Strength Calculation and Analysis of Injection Molds for Heavy‑Duty Plastic Pallets
Heavy‑duty plastic pallets are widely used in warehousing, logistics, and freight transportation. Featuring large outline dimensions, wide molding projection area, and dense internal reinforcing ribs, these pallets generate high melt pressure inside the injection cavity during production. The mold bears long‑term significant mold‑opening force, bending load, and lateral extrusion. Insufficient structural strength of the mold will lead to a series of production problems, such as flash at the parting surface, bending deformation of mold plates, cracking of cavity sidewalls, and out‑of‑tolerance flatness of products, which seriously affect mold service life and molding quality of plastic parts.
Basic Calculation Parameter Setting
This calculation adopts a mainstream heavy‑duty plastic pallet specification with outline dimensions of 1200 mm × 1000 mm and cavity depth of 150 mm, suitable for HDPE injection molding. The main mold structure uses pre‑hardened plastic mold steel with an elastic modulus of 2.1×10⁵ MPa and yield strength of 950 MPa. In accordance with large heavy‑duty mold design specifications, a safety factor of 2.0 is set, and the allowable stress under comprehensive working conditions is controlled within 400 MPa. Due to obvious pressure loss during large‑area filling, the average cavity melt pressure is set at 50 MPa. Referring to industrial injection mold standards, the allowable deflection of mold plates is limited to strictly control the deformation range, balancing strength and molding accuracy requirements.
Clamping Force Calculation
Clamping force is the core index to restrict parting surface opening and suppress flash defects, and also a prerequisite for injection molding machine selection and mold structure design. According to mechanical calculation formulas, clamping force is determined by the molding projection area, cavity pressure, and safety factor. The molding projection area of this pallet is 1200 mm × 1000 mm. After converting to standard calculation units, the basic load is calculated with 50 MPa cavity pressure, and the theoretical required clamping force is obtained after superimposing the safety factor. Based on the calculation results, the theoretical clamping demand is relatively high after adding basic molding load and safety margin. In actual selection, 10%–20% process margin should be reserved, and large‑tonnage special injection equipment should be selected to offset the mold‑opening force generated by high‑pressure melt, prevent the expansion of mold‑closing gaps, and avoid defects such as burrs and oversized dimensions at the edge of plastic parts from the source.
Strength and Stiffness Calculation of Cavity Sidewalls
Heavy‑duty pallets are deep‑cavity plastic parts. Molten plastic exerts continuous and uniform lateral pressure on cavity sidewalls during injection, and the sidewalls are prone to deformation under long‑term bending. Therefore, design should focus on stiffness check supplemented by strength check. The classic mechanical model of rectangular cavity is adopted to calculate the minimum theoretical thickness of cavity sidewalls by substituting cavity depth, melt pressure, elastic modulus of steel, and allowable deformation into formulas. After verification, the theoretical safe thickness of sidewalls meets the basic requirements of compression resistance and deformation prevention. Considering fatigue loss in long‑term mass production, the sidewall thickness is appropriately increased in actual design to enhance structural redundancy. Synchronous stress check of sidewalls shows that the final stress value is far lower than the allowable stress of steel, with no hidden dangers of plastic deformation, cracking, or local collapse. It can stably resist lateral melt pressure and ensure long‑term stability of cavity dimensions.
Strength Check of Moving Mold Plate and Base Plate
Large pallet molds have a large overall span and cannot bear high‑pressure loads only by the rigidity of the mold plates themselves. A combined structure with support pillars must be configured to share the force. In accordance with industrial heavy‑duty mold design requirements, the maximum arrangement spacing of support pillars is controlled within 400 mm. Taking the limit span as the calculation condition, calculation is carried out using a simply supported beam model under uniform load. Combined with deflection control standards, the deformation per unit length is limited. By substituting material parameters and load data, the minimum theoretical thicknesses of the moving mold plate and base plate are calculated. In actual design, the mold plates are appropriately thickened according to processing and assembly requirements to reduce the risk of bending deformation under long‑term high pressure. Strength verification shows that the overall stress distribution of the mold plates is uniform, and the maximum bending stress is within a safe range, which can effectively avoid molding defects such as mold collapse at the center, surface depression of plastic parts, and uneven wall thickness.
Structural Check of Mold Support System
The support system is a key reinforcement structure of large injection molds, which directly shares the molding pressure borne by the mold plates and reduces the effective stress span. Support pillars are arranged uniformly and symmetrically to fully cover the entire molding area of the pallet. Aiming at high‑pressure stress positions such as hollow centers and concentrated ribs of the product, the support layout is densified to disperse concentrated loads. All support pillars maintain equal end heights and close fitting to avoid excessive local single‑point stress caused by assembly errors and reduce stress concentration. A reasonable support layout can greatly reduce the bending amplitude of mold plates, balance the overall stress state of the mold, improve the structural stability and fatigue resistance of the entire mold, and extend the service life in long‑term continuous production.
Conclusion
Affected by large product dimensions and high‑pressure molding conditions, the structural strength design of heavy‑duty plastic pallet injection molds cannot rely on traditional empirical judgment and must be accurately verified through systematic mechanical calculations. Through itemized calculation and verification of key parts such as clamping force, cavity sidewalls, moving mold plates, and support systems, the reasonable thickness, arrangement spacing, and load matching standards of each structural component are clarified. Reasonable strength design can effectively resist external forces such as mold opening, bending, and extrusion caused by high‑pressure melt, reduce mold deformation and damage failures, improve the molding quality of plastic parts, and reasonably control the consumption of mold steel to avoid cost waste and poor heat dissipation caused by excessive structural redundancy. The strength calculation logic in this paper conforms to the actual production conditions of heavy‑duty pallets, and the calculation data is accurate and reliable, providing practical theoretical reference for the structural design, optimization and upgrading, and rectification and reinforcement of old molds of the same type of large‑area injection molds.
