At high speeds, gear accuracy has a vital influence on transmission noise. The higher gear accuracy, the lower noise. More and more plastic gears are used in products with low noise requirements such as electric curtains and sweeping robots. In order to reduce transmission noise, precision requirements for plastic gears are also getting higher and higher. Plastic gears are prone to non-uniform shrinkage during injection molding process, which reduces accuracy of tooth profile. It is necessary to modify tooth profile of mold cavity to improve accuracy of gear molding. When plastic gear tooth width is uneven at both ends of wall thickness or web is offset, it is prone to phenomenon that two ends of gear are different in size, which is called size head. When gear has a large and small head, one end of tooth width will bear load while the other end of tooth width does not bear load during transmission process, which will not only shorten service life of plastic gear teeth, but also cause serious noise problems.
Now take a plastic spur gear as an example, using MoldFlow analysis software to analyze causes of size and size of head, calculate tooth profile parameters of front and back mold surfaces of cavity according to shrinkage rate and variable modulus method, use wire cutting to process taper of cavity plate to eliminate size of molded gear. Result proves that this method can effectively solve problem of size of plastic spur gear, improve gear forming accuracy.

 

Research object is a small gear in a plastic double gear, material is POMDelrin100P, modulus is 0.75mm, pressure angle is 20°, number of teeth is 15, and tooth profile accuracy is ISO13289. Mold casting system uses a single point gate for pouring, as shown in Figure 1.

 

 

Initial cavity tooth profile parameter design is carried out according to variable modulus method. As shown in Table 1, tooth profile drawing software is used to draw CAD tooth profile according to cavity tooth profile parameters, as shown in Figure 2, then use line cutting method to process tooth profile cavity according to drawn CAD tooth profile.

 

 

Use cavity tooth profile parameters for mold cavity plate processing and injection molding. Use inspection tools to detect tooth profile errors in different height directions for a single gear tooth of a gear sample. When measuring, movable mold is mounted with side upward, as shown in Figure 3. Test results are shown in Figure 4, where number represents position in height direction (1 is upper end surface, 10 is lower end surface), and letter represents tooth profile position (A is tooth root, J is tooth top). It can be seen from Figure 4 that gear shows a phenomenon that upper end is larger and lower end is smaller. Helix inclination deviation (fHβ) value is 10.7μm, and accuracy is ISO1328N7. Tooth profile inclination deviations at different heights are also different. Maximum value of tooth profile inclination error (fHα) is 7.2μm, minimum value is -3.7μm, overall tooth profile accuracy is ISO1328N7, maximum value of right tooth profile inclination error (fHα) is 6.8μm, and minimum value is- 4.5μm, overall tooth profile accuracy is ISO1328N7 level.

 

In order to analyze causes of tooth profile and size of head, Mold-Flow software was used to simulate and analyze injection molding process. Results of predicted warpage deformation analysis are shown in Figure 5. From analysis results of Figure 5, it can be found that warpage deformation results of mold flow analysis are consistent with actual results. Change in outer diameter of gear before and after shrinking can be calculated by following formula to calculate shrinkage rate of upper and lower end faces.

 

Among them, S is shrinkage rate; Da,0 is diameter of gear addendum circle before shrinking, mm; D'a is diameter of gear addendum circle after shrinking, mm.
Diameter of addendum circle of gear before tooth profile shrinkage is φ13.668mm, diameter of addendum circle of upper end face after shrinking is φ13.559mm, diameter of addendum circle of lower end face after shrinking is φ13.518mm, difference in diameter of addendum circle of upper and lower end faces It is 0.041mm. After calculation, shrinkage rate of upper end surface of gear is 0.8%, shrinkage rate of lower end surface is 1.1%, difference in shrinkage rate is 0.3%, which leads to phenomenon of large and small heads.

 

In order to further analyze formation mechanism of large and small head phenomenon, product temperature field at end of pressure holding time is analyzed. As shown in Figure 6, center temperature of upper end of gear is 117.0℃, center temperature of lower end is 91.78℃, temperature at upper end is about 25℃ higher than temperature at lower end. Thermal expansion coefficient of this material is 1.1*10-4/K. When tooth tip diameter is φ13.668mm and temperature difference is 25℃, size difference between upper and lower end faces due to thermal expansion effect is 0.0375mm. This calculation result is consistent with result obtained by warping deformation, indicating that uneven temperature distribution is root cause of phenomenon of large and small heads. Tests have proved that adjustment of mold temperature, holding pressure, cooling time and other injection process parameters can not solve problem of tooth profile and size.

 

When problem of tooth shape and size can not be solved by modifying injection process parameters, the only way to correct cavity plate is to reverse it. In order to correct accuracy of tooth profile, reverse correction can be made by adjusting diameter of addendum circle, diameter of root circle, tooth thickness of upper and lower end faces, modulus and pressure angle parameters remain unchanged. According to initial cavity tooth profile parameters and measurement results of product, actual shrinkage rate of addendum circle diameter, tooth root circle diameter, common normal length on fixed mold side and movable mold side are calculated, as shown in Table 2.

 

 

Without changing initial cavity pressure angle and modulus, according to calculated actual shrinkage rate, addendum circle diameter, tooth root circle diameter and common normal length are calculated respectively. Results are shown in Table 3. CAD tooth profiles of fixed mold side and movable mold side of cavity were drawn respectively, as shown in Figure 7.

 

Tooth profile correction result is used for cavity plate processing and injection molding, and accuracy of sample is tested. Test result is shown in Figure 8. It can be seen from Figure 8 that although helix slope deviation value (fHβ) has been greatly improved, from initial 10.7μm to 2.4μm, there are still large differences in tooth profile tilt deviation between layers at different heights. Only by correcting diameter of addendum circle, diameter of root circle, length of common normal of upper and lower end faces, problem of tooth profile accuracy caused by uneven shrinkage cannot be solved.

 

 

 

In order to further correct deviation of tooth profile inclination, actual modulus of fixed mold side and movable mold side is calculated according to tooth profile inclination deviation of upper and lower end faces, modulus shrinkage rate of fixed mold side and movable mold side is calculated according to modulus of initial cavity and measured product modulus, as shown in table 4 shown. According to variable modulus method, tooth profile parameters of fixed mold side and movable mold side are calculated as shown in Table 5, and then CAD tooth profile is drawn separately, as shown in Figure 9. Taper cavity plate is processed by high-precision wire cutting. Upper and lower heads of wire cutting machine respectively control processing path of cutting wire according to tooth profile of fixed mold side and movable mold side of cavity to realize processing of taper cavity plate.

 

In order to ensure machining accuracy and surface roughness of mold cavity tooth profile, 5 times of finishing are performed after one rough machining. Corrected mold is used for injection molding, and sample is tested for accuracy. Tooth profile test result is shown in Figure 10. It can be seen from test results in Fig. 10 that problem of size and head of gear has been solved, helix inclination deviation value (fHβ) is 1.9μm, and accuracy is ISO1328N2. Difference of tooth profile inclination deviation value between layers of different heights is small. Maximum value of tooth profile inclination error (fHα) of right tooth surface is 2.3μm and minimum value is 0.3μm. Overall tooth profile accuracy is ISO1328N5. Maximum profile tilt error is -0.3μm and minimum is -3.9μm. Overall tooth profile accuracy is ISO1328N4, and gear profile accuracy has been greatly improved.
For problem of size and head of a plastic gear, MoldFlow software was used to analyze cause of problem, shrinkage rate of fixed mold side and movable mold side was calculated, tooth profile parameters of front and back mold surfaces of cavity were calculated according to shrinkage rate and variable modulus method, tooth profile correction technology of taper cavity plate processed by wire cutting was used to solve problem of small heads of formed gear, and taper cavity plate was processed by wire cutting machine. Actual results show that this method not only solves problem of tooth profile size head, but also improves tooth orientation accuracy from ISO1328N7 to ISO1328N2, and tooth profile accuracy from ISO1328N7 to ISO1328N5.