deforms little.
(4) Fixed mold core is designed as a straight cylinder according to shape of inner wall of vertical rib, molding part has a certain draft angle, and can be processed in one step by slow wire cutting.
(5) When designing runner and gate, installation position of sprue sleeve should be as low as possible, length of main channel and runner should be shortened. At the same time, without affecting appearance and molded plastic parts can automatically fall off, latent gate part is designed as a rectangle with a larger cross-sectional area and a size of 2.0mm * 0.6mm, which not only reduces pressure loss in flow channel during molding, but also avoids defects such as excessive injection molding pressure, large deformation of plastic parts, or flash edges.
(6) Fixed mold core for forming inner wall of vertical ribs and round side core for forming fan-shaped side holes are both straight-cylindrical inserts that slide relative to fixed mold cavity plate, which can play an exhaust role and avoid defects such as trapped air at the end of melt flow in the ribs, unsatisfied injection, or flow marks on the surface of plastic part.
(7) Gate position should be selected so that melt flow direction in gate and melt flow direction in cavity are basically same. This can reduce melt flow resistance and molding pressure, and reduce deformation of plastic part. Mold trial and mass production molding cycle is about 30 seconds, molding temperature is 250~280℃, and molding pressure is 35~40MPa. Actual molded plastic parts are shown in Figure 6. In later use, due to heat dissipation requirements, number of fan-shaped side holes on the side was increased, and 2 long holes were added on the side.

 

Mold has been used to mass produce plastic parts. Mold is stable and good in use, easy to repair and maintain, molded plastic parts are qualified. Base injection mold has a novel structure and can be used as a reference for molding similar plastic parts with appearance requirements.