In modern large-scale supermarkets, agricultural production bases, large-scale warehousing and logistics hubs, plastic products are commonly used for storage and turnover of goods. Plastic products have advantages such as being economical, safe and hygienic, lightweight and durable, and environmentally friendly and recyclable. They are widely used in storage and turnover of goods in industries such as technology electronics, hardware, medical, mechanical parts, refrigerated food, aquatic products, logistics, warehousing. Plastic turnover baskets are commonly used plastic products, generally large in size, with a small demolding angle, evenly distributed reinforcing ribs and regularly arranged holes on all four sides. They are widely used injection-molded products in production.

Turnover basket, as shown in Figure 1, is a rectangular plastic part with dimensions of 600.0 mm in length, 400.0 mm in width, 150.0 mm in height, and 5.0 mm in wall thickness. Bottom has four L-shaped bosses, each 5.0 mm high, providing support and stacking positioning. Surrounding area has evenly distributed reinforcing ribs and diamond-shaped holes. Turnover basket is made of high-density polyethylene (PE-HD), which is chemically stable, insoluble in any organic solvent at room temperature, resistant to corrosion from acids, alkalis, and various salts. After molding, product must have a smooth, flawless surface, with no obvious shrinkage marks, flash, weld lines, deformation, warping, scratches, or other appearance defects.

 

Figure 1. Product image of turnover basket.
Mold parting surface is designed to facilitate smooth removal of injection-molded product from mold cavity. Based on structural characteristics of plastic part, it is divided at a specific location within mold cavity as a surface; this dividing surface is called parting surface. Design and selection of parting surface affects molding quality and ease of demolding of plastic part, also involves mold structure and manufacturing cost. Therefore, design and selection of mold parting surface must be carefully considered. Mold parting surface design should ensure molding quality and precision requirements of plastic part, facilitate demolding, improve production efficiency. Mold structure should be simplified as much as possible to facilitate processing of molded parts and reduce mold manufacturing costs. Considering structural characteristics and size of turnover basket product, its parting surface is selected at maximum outline of top plane of product, with each product having one mold cavity, as shown in Figure 2.

 

Figure 2. Parting Surface Selection for Product
Gate design is based on fact that product is a single-cavity, large, and deep plastic part. A large direct-gating gate design is adopted, where gate directly injects molten material to bottom of product. Melt flows directly from nozzle through gate into cavity, resulting in the shortest flow path, low flow resistance, fast feeding speed, good molding effect, low pressure and heat loss, strong pressure holding and shrinkage compensation, simple mold structure, easy manufacturing, and low cost. Gate design is shown in Figure 3.

 

Figure 3. Product Gate Design
To facilitate processing of molded components and subsequent assembly and maintenance of mold, cavity and core of mold are separated into inserts, as shown in Figure 4.

 

Figure 4. Mold Core and Cavity Mechanism
Parting surface and gate of mold have been designed. Analyzing outer structure of plastic part (as shown in Figure 1), reinforcing ribs and diamond-shaped holes evenly distributed around product form an undercut, preventing direct ejection and demolding. Furthermore, the overall height of product reaches 150.0 mm, and inner wall of product has almost no demolding angle. If a conventional ejection design is used, ejection stroke is large, resulting in a large overall mold size, increased material costs, high component processing fees, and a long overall manufacturing time. After mold production, a large injection molding machine is required for further production, leading to low economic efficiency. Secondly, using a conventional direct ejection mechanism is prone to ejection defects such as whitening and breakage. Forced ejection can also cause product deformation and warping, affecting product quality. In conclusion, solving demolding design problem for turnover basket product is crucial.
Analysis of outer side of turnover basket product reveals that evenly distributed reinforcing ribs and regularly arranged diamond-shaped holes on all four sides form four-sided undercut areas, posing certain difficulties for outer demolding design. Demolding design for four-sided undercut sections generally requires a side core-pulling mechanism. Conventional side core-pulling designs typically include two types: lifter side core-pulling and slider side core-pulling. Considering dimensions of turnover basket product, the overall size of mold, convenience and rationality of manufacturing, a side slider core-pulling mechanism was adopted for outer demolding mechanism of turnover basket product. A conventional side slider core-pulling mechanism (as shown in Figure 5) generally involves machining a slide rail on mold plate. Injection molding machine opens mold, inclined guide pillar drives outward slider to achieve core-pulling purpose.

 

Figure 5 - Schematic diagram of a general slider structure
If this conventional slider design is adopted, dimensions of fixed and moving mold plates need to be increased. Furthermore, machining slide rails, wedging parts, wedging blocks on fixed and moving mold plates is time-consuming and labor-intensive, resulting in a larger overall mold size, further impacting mold's manufacturing efficiency and economic benefits. Considering impact of product dimensions on the overall mold dimensions, given various drawbacks of conventional design schemes, a hydraulic cylinder side core-pulling design was adopted instead of conventional slider design in side core-pulling mechanism. This design features a four-sided, two-way sliding, hydraulic cylinder-assisted slider side core-pulling demolding mechanism. Hydraulic cylinder-assisted slider design significantly reduces mold platen size, further lowers material costs, and greatly improves manufacturing efficiency.
Hydraulic cylinder installation position is shown in Figure 6. Cylinder body is fixedly mounted on the front and rear large sliders 1 and 2, respectively. Cylinder piston rod is connected to fixed platen, a slide rail is provided on fixed platen to allow sufficient movement space for piston rod during extension and retraction, preventing jamming and damage to cylinder.

 

1. Hydraulic cylinder piston rod movable slideway; 2. Hydraulic cylinder piston rod; 3. Fixing screw; 4. Hydraulic cylinder body.
Figure 6 Hydraulic cylinder installation position
Hydraulic cylinder-assisted four-sided slider demolding structure is shown in Figure 7. By extending cylinder piston rod, cylinder body moves outward, driving front and rear large sliders 1 and 2 to move outward under guidance of inclined guide pillars. Simultaneously, to ensure synchronized outward movement of left and right small sliders 3 and 4, fixed slide rail connecting rods are machined, installed at 45° center positions on both sides of front and rear large sliders 1 and 2, respectively. Slide rails are also opened at corresponding 45° center positions on the sides of front and rear large sliders 1 and 2. Under synchronized movement of slide rail connecting rods on both sides of front and rear large sliders 1 and 2, left and right small sliders 3 and 4 are driven to move outward synchronously, achieving demolding with outer four sides of product inverted, solving problem of demolding outer side of turnover basket product.

 

1. Slide rail connecting rod; 2. Slide block seat; 3. Hydraulic cylinder; 4. Inclined guide post; 5. Wear-resistant block; 6. Slide head; 7. Product.
Figure 7. Hydraulic cylinder ejecting four-sided slider demolding structure diagram
To further improve processing efficiency of side sliders and reduce cost of slider materials, sliders adopt an inlaid split structure design, consisting of a slider head and a slider seat (Figure 8), making it convenient for sliders to be processed on different machine tools. Its manufacturability is improved. Secondly, to address issue of overflow and fly-out defects caused by slider moving outward under injection pressure, corresponding beveled surfaces are machined on slider seat, moving and fixed mold plates using precision CNC machine tools. Clamping pressure of injection molding machine, beveled surfaces of moving and fixed mold plates wedge slider beveled surfaces, preventing slider from moving backward and ensuring product quality. Simultaneously, wear-resistant blocks are machined, installed on wedged surfaces of fixed and moving mold plates and slider seat to reduce frictional damage to slider seat and moving and fixed mold plates caused by wedge clamping, thus improving service life of mold components.

 

Figure 8. Schematic diagram of slider structure
This addresses issue of demolding on outside of product. Considering characteristics of inner part of turnover basket product, its demolding design typically uses a direct ejection structure with ejector pins.
However, because inner wall of turnover basket product is vertical, product lacks a draft angle, and its overall height reaches 150 mm, ejection distance needs to be at least 150 mm. If direct ejection with ejector pins is used, high ejection resistance will result in defects such as whitening and breakage, severely affecting product quality. Secondly, ejector pin design requires mold to have an ejector pin base plate, a face plate, mold feet for support and ejection space, significantly increasing material preparation and overall size of mold, greatly increasing manufacturing cost. Simultaneously, procurement and processing of these structural components further extends mold manufacturing cycle, significantly reducing manufacturing efficiency. Therefore, based on years of mold design experience, we broke with conventional mold design thinking, abandoning mold feet and ejector pin plates, creatively designed a fixed mold tie rod and pull plate, utilizing mold opening stroke to pull out product in a secondary mold opening structure, as shown in Figure 9, solving problem of inner demolding of product.

 

Figure 9: Secondary demolding structure diagram of fixed mold pull plate.
Secondary demolding process is as moving mold moves backward in injection molding machine, and slider moves outward synchronously under action of hydraulic cylinder, causing outer side of product to begin demolding. Once slider reaches a certain safe distance, first demolding is complete. Injection molding machine continues to move backward, and secondary tie rod pulls out to a set distance, driving pull plate to pull out product until it is completely removed from core, completing second demolding.
Figure 9: Secondary demolding structure diagram of fixed mold pull plate. This innovative mold design, employing a fixed mold tie rod and pull plate, utilizes mold opening stroke to pull out product through a secondary mold opening mechanism. This eliminates need for conventional mold ejection mechanisms, preventing issues like whitening, breakage, and deformation during ejection. Innovative design of fixed mold tie rod and pull plate eliminates need for ejector pin panels, base plates, and mold feet, resulting in a simpler and more optimized mold structure. The overall mold size is significantly reduced. Design utilizes precision CNC machine tools, further ensuring mold's manufacturing accuracy while significantly improving manufacturing efficiency and highlighting its economic performance.

Cooling design of mold plays a crucial role in quality of product and production efficiency. A reasonable cooling system design can effectively control warping and deformation defects, reduce cooling time, and improve production efficiency. Cooling system for turnover basket mold is designed on fixed mold, moving mold, and slide block. Specific cooling design is as follows:
Eight parallel cooling water channels are evenly arranged on fixed mold platen and cavity. Cooling water enters fixed mold cavity from fixed mold platen and flows out from the other side, forming an S-shaped loop cooling system through water pipes, as shown in Figure 10. Parallel straight cooling water channel design makes water channel processing simple and convenient.

 

Figure 10 Fixed Mold Cooling System
Core on moving mold is a high core. For this type of high core, a cooling well-type baffle cooling system design is recommended. Therefore, cooling system for moving mold of turnover basket is designed so that cooling water enters through moving mold plate. Twenty-four cooling water wells, each 20 mm in diameter and 190 mm deep, are evenly arranged on moving mold core, with baffles installed within each well. Water channels for each well are also created at the bottom of core, as shown in Figure 11(a). After entering wells through water channels, cooling water flows out from the other end, forming a loop, as shown in Figures 11(b) and (c).

 

Figure 11 Moving Mold Cooling System
Because contact area between turnover basket's side sliders and product is large, a corresponding cooling system is also needed to improve production efficiency and ensure product quality.
Cooling system on sliders is a series water channel system designed on both large and small sliders, as shown in Figure 12. Figures 12(a)~(b) show large slider, and Figures 12(c)~(d) show small slider.

 

Figure 12 Slider Cooling System
The overall mold drawing of turnover basket is shown in Figure 13.

 

1. Moving mold plate; 2. Water nozzle; 3. Mold pit; 4. Core; 5. Pull plate; 6. Pull rod; 7. Hydraulic cylinder; 8. Slider water circuit connection hose; 9. Secondary pull; 10. Large slider; 11. Fixed mold water circuit connection pipe; 12. Fixed mold pull rod; 13. Pump nozzle; 14. Fixed mold plate; 15. Fixed mold water circuit; 16. Cavity; 17. Angled guide post; 18. Small slider; 19. Slider connecting rod; 20. Product; 21. Guide sleeve; 22. Guide post.
Figure 13 Mold Structure
Working process is as follows: After mold closes, plastic is injected to fill mold cavity. After a certain period of pressure holding, cooling, mold, under movement of injection molding machine, moves moving platen 1 backward. During this backward movement, large front and rear sliders 10 begin to slide out under ejection action of hydraulic cylinder 7. Driven by slider connecting rod 19, left and right small sliders 18 simultaneously move out until sliders are completely disengaged from outside of product by a certain distance. First demolding is completed.
Injection molding machine continues to move backward, driving secondary tie rod 9 to be pulled out a certain distance within tie rod 6. Secondary tie rod 9 and tie rod 6 then drive pull plate 5 to pull out. Under pull of the pull plate 5, product 20 detaches from core 4 until a safe distance for product removal is set. Second demolding is completed. Product is removed, mold closes again, next injection and mold opening cycle begins, completing the entire mold working process.

(1) Turnover basket mold design abandons conventional mold structure schemes and innovatively adopts an external demolding design with a hydraulic cylinder ejecting a slider. A new secondary demolding structure, using a fixed mold tie rod and pull plate to pull product out from inside, simplifies and rationalizes mold structure. Precision CNC machining ensures mold accuracy and greatly improves mold manufacturing efficiency.
(2) In actual production, newly designed mold mechanisms operate smoothly, and batch production quality of products is stable. It has become a new standard for mold design schemes for this type of product. Subsequent products of same category and other specifications have adopted this scheme for design and production, bringing good economic and social benefits to enterprise.