The key to controlling shrinkage in plastic pellet small appliance casings lies in the coordinated efforts of material selection, process optimization, mold design, and structural adaptation. This minimizes volumetric shrinkage variations caused by melt cooling, crystallization, or solidification during the plastic molding process, ensuring that the final dimensions of the casing precisely match the internal component assembly space. This prevents excessive shrinkage that results in insufficient assembly clearance (making it impossible to fit components) or uneven shrinkage that results in casing deformation (making components loose after assembly). Small appliance casings are often thin-walled and complex structures, housing precision components such as circuit boards, motors, and switches. Therefore, shrinkage control requires comprehensive process control from source material to final molding, ensuring both stable and uniform shrinkage.
Precise material selection and modification are fundamental to shrinkage control. Low-shrinkage plastic pellets should be selected based on the functional requirements of the casing, and additives should be used to adjust shrinkage trends. The shrinkage rates of different types of plastic pellets vary significantly. For example, crystalline resins (such as PP and PE) shrink significantly during cooling due to the alignment of their crystals, while amorphous resins (such as ABS and PS) shrink relatively less. Therefore, amorphous or low-crystallinity resins are preferred for housings requiring high dimensional accuracy, such as those housing micromotors. If crystalline resins are required to meet requirements for impact resistance and heat resistance, fillers (such as glass fiber and mineral powder) or toughening agents are added to the plastic pellets. Fillers provide physical support and reduce volumetric shrinkage during resin crystallization, while toughening agents improve melt flow, ensuring more uniform shrinkage during cooling and avoiding concentrated shrinkage. Furthermore, the particle size distribution of the plastic pellets must be uniform to avoid uneven mixing during melting due to particle size differences, which in turn leads to localized shrinkage differences and affects the dimensional accuracy of the housing.
Precise control of injection molding process parameters can effectively suppress shrinkage fluctuations. By optimizing parameters such as temperature, pressure, and time, shrinkage during the melt cooling and solidification process within the mold can be more controlled. The barrel temperature must be set according to the melting characteristics of the plastic pellets to ensure complete melting and uniform melt temperature. This avoids local overheating that could lead to resin degradation (causing abnormal shrinkage) or incomplete melting (resulting in uneven filling and shrinkage). Matching injection pressure and holding time is crucial: injection pressure must be sufficient to push the melt into the mold cavity to avoid partial underfilling of the shell (indirectly increasing shrinkage). The holding time should be adjusted according to the shell wall thickness. As the melt begins to cool and shrink, continued holding pressure replenishes the cavity, offsetting some of the shrinkage and minimizing final dimensional deviation. Controlling cooling time is equally important. Cooling too quickly will cause the shell surface to solidify quickly, and continued shrinkage of the melt inside can easily generate internal stress (causing later deformation and compromising assembly). Cooling too slowly will extend the production cycle and may increase shrinkage. Therefore, an appropriate cooling time should be set based on the resin's thermal conductivity to ensure simultaneous cooling of the shell interior and exterior for more uniform shrinkage.
Mold design adaptability is key to controlling shrinkage. By optimizing cavity dimensions, gates, and cooling systems, the direction and magnitude of shell shrinkage during the molding process can be controlled. When designing the mold cavity dimensions, allow for shrinkage of the plastic pellets. Based on the theoretical shrinkage of the selected resin, the cavity dimensions should be appropriately enlarged to ensure that the shell's dimensions meet assembly requirements after cooling. This avoids the shell being undersized (making it impossible to fit components) due to the cavity dimensions not accounting for shrinkage. The location and number of gates should be determined based on the shell's structural layout. Symmetrical or multi-point gates are preferred to ensure uniform melt filling of the cavity from multiple directions, minimizing shrinkage variations caused by directional differences (e.g., a single-direction gate can result in less shrinkage near the gate and more shrinkage farther away). The mold's cooling system should be tailored to the shell's shape, with cooling water lines evenly distributed around the cavity. In particular, the density of cooling water lines should be increased in areas with thicker walls (such as shell ribs and buckles). This prevents slower cooling and greater shrinkage in thicker areas, which could lead to differential shrinkage compared to thinner areas. This could cause shell deformation and hinder internal component assembly.
Optimizing the shell's structural design can mitigate assembly issues caused by differential shrinkage. By adjusting wall thickness and adding auxiliary structures, shrinkage can be more easily controlled at the structural level. The housing wall thickness should be designed to be uniform, avoiding significant thickness variations (such as sudden increases or decreases in thickness). This is because uneven wall thickness leads to different cooling rates—thicker areas cool slower and shrink more rapidly, while thinner areas cool faster and shrink less. This differential shrinkage can easily cause the housing to warp or dent, leading to misalignment of internal components. Where thick walls are necessary (such as screw-mounted cylinders), a transition radius or gradual wall thickness gradient should be designed around them to smooth the thickness change and reduce shrinkage stress concentration. Hollow structures (such as thin-walled cylinders) can also be designed within thick walls to reduce material usage, accelerate cooling, and minimize shrinkage differences with surrounding thinner areas. Furthermore, internal housing components such as assembly clips and locating posts should have appropriate draft angles to prevent deformation caused by post-molding shrinkage, which could affect component fit and ensure smooth insertion into the locating structures.
Post-injection molding secondary processing can further correct for shrinkage variations and relieve internal stresses, ensuring long-term dimensional stability of the housing and adapting to the long-term assembly requirements of internal components. For housings with extremely high shrinkage control requirements (such as those housing precision circuit boards), annealing is performed after molding. This involves placing the housing in a constant-temperature oven at a temperature slightly below the resin's glass transition temperature for a period of time. This allows internal stresses to gradually release and stabilizes the resin's molecular alignment, minimizing secondary shrinkage during use (preventing component loosening due to secondary shrinkage). Some housings also undergo surface shaping. Using a specialized fixture, the housing is secured to a standard-sized tooling fixture and gently heated to fit the mold. This corrects shrinkage deviations introduced during the molding process and ensures that critical assembly dimensions (such as apertures and spacing) are precisely met.
End-to-end quality monitoring and parameter feedback mechanisms are a closed-loop process to ensure stable shrinkage. Through real-time monitoring and adjustment, uncontrolled shrinkage is avoided during mass production. Before mass production begins, a small number of samples are produced through trial molds to test the shrinkage of key housing dimensions. Based on the actual shrinkage, the mold cavity size and injection molding parameters (such as fine-tuning the hold time and cooling temperature) are adjusted until the sample dimensions meet assembly requirements. During batch production, shell dimensions must be regularly sampled and inspected. Shrinkage trends must be monitored using dimensional measuring instruments (such as calipers and projectors). If abnormal shrinkage is detected (e.g., dimensional deviations outside the allowable range), the cause must be promptly investigated. This could be due to batch differences in plastic pellets (e.g., melt index variations), injection molding machine parameter drift (e.g., barrel temperature fluctuations), or mold wear (e.g., increased cavity size). Targeted adjustments should be made (e.g., pellet batch changes, machine parameter calibration, mold repair) to ensure that the shrinkage of each batch of shells is stably controlled within the range suitable for internal component assembly.
