Key points for customizing plastic shells of injection molds

In modern manufacturing, custom injection molded plastic housings are widely used in electronics, automotive, and home appliance industries. The customization process requires precise control over six core stages to balance product performance, cost, and production efficiency.

1. Product Design

Requirement Clarification

Deeply communicate with customers to confirm product functions (e.g., electronic housings need to fit internal component layout and meet heat dissipation needs), appearance (color, gloss, texture), and dimensional tolerances. Key dimensional tolerances for consumer electronics are typically controlled within ±0.1mm, while those for daily necessities can be relaxed to ±0.3mm. Additionally, clarify the service environment (temperature, humidity, chemical exposure) to lay a foundation for subsequent design and material selection.

Structural Design

Prioritize uniform wall thickness to avoid shrinkage marks and warping caused by uneven cooling. For local thick-walled areas, add reinforcing ribs—their thickness should not exceed 70% of the main wall thickness, and their distribution should avoid critical appearance areas. Determine draft angles based on surface requirements: 0.5°-1° for visible surfaces and 1°-3° for non-visible surfaces, with separate optimization for complex structures. All sharp corners must be rounded with a radius of ≥0.5mm to reduce stress concentration and mold fatigue cracks.

2. Material Selection

Performance Matching

Select materials based on application scenarios: For high-temperature environments (e.g., electric kettle or oven housings), use PP with a heat resistance of over 120°C; for automotive components (e.g., door handles, interior trim housings), choose ABS for its high impact resistance (Izod impact strength: 20-40 kJ/m²) and good dimensional stability; for transparent parts (e.g., lens housings, display covers), opt for PC, which offers high light transmittance (over 90%) and excellent impact resistance.

Cost & Environmental Compliance

On the premise of meeting product quality, select cost-effective materials—compare market prices of alternative materials and consider factors like processing difficulty (e.g., PC has higher melting temperature and requires more energy than PP). Prioritize eco-friendly materials: Food-contact housings must comply with standards such as FDA 21 CFR Part 177; electronic housings should use recyclable grades to align with global environmental regulations (e.g., EU RoHS).

3. Mold Design & Manufacturing

Mold Structure Design

Set the parting line at the minimum contour of the product to minimize flash and simplify mold processing. For multi-cavity molds (common for small to medium-sized housings), ensure cavity symmetry and consistent distance from the sprue to each cavity to guarantee uniform filling and stable product quality. Use inserts for complex or frequently modified structures (e.g., threaded holes, local protrusions) to improve mold reusability. Side core-pulling mechanisms (slides, lifters) must use wear-resistant materials (e.g., H13 steel) and precision guides to prevent jamming during operation.

Mold Manufacturing Process

Adopt 5-axis CNC machining centers for mold cavity processing, achieving a surface roughness of Ra0.2μm or lower to meet high-precision housing requirements. Use EDM (Electrical Discharge Machining) for complex features like deep holes (depth-to-diameter ratio >5:1) and narrow slots that are difficult to machine with traditional methods. For housings with uneven wall thickness, 3D-print molds with conformal cooling channels—this reduces cooling time by 20%-30% and minimizes product warping.

4. Injection Process Control

Parameter Setting

Adjust injection pressure based on material fluidity and product structure: Use 40-80 MPa for high-fluidity materials (e.g., PP) and 80-120 MPa for low-fluidity materials (e.g., PC) to prevent under-filling or flash. Control injection speed in stages: Slow speed for filling thin-walled areas (to avoid air entrapment) and moderate speed for thick-walled areas (to improve efficiency). Set holding pressure (60%-80% of injection pressure) and holding time (5-15 seconds, depending on wall thickness) to ensure dimensional accuracy and reduce shrinkage.

Temperature & Cooling Management

Control barrel temperature within the material’s melting range (e.g., 180-220°C for PP, 240-280°C for PC) to ensure full plasticization without degradation. Maintain mold temperature stability—use water-cooling systems for general materials (e.g., 40-60°C for ABS) and oil-heating systems for materials requiring high mold temperatures (e.g., 80-120°C for PC). Optimize cooling channel design: Place channels 10-15mm from the cavity surface and ensure uniform distribution; use conformal channels for complex housings to achieve consistent cooling.

5. Surface Treatment

Spraying

Choose appropriate coatings based on needs: Use acrylic coatings for matte/glossy effects and polyurethane coatings for enhanced wear resistance (pencil hardness ≥2H). Control spray thickness (20-50μm) and environment (temperature 20-25°C, humidity 40%-60%) to avoid orange peel or pinholes.

Electroplating

Apply electroplating (e.g., nickel-chrome) to achieve metallic luster for electronic or automotive housings. Follow strict pre-treatment steps (degreasing, etching, activation) to ensure coating adhesion (cross-cut test ≥4B). Control plating thickness (5-15μm) to balance appearance and conductivity.

Silk-Screening/Pad Printing

Use silk-screening for flat or regular curved surfaces (e.g., logos on remote control housings) and pad printing for irregular surfaces (e.g., buttons on curved instrument panels). Select ink compatible with the plastic material (e.g., solvent-based ink for ABS, UV-curable ink for PC) and cure at 60-80°C to ensure print durability.

6. Quality Inspection

Dimensional Inspection

Use coordinate measuring machines (CMM) with an accuracy of ±0.005mm to test key dimensions (e.g., hole positions, wall thickness) of samples. Conduct statistical process control (SPC) during mass production—sample 5% of each batch and monitor dimensional stability via control charts.

Appearance Inspection

Use automated optical inspection (AOI) equipment for high-volume production to detect defects like scratches, bubbles, and weld lines with a detection rate of over 99%. For high-end products, conduct manual visual inspections under standard light (D65) to check color consistency (ΔE < 1.5) and gloss uniformity.

Performance Testing

Conduct scenario-specific tests: For electronic housings, perform insulation resistance tests (≥100 MΩ at 500V DC) and temperature cycling tests (-40°C to 85°C, 50 cycles); for automotive housings, carry out impact tests (1.5m drop onto concrete) and weather resistance tests (1000h UV exposure); for food-contact housings, perform migration tests to ensure compliance with safety standards.