How to control the assembly and disassembly force of injection molded parts by adjusting the machine
The assembly and disassembly force of injection molded parts is a key indicator of product assembly performance, directly impacting user experience and product reliability. Excessive assembly and disassembly force can easily lead to assembly difficulties or component damage, while insufficient force can cause loose connections and functional failure. Precise adjustment of injection molding process parameters effectively controls the product’s dimensional accuracy, surface roughness, and microstructure, thereby achieving stable control of assembly and disassembly force. This process requires systematic optimization of injection molding parameters based on material properties, mold structure, and product design requirements to ensure that the assembly and disassembly force remains within the design tolerance (typically 5-50N, depending on product specifications).
Holding pressure and holding time are key parameters for controlling assembly and removal forces, significantly impacting the dimensional accuracy and density of the part. Increasing holding pressure allows the melt to fully compensate under high pressure, bringing the part’s dimensions closer to the mold cavity ( within ±0.01mm). This increases the actual interference fit at the interference fit, leading to a corresponding increase in assembly and removal forces. For example, for a polyethylene (PE) fastener, increasing the holding pressure from 50 bar to 70 bar increases the interference fit between the clip and the slot from 0.05 mm to 0.08 mm, and the assembly and removal force increases from 20 N to 35 N. The holding time must be aligned with the material’s cooling rate. For crystalline plastics (such as PP), the holding time should be sufficiently long (10-20 seconds) to ensure sufficient crystallization. For amorphous plastics (such as PS), the holding time can be shortened (5-10 seconds). If the assembly and removal forces are too high, gradually reduce the holding pressure (by 5-10 bar at a time) and shorten the holding time (by 1-2 seconds at a time) until the assembly and removal forces meet the specified standards. It should be noted that the holding pressure should not be lower than 50% of the injection pressure, otherwise sink marks will easily occur.
Adjusting mold temperature indirectly affects the part’s cooling rate and crystallinity, which in turn indirectly affects assembly and disassembly force. Increasing mold temperature slows part cooling. Crystalline plastics increase their crystallinity (for example, PP increases its crystallinity from 50% to 70%), resulting in more regular molecular alignment and increased part hardness, leading to a corresponding increase in assembly and disassembly force. For amorphous plastics, increasing mold temperature reduces internal stress, making the part softer and slightly decreasing assembly and disassembly force. For example, in the mating of a polyoxymethylene (POM) gear and sleeve, increasing mold temperature from 60°C to 80°C increases the gear tooth surface hardness from Shore D75 to D80, and the assembly force increases from 40N to 55N. During actual machine adjustment, fine-tuning mold temperature (±5°C increments) can achieve precise control of assembly and disassembly force. If the assembly and disassembly force is too high, reduce the mold temperature (for crystalline plastics) or increase it (for amorphous plastics); if the assembly and disassembly force is too low, do the opposite. Mold temperature uniformity is also crucial, with temperature differences between different areas maintained at ≤2°C to prevent fluctuations in assembly and disassembly force due to localized dimensional deviations in the part.
Injection speed and melt temperature affect the surface quality and internal stress of the part, thus altering the stability of assembly and disassembly forces. Excessively fast injection speeds (>80 mm/s) can lead to severe melt shear, resulting in weld marks or flash on the part surface and increased assembly resistance. Excessively slow injection speeds (<30 mm/s) can lead to underfilling, resulting in undersized parts and reduced assembly and disassembly forces. For example, in the snap-fit structure of an ABS housing, when the injection speed was reduced from 60 mm/s to 40 mm/s, the weld mark strength at the base of the snap-fit decreased from 25 MPa to 18 MPa, and the assembly and disassembly force fluctuation range increased from ±3 N to ±5 N. The melt temperature must be controlled within the material's optimal processing range (e.g., 280-300°C for PC). Excessively high temperatures can cause material degradation, the appearance of silver streaks on the surface, and an increased coefficient of friction. Excessively low temperatures can lead to poor melt flowability, a rough surface (Ra values exceeding 1.6 μm), and increased assembly and disassembly forces. Through multi-stage injection speed (such as low-speed filling the gate, high-speed filling the cavity, and low-speed pressure holding) and precise temperature control (±1°C), the surface roughness of the product can be controlled within Ra0.4~0.8μm, significantly reducing the fluctuation of assembly and disassembly forces.
Properly setting the cooling time can reduce part warping and deformation and ensure consistent assembly and removal force. Insufficient cooling time can result in excessively high temperatures during demolding (e.g., exceeding Tg + 20°C), which can easily cause post-shrinkage, leading to dimensional instability and assembly and removal force deviations exceeding ±10N. Excessive cooling time can reduce production efficiency and increase energy consumption. For example, in a polyamide (PA6) plug mold, increasing the cooling time from 20 seconds to 30 seconds reduced the roundness deviation of the plug pin hole from 0.03mm to 0.01mm, and the pin-hole clearance fluctuation decreased from ±0.02mm to ±0.005mm, improving assembly and removal force stability by 60%. When adjusting the machine, the minimum cooling time should be calculated based on the part wall thickness (typically 1.5 to 2 times the wall thickness) and verified through mold trials: when the assembly and removal force deviation for 10 consecutive molded parts is ≤±2N, the cooling time setting is appropriate. For complex structural parts, uneven cooling can be used (extending cooling for 3 to 5 seconds in thick-walled areas) to reduce the impact of local deformation on assembly and disassembly forces.
Back pressure and screw speed indirectly influence assembly and removal force by regulating the plasticization quality of the melt. Increasing back pressure (5-15 bar) results in more shear and more uniform mixing of the melt, increasing product density (for example, the density of a PP product increases from 0.90 g/cm³ to 0.92 g/cm³), enhancing dimensional stability, and reducing assembly and removal force fluctuations. However, excessive back pressure increases the melt temperature (by 2-3°C for every 5 bar increase in back pressure), increasing the risk of degradation. The screw speed must be matched to the back pressure. Excessive speed (>150 r/min) can lead to melt overheating, while too slow speed (<50 r/min) can lead to inadequate plasticization. For example, in the case of a clip on a polycarbonate (PC) lampshade, when the back pressure increased from 8 bar to 12 bar and the screw speed decreased from 100 r/min to 80 r/min, the standard deviation of the clip dimensions decreased from 0.02 mm to 0.01 mm, and the standard deviation of the assembly and removal force decreased from 1.5 N to 0.8 N. If the assembly and disassembly force consistency is poor, gradually increase the back pressure (by 2 to 3 bar each time) while reducing the screw speed (by 10 to 20 rpm each time) until the ideal state is achieved. It is important to note that the back pressure and screw speed adjustments must be performed simultaneously to avoid fluctuations in melt quality caused by separate adjustments.
