Figure 1 shows coil bobbin used for winding, which is molded with high-strength and high-rigidity PC+ABS-GF10% alloy material, with a shrinkage rate of 0.3% and a mass of about 795g. Characteristic shape of product is relatively simple, but performance requirements are higher. Cylindrical surface B is used to wind coil, and its perpendicularity tolerance relative to two flange surfaces is +0.05mm.

 

Perform a run-out test on product after molding. Test method is shown in Figure 2. Bottom surface of flange is divided into 16 parts, and then 9 parts are measured on 16 equal parts of cylindrical surface with a dial indicator. Runout tolerance is within 0.2mm as qualified. Inner dimensions of two flange surfaces are (94±0.2) mm, which will become smaller due to shrinkage and deformation during molding. Therefore, when mold is manufactured, size must consider tolerance and add shrinkage rate, that is, 94.2×1.003=94.48mm, so as to avoid need for welding of mold due to excessive size of the later.
Molding machine is HTF470W1, and target molding cycle is planned to be 65s. Plastic molding temperature is 240~270℃, and mold temperature is controlled at about 80℃. Die steel is made of pre-hardened steel BPM2738 from Buderus, Germany. Nickel content is about 1%, hardness is uniform, 28~32HRC, it has excellent processing and polishing performance. Because plastic contains 10% glass fiber, BPM2738 needs to be nitridated, hardness after nitriding in ammonia gas can reach 650HV, which can obtain a harder surface layer, better wear resistance and corrosion resistance, thereby extending use of mold life.

 

In order to realize automatic production of the whole process, mold adopts a latent gate. According to characteristics of product, gate position is set on the side wall ofφ54mm hole in the middle of product, while avoiding side ribs to avoid insufficient filling of cavity. In order to balance melt filling to ensure performance of molded product, 8 gates are set, as shown in Figure 3. During injection molding process, pressure loss of runner is relatively large. Because specific surface area of circular section is small (ratio of surface area of runner to its volume), contact surface between melt and mold parts in runner is the smallest, pressure and temperature loss is the smallest, which is beneficial to flow and pressure transmission of melt, so a circular cross-section runner is selected. If diameter of runner is too small, filling amount per unit time will be reduced, filling time will be prolonged. Molded products often have defects such as lack of material and ripples, which affect molding quality of products; excessive diameter of runner will not only increase accumulation of air and product will easily produce bubbles, but also amount of melt will increase and cooling time will be prolonged.
According to plastic properties, maximum melt flow ratio and experience, diameter of runner is φ5mm. Gate is channel connecting runner and cavity, which controls melt flowing into cavity. After injection is completed, cavity is sealed so that melt that has not cooled and solidified in cavity does not flow back. Gate size has a great influence on molding of product. If gate size is too large, melt fluidity will be too low, which will increase internal stress of molded product, cooling of gate condensate will be slow, which will easily cause appearance marks after cutting; Small gate is beneficial to change apparent viscosity of melt non-Newtonian fluid, increase shear rate, thereby increase temperature of cavity melt to increase fluidity, at the same time, it can also control and shorten feeding time, reduce internal stress of molded product, shorten molding cycle. But if gate is too small, pressure loss at the gate is too large, resulting in insufficient melt filling. Based on previous experience, gate size is designed as φ1mm.

In injection mold, design of cooling system has an important impact on molding quality and production cost of product. Cooling time of product accounts for about 2/3 of entire molding cycle. When cooling system design is unreasonable, production cycle will be too long and production cost will increase. On the other hand, uneven cooling effect will affect surface gloss of molded product and make surface gloss of product inconsistent. At the same time, molded product will warp and deform due to residual stress generated during filling and holding pressure at molding stage, which affects dimensional stability of product. Effective cooling circuit design can reduce cooling time, improve product production efficiency, reduce residual stress, ensure product size and shape, thereby improving product molding quality and meeting performance requirements.

 

1. Sprue sleeve 2. Fixed mold cooling insert 3. Fixed mold insert 4. O-ring 5. Product 6. Moving mold insert 7. Moving mold cooling insert
Depth of cylindrical rib in the middle of product is 50mm. It is necessary to design insert molding, divide parts of molding rib for processing, exhaust and polishing. In order to eliminate step difference of side wall of inner hole at A, fixed mold insert 3 and movable mold insert 6 adopt taper positioning on parting surface, circular branch runner is opened along parting surface. Length of sprue is 145mm, sprue sleeve 1 needs to be cooled, otherwise sprue condensate is easy to break and it is not convenient for robot to take out, ribs formed by fixed mold insert 3 and movable mold insert 6 also need to be cooled. In order to design cooling water circuit in fixed mold insert 3 and movable mold insert 6, fixed mold cooling insert 2 and movable mold cooling insert 7 are respectively set, material is Be-Cu or Al, cooling water circuit adopts form of double-head spiral groove, as shown in Figure 4.

 

In order to cool mold uniformly, control temperature difference between inlet and outlet of mold cooling water circuit within 5°C, 4 water paths are set up in movable and fixed mold plates and inserts according to shape of product. Water path is in the form of a straight water path and a water tower. Diameter of water path is φ12mm, waterway spacing is 50mm. In order to ensure cooling effect of mold, there should be no gap between water barrier, waterway plug and B, as shown in Figure 5. Water barrier should be as long as possible, front end should be notched to ensure water flow. Material of water barrier is generally stainless steel to prevent rust from affecting cooling effect of mold.

 

Slider cooling is shown in Figure 6, according to shape of product, a waterway is set along periphery of product. Distance from waterway to product surface is 10-15mm. There are 4 layers of waterways along mold opening direction. Diameter of waterway is φ10mm and waterway spacing is 20-30mm.

 

Sliding block drive generally adopts inclined guide column, square inclined guide block and hydraulic cylinder. In order to ensure smooth movement of slider, size of slider is designed to meet requirements of L/H≥1.8, L/W≥1.5, 150mm≤W≤450mm, slider should add an intermediate guide block C; when 450mm≤W, slider should add 2 guide sliders. After mold is opened, sliding block must have a 2/3L supporting surface. In order to prevent sliding block from being unable to lock, it must meet H2≥2/3H. When using A-type slider, L/H≥1.5, when using B-type slider, L/H<1.5 and L/H1≥1.8, as shown in Figure 7.

 

1. Slide block 2. Guide plate 3. Guide rod 4. Wear plate 5. Middle guide block 6. Slider pressing plate 7. Positioning block 8. Lifting ring auxiliary block 9. Taper positioning block
Undercut of slider is 47mm. In order to facilitate removal of product, stroke of slider is designed to be 60mm. If slider is designed according to conventional design, its size and mold shape will increase by 100mm on one side. In order to reasonably reduce size of mold and reduce cost of mold manufacturing, slider drive adopts form of guide rods and chutes, as shown in Figure 8.

 

Guide rod 3 is fixed on slider 1, guide rod 3 slides in sliding groove of guide plate 2. Depth of guide rod 3 embedded in slider 1 is 1.5 times diameter of guide rod. In order to ensure strength of guide rod, diameter of guide rod (empirical value) is generally greater than 1/6 of length of guide rod beyond slider (that is, 130/6≈ 22mm), take φ30mm, material of guide rod and oblique guide post are both SKD61 (high-frequency quenching). Shape and size of guide plate chute is shown in Figure 9, which consists of four areas A, B, C, and D. In order to prevent product from sticking to fixed mold and causing internal strain, when guide rod 3 slides in area A, slider remains stationary, which is equivalent to delay structure of slider. Core pulling angle of B area slider transitions from 10° to C area slider core pulling angle of 20°, D area slider stops moving.
Width of chute at core-pulling stage in A, B, and C areas is 31mm. Length of guide post is generally 15mm higher than height in clamping direction of A, B, and C areas. Guide plate is fixed on the side of fixed mold to protect guide post and support fixed mold. Guide plate is about 10mm longer than guide post, so groove width in D area needs to be treated with clearance, groove width is designed to be 40mm. Slider is usually set on operating and reverse side of mold, guide rod and guide plate are set on sky side (side where injection molding machine is installed is sky side). In order to facilitate lifting of mold, a lifting ring auxiliary block 8 is required on sky side. Lifting ring auxiliary block and front thread are designed as an integral type to prevent thread from breaking during mold lifting process, causing safety accidents.

 

Slider is a common Haval slider, as shown in Figure 10. In order to eliminate step difference of inner cylinder of molded product, a tapered positioning block 9 is used for positioning between two sliders. In order to facilitate later polishing and adjustment of step difference of inner cylinder of molded product, a positioning pin and a fixing screw should be set between two sliders. Slider pressing plate 6 is fixed on movable mold plate. Due to gap between screws and holes of fixed pressing plate, in order to prevent change of screw locking position from affecting smooth movement of slider, a positioning block 7 is provided between slider pressing plate and movable mold plate. Because two Haval sliders share one slider pressing plate, pressing plate has a longer length, positioning pins can be set at both ends to ensure smooth movement of slider. Slider pressing plate is made of P20 and is nitrided. At the same time, insertion angle of slider and fixed mold insert is 3° to prevent slider from straining, an oil groove is opened in sliding part. Shape of coil products is mostly circular, shape of slider is also designed to match fixed mold plate with a matching angle of 10°.

 

1. Positioning ring 2. Fixed mold seat plate 3. Fixed mold insert 4. Fixed template 5. Slider 6. Slider limit block 7. Movable template 8. Cushion block 9. Movable mold seat plate 10. Reset switch 11 .Support column 12. Push plate 13. Push rod fixed plate 14. Push plate guide column 15. Push rod 16. Moving mold insert 17. Fixed mold cooling insert 18. Moving mold cooling insert 19. Guide plate 20. Slider Pressure plate 21. Wear plate 22. Ring auxiliary block 23. Positioning block 24. Guide rod
Mold structure is shown in Figure 11. Working process is: molten plastic flows into mold runner through injection molding machine nozzle, then enters cavity. After heat preservation, pressure retention, and cooling, mold is opened. When mold is opened, fixed mold plate 4 and movable mold plate 7 are opened, guide plate 19 drives guide rod 24 fixed on slider 5 to slide to complete core pulling action; ejector rod of injection molding machine pushes push plate 12 to drive push rod 15 to push out product, finally robot takes out product, and mold opening action is completed; Ejector rod of injection molding machine is reset, and push rod 15 is reset under action of return spring. Injection molding machine receives signal of stroke reset switch 10 to close mold. Guide rod 24 first enters D zone of guide plate 19, guide rod continues to close after entering guide sleeve. Guide rod 24 sequentially passes through C, B, and A areas of guide plate 19, thereby pushing slider 5 to reset. Fixed mold plate 4 locks slider to form a closed cavity. Mold clamping is completed and next injection molding is prepared.
According to structural performance and characteristics of coil bobbin, ordinary runner is used to turn 8 latent gates for pouring and Haval slider is formed. Slider is moved by guide rod moving in guide plate chute. Slider of circular coil bobbin is driven in this way, which can not be restricted by conventional size. Under premise of ensuring strength of mold, mold size can be effectively reduced to meet installation requirements of injection molding machine and reduce mold manufacturing cost.