Small buckle structure is shown in Figure 1. Material is PP, and there is a long inner hole on the side, which needs to be formed by a side core pulling mechanism. Due to large number of flat surfaces of plastic parts, size of buckle and through hole is more important, and accuracy of other dimensions is relatively low.

 

Figure 1 Plastic part structure

R3.3 mm rounded corners of original plastic parts will produce sharp corners after parting, which will affect service life of mold. After communication with customer, R3.3 mm is changed to R1.9 mm. Comparison before and after modification is shown in Figure 2.

 

Figure 2 Comparison of rounded corners before and after modification
As shown in Figure 3, R angle of buckle at this position is not easy to form, and R angle around buckle position cannot be demolded in side core pulling direction. After communication with customer, R angle around buckle position is cancelled.

 

Figure 3 Comparison before and after modification of buckle part

After snap is assembled, it is located on outer surface of vacuum cleaner, so accuracy of its outer surface is relatively high. Through analysis of its shape, surface quality and output requirements, it is produced by a 2-cavity injection mold of hot runner + point gate.

Molded parts are shown in Figure 4. In order to facilitate assembly and subsequent replaceability, molded parts adopt an inlaid structure. This design will increase processing difficulty of cavity plate, but reduce processing difficulty of movable and fixed mold plates. Long inner hole of plastic part needs to be formed and demolded with a side core-pulling structure. Mold is a 2-cavity structure, and 2 side core-pulling mechanisms are required, but as long as 2 molding plastic parts are set in same direction, 2 side core-pulling mechanisms can be combined into one, and inner cavity of 2 plastic parts is formed at the same time.

 

Figure 4 Formed Parts
Molding of bottom buckle of plastic part is more important, and its molding parts are also push-out parts. As shown in Figure 5, core insert is nested inside spring-type core, core insert is a fixed part, spring-type core can follow movement of push-out mechanism, spring-type core is processed into required shape and size from round bars, then cut to opening according to cutting line. Upper end is designed with a conical surface, which can ensure that it is closely matched with cylindrical sleeve of core plate during injection molding, leaving no gap.

 

Figure 5 Spring core

Gating system is shown in Figure 6. In order to make plastic part easy to form, it is very important to select gate form. It is not only necessary to ensure that cavity is filled with molten plastic within specified time, but also to ensure that gate condensate can be easily removed during demolding without affecting surface quality of plastic part.

 

Figure 6 Gating system
Because mold has a structure of 2 cavities, hot runner + point gate is used for feeding, and gate diameter is φ0.2 mm. Characteristics of this structure are: ① Material saving, because hot runner will not produce condensate; ② Molding quality of plastic parts is improved, melt in hot runner system can be kept at a constant temperature, and probability of defects in plastic parts is small; ③ Needle valve type hot runner point gate is used for feeding, gate diameter is small, and gate traces of plastic parts can be ignored.

When plastic melt fills cavity, there are not only air and water vapor in cavity, but also low molecular volatile gas, gas generated by volatilization of plastic additives, etc. If these gases cannot be discharged from cavity in time, it will affect plasticity. Molding quality of parts, therefore, exhaust system design is very important. Mold adopts following 3 common exhaust methods.
(1) Parting surface exhaust: set parting surface exhaust of mold on the core, and open several gaps within the range of the overflow value of the molten plastic around the parting surface of the core.
(2) Exhaust of inserts: an exhaust port is designed inside mold insert, exhaust port leads to outside of mold frame, and gas is discharged from exhaust port during injection.
(3) Core insert exhaust: Process a gap in core insert or hole of core insert that does not exceed overflow value of molten plastic, so that gas is discharged from gap when molten plastic is injected at a high speed.

Water temperature of water inlet of mold should be consistent with temperature of mold, and temperature difference between inlet and water outlet should be kept at ±3℃, otherwise plastic parts will shrink too fast, shrink unevenly, and cause cracks or baking defects. Cooling water circuit design is shown in Figure 7. After mold flow analysis, temperature of water inlet of fixed mold is 59.83 ℃, temperature of water outlet is 59.98 ℃, temperature of water inlet of movable mold is 50 ℃, and temperature of water outlet is 50.78 ℃. Water inlet temperature is 60 ℃ and water outlet temperature is 60.11 ℃. Temperature difference between inlet and outlet of these three places does not exceed ±3℃, so design of waterway is reasonable.

 

Figure 7 Cooling water circuit design

Design of side core-pulling mechanism should complete core-pulling and parting under condition of ensuring that molded plastic parts are not deformed, and cannot cause defects in plastic parts due to wear of its parts. Side core-pulling sliders are required to have high rigidity and strength. There should be a gap between slider and cavity plate for cavity exhaust, but size of gap should not exceed overflow value of melt.
According to design of cavity plate and core, two sliders of side core-pulling mechanism are set on same side, one slider drives two side core-pulling parts to achieve simultaneous core-pulling and parting. Because core-pulling distance is long, a hydraulic cylinder is selected for core-pulling, and a limit mechanism is set at the end of side core-pulling to avoid excessive core-pulling of hydraulic cylinder. Slider and side core are designed to be inlaid and connected with screws, and their structure is shown in Figure 8.

 

Figure 8 Side core pulling mechanism

Design principles of demolding mechanism: ① Structure should be reasonable, and movement should be accurate, reliable and flexible; ② There should be sufficient strength and rigidity; ③ Appearance and molding quality of plastic parts should be ensured; ④ Plastic parts should be kept on the side of movable mold as much as possible. After side core pulling is completed, when mold is opened, mold pushes push plate through ejector rod of injection molding machine, push plate drives push rod and core insert to move. After movement reaches set stroke, plastic part is completely pushed out, as shown in Figure 9 shown.

 

Figure 9 Demoulding of plastic parts
Since spring core is a paired conical cylindrical sleeve, which is made of spring steel, it is prefabricated into a spring-open state during assembly. During push-out process, it slowly bounces around along cutting line in Figure 5 to realize release of buckle part mold. When core is reset, push rod and core insert are reset under action of return spring.

Mold structure is shown in Figure 10. Material of core and cavity plate is imported 1.2738 steel, which has excellent polishing properties, good plasticity, toughness and wear resistance, no large deformation due to heat treatment, fewer heat treatment procedures, shortening mold manufacturing cycle; at the same time, it has good surface nitriding ability, and its surface can be hardened to improve wear resistance of core and cavity plate. Core and cavity plate must be softened and annealed before quenching.

 

Figure 10 Mold structure
1. Hydraulic cylinder 2. Cooling pipe 3. Wear-resistant sheet 4. Guide post 5. Sprue sleeve 6. Fixed die fixing plate 7. Heating socket 8. Wet socket 9. Return spring 10. Moving die seat plate 11. Push Plate 12. Push rod fixing plate 13. Spacer block 14. Moving die fixing plate
Material used for side core-pulling inserts is 1.2344 steel. Because parts are often extracted or inserted, high wear resistance, high temperature resistance and thermal fatigue resistance are required. This steel has following advantages: ① Uniform material, excellent machinability and polishing; ② High toughness and high plasticity; ③ High wear resistance at high and low temperature; ④ Excellent overall hardenability; ⑤ Little change in strength at high temperature, good thermal fatigue resistance.
Spring core needs to be formed and pushed out, and it needs high wear resistance, corrosion resistance, hardness and fatigue resistance. Based on above requirements, insert material is Progressive spring steel.