Injection Molded Parts Quality & Acceptance Standards | Full Guide
Complete acceptance standards for conventional injection molded parts: surface classes (A/B/C), inspection conditions, defects, tolerances, gate vestiges & deformation rules.
Introduction
When you order plastic injection molded parts, clear quality and acceptance standards prevent disputes, ensure consistency, and guarantee your parts meet functional and appearance requirements. Every buyer and product designer needs a standardized checklist to inspect finished parts.
This guide covers practical, industry-proven acceptance rules for conventional injection molded components — including surface classification, inspection methods, defect limits, dimensional tolerances, and gate requirements.
1. Surface Classification for Injection Molded Parts
First, define your part’s visible surfaces to apply the right quality level.
Class A Surfaces
- Highest aesthetic and finish requirements
- Normally visible during daily use
- Used in automotive, consumer electronics, premium goods
- Must be smooth, defect-free, and high-quality
Class B Surfaces
- Exposed but not prominent
- Side, back, or bottom areas not in direct view
- Moderate appearance standards
Class C Surfaces
- Hidden, internal, or assembly-contact surfaces
- Lowest cosmetic requirements; focus on function
2. Standard Inspection Conditions for Injection Molding Quality
Use these universal inspection rules for consistent results:
- Function first: All standards apply only when function is normal.
- Visual distance: 30 cm for 3–5 seconds; recheck at 50 cm for 3–7 seconds.
- Lighting: Factory fluorescent light.
- Vision: ≥ 0.7 eyesight.
- Angle: Perpendicular + 45° above/below.
3. Key Acceptance Criteria for Common Defects
Below are the standard defect limits for conventional injection molded parts.
表格
| Defect Type | Class A | Class B | Class C |
|---|---|---|---|
| Cracks | Not allowed | Not allowed | Allowed if no function/appearance impact |
| Short Shots | Not allowed | Not allowed | Minor allowed if safe & functional |
| Silver Streaks | Match engineering sample | Match engineering sample | Match engineering sample |
| Sink Marks | No visible/tactile marks | No visible/tactile marks | No visible/tactile marks |
| Dark Spots / Stains | ≤2 spots, >100mm apart, <0.4mm² | ≤3 spots, >100mm apart, <0.5mm² | ≤4 spots, >100mm apart, <0.9mm² |
| Scratches | ≤8mm long, ≤0.05mm wide | ≤10mm long, ≤0.1mm wide | ≤2.5mm long, ≤0.15mm wide |
| Ejection Marks | Not allowed | Not allowed | Allowed if no visual/function issue |
4. Flash (Burrs) Acceptance Rules
- No flash allowed on button holes, switch holes, moving mating areas.
- Internal structural flash must not affect assembly.
- Exposed/unsafe areas must be smooth and non-scratchy.
5. Deformation Standards
- Base shell & face shell deformation: ≤ 0.3 mm when measured on a flat platform.
- Other parts: acceptable if they do not affect assembly or function.
6. Dimensional Tolerance Standards
Use this table for general-purpose plastic parts:
表格
| Dimension Range (mm) | Tolerance (mm) |
|---|---|
| 0 – 10 | ±0.05 |
| 10.1 – 50 | ±0.10 |
| 50.1 – 100 | ±0.15 |
| 100.1 – 200 | ±0.20 |
| Over 200 | ±0.25 |
7. Gate Vestige Standards
- Exposed / safety surfaces: Gate must be flat, no sharp edges.
- Hidden but assembly-critical: ≤ 0.5 mm, no function interference.
- Internal / non-exposed: ≤ 1.5 mm, free break shape allowed.
Conclusion
These are general acceptance standards for conventional injection molded parts. For your specific project, you must create a customized checklist covering:
- Packaging and labeling
- Surface treatment (texture, painting, plating)
- Assembly fit
- Performance testing
- Cosmetic intent
Clear standards help suppliers deliver consistent quality and avoid costly rejects.
FAQ
What are Class A surfaces in injection molding?
High-visibility cosmetic surfaces requiring perfect finish.
How to inspect injection molded parts?
Use 30cm distance, factory lighting, and 45° viewing angles.
What is the standard sink mark limit?
No visible or tactile sink marks on Class A/B/C surfaces.
What is typical dimensional tolerance for plastic parts?
From ±0.05 mm (small parts) to ±0.25 mm (large parts).
