1 Analysis of plastic parts

Stand of desk fan is shown in Figure 1, where A is position of rotating shaft, which is used to assemble head of desk fan, and B is used to assemble base. Bracket has a ring of assembly rib, and back is equipped with a decorative plate to form a complete support frame. Bracket plays role of supporting head of table fan, so there are more reinforcing ribs inside. When designing mold, attention should be paid to position of plastic ribs, gate design and other issues to avoid defects such as shrinkage marks or weld lines on appearance surface. Figure 1 Bracket
The overall dimensions of plastic parts are 318 mm * 87 mm * 224 mm, injection material is ABS, shrinkage rate is 0.5%, and bottom wall thickness is 2.5 mm. In process of structural design of plastic parts, relevant requirements such as mold structure design, mold parts processing, and injection production should be considered.
In view of the fact that internal ribs of plastic part are prone to shrinkage of appearance surface, CAE analysis of thickness of plastic part is shown in Figure 2. Thickness of main body of plastic part is checked by CAE analysis, and it is required to ensure consistency as much as possible; if plastic part needs to have different thicknesses due to structural reasons, at least 10 times thickness of changing handover position should be subjected to gradual transition processing, then corners should be rounded to ensure injection. Melt is smooth, and appearance of plastic parts has no injection defects. Figure 2 CAE analysis of plastic part thickness
Ribs that directly contact appearance surface are treated as follows: shrinkage defects are not allowed in direct contact position with appearance surface, thickness of root of ribs is designed to be 0.4 times thickness of main body of plastic part, and to ensure strength of ribs, thickness of small end of ribs ≥ 0.8 mm, otherwise it will be difficult to fill ribs during production process. Non-critical surfaces can accept slight shrinkage defects, and thickness of contact position between root of rib and plastic part should be ≤ 2/3 of thickness of main body of plastic part, making rib easier to inject.
For force-bearing rib, such as stop rib at contact position between inside of plastic part and base, it is not in direct contact with appearance surface of bracket, and thickness of small end of rib should be ≥ 1 mm to ensure that plastic part does not deform under stress .

2 Mold structure design

2.1 Parting surface design

In addition to its practical function, plastic part is also an appearance decoration. There should be no obvious parting line on the surface. Parting line needs to be moved to edge of R angle. Therefore, selection of parting surface of mold is particularly important. In order to eliminate possible defects of plastic parts, core-pulling slider in fixed mold, fixed mold insert, fixed mold plate at rotating shaft of molded plastic part should be assembled and polished together. In addition, when mold is parted, R angle of parting package is designed on the side with less material to ensure that distance between parting surface and appearance edge is 0.4 mm, movable and fixed mold inserts have a straight body position of 0.1 mm each. Fixed mold side is made 0.05 mm smaller to prevent flashes on parting surface of plastic part, as shown in Figure 3. Figure 3 Parting of moving and fixed molds

2.2 Design of gating system

Table fan bracket belongs to the first-class appearance parts, and there are no defects such as strain, deformation, shrinkage marks, etc., so design of gating system is particularly critical. Due to large production volume of table fan, mold adopts hot runner for uniform feeding at 3 points to reduce cost waste caused by condensate; in order to improve appearance and surface quality of plastic parts, a cold slug well must be designed before gate to prevent condensate from entering cavity. ; Molten plastic can not directly fill cavity, mold adopts S-shaped runner, and side gate (square) is poured, as shown in Figure 4. Figure 4 Bracket gate design

2.3 Design of forming slider

Design of forming slider mainly includes two parts: design of core-pulling slider in fixed mold at two rotating shaft positions and design moving mold slider for forming internal structure of plastic part.
1, Design of inner core-pulling sliders of fixed mold at two rotating shaft positions is shown in Figure 5. Inner core-pulling sliders 4 are symmetrically arranged in slider grooves of fixed mold insert 1, and slider bars are installed in fixed mold insert. At the bottom, locking block 2 is fixed on hot runner plate 6, and T-shaped block 3 is installed on locking block. When hot runner plate and fixed platen are separated, locking block is driven to move, the two inner core-pulling sliders are moved to center of mold and released from plastic parts buckle. Figure 5 Core pulling slider in fixed mold
1. Fixed mold insert 2. Locking block 3. T-shaped block 4. Internal core pulling slider 5. Fixed mold fixed plate 6. Hot runner plate 7. Fixed template 8. Slider
Key points of design of core-pulling slider in fixed mold: ①Slider should be close to edge line of R corner of plastic part when parting, and parting line should be hidden at R corner position to ensure appearance of plastic part; ②Demoulding angle of appearance surface in core pulling direction is at least 4°~5°. For sake of appearance, there may be a need for sunburst in follow-up, and it must be ensured that slider core will not affect appearance surface; ③ Slider groove is deep, and its bottom limit post is difficult to assemble, so limit glass beads are installed on the side of slider bar on fixed die insert to control slider stroke; ④While calculating slider stroke, opening distance between hot runner plate and fixed platen is also determined.
2, Design of movable mold slider for internal structure of molded plastic part. Plastic part A is formed by fixed mold slider and movable mold slider, as shown in Figure 6. There is a groove at plastic part A to ensure uniform thickness of glue level and prevent appearance of plastic part from shrinking. Die slider is provided with a boss, as shown in K in Figure 6. When closing mold, it is necessary to reset movable mold slider first, then reset fixed mold slider, otherwise boss will be damaged. Figure 6 Structural features of mold

2.4 Principle of mold action

The overall appearance of mold is shown in Figure 7. Since core-pulling slider in fixed mold and movable mold slider have a sequence in opening and closing mold, mold base needs to be provided with a hook structure. Figure 7 Overall appearance of mold
Mold clamping process: Lock on fixed platen is in a pop-up state when mold is closed. Under action of clamping force of injection molding machine, when short tie rod on hot runner plate is against lock, hot runner plate and fixed plate cannot continue to be closed, as shown at N in Figure 8. Moving and fixed molds continue to close mold. When moving and fixed mold plates are in place, limit block on movable mod plate will press lock back into lock groove, short tie rod on hot runner plate loses resistance and continues to close mold until hot runner plate and fixed template are in place. Figure 8 Mold clamping process
Mold opening process: Mold is opened under pulling force of injection molding machine, but limit block on moving mold plate is fastened to lock on fixed mold plate, auxiliary force of mold clamp between moving and fixed plates is added, so that moving and fixed plates cannot be opened, as shown at D in Figure 9. Hot runner plate and fixed mold plate start to separate from fixed plate, until hook on hot runner plate presses lock on fixed plate back into lock groove. At this time, movable and fixed plates are opened, and plastic parts are safely taken out until mold opening operation ends. Figure 9 Mould opening process
Following points should be paid attention to when designing mold structure.
(1). Inclination of contact surface of hook and lock should be at least 5°, position of contact point should be designed to be rounded to ensure that parts move smoothly and will not be inserted, as shown in Figure 10. Figure 10 Partial enlargement of hook and lock
(2) Spring compression at the bottom of lock is about 40% of free compression state of spring, to ensure that lock has sufficient elastic force during working process of mold, and follow-up maintenance should be checked frequently to ensure smooth movement of mechanism.
(3) Distance R between hook and lock (see Figure 10) is determined according to opening distance between hot runner plate and fixed platen.
(4) Retractor components are evenly arranged on operating and non-operating sides of mold, nitrogen gas spring and mold opener should also be evenly arranged to ensure a balanced force.

2.5 Cooling and ejection system design

Cooling water circuit of mold should be arranged evenly along shape of plastic part. Surface of plastic part has drying patterns. It is sufficient to use normal temperature cooling water. If plastic part has high gloss requirements, it can be connected to a water temperature machine to ensure appearance of plastic part.
When push-out system is designed, holding force is large at deep position of movable mold rib, and direct push-out can be used. Note that push rod should be designed inside plastic part and cannot cross parting surface, otherwise parting surface flash will be difficult to control; Since there are many reinforcing ribs inside plastic parts, push rods above φ8 mm are selected and arranged at intervals on both sides of ribs to prevent white top defects when plastic parts are pushed out.