Abstract: This paper details main structural design of an injection mold for plastic pallets with built-in steel tubes, including gating system, molding components, side core pulling, ejection system, cooling system, and venting system. A general assembly drawing of mold structure is provided, and working principle of mold is explained. Due to complex mold structure, which is a four-sided slider core-pulling structure, a single-sided core-pulling mechanism of "large slider + hydraulic cylinder" is used for top undercut area; a two-sided core-pulling mechanism of "large slider + hydraulic cylinder" is used for operating and non-operating undercut areas; a core-pulling structure of "slanted guide post + large slider" is used for bottom undercut area; and a novel cross-auxiliary rod lifter core-pulling structure is used for inner undercut area, thus solving problem of difficult pallet demolding. Furthermore, stress of pallet under static and dynamic loads (forklift handling) was simulated using finite element analysis software Abaqus. Analysis results show that maximum stress of pallet under both conditions did not exceed yield strength of material, indicating that pallet strength meets usage requirements. Finally, actual production was carried out, and results show that designed mold structure is reasonable, safe, and reliable, and quality of molded pallet is good, providing a reference for design of similar pallets.
As a portable horizontal platform device for storage, handling, loading and distribution, pallets have been widely used in production, transportation, warehousing and distribution, and are considered one of two key innovations in logistics industry in 21st century. In 2022, General Office of State Council issued "14th Five-Year Plan for Modern Logistics Development", which accurately focuses on key directions of modern logistics development, which also brings new opportunities to development of pallet industry. In 2022, China's annual pallet production was about 370 million pieces, and pallet market stock was about 1.7 billion pieces. In order to protect environment and reduce deforestation, use of plastic pallets instead of wooden pallets has become a future development trend.
Plastic pallets come in many varieties. According to different structures, they can be divided into single-sided and double-sided pallets; according to load-bearing capacity, they can be divided into light-load, medium-load and heavy-load; according to material, they can be divided into traditional plastic pallets and built-in steel pipe type plastic pallets. Plastic pallets are mainly injection molded using thermoplastic plastics such as high-density polyethylene (PE-HD) and polypropylene (PP). Compared with other materials, PE-HD pallets and PP pallets have good corrosion resistance, heat resistance, high impact resistance, long service life, and high cost performance. Taking built-in steel pipe plastic pallet as an example, this paper details main structural design of mold, including design of gating system, molding parts, side core pulling, ejection system, cooling system, and exhaust system. Mold assembly drawing is also drawn, and working principle of mold is explained. Then, load conditions of pallet under static and dynamic load (forklift handling) conditions are simulated using Abaqus software, so as to provide a reference for actual production and application.

Parts of built-in steel pipe plastic pallet are shown in Figure 1. Plastic material is HDPE-KS 10100 UE, which has a shrinkage rate of 1.8% and has good corrosion resistance, heat resistance, high impact resistance, long service life, and high cost performance. Pallet is mainly composed of two parts: pallet panel and pallet feet. Pallet panel is bearing surface of pallet, which is composed of multiple longitudinal and transverse reinforcing beams and a central plate. Pallet feet are set around pallet panel and play a supporting and fixing role. Pallet structure has following requirements: (1) Maximum size of pallet is 1300 mm * 1100 mm * 150 mm. Wall thickness of plastic part is unevenly distributed. Main wall thickness is 3 mm, wall thickness of rib is 4 mm at small end and 5 mm at large end, and pallet feet have a stress requirement, with a maximum wall thickness of 6 mm. Product has no texture area, and minimum draft angle of the rib is 1°. (2) Plastic part has a complex shape. There are multiple steel pipe holes, ribs, and indentations around top, bottom, operating, and non-operating areas. There are also 4 indentations in pallet feet of product inside inner area. Structure is more complex when mold is demolded. (3) Plastic part has many deep cavities and deep ribs, which can easily cause problems such as air trapping and short shot in ribs. Design of venting structure should be fully considered when designing mold. (4) Product has many holes, and gate design needs to consider filling balance to avoid problems such as mold expansion and flash. (5) Plastic part is a functional part, slight gate marks and weld lines are allowed. Maximum deformation of product should be within ±5 mm, and there should be no obvious shaking when product is stacked. Strength requirement is that it can withstand a maximum load of 4,000 kg when goods are placed at rest; and a maximum dynamic load of 2,000 kg when forklift is handling it.

 

Fig. 1 Steel pipe plastic tray model

Plastic trays are functional parts and do not have strict requirements for appearance. Shrink marks, weld lines, and gate marks are allowed. However, as a load-bearing carrier, it has high requirements for product deformation and mechanical properties. If product deforms significantly after injection molding, it will affect stacking, cause shaking, and affect safety; poor mechanical properties will lead to cracking during use. Plastic pallets have uneven wall thickness and complex structure. In actual production, problems such as unbalanced filling, flash, short shot, and deformation often occur For such pallet plastic parts, author designed a casting method of "20-point open hot runner + large gate direct injection". Diameter of hot runner is 16 mm and diameter of hot gate is 5 mm. At the same time, cold material section of gate is lengthened and cold material section is 30 mm high, which can effectively avoid hot runner heating coil being too close to product surface, affecting cooling of gate, thus causing problem of shrinkage at gate position. Mold casting system design is shown in Figure 2.

 

Fig. 2 Design of mold pouring system

Moving and fixed mold sides of pallet have many deep ribs and deep cavity structures, which are prone to wear during injection molding, resulting in flash. Therefore, moving and fixed molds of pallet adopt a block structure. Use of a block-type structure facilitates machining and heat treatment, saves material costs, and makes it easier to replace blocks and maintain mold. Figure 3 shows block-type design used in fixed mold. Mold core size is 1680 mm * 1480 mm * 235 mm. Mold base is made of S50C steel with a hardness of 28~32HRC. Mold core is made of 718H mold steel with better performance. Compared with S50C, it has a higher surface hardness (30~34HRC) and better wear resistance. Fixed mold core is designed with 32 forming part blocks, which are fixed to mold base by internal hexagonal head screws.

 

Fig. 3 Design of fixed mold forming parts
(a) Fixed Mold base (b) Fixed mold core inlay block
Figure 4 shows design of moving mold forming parts. Mold base is made of S50C steel, and mold core is made of 718H mold steel. There are four large slider core-pulling mechanisms around tray, mainly used for inserting steel pipes into molded plastic parts. Slider core-pulling mechanism is subjected to a large injection pressure in mold cavity during each injection process, it is in constant contact and friction with product and mold insert surfaces during each reciprocating motion. Therefore, it needs to have high strength and wear resistance. Slider core-pulling part is made of Cr12 mold steel with a hardness of 58~62HRC. It has high strength and wear resistance, which can improve service life of core-pulling mechanism. Moving mold core is designed with 39 molding part inserts, which are fixed to core mold base by internal hexagonal head screws. The parting surface extends through top surface of slider. Parting surface is smooth and easy to process. Top surface of slider is used as sealing surface. Sealing distance is 72 mm. Side locking surface of slider matches mold cavity, and sealing effect is good.

 

Fig 4. Design of moving mold forming parts
(a) Moving mold base (b) Core insert block design

Due to uneven shape of outer surface of tray sidewall, plastic part cannot be directly ejected from mold. This mold features a side-parting core-pulling mechanism designed for five undercut areas: top side, bottom side, operating side, non-operating side, and inner side. This mechanism converts opening direction perpendicular to parting surface into lateral movement, allowing mold forming parts in uneven side structure of plastic part to detach from product before ejection, thus facilitating demolding. Compared to conventional tubeless trays, design challenge of this tray lies in 18 long core-pulling mechanisms on the side, with a maximum core-pulling stroke of 1330 mm. Because core-pulling structure is slender, it is prone to displacement, bending deformation, other problems during injection molding. Therefore, local perforation treatment is required at core-pulling positions to increase positioning support of core-pulling mechanism.

Figure 5 shows design of side core pulling mechanism for undercut area on the top side. As shown in Figure 5a, there are 6 steel pipe holes, 2 rib undercuts and 1 concave undercut on the side of tray in top side area. Length of steel pipe is 1330 mm. If two sides are designed with a pulling structure, stroke can be shortened by half, reducing wear of core and making it more stable. However, this area is top side of mold, and there is not enough space on the bottom side of mold for core pulling movement. Therefore, only a single-sided "large slider + hydraulic cylinder" side core pulling mechanism can be designed. Maximum stroke of core pulling is 1330 mm. Wear-resistant plates are designed on the bottom of slider and side locking surface. Wear-resistant plates are fixed to slider seat by screws. Wear-resistant plates can reduce contact friction between slider and its mating surface, improve service life of slider. Limit block is fixed to slide rail pad by screws, which limits stroke of the entire slider core pulling. Side core pulling mechanism is fixed to screw and connected to slider seat by a fixing block. Driven by a hydraulic cylinder, large slider moves outward along slide rail until undercut at steel pipe position is completely disengaged.

 

Fig. 5 Design of lateral core pulling mechanism in inverted buckle area of sky side

Design of side core pulling structure for operating and non-operating side undercut areas is shown in Figure 6. As shown in Figure 6a, there are 6 steel pipe holes, 2 tray foot undercuts, 2 rib undercuts, and 2 concave undercuts in operating side area of product. Non-operating side area is symmetrical to operating side area, so its features are exactly same. Therefore, a "large slider + hydraulic cylinder" side core pulling mechanism is designed for operating and non-operating sides on mold. Compared with single-sided core pulling, advantage of side core pulling at steel pipe position is that maximum core pulling stroke is shortened to 570 mm, and core pulling strength is better. This also reduces friction during core-pulling process, resulting in a longer service life.

 

Fig. 6 Design of lateral core pulling mechanism in reverse buckle area on operating and non-operating sides

Ground-side reverse buckle area core-pulling mechanism is shown in Figure 7. As shown in Figure 7a, there are two rib reverse buckles and one concave reverse buckle feature in ground-side area. Therefore, a "slanted guide post + large slider" core-pulling method is adopted. Limiting block is fixed on core side, and its limiting stroke is 10 mm. During mold opening process, driven by slanted guide post, slider slides outward synchronously. Under restriction of limiting block, large slider slides a maximum of 10 mm along slide rail, then stops moving outward, and side reverse buckle disengages.

 

Fig.7 Design of side core pulling mechanism for ground side inverted buckle area

There are 4 inverted buckles on inner side surface of product, with an ejection angle of 18°. If a conventional lifter mechanism is used, ejection angle is too large, lifter is unstable, it is easy to cause shaking and jamming during ejection. Therefore, a new type of cross auxiliary rod lifter core pulling structure is designed. Design diagram of this structure is shown in Fig.8. This structure mainly consists of 1 lifter component, 1 auxiliary rod and 1 slide. Slide is installed on the face pin plate, lifter rod guide slide slides on slide, two ends of auxiliary rod are fixed to moving mold plate and moving mold base plate respectively, auxiliary rod passes through slide, angle of lifter rod and auxiliary rod are same. During ejection, ejector rod drives bottom pin plate, bottom pin plate drives slide and guide slide to move upward along auxiliary rod, lifter rod and lifter are moved obliquely upward and laterally by action of slide guide rail to demold.

 

Fig. 8 Design of side core pulling structure in inner inverted buckle area

During injection molding process, product is cooled and formed in mold. Due to volume shrinkage of plastic, it will generate a clamping force on core. Especially in some deep cavity and deep rib positions, due to unreasonable design of ejection mechanism, appearance of product often shows whitening and bulging. For this type of tray product, core has a lot of deep ribs and deep cavity structures, and shrinkage of deep cavity will generate a large clamping force on ejector block, resulting in product being stuck in mold. Therefore, ejection mechanism of this mold adopts a combination of "round ejector pin + lifter" for ejection. Design of ejection mechanism is shown in Fig. 9.

 

Fig. 9 Design of ejection mechanism
For four undercut positions inside the product, lifter method is adopted. Lifter design can not only be used as a side core pulling, but also play an ejection role for product. At the same time, for deep cross-rib structure, mold is designed with 204 circular ejector pins, each 10 mm in diameter, positioned at cross-rib locations on product. When product is ejected, ejector rollers on the injection molding machine exert force on the bottom pin plate, causing ejector components to eject. During ejection and resetting, ejector rollers move bottom pin plate along reset rod and guide post to reset.

In injection molding process, injection time accounts for approximately 5% of molding cycle, holding time for approximately 10%, demolding time for approximately 15%, and cooling time for approximately 70%. A well-designed cooling system can not only shorten product manufacturing cycle, improve production efficiency, and reduce production costs, but also ensure uniform temperature distribution between mold cavity and core sides, reducing temperature difference between front and rear molds and minimizing product deformation caused by uneven cooling. For this tray, cavity side uses a 20-unit "straight-through + partition + water collector" cooling water circuit design. Diameter of straight-through water pipe is 11.5 mm. For deep cavities and ribs on nine tray feet, straight-through water channels are too far from bottom of cavities to provide adequate cooling. Therefore, a combination of partitioned water channels with a diameter of 24 mm was added. Core side employs a 22-unit "straight-through + partitioned + water collector" cooling water circuit design, with straight-through water channels having a diameter of 11.5 mm and partitioned water channels having a diameter of 24 mm. Due to limited space on 18 long core-pulling structures on surrounding large sliders, neither partitioned nor straight-through water channels could fit. Therefore, a jet-type cooling water circuit was designed on core-pulling mechanism. The entire slider structure uses a total of 28 "straight-through + jet-type" cooling water circuit designs, with straight-through water channels having a diameter of 11.5 mm and jet-type water channels having a diameter of 6 mm. For four large sloping tops on core side, a 4-unit straight-through water channel design is used, with a water channel diameter of 9 mm. Cooling system design is shown in Figure 10.

 

Figure 10. Design of cooling system

During injection molding, a large amount of air exists in feeding system and mold cavity. If mold does not have a suitable venting system, high internal stress will be generated inside product, and obvious flow marks, air marks, and weld lines will appear on product surface, affecting performance of plastic part. If gas is compressed and cannot be discharged during injection, mold cavity will generate instantaneous high temperature, which will cause melt to decompose and discolor, even produce defects such as carbonization, scorching, and short shots. Therefore, this mold has a primary venting groove with a width of 5 mm, a depth of 0.15 mm and a secondary venting groove with a width of 5 mm and a depth of 0.45 mm designed every 30 mm around parting surface of fixed mold. Same primary and secondary venting grooves are also designed on cavity and core insert. At the same time, gap between core inserts can also allow some gas to be discharged. Design of venting system is shown in Figure 11.

 

Figure 11. Design of exhaust system

Figure 12 is mold assembly drawing. The overall dimensions of built-in steel tube pallet mold are 1880 mm * 1880 mm * 1188 mm, which belongs to category of large-scale logistics plastic part injection molds. Mold design adopts a 20-point open hot runner large gate gating system. Hot runner needs to be designed with runner balance to ensure that hot nozzles discharge simultaneously, so as to make plastic part fill evenly, which can reduce injection pressure and clamping force and avoid flash. Working principle of mold is as follows:

 

Fig. 12 Mold assembly drawing
1—Moving mold plate; 2—Bottom needle plate; 3—Face needle plate; 4—Mold foot; 5—Cross auxiliary pole; 6—Core fixing plate; 7—Guide sleeve; 8—Guide pillar; 9—Cavity fixing plate; 10—Hot runner fixing plate; 11—Fixed template; 12—Hot runner; 13—Positioning ring; 14—Diagonal top rod; 15—Steel pipe; 16—Support column 2; 17—Diagonal guide pillar; 18—SA03 large slider; 19, 31, 36, 37—Hydraulic cylinder 1; 20—Sliding block pressure bar 1; 21—Slide rail 1; 22—Slide guide block 1; 23—SA02 large slider; 24—Reset lever; 25—Cavity insert 26—Wear resistant plate; 27—SA01 large slider; 28, 33—Sliding block guide block; 29—Slide rail 2; 30—Sliding block pressure bar 2; 32—Core inlay block; 34—Slide rail 3; 35—Sliding block pressure bar 3; 38—Limit block; 39—Support column 1; 40—Dome pole
(1) First, use a crane to lift mold from top to bottom between four columns of injection molding machine, accurately align mold with injection molding machine nozzle through mold positioning ring. Use clamps to firmly fix fixed mold base plate, moving mold base plate and injection molding machine fixed plate, and mold clamping is completed. Then connect mold inlet and outlet water channels to mold temperature controller and set corresponding water channel temperature. Plastic solid particles are plasticized into melt under heating of injection molding machine and internal screw rotation shearing. Under screw pressure, it is injected into mold cavity through hot runner main nozzle, branch channel, vertical straight channel and gate. Because hot runner is an open hot runner, there is no need for a solenoid valve to control opening of valve needle, and gate is always open. After 5 seconds of injection time and 6 seconds of holding time, melt successfully fills mold cavity. Subsequently, under operation of mold cooling system, heat of product is conducted to cooling water through circuit pipe wall, and heat is continuously carried out to outside of mold by flow of cooling water.
(2) After product temperature reaches ejection temperature of plastic, driven by hydraulic cylinder of injection molding machine, moving mold part is driven by moving mold fixing plate and opens mold from parting surface along tie rod. During mold opening process, SA03 large slider, driven by inclined guide post, moves 10 mm in undercut direction with limit block to complete side core pulling action, side parting opens, one side undercut is completely disengaged, and plastic part stays on moving mold.
(3) After mold opens, other three large sliders SA01/02/04, driven by hydraulic cylinders, move laterally along guide rails in their respective undercut directions. Once all sides are open, four undercuts on plastic part are completely dislodged. Then, under action of injection molding machine's ejector rod, bottom and top pin plates move along guide pillars, driving lifter rods LA01/02/03/04 and ejector pins to slowly eject product while performing lateral core pulling.
(4) After robot arm removes part, the ejector plate, via reset rod, drives lifter rods LA01/02/03/04 and ejector pins to reset first. Then, slider core puller SA01/02/04 resets via hydraulic cylinders, and slider SA03 resets via inclined guide pillars, closing mold. Repeating above mold actions allows for mass production.

According to actual application scenario, pallet should meet two working conditions shown in Figure 13:

 

Fig. 13 Tray dynamic and static load conditions
(1) Static load condition. Pallet is placed on a horizontal and rigid plane, and goods are evenly spread on pallet. Maximum load mass that pallet can bear is 4,000 kg.
(2) Dynamic load condition. When using forklifts and other handling equipment, pallet is in dynamic operation, and goods on pallet are evenly spread. Maximum load mass that it can bear is 2,000 kg.

Models of pallet, steel pipe, forklift forks and heavy objects are imported into Hypermesh for mesh generation. Since pallet is an axisymmetric model, in order to reduce number of meshes and shorten calculation process, 1/4 of pallet model is divided into tetrahedral meshes with a mesh size of 6 mm. After division, number of tetrahedral meshes is 237,994. Steel pipe, forklift spikes and heavy object models are divided into hexahedral meshes with a mesh size of 30 mm. After division, number of hexahedral meshes are 976, 118 and 2,772 respectively. Mesh after division is shown in Figure 14.

 

Fig. 14 Model grid division

Before performing static and dynamic load analysis, it is necessary to define relevant properties of materials used for pallet and steel pipe in Abaqus material database. Basic material properties are listed in Table 1.

Table 1 Recommended process parameters

When using Abaqus to perform finite element analysis on pallet load, boundary conditions and load settings under two working conditions are very important. Settings are shown in Figure 15. As shown in static load condition, there is contact compression between load and pallet. Therefore, contact analysis needs to be performed on the top surface of pallet and bottom surface of load, and contact surface should be defined as finite slip. Steel pipe installed in pallet provides support, so steel pipe and pallet are bound together. Pallet is placed horizontally on the ground, and bottom surface of pallet is fully constrained. Simultaneously, pallet needs to bear a 4000 kg load, so a downward gravity force along Z-axis also needs to be applied. As shown in dynamic load condition, in addition to contact compression between load and pallet, there is also contact compression between forklift forks and pallet. Therefore, contact analysis should be performed between forklift forks and pallet, and contact surface should be defined as finite slip. Steel pipe and pallet are bound together. Simultaneously, pallet needs to bear a 2000 kg load during operation, so a downward gravity force along Z-axis also needs to be applied.

 

Figure 15 Boundary conditions and load settings
(a) Static load (b) Dynamic load

Static and dynamic load conditions were analyzed separately, and stress cloud diagrams under pallet load are shown in Figure 16. From stress results obtained under static load condition in Figure 16a, it can be seen that maximum stress of pallet under static load is 9.988 MPa, maximum stress is located in support area around pallet. From stress results obtained under dynamic load condition in Figure 16b, it can be seen that maximum stress of pallet under dynamic load is 5.647 MPa, maximum stress is located in contact area between forklift forks and pallet. As shown in Figures 16c-16d, maximum stress of steel pipe under static load is 4.858 MPa, while maximum stress under dynamic load is only 0.5915 MPa. According to PE-HD material parameters, yield strength of material is 27 MPa. Maximum stress value under static load and dynamic load conditions is much smaller than yield strength of material. Therefore, it can be concluded that pallet structure design is relatively reasonable, no further optimization is needed.

 

Fig. 16 Tray stress cloud chart

Designed mold was verified in factory. After actual mold trial, mold structure design was reasonable, it could run smoothly, product appearance quality was good, product design strength was high, and it could meet actual use requirements. Final product is shown in Fig. 17.

 

Fig. 17 Plastic tray products
(a) Front of tray (b) Back of tray

(1) Based on structure and characteristics of built-in steel pipe plastic tray, a two-plate injection mold with "one mold and one cavity" and "20-point open hot runner to large gate direct casting" was designed. To address difficulty of core pulling during lateral demolding of pallets, a clever lateral core pulling mechanism was designed around pallet. For top-side undercut area, a single-sided core pulling mechanism using a large slider and hydraulic cylinder was employed; for operating and non-operating side undercut areas, a two-sided core pulling mechanism using a large slider and hydraulic cylinder was used; for bottom-side undercut area, a core pulling structure using inclined guide pillars and a large slider was employed; and for inner area, a novel cross-auxiliary rod lifter core pulling structure was used. This effectively solved problem of pallet demolding difficulties, reduced processing difficulty and manufacturing cost of mold.
(2) Based on Abaqus software, stress on pallet under static and dynamic loads (forklift handling) was simulated. Analysis results show that maximum stress of pallet under both conditions did not exceed yield strength of material, indicating that pallet strength meets usage requirements.
(3) Actual production verification shows that designed injection-molded pallet structure is reasonable, filling is relatively balanced, it can be molded in one step, and its strength meets usage requirements.