What matters should be paid attention to in plastic injection processing of household appliances?

The injection molding of plastic parts for household appliances should focus on three core objectives: up-to-standard appearance, reliable performance and stable production, and focus on controlling material selection, mold state, process parameters and post-treatment, so as to avoid product scrapping or service failure due to negligence of details.

First, material selection: match the usage scenarios and performance requirements of household appliances.

The characteristics of plastic materials directly determine the durability, safety and appearance of household appliances, so it is necessary to give priority to screening according to the use scenarios.

Select materials according to functional requirements

High-temperature resistant scenes (such as inner cover of rice cooker and oven parts): PBT, PA66 and other high-temperature resistant plastics are preferred, and the long-term use temperature should be 120-180℃ to avoid high-temperature deformation.

Impact-resistant scenes (such as washing machine shell and refrigerator drawer): Choose materials with good toughness such as ABS and HIPS, and the notched impact strength should be ≥ 20kJ/m to prevent falling or collision damage.

Food contact scenes (such as soymilk cup and microwave oven liner): Food-grade plastics (such as PP and PC) must be selected, which must meet the food safety standards of GB 4806.7 to avoid the precipitation of harmful substances.

Pay attention to material processing characteristics

Materials with poor fluidity (such as PC, POM): It is necessary to choose models with high melting index to avoid insufficient filling or bubbles during injection molding.

Moisture-absorbent materials (such as PA and PET): Before processing, they must be dried (at 80-120℃ for 2-4 hours), and the water content should be controlled below 0.05% to prevent crazing or brittle cracking after molding.

Second, mold control: to ensure the accuracy and appearance of plastic parts.

Mold is the "master" of injection molding, and its precision, surface state and structural design directly affect the quality of plastic parts, so it needs to be inspected and maintained.

Die precision and surface treatment

The dimensional tolerance of mold cavity and core should be controlled within ±0.02mm to ensure that the size of plastic parts meets the requirements of household appliances assembly (such as snap fit clearance and aperture deviation).

The mold surface of exterior parts should be polished to mirror surface (Ra≤0.02μm) to avoid the surface roughness of plastic parts caused by residual processing lines; At the same time, check whether the cavity has scratches and rust to prevent the plastic parts from indentation or impurities.

Mold structure optimization

Reasonable design of gate position: appearance parts (such as TV shell) need to set the gate on the invisible surface to avoid the influence of gate traces on the appearance; Large parts (such as refrigerator door panels) need to use multi-point gates to prevent uneven filling from causing shrinkage marks.

Reserved vent slot: vent slot with a depth of 0.01-0.02mm shall be opened at the position where the melt is finally filled (such as corners and uneven wall thickness) to avoid bubbles or scorching caused by trapped gas.

Check the ejection mechanism: the position of the ejector pin should avoid the appearance surface, and the ejection force should be uniform to prevent the plastic parts from being white, deformed or cracked when ejected.

Third, process parameter setting: avoid common molding defects

The matching degree of injection molding process parameters (temperature, pressure and time) is the key to reduce defects, which needs to be adjusted according to materials and plastic parts structure.

Temperature control

Barrel temperature: set according to material characteristics (for example, ABS barrel temperature is 200-240℃ and PP is 180-220℃) to avoid poor melt fluidity caused by too low temperature or material decomposition caused by too high temperature (for example, PC is easy to change color when it exceeds 300℃).

Mold temperature: the mold temperature of exterior parts needs to be increased (such as ABS mold temperature of 50-80℃) to reduce internal stress and avoid warpage of plastic parts; Thick-walled parts (such as air conditioning base) need to control the mold temperature at 40-60℃ to prevent uneven cooling from causing shrinkage cavity.

Pressure and velocity

Injection pressure: thin-walled parts (such as the control panel of washing machine) need to increase the injection pressure (80-120MPa) to ensure that the cavity can be filled quickly; The pressure of thick-walled parts should be reduced (50-80MPa) to avoid the increase of internal stress caused by excessive pressure keeping.

Injection speed: complex structural parts (such as household appliances buttons) need to adopt sectional speed (slow first and then fast) to prevent the melt from washing the cavity and causing flash; Appearance parts should be injected at a uniform speed to avoid flow marks.

Holding pressure and cooling time

Holding time: the holding time of thick-walled parts needs to be extended (10-20 seconds) to supplement melt shrinkage and reduce shrinkage; The holding time of thin-walled parts should be controlled at 2-5 seconds to avoid cracks in plastic parts caused by excessive holding.

Cooling time: according to the thickness setting of plastic parts (the cooling time will increase by 1-2 seconds for every 1mm increase in thickness), ensure that the plastic parts are completely solidified and shaped, and avoid deformation after demoulding (for example, PP parts are easily bent due to insufficient cooling).

4. Post-treatment and quality inspection: ensure that the finished products meet the standards.

Treatment and inspection after injection molding is the last quality pass, which needs to cover appearance, performance and safety.

Post processing

Deburring: appearance parts need to be manually or mechanically stripped of gates and flash edges to ensure smooth edges and avoid scratching users; Assembly parts (such as around screw holes) need to be deburred to prevent the assembly accuracy from being affected.

Eliminate internal stress: anneal plastic parts (such as PC transparent parts) that are prone to warping (temperature 80-120℃, heat preservation for 1-2 hours) to release internal stress and reduce the risk of cracking during use.

quality detection

Appearance inspection: Check whether the plastic parts have defects such as lack of materials, flash, crazing, color difference, etc. The color difference of the appearance parts should be controlled at ΔE≤1.5(CIE LAB standard).

Dimension detection: use calipers, micrometers or coordinate measuring instruments to detect key dimensions (such as assembly hole diameter and buckle height) to ensure that the design tolerance is met.

Performance sampling inspection: the impact strength, heat resistance (such as high temperature baking test) and aging resistance (such as ultraviolet aging test) of plastic parts are sampled regularly to ensure that the long-term use requirements of household appliances are met.