【Abstract】According to structural characteristics of plastic part, mold structure and molding process are combined with inclined sliding rod core pulling and lateral molding slider. Introduced selection of parting surface of moving and fixed molds, as well as design of gating system and mold assembly structure.

Plastic part shown in Figure 1 is a barrel-shaped container for storing and transporting insurance food. From perspective of food safety, attention should be paid to structural design of container to ensure that food is not contaminated and separated during entire process of storage, storage, transportation and use.

 

Figure 1 3 plastic parts that make up food preservation bucket
It can be seen from Figure 1 that three plastic parts that make up food preservation bucket are: bucket body, bucket lid, and buckle. Figure 2 shows assembly process of lid and buckle ring after placing food in barrel: ①Put food in barrel; ②Rotate lid of barrel 30°to cover; ③After closing lid, rotate lid 60° to fit three built-in shoulders of barrel and tighten; ④After cap is closed, buckle is slowly pushed in from side of barrel with a small convex circle (handle is guided by a bevel); ⑤After buckle is pushed in place, two small convex circles on its inner side are pressed into inner hole of handle to complete assembly. Because buckle is designed for one-time use, it uses hard polystyrene (PS) plastic with a concave arc structure on back. If inspector finds that buckle is broken or missing, food may have been separated and contaminated. When taking food, takeover party only needs to cut off buckle and rotate lid to take out food.
Materials used are PP engineering plastics commonly used in beverage, food and other packaging. Because PP plastic material is a safer engineering plastic that is widely used in packaging, storage and repeated use, it has an excellent melt index, has good melt fluidity and moldability during injection molding. Wall thickness of plastic parts is uniform (both are about 4mm), which meets process requirements of injection molding. 

 

Figure 2 Assembly process after placing food in fresh-keeping barrel

Structural graphics and related dimensions of plastic part of lid are shown in Figure 3. In view: plastic part has a disk-shaped hollow shape, upper part is a handle design, center part is a large arc-shaped downward molding. Main purpose is to increase lifting strength of handle. Inner bottom surface of plastic part is designed with three 120° equally divided 45° structures that are locked to shoulder surface of food bucket, and two ϕ 4mm vent holes are provided on the surface of bucket lid.

(1) Polypropylene plastic PP is a non-toxic, odorless, colorless white waxy transparent pellet of acrylonitrile. It does not absorb water, has good gloss, and is easy to color. However, it is not recommended to be colored when used in food storage and packaging. Except for strong oxidants, it has good chemical stability; it is insoluble in water, hardly absorbing water, and has good water stability. Melting point of polypropylene plastic is 164℃～170℃, and it has good heat resistance. Heat distortion temperature is 150℃, and it can be used for a long time below 110℃.
(2) Structural craftsmanship of plastic parts.
a. Dimensional accuracy analysis of plastic parts. Dimensional accuracy of shape and inner wall of plastic part is not high, allowable tolerance size deviation varies widely, and it is easy to control in  actual production process. Free size can be checked for tolerance value according to MT7.
b. Surface quality analysis of plastic parts. Inner and outer walls of plastic part should be smooth and free of burrs. Technical requirements in part drawing of plastic part (see Figure 3) do not require high requirements for all shapes of plastic part, surface roughness value Ra3.2μm is enough.
c. Structural process analysis of plastic parts.Dimensional accuracy of shape and inner wall of plastic part is not high, wall thickness of allowable tolerance size is relatively uniform (about 2.5mm), which meets requirements of injection molding.

 

Figure 3 Lid of food preservation pail (PP)

When selecting parting surface, according to selection principle of parting surface, maximum contour of plastic part should be used as parting interface of moving and fixed molds. At the same time, it facilitates exhaust of mold cavity and keeps plastic parts in movable mold part as much as possible. When designing injection mold, consider that bottom edge of plastic part is a circular plane. Therefore, circular plane at the bottom of plastic part is selected as horizontal parting surface between moving and fixed molds, part of shape where handle of plastic part matches with buckle ring is partly parted in the form of partial lateral parting, as shown in Figure 4.

Since plastic parts are of thin-walled structure and large in volume, gating system should adopt point gate feeding method. Due to long process of plastic material flow in mold, filling speed of plastic melt in mold is affected to a certain extent. Therefore, feed form of point gate with 3 equal points is adopted to avoid formation of melt marks or insufficient filling of melt. Schematic diagram of gating system is shown in Figure 5.

 

Figure 4 Schematic diagram of selection and determination of parting surface

 

Figure 5 Schematic diagram of uniform distribution of three-point gate

Part where mold needs to be formed is mainly composed of two parts: ①Inner bottom surface of plastic part is designed with three 120° equally divided 45° and shoulder surface of food barrel is locked with each other, inclined guide forming sliding rod must be adopted; ② is a ϕ 6mm recess formed laterally at one end of handle that matches buckle ring. Mold forming form of three-incline guide forming slide bar is shown in Figure 6. Figure 7 shows mold forming form of ϕ 6mm recess formed laterally at one end of handle that matches buckle ring. Partial mold structure of lateral molding is shown in C-C (partial) section of assembly drawing. Because 2-ϕ 6mm dimple hole at one end of handle is shallow (1.5mm), molded plastic part has a small tightening force on pulling core, so a simple mold structure that uses a round spring to push out mold instead of using a sliding block and an oblique guide pillar. As shown in section C-C (partial) in Fig. 8, lateral core-pulling mechanism is mainly composed of an oblique wedge 38, a circular spring 39, a slider 40, and a laterally formed ϕ 6mm round core 41. In design of ejection mechanism after plastic part is molded, connecting seat 28 of three-oblique guide molding slide rod and reset rod 27 are fixed on push rod fixing plate 29 at the same time, so reset of plastic part after injection molding is pushed by reset rod. Reset mechanism composed of push rod fixing plate, etc. completes reset of three-oblique guide forming sliding rod, as shown in figure.
Structural form of connecting seat 28 of three-incline guide forming sliding rod is shown in G-direction view of top view, and view is below B-B cross-sectional view. A de-casting plate is set between fixed mold plate and fixed mold base plate. In order to realize automatic separation and fall off of gating system after mold is opened, a de-cast plate 3 is set between fixed mold plate 15 and fixed mold base plate 1. As shown in A-A (rotating) cross-sectional view of mold assembly drawing.

 

Figure 6 Schematic diagram of core-pulling forming of three inclined sliding rods

1. Fixed mold cavity 2. Movable mold large insert 3. Three inclined forming slide bar 4. Movable mold

 

Figure 7 Schematic diagram of side core-pulling forming of hole

Mold structure is shown in Figure 8. After injection is completed and mold is fully opened, size of suspended part between moving and fixed molds is larger (>290mm). In order to ensure continuous and safe injection molding production, following measures and structural schemes are mainly adopted:

 

Figure 8 Mould assembly structure diagram
1. Fixed mold base plate 2. De-casting plate 3. Fixed mold backing plate 4. Fixed mold forming large insert 5. Z-type pull rod 6. Sprue ejector ejection column 7. 39. Round spring 8. Sprue sleeve 9. Positioning ring 10, 17, 33, 34, 36, 37. Hexagon socket screw 11. Fixed mold forming semi-circular ball insert 12. Movable mold forming semi-spherical insert 13. Oblique guide forming slider 14. Mould forming large insert sleeve 15. Fixed template 16. Movable template 18. Open mould fixed distance sleeve 19. Guide post 20, 21. Fixed mold guide sleeve 22. Dynamic mold guide sleeve 23. Fixed distance tie rod 24. Movable mold backing plate 25. Cushion block 26. Movable mold seat plate 27. Reset lever 28. Inclined guide sliding rod seat plate 29. Push rod fixing plate 30. Push rod pad 31. Sliding pin of inclined guide slide bar 32. Movable mold ϕ 44mm round core 35. Cylindrical pin 38. Oblique wedge 40. Side guide slider 41. Lateral molding round core
(1) After mold is opened, dead weight of fixed mold part is relatively large. Four reverse long guide pillars 19 that bear gravity are designed as second-order guide pillars of ϕ 40mm. Avoid deformation of guide post caused by excessive weight of mold, quality problem of uneven wall thickness at four arcs of plastic part.
(2) In order to realize semi-automated and automated production of mold, automatic fall-off of pouring system is key. This mold is composed of a mold-opening fixed distance sleeve 18, a fixed distance pull rod 23, a Z-type pull rod 5, and a sprue bullet top ejection column 6 to form a set of ejection mechanisms that automatically fall off gating system.
(3) Exhaust of movable mold part of mold is discharged from parting plane matching gap between fixed mold forming large insert 12 and movable mold forming large insert 14, exhaust is at the end of plastic flow. Gap value is about 0.03~0.05mm, which meets molding exhaust conditions of PP plastic.