This study examined cracking problem of a certain automobile dust cover base after press-fitting. Moldflow was used to analyze mold flow and, combined with crack characteristics, to identify cause of cracking. Optimal gate location was determined, gating system was optimized, and mold cooling system design was verified. The overall mold design was completed using UG. After trial production and assembly, plastic part met operational requirements. After mass production, molded parts achieved stable and reliable quality, resolving cracking issue in dust cover base.

After trial production of a certain model of dust cover base, press-fitting was performed. After assembly, as shown in Figure 1(a), small cracks developed at assembly site, as indicated by circle in Figure 1(b). Cause of cracking required investigation and analysis.

 

Figure 1 Dust Cover Base

Plastic part is made of polypropylene resin T30S. Material quality inspection report is shown in Table 1. Company tested its key performance indicators and found all to be satisfactory, thus ruling out material issues.

Table 1 Material Test Report

Part's wall thickness was 3 mm and generally uniform. Part's structure was an improvement over previous model, and mating components were consistent with previous model. Previous model had no cracking issues, thus ruling out part design issues. Finally, company tested various processes, but none resolved issue, preliminarily ruling out process issues.

Part's appearance and dimensions met requirements, but its performance did not meet requirements. Having ruled out material, part design, and production process issues, initial diagnosis was an improperly designed mold gating system. Original gate was located at the bottom of part to be molded, as shown in Figure 2. This distance from assembly likely hindered filling and pressure retention. Since there was no fixed principle for determining optimal gate location, Moldflow analysis was performed to verify design.

 

Figure 2: Gate Location of Old Solution

To improve mesh quality, text and symbol features on the bottom surface of part were removed before meshing. With a 2.3 mm edge length, mesh repair resulted in a maximum aspect ratio of 4.67, an average aspect ratio of 1.54, and a matching percentage of 90.1%, meeting analysis requirements. Part material was T30S polypropylene (PP), which was not available in material library. Polypropylene PPC 4660, with a similar melt index, was selected.

Many factors must be considered when determining gate location, such as need for gate to be located in thick wall, away from thin-wall features, to ensure balanced filling and consistent flow. Moldflow offers two gate location analysis algorithms: an advanced gate locator based on minimum flow resistance and a gate area locator that considers flow balance. Analysis results from these two algorithms are not significantly different. The best gate fit for this part is located in rib, but mold has a half slider. Therefore, gate could be located in upper annular area or on slider parting surface. Ultimately, considering mold structure and gate location requirements, minimizing flow resistance and ensuring balanced filling, new gate location was chosen in lower-middle area of molded part. Fit in this area reached 0.940 9, better than 0.606 9 of original gate location, as shown in Figure 3.

 

Figure 3 Optimal Gate Location Analysis

Based on new gate location, a new gating system was designed. Old runner had a φ6 mm diameter. New main runner has a vertical runner at the end, which connects to runner. Both runners have a φ8 mm diameter and are located on slider. Gate dimensions for both new and old designs are identical: 3 mm wide and 1.2 mm deep. Two designs are shown in Figure 4.

 

Figure 4 Old and new casting systems
Fixed mold and slider are cooled by cooling water channels, where fixed mold cooling water channel diameter is φ8 mm, slider cooling water channel diameter is φ6 mm, core is cooled by water baffles with a water hole diameter of φ20 mm. Cooling is sufficient and uniform.

After gate position analysis is completed, molding analysis is run to find optimal filling time and perform initial filling and holding pressure analysis. Then, according to recommended screw curve, multi-stage injection process is set according to actual injection molding machine for analysis. Considering that injection pressure, holding pressure and time are key factors affecting performance of molded plastic parts, based on better process, filling rate is changed to extend filling time from 1.2 s to 8 s and holding pressure is increased from 15 MPa to 60 MPa. Mold flow analysis of different processes is carried out multiple times. Injection, holding, and cooling process conditions that yielded the best analysis results were as follows: mold temperature of 30℃, melt temperature of 200℃, and injection, holding, and cooling time of 35 seconds. Multi-stage injection process parameter settings are shown in Table 2, with a holding pressure of 45 MPa, a holding time of 6.5 seconds, and a decay time of 4 seconds. Holding curve is shown in Figure 5. Analysis results of old solution showed little improvement, and results were essentially same as those obtained by company, which showed that plastic parts would crack after assembly despite various process adjustments.

Table 2 Injection Process Parameters

 

Figure 5 Holding Pressure Curve

Optimal filling times for new and old solutions were 3.35 and 2.9 s, respectively, both achieving complete filling. While problem is difficult to identify based on time alone, overlap with surface orientation reveals that rib in old solution acts as a flow diverter. Molten material on this side first reaches bottom of cavity, then changes direction by nearly 90°, flowing from a nearly horizontal direction to diagonal angle opposite rib. This flow joins vertical molten material to complete cavity end, filling cavity from left, right, and top. In theory, gate location should ensure co-directional flow, so this filling method is detrimental to part quality. Actual part cracking occurs in this area. Because gate location prevents molten plastic from reaching bottom of cavity simultaneously, it is difficult to correct problem through process changes. New solution also exhibits issues, with weld lines forming, as shown in Figure 6. However, measures can be taken to improve weld line strength.
Surface orientation results show that molecules of old solution mainly flow in axial direction, while new solution mainly flows in circumferential direction. For isotactic polypropylene, tensile strength parallel to flow direction is greater than that perpendicular to flow direction. Assembly part of plastic part mainly bears circumferential stress, so old solution is not conducive to strength of plastic part.

 

Figure 6 Filling time contour and surface orientation

Weld line of old solution is shown in Figure 7 (a), and there is no weld line at assembly part of plastic part. New solution produces a weld line at assembly part, as shown in Figure 7 (b). If we rely solely on analytical experience, we will generally choose old solution, but practice has proved that old solution is not feasible. New scheme can ensure strength of weld line from three aspects: ① T30S polypropylene is a crystalline polymer, and its fusion seam strength is close to strength of polymer body, so strength of weld line can be guaranteed in theory; ② Optimize injection process to increase convergence angle of weld line in assembly part. A large convergence angle can ensure strength of weld line. When analysis parameters are defaulted, convergence angle is about 20°. According to recommended screw curve, after multi-stage injection process is optimized, convergence angle of weld line is greater than 75°, as shown in Figure 7 (c); ③ Increase gate and runner size to form weld line as close to gate as possible, which can improve strength of weld line.

 

Figure 7 Weld line

Considering that above analysis cannot prove that new scheme has significant improvement, residual stress is further analyzed. Residual stress can be improved by optimizing process parameters. By optimizing multi-stage injection and segmented pressure holding process parameters, new scheme has a better effect. Following is an analysis of residual stress of new and old schemes.
Moldflow analysis revealed that residual stress in assembly area of old solution was difficult to improve. This was because this area was the last to be filled and was far from injection point, which adversely affected melt temperature and pressure transmission. Results before and after optimization are shown in Figures 8 and 9. Before optimization, maximum value reached approximately 30 MPa, with residual stress in a small, final area reaching approximately 8 MPa, and residual stress distribution shifted abruptly. After optimization, maximum value and distribution improved somewhat, but effect was limited. Cracking also occurred at this location, consistent with Figure 1 and characterized by brittle cracking. Furthermore, dark marks appeared in actual part at this location, which is generally consistent with moldflow analysis results. Regardless of process improvements, residual stress remained insignificant, and stress marks persisted, as shown in Figure 8. This is consistent with company's actual production results, confirming that residual stress was root cause of the cracking.
Automated analysis of residual stress in assembly area of new solution also revealed unsatisfactory results, with high and uneven residual stress distribution, and noticeable stress marks at weld line. However, after optimization, residual stress is about 17 MPa. Although it is reduced by 32% at weld line position, it is evenly distributed and stress mark disappears, as shown in Figure 9. Since increase of residual stress will reduce strength, new solution is better than old solution.

 

Figure 8 Residual stress of plastic part of old solution

 

Figure 9 Residual stress of plastic part of new solution
In summary, new solution has advantages in three aspects: filling, pressure holding, and residual stress, but it also has defects, namely weld lines. If it is difficult to accept new solution based on experience alone, since original mold cannot solve cracking problem, initial judgment of cracking cause is that residual stress is too large. Mold flow analysis results show that residual stress of new solution is better than old solution. Try to use new solution to mold plastic parts. When implementing new solution, in order to prevent gate position from being changed again, company only processes one gate for plastic part molding during mold part processing, then processes another gate for plastic part molding after trial production is successful.

Plastic part structure is shown in Figure 10. φ53 mm and φ59 mm cylindrical surfaces in Figure 10 were drafted. Due to relatively large shrinkage of PP, a draft angle of 3° or greater is ideal. However, tolerance of mating portion is only 0.2 mm, so a draft angle of 1° is used. To maintain strength without reducing part wall thickness, a reduced-resistance drafting method is used on inside, while an increased-resistance drafting method is used on outside.

 

Figure 10 Plastic Part Structure

Main parting surface is selected at the bottom end of part to be molded, lateral parting surface is selected in the center of part to be molded. Cavity plate and core are shown in Figure 11.

 

Figure 11 Cavity plate and core

 

Figure 12 Undercut size calculation
Slider stroke is generally designed to be undercut size plus 2~5 mm, so slider stroke is designed to be 26 mm and angle is designed to be 20°. Final Hough slider is shown in Figure 13.

 

Figure 13 Hough slider
Slider is positioned by screws and blocks, and slider is pressed onto push plate with a pressure strip. Locking block is directly designed with original fixed template, as shown in Figure 14.

 

Figure 14 Locking block

When processing gate, only one side is processed first, then a small amount of trial production is carried out. After molded plastic parts are assembled and qualified by customer, the other side is processed to avoid re-molding. Completed casting system is shown in Figure 15.

 

Figure 15 Improved Gating System

Molded parts are ejected using a push plate, which is secured to reset lever with an M8mm screw. During part ejection, push plate pushes separated slider and part together. Reset spring then quickly resets part, completing demolding. Reset spring is 125mm long, with a 10mm preload, and ejection stroke is designed to be 50mm.

Mold structure is shown in Figure 16. Because injection molding machine used for mold flow analysis has same parameters as company's injection molding machine, injection molding was performed according to mold flow analysis plan. Process parameters are as follows: accumulator length 116mm, barrel temperature 170℃ for first stage, 190℃ for second stage, 200℃ for third stage, 195℃ for fourth stage, and nozzle temperature 190℃. A four-stage injection process is used (see Table 2), with a total injection time of 3.5 seconds. A three-stage holding pressure was used: first stage was 45 MPa for 6.5 seconds; second stage was 34 MPa for 2 seconds; and third stage was 15 MPa for 2 seconds. Cooling time was 15 seconds. A qualified plastic part is shown in Figure 17. Circle indicates gate location. Part passed strong light inspection, showed no dark marks on assembly area, passed dimensional inspection, and showed no cracking during trial assembly.

 

Figure 16 Mold Structure
1. Fixed mold base plate 2. Inclined guide pins 3. Fixed mold plate (also locking block) 4. Cavity plate 5. Screws 6. Wear blocks 7. Slide block 8. Wear blocks 9. Stopper 10. Screws 11. Screws 12. Moving mold plate 13. Spacer 14. Return lever 15. Spring 16. Push rod retaining plate 17. Push plate 18. Moving mold base plate 19. Seal 20. Push plate 21. Wear blocks 22. Water barrier 23. Core 24. Slide block 25. Wear blocks 26. Seal 27. Retaining ring 28. Sprue bushing 29. Slider pressure plate

 

Figure 17 Qualified plastic part