【Abstract】In view of shrinkage and deformation of gear and unrequired precision of gear shaft hole during injection molding of plastic gear, by optimizing design of gating system, demolding mechanism and cooling method, precision plastic gears that meet requirements are produced to improve performance of gears.

With rapid development of industry, people have a great demand for cameras, printers and copiers in daily life and office. Plastic gears have gradually replaced metal gears. These equipments have relatively high requirements for gear meshing accuracy. In design of parting surface of gear plastic part, tooth-shaped inserts with same shape of fixed mold cavity and movable mold core design adopt a layout of and 1 cavity. In order to avoid disadvantages such as underfilling and deformation of injection molding, mold adopts a three-plate mold point gate 3-point balanced injection method. In order to smoothly demould gear plastic parts without leaving marks, a round cradle pin is used to balance out. By optimizing design of these three systems, accuracy and performance of gears are guaranteed.

Plastic gear is shown in Figure 1. Selected material is polyoxymethylene (POM), which has good mechanical properties, molding shrinkage rate is 2%~3%. When material temperature is high, discoloration will occur. Cooling system should be designed to circulate evenly and well to ensure that temperature of injection mold is between 80℃ and 100℃.
As shown in Figure 2, maximum outer diameter of plastic gear is pulley, based on middle ϕ 4mm shaft hole, coaxiality requirement is ϕ 0.05mm, pulley and middle hole ϕ 4mm are required to have a full runout geometric tolerance of 0.05mm. Selection of main parting surface is based on top surface ϕ 11mm maximum contour, light-colored area is set in fixed mold part, dark area and intermediate shaft forming core are set in movable mold part, which can ensure accuracy of plastic gear. Features with form and position tolerances and dimensional tolerances are determined according to MT5 grade, for free sizes, tolerance value can be obtained according to MT7 look-up table.

 

Figure 1 Three-dimensional drawing of plastic gear

 

Figure 2 Two-dimensional drawing of plastic gear

(1) Diameter of small end of main runner is ϕ 0.5~ ϕ 1mm based on nozzle diameter of injection molding machine, cone angle of main runner is 2°~6°, length is ≦60mm, diameter of small end of plastic gear runner is ϕ 2.5 mm, cone angle is 2°, and actual length is 37mm.
(2) Runner adopts commonly used U-shaped section, H=1.25R, R=0.5B, plastic gear runner H is 3.0mm, B is 3.9mm, and R is 1.7mm, so that plastic is in a molten state. It quickly flows into runner and is evenly filled, ensuring good pressure and uniform filling.
(3) Types of gates usually include side gates and point gates. Side gates can only be designed on main parting surface. Design and processing are convenient, but there are gate marks. In order to obtain high-precision and high-finish plastic, three-gate pouring method is selected. Advantage of point gate is that there is no need to trim burr, gate will fall off automatically, automated production can be realized. Plastic in molten state flows radially from gate to surroundings, 3 weld lines are formed at flow junction. At weld line position, flow front tends to be parallel, a low shrinkage area is formed along weld line, which is not easily deformed, thereby obtaining a high-precision gear.
In summary of above analysis, combined with MoldFlow mold flow analysis software to determine pouring point, plastic gear pouring system design, as shown in Figure 3.

Figure 3 Design of plastic gear pouring system

When mold is injected on injection molding machine, material of plastic part is POM, which is a heat-sensitive material, and it is easy to change color when temperature of material is high. Therefore, in order to control material temperature and molding time during injection molding, a cooling system should be reasonably set up to ensure that mold temperature is maintained at 80℃ to 100℃ during injection. Stripper plate is equipped with circulating water channels, fixed and movable mold plates are each set with 2 symmetrical and straight water channels. Cross-sectional diameter of water channel is ϕ 8mm. Mold cooling system design is shown in Figure 4. An exhaust groove is set on fixed mold plate and on tooth surface to eliminate trapped air. Groove depth is less than plastic overflow value, and groove depth is 0.2mm.

 

Figure 4 Cooling system and exhaust slot design

In order to ensure high precision and high smoothness of gears, it is designed as a mold structure with one cavity. In order to save costs, facilitate manufacturing and processing, core and cavity are designed as a combination of inserts into fixed mold plate and movable mold plate, shoulders are fixed. Fixed mold plate and movable mold plate inserts, inserts use high-quality die steel 718H, as shown in Figure 5.

 

 

 

Figure 5 Forming parts of fixed and movable gear molds
a — —fixed mold insert b — —movable mold insert

Commonly used push-out mechanisms include ejector rods, cylinders and pusher plates. Push plate is suitable for shell and plastic parts that are not allowed to leave traces on outer surface. Cylinder is commonly used to push out cylindrical, ring-shaped and holed plastic parts. This kind of demolding force is uniform and plastic part is small in deformation, but its accuracy is low. It can be used for gears below level 4. There is a gap in the middle of cylinder, which increases ejection error. Ejector rod can eject plastic parts uniformly. This kind of demolding can ensure runout tolerance of plastic parts, accuracy can reach 3 levels or more. According to characteristics of plastic part, ejector pin is selected to push out, as shown in Figure 6.

 

Figure 6 Mandrel

Mold assembly structure is shown in Figure 7.
When mold is opened, in order to prevent gear plastic part from being strained or left on the side of fixed mold, a spring 16 is designed on small pull rod to separate Ⅰ-Ⅰ parting surface from Ⅱ-Ⅱ parting surface first, gate is separated from gear plastic part under force of tie rod 19   make Ⅰ-Ⅰ parting surface preferentially separate from Ⅱ-Ⅱ parting surface; movable mold moves to top rod side of injection molding machine. When left side of step of small tie rod 11 touches right side of fixed mold plate 4, movement of Ⅰ-Ⅰ parting surface is finished; Ⅱ-Ⅱ parting surface is separated as main parting surface. Due to packing force of gear plastic part, plastic part remains on movable mold core. Positioning pull plate on upper side of mold limits distance of Ⅱ-Ⅱ parting surface (Separation distance of Ⅰ-Ⅰ, Ⅱ-Ⅱ can allow material head and gear plastic parts to automatically blank); subsequently, Ⅲ-Ⅲ parting surface is separated, gate material falls off; movable mold continues to move to the right, ejector pin of injection molding machine pushes ejector bottom plate to move, ejector pin ejects gear plastic part from movable mold core, and plastic part is automatically demolded.
When mold is closed, spring 13 on reset rod 12 preferentially resets ejector pin and drives ejector pin 10 to return to initial state. Under action of mold guide post and positioner, fixed mold side and movable mold side are closed. Finally, clamping of mold is completed.

 

 

Figure 7 Assembly drawing of gear plastic parts
1. Positioning ring 2. Fixed mold base 3. Stripping plate 4. Fixed mold 5. Movable mold 6. Movable mold base 7. Mandrel plate guide sleeve 8. Mandrel plate guide column 9. Support column 10. Top Rod 11. Small tie rod 12. Reset rod 13. Reset rod spring 14. Movable mold insert 15. Waterway 16. Small tie rod spring 17. Plastic part 18. Fixed mold insert 19. Pull rod 20. Pump mouth

Mold adopts a three-plate mold point gate method for pouring, 1 cavity, 3 parting surfaces are designed, water channels are designed on stripper plate, fixed mold plate and movable mold, exhaust groove is designed on parting surface, which has good exhaust, solves problem of shrinkage and deformation of gear plastic part. When designing ejection mechanism, Ⅰ-Ⅰ parting surface is given priority to Ⅱ-Ⅱ parting surface. Pull rod and positioning plate are designed to limit distance of mold opening. Ejector rod is used to push out to ensure run-out tolerance and accuracy of plastic part. When mold is actually produced, mold mechanism is stable and reliable, quality and accuracy of produced product meet requirements of use.