In recent years, driven by rapid development of automotive industry, cross-axis transmission gear train that uses steel worms and plastic helical gears (or plastic worm gears) to mesh has been widely used in automotive window shutters, wiper motors and electric seat adjustment systems. Compared with traditional steel worm-steel worm gear combination, this kind of light-load cross-axis transmission gear train has advantages of light weight, low noise, low manufacturing cost and good shock absorption characteristics. In addition, combination of steel worm and plastic helical gear does not require high lubrication conditions and has good electrical insulation performance. Helical gear screw assembly shown in Figure 1 has been applied to a vehicle drive. Solution to common problems of helical gear screw assembly in injection molding is introduced from perspective of injection mold design of helical gear screw assembly, which provides a reference for mold designers.

 

Figure 1 Plastic helical gear with metal screw insert
1 Process analysis of plastic helical gear with metal screw insert
Plastic helical gear shown in Figure 1 is a transmission part for adjusting a car seat, and screw is a metal insert. Tooth profile part is an external spiral cylindrical helical gear, normal modulus m=1.04 mm, number of teeth z=25, helix angle β=6.51°, normal pressure angle α=12°, and material is Delrin 100P. Helical gears are used to transmit motion, which can not only obtain better transmission capacity, but also improve lubrication conditions and reduce transmission noise. Gear adopts a metal screw as core shaft, head helical gear is formed by injection process covered with POM plastic, and axial direction is basically symmetrical.
Due to long metal screw insert of helical gear assembly, tooth profile requires high dimensional accuracy, and molding is difficult, which is mainly reflected in following four aspects.
(1) Two journals are respectively formed on fixed mold side and movable mold side, and coaxiality of φ0.03 mm is difficult to guarantee.
(2) Metal screw insert is long (some models above 400 mm), it is difficult to position during injection molding, metal screw is easy to bend and deform, which causes axial runout of metal screw relative to A and B benchmarks to be 0.2 mm out of tolerance.
(3) In mass manufacturing process of metal screws, it is difficult to maintain same length dimension. Tolerance zone of 0.3 mm has a greater impact on distance between steel ball and end face of helical gear (20±0.03) mm. General mold structure is difficult to ensure its accuracy requirements, and mold structure needs to be specially designed.
(4) Due to presence of metal screw inserts, shrinkage of plastic helical gears is complicated. Shrinkage characteristics of this helical gear are different from ordinary cylindrical helical gears, and special treatment is required when designing cavity of formed helical gear.

Because tooth profile has a helix angle, helical gear plastic parts are difficult to demold. Commonly used demoulding method is to rotate tooth-shaped cavity plate or rotate gear plastic part to demold. Plastic helical gear discussed has a more complicated mold structure due to metal screw insert. According to product structure characteristics, tooth-shaped cavity plate insert is used for demoulding. Mold structure is shown in Figure 2. Mold adopts a three-plate structure with 1 mold, 2 cavities, and 3-point balanced gate pouring. Cavity plate insert of helical gear is designed on fixed mold. When mold is opened, helical gear is used to impose side of cavity plate insert. Thrust force makes cavity plate insert rotate to realize demoulding. By setting elastic ejector rod and sliding block delay locking mechanism under metal screw insert, accuracy requirements of height dimension of molded helical gear plastic part are guaranteed. Practice has proved that this demolding method can meet dimensional accuracy requirements of helical gear plastic parts. Tooth profile accuracy of produced helical gear assembly can reach national standard GB/T 10095—2001 level 8 accuracy, pitch accuracy can reach level 9, and spiral line accuracy can reach level 10. All three indicators meet accuracy requirements of drawings.

 

Figure 2 Mould structure
1. Movable mold seat plate 2. Push plate 3. Push rod fixed plate 4. Movable template 5. Core 6. Slider 7. Slider insert 8. Spring 9. Locking block 10. Fixed plate 11. Gate Insert 12. Bent pin 13. Magnetic clamping assembly 14. Push plate guide column 15. Spring 16. Reset rod 17. Cavity plate insert 18. Simple ball bearing 19. Gate insert 20. Fixed distance tie rod 21 .Cemented carbide inserts 22. Pull rod 23. Fixed mold seat plate 24. Push plate 25. Guide column 26. Spring 27. Fixed distance pull rod 28. Fixed template 29. Metal ring insert 30. Metal screw insert 31. Elastic top sleeve 32. Elastic ejector rod 33. Spring 34. Cover 35. Ejection extension block 36. Positioning ring sheath
Working principle of helical gear mold is as follows.
(1) Use a manipulator to put metal screw insert 30 into movable mold, so that tail of screw contacts elastic ejector rod 32. When metal screw insert is placed, movable mold part is driven by injection molding machine seat plate to start clamping. Guide post 25 first enters guide sleeve hole installed on movable mold plate 4 to guide clamping of movable mold and fixed mold. Under action of bending pin 12, sliding block is clamped and metal screw insert 30 is initially clamped and positioned. At this time, metal screw is not completely fixed and can move up and down. Continue to close mold. Due to action of spring 26 and magnetic clamping assembly 13, movable die plate 4 and fixed die plate 28 are closed, slider insert 7 is closed under action of locking block to center screw insert, so that screw insert is inserted into hole of gate insert 19. After cemented carbide insert 21 contacts steel ball at head of screw insert, screw insert is pushed down to compress spring 33. Under action of spring 33, it is ensured that cemented carbide insert 21 is kept in contact with steel ball at head of metal screw insert. Continue to close mold. When pushing plate 24 contacts sliding block locking block 9, spring 8 is compressed to completely lock the two sliding blocks, and metal screw insert 30 is fixed to prevent metal screw insert 30 from moving downward during injection process.
(2) After completion of moving and fixed mold clamping, perform injection, pressure holding, cooling, etc., when injection is completed, movable mold part is driven by injection molding machine base plate to start parting. Part I: Under action of spring 26, pusher plate 24 and fixed mold plate 28 are separated. Under action of pull rod 22, sprue aggregate is fixed on pusher plate 24, point gate is pulled off and separated from molded plastic part. Part II: After mold is opened for a certain distance, under action of fixed distance tie rod 20, pusher plate 24 and fixed mold seat plate 23 begin to separate, sprue condensate is separated from sprue bushing and pull rod 22 to complete demolding of runner condensate. Part III: Mold continues to open. Under action of magnetic clamping assembly 13 and fixed-distance pull rod 27, fixed mold plate 28 and movable template 4 are separated. At this time, slider locking block 9 is separated from slider 6 Under action of bending pin 12, slider insert 7 continues to hold metal screw insert 30 in a clamped state, molded plastic helical gear stays in movable mold under pulling force of annular groove set on slider insert 7. During mold opening process, plastic helical gear exerts a lateral thrust on helical gear cavity plate insert 17, so that cavity plate insert 17 rotates synchronously with assistance of simple ball bearing 18, and helical gear plastic part comes out of cavity plate insert 17. Continue to open the mold, slider 6 is separated to both sides under action of bending pin 12, slider insert 7 is separated from molded plastic part. At this time, movable mold and fixed mold are completely separated.
(3) After mold is opened for a certain distance, base plate of injection molding machine stops moving, ejection hydraulic cylinder of injection molding machine starts to move, pushing ejection extension block 35 to push push plate 2 forward. At this time, elastic top sleeve 31 fixed on push plate 2 drives metal screw insert 30 to push plastic helical gear out of mold, ejector hydraulic cylinder of injection molding machine retracts and resets, push plate 2 is reset under action of return spring 15. Manipulator takes out helical gear assembly, the entire mold opening action is completed.

When designing plastic gear injection molds, reasonable design of gating system is very important for injection molding. It not only affects appearance quality and dimensional accuracy of molded plastic parts, but also has a greater impact on mechanical properties of plastic parts. Mold selects a 3-point balanced pouring method, and 3-point gates are evenly distributed on same circumference, as shown in Figure 3. When 3-point balanced pouring is used, plastic melt flows radially from gate to surroundings, fiber orientation difference is smaller than that of single-point eccentric gate, precision of formed gear is relatively higher. Use of balanced pouring and unobstructed pressure holding for precision plastic gear molding die are two key factors in design of runner system. On the one hand, only balanced pouring can ensure dimensional accuracy of molded plastic gears, especially for multi-cavity gear molds; on the other hand, keeping pressure unblocked is a necessary condition to ensure that molded plastic gear has excellent mechanical properties. Generally, higher injection pressure and holding pressure are required in production of plastic gear injection molds, so that molded plastic parts have higher density and dimensional stability.

 

Figure 3 Point gate pouring method

Mold cavity is designed as a combined structure, which is mainly composed of helical gear cavity plate inserts, gate inserts, carbide inserts, simple bearings composed of balls and cages, and fixed plates, as shown in Figure 4. There is a clearance fit between helical gear cavity plate insert and fixed plate (gap is 0.02~0.03 mm), which facilitates rotation of helical gear cavity plate insert when demolding. Helical gear cavity plate insert is positioned by cone surface of outer ring and the core, cone surface of inner ring and gate insert. Precise positioning of the two tapered surfaces ensures coaxiality requirement of φ0.03 mm at journal of molded plastic part. A simple bearing is designed between shoulder of cavity plate insert and fixed plate to reduce friction force when cavity plate insert rotates. There is a gap of 0.5 mm between simple bearing and helical gear cavity plate insert and fixed plate. When mold is opened, cavity plate insert can be pulled out by 0.5 mm to separate cavity plate insert from gate insert, avoiding friction between cavity plate insert and gate insert hindering rotation of cavity plate insert. A cemented carbide insert is designed on gate insert to prevent steel ball at head of screw insert from being worn during mass production to ensure stability of molded plastic part (20±0.03) mm in size.

 

Figure 4 Combined cavity structure

Slider adopts an insert-type structure, and slider is driven by a bending pin. In order to ensure stability of molded plastic part (20±0.03) mm in size, eliminate length and size error of metal screw insert, locking method of slider is specially designed, and delay locking structure is adopted, as shown in Figure 5.

 

Figure 5 Slider structure
In order to cooperate with demolding of helical gear plastic part, an annular groove is designed under journal, as shown in Figure 6. Existence of annular groove can ensure that molded plastic part can be left in movable mold when mold is opened, so that toothed part can be demolded smoothly.

 

Figure 6 Annular groove on plastic part
When mold is closed, in order to insert screw insert into hole of gate insert 19, slider must be reset first to fix screw insert, so as to ensure that center position of screw insert does not shift, cavity wall will not be damaged when mold is closed, so mold uses a bent pin to drive slider. Using bent pin to drive slider can delay sliding of slider. During mold opening process, when plastic helical gear drives cavity plate insert to rotate and demold, slider insert 7 will not escape from annular groove of molded plastic part.
Main reason for delay locking method of slider is that a metal ring is welded to screw insert (see Figure 7), point gate is located above metal ring. When melt is injected, melt will have a greater impact on metal ring, causing metal screw insert to move downwards, causing displacement and bending deformation of metal screw, which affects accuracy and stability of molded plastic part (20±0.03) mm size. There are two ways to solve this problem.

 

Figure 7 Metal ring welded on the screw
(1) Control length and dimension tolerance of metal screw to ±0.03 mm to meet dimensional requirements of helical gear assembly.
(2) In mold structure, helical gear assembly ensures that steel ball at head of metal screw insert and cemented carbide insert 21 are always in contact during melt injection, and metal screw insert does not move downwards.
In mass production process of metal screw, it is difficult and uneconomical to control length and dimension tolerance to ±0.03 mm. Machining error of ±0.15 mm is acceptable, so it can only be considered from mold structure. To prevent helical gear assembly from moving downwards during melt injection, a strong spring is generally added under metal screw insert to eliminate length error of screw insert with elastic force of spring, at the same time overcome impact force on metal ring when melt is injected, as shown in Figure 8.

 

Figure 8 Add a spring mechanism under the screw
After adding a strong spring below metal screw insert, spring exerts a greater thrust on metal screw insert when mold is opened. This thrust may cause annular groove on helical gear assembly to assist demolding to crack, may also cause screw insert to bend and deform, which affects runout accuracy of screw. In order to solve these problems, mold adopts a light-loaded spring to reduce length error of screw insert, avoiding bending deformation of metal screw insert caused by excessive spring force, and at the same time, strong locking of slider to screw insert is used to overcome impact force of melt on metal ring during injection stage, and to avoid screw insert from moving downwards.
Locking block 9 is fixed between fixed mold plate 28 and pushing plate 24. Slider insert 7 is not locked during clamping stage of movable mold plate 4 and fixed mold plate 28, so that screw insert can move downward freely in the embrace of slider before movable die plate 4 and fixed die plate 28 are completely closed. Only when pushing plate 24 is in contact with sliding block locking block 9 can the two sliding blocks be completely locked and metal screw inserts are fixed to prevent metal screw inserts from moving downwards during injection process.
In mold, commonly used nylon mold clamping device and other mechanical clamping devices are not used. Instead, a magnetic clamping component with no clamping resistance is used to ensure that movable mold plate 4 and fixed mold plate 28 are completely closed before movable mold plate and pusher plate 24 are closed, so as to avoid defects such as head of screw insert being bent and neck of screw insert being scratched due to fact that movable mold plate and fixed mold plate have not been completely closed after screw insert is fixed by slider.