The injection molding accuracy of home appliance plastic parts and the stability of the assembly clearance between metal components require coordinated control throughout the entire process, from material selection and mold design to process control and post-processing. Precision in each step directly impacts the final assembly result. First, during the material selection stage, the shrinkage characteristics of the plastic must be fully considered. Different types of plastic (such as PP, ABS, and PC) have significantly different shrinkage rates after molding due to differences in molecular structure. Even the same material can experience shrinkage changes after adding modified ingredients such as glass fiber and mineral fillers. To match the fixed dimensions of metal components, materials with stable and low shrinkage fluctuations should be prioritized. Alternatively, material modification (such as adding a specific ratio of glass fiber to reduce shrinkage) should be employed. Pre-calculating and compensating for post-molding shrinkage can prevent uneven shrinkage that could lead to dimensional deviations in the part and compromise the assembly clearance with the metal component.
Molds, as the core component of injection molding, ensure that their machining precision is fundamental to ensuring the dimensional accuracy of plastic parts. The mold's cavity and core must be manufactured using high-precision machining equipment (such as CNC machining centers and EDM machines). Key dimensions (such as aperture, wall thickness, and mating surface tolerances) must be controlled within micron limits. The fit of the parting surface must also be strictly controlled to avoid plastic overflow due to excessive mold clearance or part shorts due to overly tight fit. Furthermore, the design of the mold's temperature control system is crucial. Cooling water channels must be strategically arranged based on the plastic part's structure (e.g., thin-walled and thick-walled areas) to ensure uniform temperature across the mold. This prevents internal stresses in the plastic caused by localized cooling rate differences during the molding process, which can lead to warping and deformation after cooling. For example, uneven cooling of thick-walled parts can easily result in a central shrinkage depression, directly affecting the clearance with metal shafts. Uniform cooling ensures more consistent plastic shrinkage and more stable part dimensions.
Precise control of injection molding process parameters is crucial for achieving part precision. During the injection molding process, parameters such as melt temperature, injection pressure, holding time, and cooling rate must be dynamically adjusted based on material properties and part structure. If the melt temperature is too high, the plastic molecular chain becomes too fluid, which can lead to increased shrinkage after molding. If the holding pressure is insufficient or the holding time is too short, the plastic cannot be fully compacted within the mold cavity, which can also cause excessive shrinkage. In actual production, mold optimization is performed to determine the optimal process parameter range. A closed-loop control system monitors temperature and pressure changes during the injection molding process in real time. Any deviations from the set values are automatically adjusted to avoid dimensional fluctuations in batches of parts due to human error. For example, for plastic screw holes designed to mate with metal screws, precise holding time control is required to ensure a full, precisely dimensioned inner wall. This ensures smooth screw insertion without causing loosening due to oversized holes.
Post-processing of home appliance plastic parts also plays a complementary role in ensuring assembly gap stability. After injection molding, some parts may experience slow deformation during use due to incomplete internal stress release, impacting long-term assembly accuracy. Annealing is then necessary. This involves subjecting the part to a specific temperature for a period of time to allow the plastic's internal molecular chains to realign, releasing internal stress and stabilizing the part's dimensions. For parts requiring high-precision fit (such as plastic gears meshing with metal gears), minor secondary processing, such as cutting and polishing, is required to precisely adjust critical mating surfaces and ensure the clearance with the metal component is within the design tolerances.
In addition, tolerance matching during assembly must be carefully considered in advance. During the design phase, appropriate assembly clearances must be reserved based on the plastic's thermal expansion coefficient and the metal's physical properties. For example, when a home appliance generates heat, plastic parts undergo dimensional changes due to thermal expansion and contraction. If the initial clearance is too small, it can cause the part to become stuck; if it is too large, it can affect assembly stability. Furthermore, through full-process dimensional inspection (such as using a coordinate measuring machine to sample key dimensions of each batch of parts), dimensional deviations can be promptly identified and retroactively adjusted in previous steps. This creates a closed loop of "design-production-inspection-optimization," ultimately ensuring that the injection molding precision of home appliance plastic parts consistently matches the assembly requirements of the metal components, achieving long-term clearance stability.
