Figure 1 shows a precision double plastic helical gear in an automobile potentiometer. Large and small gears are cylindrical helical gears with relative phase requirements. Gear accuracy is required to be Q-ISO1328 9-10. Difficulty of molding mainly includes following 3 points:: ① Coaxiality of large and small helical gears is relatively high, but it is difficult to guarantee due to influence of demolding mechanism; ② Due to limitation of processing technology of helical gear cavity, relative phase angle of two helical gears is difficult to accurately control; ③Helix angle β of pinion helical gear is 23°, which makes it difficult to demold.

 

Figure 1 Precision double plastic helical gear
Due to large production batch and high dimensional accuracy of plastic part, in order to improve production efficiency, a three-plate mold structure with one mold, two cavities and a point gate is adopted. Material of plastic part is POM, and MoldFlow is used to analyze structure and wall thickness of plastic part. Shrinkage rate used in mold design is 2%.

When using injection technology to produce plastic gears, a reasonable gating system design is very important to injection mold, which not only affects dimensional accuracy and appearance quality of plastic parts, but also has a greater impact on mechanical properties of plastic parts. Point gate three-point balanced pouring is now used, and three point gates are evenly distributed on same circumference. As shown in Figure 2, molten plastic flows radially from center of point gate to periphery of cavity, difference in fiber orientation distribution is smaller than that of single-point eccentric gate, which improves gear forming accuracy. Balanced pouring and unobstructed pressure holding are basic requirements of runner system design. On the one hand, balanced pouring can ensure that plastic gear has good shape and dimensional accuracy, especially for multi-cavity gear molds; on the other hand, unimpeded pressure holding is a necessary condition to ensure that plastic gears have excellent mechanical properties. Generally, plastic gear injection molds need to use higher injection pressure and holding pressure during production, in order to make molded plastic parts have higher density and dimensional stability.

 

Figure 2 Gate
Point gate is arranged in groove of wheel hub, traces of gate aggregate will not be higher than end surface of wheel after gate condensate is broken. It does not need to be trimmed and does not affect appearance of plastic part, which is conducive to realization of automated production. Precision plastic gear mold gate processing accuracy needs to ensure that cross-sectional area error is controlled within 5%. Point gates are often processed by precision wire cutting and precision EDM. Front end of gate is generally processed by slow wire cutting or grinding machines. Taper of gate head is processed by EDM, as shown in Figure 3.

 

Figure 3 Shape of gate head

In order to ensure accuracy and stability of double plastic gear transmission, tooth jump tolerance and coaxiality requirements of large and small gears, mold structure needs to be considered during design. Cavity plate installation methods of precision double plastic helical gear mold generally have following 3 kind.
(1) Design large and small helical gear cavities on movable and fixed mold sides respectively. This structure usually adds a taper positioning device between moving and fixed molds to improve coaxiality of large and small helical gear cavities, but accuracy needs to be guaranteed by multiple mold parts, there is a large cumulative error after mold is assembled. This kind of structure has high requirements on precision of mold parts and assembly precision, and it is difficult to maintain mold, so it is rarely used.
(2) If large and small helical gear cavities are all designed on fixed mold side, coaxiality does not need multiple parts to ensure, which can better solve problem of coaxiality error, but plastic parts are easy to bond to fixed mold, and it is difficult to demold. This structure is rarely used in production.
(3) Large and small helical gear cavities are all designed on movable mold side, which not only eliminates impact of mold clamping accuracy on plastic parts, but also solves problem of plastic parts sticking to mold, facilitates maintenance of mold during subsequent mass production. It is widely used.
Mold adopts a structure in which large and small helical gear cavities are all designed on movable mold side, as shown in Figure 4. Shaft hole insert of helical gear is designed in fixed mold. Main reason is that plastic part is suitable for movable mold. If shaft hole insert is designed in movable mold, it is equivalent to push-out structure of tube. Mold parts have a large matching gap and a large accumulated error. It is difficult to ensure coaxiality requirements of inner hole and pinion gear. When shaft hole insert is fixed to fixed mold, its length is small, accuracy of parts is easy to ensure. In addition, shaft hole insert is fixed with cavity plate insert, matching gap is small. Adding a tapered surface positioning between cavity plate and core can ensure coaxiality requirements of shaft hole and gear.

 

Figure 4 Dynamic mold structure
1. Cavity plate 2. Shaft hole insert 3. Large gear cavity plate 4. Small gear cavity plate 5. Insert sleeve 6. Cavity fixing plate 7. Connecting sleeve 8. Screw 9. Nut 10. Push rod 11. Nut fixing sleeve 12. Steel ball

Due to inconsistent helix angles of large and small helical gears, different actions are required to complete demolding. Demolding of pinion helical gear is realized by transmission principle of screw rod and nut, movement of mold opening direction is converted into rotation movement of pinion helical gear cavity plate through screw nut pair. Small helical gear cavity plate and screw are fixed by a connecting fixing sleeve. Screw and cavity plate rotate synchronously during demolding process to avoid deformation of small helical tooth due to excessive demolding resistance and affect accuracy of gear helix angle β. Following four issues should be paid attention to when designing demolding mechanism: ①Fit gap between screw rod and nut should be appropriate, and general fit gap is 0.05～0.1 mm; ②Screw angle of screw must be ≤45º, otherwise screw and nut will easily get stuck when rotating. ③Lead of screw rod must be same as lead of pinion helical gear cavity plate, otherwise tooth profile will be easily deformed when pinion helical gear is demolded, dimensional accuracy will be difficult to ensure; ④Rotation direction of screw must be same as rotation direction of pinion helical gear cavity plate.
A simple bearing consisting of a steel ball and a cage is designed between screw shaft shoulder and nut fixing sleeve to reduce friction when screw rotates. There is a gap of 0.2 mm between steel ball and screw shaft shoulder and nut fixing sleeve, which is used to freely adjust fit gap between screw rod and nut, to compensate for machining error of screw rod and small helical gear cavity lead.
Due to small helix angle and tooth width of large helical gear, material is also POM (shrinkage rate is relatively large, and demolding performance is better). Considering ejection of large helical gear through rotation of plastic part while pushing out.

Positioning method of core and cavity plate of precision gear mold is one of key points that affect accuracy of gear. Common positioning methods of core and cavity plate mainly include following two.

1. Precision positioning component is used for positioning between cavity plate and core, as shown in Figure 5, common processing technology: ①Cavity plate and shape of core are set with a machining allowance, and mounting holes of precision positioning component are processed separately; ② Install precision positioning components into cavity plate and core respectively, close cavity plate and core, then lock them with screws; ③Adjust shape of cavity plate and core to design size; ④Process molded part based on shape of cavity plate and core.

 

Figure 5 Positioning of precise positioning components

2. Conical surface positioning is used between cavity plate inserts and core inserts. As shown in Figure 6, this method has a small cumulative error and high positioning accuracy, so precision double plastic gear mold adopts a tapered positioning structure.

 

Figure 6 Cone surface positioning and waterway

Parting surface of most gear molds is flat and can be precisely processed by a surface grinder, while core insert is usually designed to be cylindrical, can be processed by a precision internal and external cylindrical grinder. Therefore, gear mold parts have characteristics of high machining accuracy, low surface roughness, and small matching clearance. Exhaust design of gear mold is very important. In production of gear injection molds, appearance defects such as insufficient filling, burns at filling end, high internal stress and deformation, obvious surface flow marks and weld marks, etc. often occur. To solve these defects, in addition to adjusting injection process and confirming whether mold gate design and processing are reasonable, it is also necessary to consider whether mold's exhaust system is smooth and reasonable. General measure to solve such problems is to open an exhaust groove to lead out the high-pressure compressed gas at filling end. Commonly used tooth surface exhaust groove structure is shown in Figure 7.

 

Figure 7 Tooth surface exhaust structure
Main functions of design of exhaust groove are: ①Exhaust air the cavity when molten plastic is injected; ②Exhaust various gases generated during heating process of plastic, the thin-walled part of plastic part and filling end far away from gate Location. For high-precision plastic parts and small plastic parts, it is also necessary to pay attention to design of exhaust groove. In addition to avoiding surface burns and insufficient injection filling, it can also eliminate various undesirable defects and reduce mold dust pollution. Usually, method to determine whether cavity exhaust is unblocked is to inject melt at the highest injection speed of injection molding machine, there is no focal spot left on the surface of plastic part to confirm that cavity exhaust is sufficient.

Mold temperature control of precision gear injection mold has an important influence on gear shape and dimensional accuracy. Generally, if mold temperature is too low, melt will not flow smoothly, affect filling, also reduce surface quality of molded plastic parts, make surface dull, weld marks are more obvious, weld strength is reduced. At the same time, it will increase flow shear force, which will increase internal stress of plastic parts and cause warping deformation. For polyoxymethylene (POM) crystalline plastics, too low mold temperature will affect crystallinity of plastic parts and have a greater impact on performance and quality. Too high mold temperature will lead to longer cooling and solidification time, affect production efficiency. At the same time, it will also cause greater shrinkage of plastic parts, affect dimensional accuracy, and are prone to defects such as shrinkage, overflow, and demold deformation. Uneven mold temperature can easily cause uneven shrinkage of plastic parts, large internal stress differences, and warpage deformation.
Precision duplex plastic helical gear adopts POM material, and mold temperature is one of main factors affecting its crystal strength. Generally, mold temperature of precision gear mold should be controlled at 80~90 ℃, which is conducive to growth of crystals, crystal state is more complete, fluidity is good for filling, so that a gear with higher strength and more stable size can be obtained.
Cooling water circuit directly surrounds outer circle of cavity plate insert and core insert (see Figure 6), which can improve temperature control ability of mold, ensure uniformity and stability of mold temperature of molding part, reduce impact of uneven mold cooling on size and mechanical properties of plastic part. In order to prevent premature solidification of runner melt from affecting pressure holding of cavity, a cooling circuit is also set on pusher plate and fixed mold seat plate to ensure stability of mold temperature. Due to high mold temperature of gear mold, thermal balance of entire mold must be considered in cooling circuit design to avoid uneven heating of mold, resulting in inconsistent thermal expansion and contraction of mold, leading to premature failure of mold positioning surface.

Mold structure is shown in Figure 8. When injection is completed, movable mold part is driven by slider of injection molding machine to retreat and start parting.

 

Figure 8 Mold structure of precision double plastic helical gear
1. Fixed mold plate 2. Upper cavity plate 3. Upper cavity plate insert 4. Plastic part 5. Large helical gear cavity plate 6. Small helical gear cavity plate 7. Lower core insert 8. Movable mold plate 9 . Cavity plate fixing plate 10. Positioning column 11. Connecting fixing sleeve 12. Moving mold backing plate 13. Nut 14. Nut fixing sleeve 15. Moving mold backing plate 16. Steel ball cage 17. Screw rod 18. Mold opening auxiliary component 19. Spring 20. Push rod 21. Spring 22. Die buckle 23. Spring
(1) The first parting: when mold is opened, stripper plate and fixed mold plate 1 start to be separated under action of spring 21, main runner aggregate is fixed on stripper plate under the action of pull rod, point gate aggregate is pulled off and separated from molded plastic part.
(2) The second parting: mold continues to open, under action of die buckle 22 and equal height screw, stripper plate is separated from fixed die seat plate, sprue condensate is separated from sprue sleeve and pull rod to complete demolding of runner condensate.
(3) The third parting: mold continues to open. Under action of mold opening auxiliary component 18, fixed mold plate 1 and movable mold plate 8 are separated, movable mold backing plate 12 and movable mold backing plate 15 are not separated at this time, so as to ensure that plastic part stays in movable mold.
(4) The fourth parting: mold continues to open. Under action of spring 23, movable mold backing plate 12 and movable mold backing plate 15 are separated to position set by fixed distance rod, nut 13 drives screw rod. 17 rotates, thereby driving pinion helical gear cavity plate 6 to rotate and demold.
(5) The ejector system of the injection molding machine pushes the molded plastic part out of the large helical gear cavity, and the manipulator removes the plastic part, and the mold opening action is completed. In order to prevent the tooth profile of the large helical gear from being deformed when it is pushed out, the speed of pushing out needs to be slowed down.
(6) Mold closing. Mold clamping action is reverse of mold opening action. When clamping mold, it should be noted that upper cavity plate insert 3 and top surface of push rod 20 are mating surfaces, which are easily damaged when subjected to force. This problem can be solved by setting spring 19 to reset push rod 20 first.

In order to facilitate mold manufacturing, subsequent production and maintenance, each forming part of movable and fixed molds is designed as an insert structure. In order to meet requirements of molding accuracy of plastic parts, it is necessary to strictly control fit gap between inserts, which is basis for ensuring tooth runout and coaxiality. Static fitting clearance of inserts is 0.001~0.003 mm, fitting clearance of sliding or rotating inserts should take into account effect of thermal expansion during production process, which is 0.005~0.015 mm. Whether injection mold can produce qualified plastic parts efficiently and stably depends on rationality of mold structure design on the one hand, and whether manufacturing accuracy of cavity plate can meet requirements on the other hand.
Due to unique shrinkage of plastic injection molding, shape of gear cavity is a non-standard internal and external gear. Processing and manufacturing of gear cavity plate is key to mold manufacturing. At present, methods of machining gear cavity plates include electric spark discharge, slow wire cutting, electroforming, and extrusion. Relative phase angle of the two helical gears has high requirements, which need to be considered when machining mold parts. Large helical gear cavity plate is a circular ring with a reference plane, which can be processed by precision slow wire cutting, and end face phase accuracy is easier to ensure. Structure is shown in Figure 9.

 

Figure 9 Large helical gear cavity plate
Because of large helix angle, small helical gear cavity plate cannot be processed by precision wire cutting, and can be processed by precision EDM with a rotating shaft. Relative phase angle of tooth profile cannot be accurately controlled during EDM. To solve this problem, forming part of helical gear can be processed first, then relative phase angle of end face tooth profile and reference can be accurately measured to determine installation position of gear cavity plate insert to ensure relative phase angle is accurate. 
Small helical gear demolding is to realize rotation of cavity plate by screw nut pair. Small helical gear cavity plate and screw are connected by a fixed sleeve, height is fixed in mold opening direction when mold is closed and injected. In order to ensure smooth movement of screw and nut pair, there is a gap of 0.2 mm between screw and nut. Gap can be solved by setting 2 positioning posts (evenly distributed on same circumference) on connecting fixing sleeve and gear cavity fixing sleeve for precise positioning.