When customizing plastic nut accessories, the design of the thread structure is a core element determining the connection strength. Plastic materials differ fundamentally from metals; plastic has a low modulus of elasticity, significant creep characteristics, and is easily deformed by temperature and stress. Therefore, optimizing thread geometry, fit methods, and molding processes is necessary to achieve connection reliability comparable to or even higher than metal fasteners. This process requires comprehensive consideration of multiple dimensions, including thread profile, pitch, thread height, clearance, and material properties.
The choice of thread profile directly affects stress distribution and connection stability. Common triangular threads, due to their larger thread angle, generate a larger radial force during tightening, creating a tight mechanical engagement between the fasteners, making them suitable for applications subject to axial tensile and shear forces. Trapezoidal threads, with their smaller thread angle and higher root strength, are more suitable for heavy-load or frequently disassembled applications. For plastic nuts, if both strength and fatigue resistance are required, a modified circular arc thread profile can be considered. This reduces stress concentration through a smooth transition and distributes the load using the curved contact surface, preventing localized cracking. Furthermore, the thread profile design must match the material properties of the connector. For example, when mating with a metal bolt, the thread profile of a plastic nut should be slightly wider than that of the metal thread to compensate for the creep deformation of the plastic and prevent thread breakage due to interference fit.
Matching the pitch and thread height is a key parameter for improving connection strength. If the pitch is too large, the number of threads per unit length decreases, increasing the load on each thread and making it prone to root breakage or stripping. If the pitch is too small, insufficient plastic flow may lead to thread forming defects, such as missing teeth or burrs. Therefore, an appropriate pitch range must be selected based on the melt flow index (MFI) of the plastic and the nut size. The thread height design also needs to balance strength and formability. Too high a thread height will weaken the nut wall thickness and reduce overall strength; too low a thread height will result in insufficient contact area, affecting the tightness of the connection. Typically, the thread height of a plastic nut can be 70%-80% of that of a metal thread, with rounded corners used to enhance root strength.
Controlling the fit clearance is a challenge in plastic nut design. Because the coefficient of thermal expansion of plastics is much higher than that of metals, the clearance between plastic nuts and metal bolts will change under temperature changes or long-term stress. If the initial clearance is too small, the threads may jam or crack due to obstructed expansion at high temperatures or under stress; if the clearance is too large, the connection will loosen, reducing sealing and vibration resistance. Therefore, appropriate elastic deformation space must be reserved in the design, for example, by reducing the thread pitch diameter or using a tapered fit, so that the nut elastically contracts when tightened, forming a self-locking effect. At the same time, micro-textures or coatings can be applied to the thread surface to increase friction and compensate for the risk of loosening caused by the clearance.
The thread molding process has a significant impact on the connection strength. During injection molding, the flow direction of the melt in the mold cavity affects the molecular orientation of the threads; the strength is higher along the flow direction, while weak areas are more likely to appear perpendicular to it. Therefore, the gate position and runner design need to be optimized to ensure that the melt evenly fills the thread area and reduces orientation differences. In addition, the thread core of the mold must have high precision and wear resistance to avoid thread size deviations due to core wear, which would affect the fitting accuracy. For nuts requiring high strength, secondary molding or post-processing techniques, such as heat treatment or surface strengthening, can be used to eliminate internal stress and improve the crack resistance of the thread root.
The coordinated design of material properties and thread structure is crucial. Different plastics have significantly different mechanical properties. For example, nylon (PA) has high strength and wear resistance, making it suitable for heavy loads; polypropylene (PP) has good toughness but lower strength, requiring modification by thickening the thread teeth or adding fiber reinforcement; polyoxymethylene (POM) has good self-lubricating properties, making it suitable for high-frequency assembly and disassembly scenarios. During design, thread parameters need to be adjusted according to material properties. For example, for brittle plastics, the thread angle can be reduced or the fillet radius increased to avoid stress concentration; for highly fluid plastics, the pitch can be appropriately reduced to improve molding accuracy.
Environmental factors have a significant impact on the strength of threaded connections. In humid or corrosive environments, plastics may absorb moisture and expand or undergo chemical degradation, leading to loosening of the thread fit. In this case, it is necessary to improve the material's environmental resistance by adding moisture-proofing agents or modifiers, or to set sealing structures on the thread surface, such as O-ring grooves or labyrinth seals, to prevent media intrusion. In high-temperature environments, creep and relaxation of plastics intensify, necessitating measures such as increasing thread preload or employing self-locking structures (e.g., Spiral threads) to compensate for the risk of loosening under long-term stress.
The thread structure design of customized plastic nut accessories must be based on material properties. By optimizing parameters such as thread profile, pitch, thread height, fit clearance, and molding process, a balance between uniform stress distribution, elastic self-locking, and environmental resistance can be achieved. This process requires continuous iterative adjustments, combining simulation analysis and actual testing, to ultimately obtain a high-strength threaded connection solution that meets specific operating conditions.
