Car armrest box (hereinafter referred to as armrest box) has storage, decoration and support functions. Quality of armrest box has a direct impact on quality of auxiliary instrument panel assembly. Now injection mould for forming armrest box adopts a special structure, matching suitable injection equipment to reduce development and production costs of plastic parts, and mold design process is analyzed.

 

Figure 1 Armrest box

Figure 1 shows car armrest box, with an overall size of 251mm*190mm*220mm and a volume of 519.386cm3. Molding mold is required to adopt 1-cavity structure. Material of plastic part is modified plastic PP-TD20KDPP1318, which has characteristics of high rigidity, low thermal expansion coefficient, good melt fluidity, and easy forming; tensile strength tested according to  ISO 527-2 test standard is 21MPa and flexural modulus It is 1700MPa, with good compression and deformation resistance; material has good impact resistance and wear resistance.

Side wall of inner cavity of plastic parts and flange on inner side are appearance surface, which belongs to A surface (visible surface of plastic part). A surface is not allowed to have such defects as push rod marks, gate marks, tiger skin patterns, and obvious shrinkage marks. There are installation requirements for opening of plastic part, and attention should be paid to deformation control. There are reinforcing ribs on outer sides of plastic part, and surface is prone to shrink marks. Stripping of reinforcing ribs requires design of slider lateral core pulling, and design needs to be considered.

Wall thickness of most areas of plastic parts is 2.5mm, and thickness of side wall stiffener is 0.8~0.9mm, which meets anti-shrinkage design requirement that stiffener thickness is 30% of main wall thickness.
Demoulding slope of side wall of plastic part is 2°~3°, and demoulding slope of four corners is 3°~5°. Probability of surface strain at four corners of plastic part is greater than that of side surface, so a large draft angle at the corner is beneficial to ensure surface quality of plastic parts. Surface is decorated with dermatome, dermatome number is M102 (25%), and demolding slope is required to be ≥1.8°.

 
Figure 2 Location of weld line
If armrest box is fed from side, side wall of plastic part is very high, it is easy to produce weld marks, air trapping on opposite side and bottom of gate. Because one side of gate is advanced with materials, it will cause mold core to shift to the other side and become unstable in severe cases, which will lead to difficulty of molding the other side of plastic part to be molded, resulting in insufficient filling. Bucket and cup plastic parts are suitable for bottom feeding. Length of material flow in each direction of plastic parts is relatively uniform. When filling, melt flows along circumference of cavity at the same time. Pressure on the surroundings is more balanced, exhaust is best, and welding marks are less. ,as shown in picture 2. Through above analysis, gate position is designed at the bottom of plastic part, as shown in Figure 1(b). Location of weld mark in Figure 2 adopts a straight gate with a large end diameter of ϕ5mm, and corresponding surface of direct feed gate is easy to produce marks. Method of adding a water jacket at gate position can solve or dilute gate marks. There is a rubber pad at the bottom of plastic parts when assembling, so that marks on the surface of plastic part will not affect appearance of plastic part.

 
Figure 3 Deformation analysis
Deformation of shell plastic parts generally occurs at the edge of plastic part. Edge of plastic part has a flange structure and side wall has stiffeners to reduce deformation of openings. From CAE analysis results of warpage deformation, deformation of plastic part meets assembly and tolerance requirements, as shown in Figure 3.

 
Figure 4 Analysis of clamping force
Mold flow analysis clamping force is 905.3kN. As shown in Figure 4, side projected area of plastic part is larger than area in clamping direction, and strength of frame edge of side part of mold needs to be considered.
 
Figure 5 Schematic diagram of mold structure of Scheme 1
Solution 1: Feed from back of plastic parts to be molded and push it out from back. Mold is designed as an inverted structure. External dimension of mold is 1000mm*950mm*970mm, as shown in Figure 5. According to above analysis data, program is suitable for selecting 8000kN equipment, maximum clamping height of equipment is 1000mm, and minimum clamping height is 730mm. Weight of plastic parts is about 500g. Production cost of 8000kN equipment is too high. Consider reducing size of mold, can choose 4700kN equipment. If 4700kN equipment is used, height of mold must be reduced by 195mm to meet requirements. Analyzing size of each part of mold, only mold ejection mechanism can design different ejection methods, and built-in or non-cushion structure can reduce height of ejection mechanism by 200mm .
 
Figure 6 Schematic diagram of mold structure of Scheme 2
Solution 2: According to above analysis, change original mold flip structure, use position of ribs on flange of openings of plastic part to design push plate ejection mechanism, ribs are better stressed, plastic part deforms little when pushing out, and mold structure is simple. External dimension is 950mm*780mm*775mm, and its structure is shown in Figure 6.
Clamping size of 4700kN equipment is: clamping width of tie rod is 800mm, and maximum clamping height is 780mm. Overall dimensions of mold are fully matched with 4700kN equipment. Estimated based on output of 500,000 pieces, more than 1 million yuan in production costs will be saved compared to first plan.

Height of core molded plastic part plus height of pusher plate part results in a larger overall core height, and core structure design is more complicated. If core and fixed plate adopt an integral structure, mold parts have good rigidity, but material waste is large; if core and fixed plate are designed as a separate structure, it is easy to process and save materials, but there is a risk of core overturning and displacement. Once core is offset, molded plastic part will be thick on one side and thin on the other side, so core positioning is very important. Size of core is small and unstable in left and right direction, which is the key to positioning design. Most of heat is concentrated on core during molding. Core cooling should also be considered simultaneously. Quality of cooling effect not only affects molding quality of plastic part, but also affects production efficiency.
 

 

Figure 7 Core structure

 

Figure 8 Cooling water channel
Core structure is shown in Figure 7. In order to solve core positioning and cooling problems, following design methods are adopted: ①Lengthen depth of core inserted into fixed plate to improve positioning accuracy; ②Wide width of core inserted into fixing plate to make core more stable, so that bottom surface of pusher plate has enough area to press core to prevent tilting; ③Increase type of core fixing screw to increase tightening force; ④Pusher plate and side of core are designed to be inclined (can not avoid air), and surrounding is matched with a positioning design with a slope of 3°. Positioning surface is designed to have a smaller slope, shape to push matching surface to a larger slope to ensure accurate positioning of push plate mating surface and not easy to wear; ⑤Core and pusher plate positioning surface is designed with wear-resistant blocks to facilitate assembly, adjustment and maintenance; ⑥Core is designed with a cooling water channel, and circulating cooling method is separated by a partition plate. Difference in mold cooling efficiency will lead to inconsistent temperatures on both sides of molded plastic part. Temperature on the side with poor cooling efficiency is higher than the other side, plastic part will bend in high temperature direction. Balanced cooling of plastic part is the key. Cooling water channel is shown in Figure 8. .

Plastic part has a deep cavity structure with thin walls and large side area. After molding, plastic part has a large packing force on core and requires a large demolding force. Therefore, design of ejection mechanism is very important. Pusher plate is used to separate plastic part from core. Due to large contact area of pusher plate, pushing force is reduced, and problems such as deformation or obvious marks caused by push rod to push plastic part are avoided.
 

 
(B) Back
Figure 9 Push plate
Push plate not only completes demoulding of plastic part, but also has functions of guiding movement, bearing and positioning of two large sliders. Strength and stability of push plate are related to safety of movement of mold parts. Design of pusher plate is mainly considered from following aspects: ①Positioning of cavity plate is designed to solve positioning problem with cavity plate; ②Guide to the slider and load-bearing design to ensure smooth sliding of slider, wear resistance and durability; ③Prevent displacement of pusher plate and core fixing plate, design and position stop of core fixing plate; ④Ensure a stable position with core, structure is designed to be positioned and matched with core of double inclined plane; ⑤Quality of pushing mechanism is heavier to ensure that pusher plate is moving. Above method can ensure that pusher plate and sliding block, cavity plate, core, and core fixing plate form a stable whole, forming a double-stop stable structure to ensure quality of mold, as shown in Figure 9..

Height of ribs on both sides of plastic part is 5~23mm, and slider structure needs to be designed. Slider should cover both sides of plastic part. Ribs are deep and demolding angle cannot be increased. Ribs are easy to stick to slider. When injection pressure is high, surface of ribs is easy to pull white or strain. A built-in ejection mechanism is designed at molding ribs of slider to assist demoulding to increase ejection force.
 
(A) Slide block locking surface
 
(B) Slider molding surface
Figure 10 Large slider structure
Sliding block is large in size and heavy in mass. It is necessary to design sliding guide groove structure, as shown in Figure 10. Sliding block bar should be sunk into pusher plate, and limit spring of large sliding block must ensure that sliding block is stable when mold is opened, and no instability occurs. Spring force calculation verification should be carried out: generally spring preload is checked by 3 times mass of slider when slider is on the upper side of mold, mass of slider is 125kg, and spring specification is SWF30-125 , spring constant is 11.8, compression amount in open mold state is 10mm, and 4 springs are designed. After mold test, slider limit is stable and reliable. During mold test, local ribs of slider are poorly exhausted, and inserts are used to exhaust defective position.
Commonly used slider drive methods are: inclined guide column drive and hydraulic cylinder drive. Drive structure of inclined guide column is simple, and opening speed is fast. Opening of slider does not take up molding cycle time, which has advantages for small distance core pulling. Hydraulic cylinder drive: After mold is opened, hydraulic cylinder piston rod works, and sliding of slider takes up molding cycle time. At the same time, large volume of hydraulic cylinder is also available, which has advantages for large stroke core pulling. Mold replaces oblique guide pillar drive structure.
There are two options for oblique guide pillar drive structure: reverse oblique guide pillar drive and forward oblique guide pillar drive.
(1) Reverse inclined guide column drive. Oblique guide post is fixed on core fixing plate, and oblique guide post drives slider through power of pusher plate. This structure has 2 advantages: ①Slider always cooperates with oblique guide post, and there is no safety problem of slider limit. ②Push plate and sliding block move synchronously, and plastic part does not have problem that push plate cannot push plastic part when filling is insufficient. However, there are two disadvantages: ①Because pusher plate must have a certain thickness, oblique guide column is long, working stress point is far from fixed point, and stress state is not good; ②In order to realize complete separation of lateral ribs and meet requirements of space for taking parts, pushing distance of a board is very long (180mm), and weight of floating push plate slider structure combination is heavier. The longer pushing distance, molding accuracy of mold parts will decrease and service life will be correspondingly shortened. .
(2) Positive oblique guide column drive. Structural advantages: Inclined guide column is fixed on cavity side, movable and fixed molds are changed to open to make slider slide, and structure is compact. However, this structure also has two disadvantages: ①Plastic part cannot be ejected by pusher plate when mold cavity is not full, so that plastic part cannot be demolded; ②Because inclined guide post is in fixed mold, when mold is opened, under action of inclined guide post, large sliding block not only receives opening force inclined in both directions, but also receives pulling force in mold opening direction. Pulling force causes pusher plate to move forward, so that pusher plate opens before slider opens, which will cause stick-slip or strain problems of plastic part.
 
Figure 11 Movable mold structure
To sum up, forward inclined guide pillar drive scheme is generally better, it is adopted, and following measures are taken to improve above shortcomings: ①Design an auxiliary ejection structure at the bottom of cavity plate to push out plastic parts, and design wear-resistant blocks on both sides of slider to ensure stability and embedding of slider and cavity plate; ②Push plate is pushed out and returned by piston rod of hydraulic cylinder to limit push plate to move forward when inclined guide column drives sliding block. Problem of mold opening sequence of pusher plate is solved, and movable mold structure is shown in the following 11.
 
Figure 12 Fixed mold structure
Fixed mold structure is shown in Figure 12. Cavity plate adopts an integral structure. Materials of cavity plate, slider and pusher plate are XPM pre-hardened steel, and other auxiliary plates are S50C. Push plate hydraulic cylinder adopts quick plug-in fittings, which is convenient to use and prevents oil leakage. Hydraulic oil circuit is serialized and integrated in push plate, which reduces external hydraulic pipes of mold, which is safer and environmentally friendly.

Mold is a pusher plate-type ejection two-plate mold structure. Large sliding blocks on both sides are driven by inclined guide posts. 4 hydraulic cylinders drive pusher plate to push out and return, as shown in Figure 13.
 
Figure 13 Mould structure
1. Fixed mold base plate 2. Cavity plate 3. Wear block 4. Pressure bearing block 5. Push plate 6. Hydraulic cylinder 7. Hydraulic cylinder positioning key 8. Link card key 9. Signal socket 10. Movable mold base Plate 11. Guide post 12. Guide sleeve 13. Slider limit block 14. Slider guide bar 15. Positioning block 16. Core 17. Spring 18. Screw 19. Slider self-lubricating wear plate 20. Travel switch dial Block 21. Stroke switch lever 22. Inclined guide column 23. Large slider 24. Inclined guide column fixing seat 25. Support column 26. Slide block top block 27. Slide block locking wear block 28. Positioning ring 29. Push block 30. Joint 31. Square guide post 32. Water nozzle 33. Seal ring 34. Crimping plate 35. Push rod fixing plate 36. Push rod 37. Push plate guide rod fixed plate 38. Push plate guide rod 39. Lock Module
Mould opening sequence: after mould clamping, first open mould lock module 39; insert mould safety protection signal plug into signal socket 9; connect neutron (hydraulic core pulling device of injection molding machine) and mould return hydraulic cylinder quick connector 30. Connect mold cooling water channel to water nozzle 32. When mold is opened, movable mold plate moves backwards, cavity plate 2 is separated from large slider 23, pusher plate 5, core 16, and plastic part remains on core 16. Large slider 23 moves downward under action of inclined guide post 22. After movable mold plate moves in place, neutron works and drives piston rod of hydraulic cylinder 6 to move. Through action of link key 8, pusher plate 5 is driven to move forward. Plastic part is pushed out from core 16 and plastic is taken out. Mold opening action is completed. If it is the first time to debug mold or when plastic part is insufficiently filled, an auxiliary ejection mechanism can be used. Ejector pin of injection molding machine contacts push rod fixing plate 35 to drive push rod 36, and push block 29 is driven forward by push rod 36 to eject plastic part. Ejector rod of injection molding machine retreats, mechanical ejection system returns under action of spring 17, and no auxiliary ejection is required for normal production. When mold is closed, piston rod of hydraulic cylinder retracts and pulls pusher plate 5 to reset, pusher plate is attached to core fixing plate, travel switch lever 21 moves down to contact travel switch to close, and injection molding machine mold clamping action starts. Movable template moves forward, large sliding block slides inward under action of inclined guide post 22, and mold clamping is completed.