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Test Specification for Dimensional Stability of Precision Injection Molding

2025-07-15 12:00:42 Precision Injection Molding
In the field of precision manufacturing, dimensional stability of injection molded parts is a core element ensuring product assembly accuracy and functional reliability. With the development of high-end industries such as 5G communications and medical equipment, dimensional tolerance requirements for injection molded parts have reached the micrometer level. Establishing standardized dimensional stability testing procedures has become crucial for quality control. This specification details testing requirements and implementation methods based on industry practices and international standards, providing technical support for precision injection molding production.
I. Analysis of Influencing Factors

  1. Material Properties

Crystalline plastics such as POM (Polyoxymethylene) have a shrinkage rate of 1.5%-3% due to ordered molecular arrangement; amorphous plastics like PC (Polycarbonate) have a shrinkage rate of 0.5%-1.5%. Hygroscopic materials such as nylon will expand by 0.2%-0.5% in environments with humidity exceeding 60%, requiring attention to environmental humidity control.

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  1. Molding Process Parameters

If the barrel temperature exceeds the recommended range by 5-10°C, the shrinkage rate will increase by 0.1%-0.3%; a 10MPa fluctuation in holding pressure can cause dimensional deviation of precision parts to increase by more than 0.02mm. Insufficient injection and holding time will lead to inadequate filling and insufficient shrinkage compensation, affecting dimensional stability.

  1. Mold Precision

The machining tolerance of cavities and cores of high-precision molds should be controlled within ±0.01mm, ensured by processes such as wire electrical discharge machining. After long-term use, if the parting surface wears by more than 0.03mm, flash will occur; unreasonable design of cooling water channels will cause uneven cooling and local dimensional deviations of plastic parts.

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II. Testing Preparation Requirements

  1. Sample Preparation

Test samples should be selected from key functional areas. For complex structural parts, multi-cavity molds should be used for injection molding to cover dimensional changes in different cavity positions, with no less than 30 samples. Injection molding should use closed-loop control injection molding machines, with precise parameter setting and complete recording of process data.

  1. Environmental Control

The testing environment should comply with ISO 291 standards, with temperature at 23°C±2°C and relative humidity at 50%±5%. For PC materials, a temperature change of 5°C causes a deviation of 0.05%-0.1%; for nylon, a humidity change of 10% leads to a fluctuation of 0.1%-0.3%. A constant temperature and humidity chamber should be equipped and calibrated monthly to ensure temperature uniformity ≤±0.5°C and humidity uniformity ≤±3%.

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  1. Equipment Calibration

Measuring equipment should include coordinate measuring machines with accuracy ≥0.001mm and micrometers with accuracy ≥0.01mm. Before testing, calibration should be performed with grade 0 standard gauge blocks. Coordinate measuring machines should undergo full inspection every 3 months, and regular measuring tools should be calibrated monthly, with calibration records archived.
III. Testing Implementation Procedures

  1. Initial Measurement

After demolding, samples should be placed in the testing environment for 2 hours. Key dimensions should be measured with coordinate measuring machines, with each dimension measured 3 times to take the average. Data should be retained to four decimal places, and environmental parameters recorded to establish a database.

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  1. Aging Testing

Samples should be placed for 48 hours and re-measured to calculate the change rate for evaluating short-term stability; for parts used long-term, tracking re-measurements should be conducted at 7 days, 30 days, and 90 days to draw trend curves.

  1. Environmental and Mechanical Testing

  • High-temperature testing: Parts are measured after 24 hours in a 70°C oven and subsequent cooling.

  • High-humidity testing: Parts are measured within 2 hours after 48 hours in a 40°C, 90% humidity environment.

  • Thermal shock testing: Parts are inspected after 10 cycles between -40°C and 85°C.

  • Mechanical loading testing: Load-bearing parts are loaded with 50%-80% of the rated load for 10 minutes, with dimensional changes measured within 30 minutes after unloading.

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IV. Evaluation Standards and Technical Optimization

  1. Data Analysis

Data processing involves calculating standard deviation (key dimensions ≤0.01mm) and coefficient of variation (≤0.1%). Data from 25 consecutive production batches should be collected to evaluate process stability; if insufficient, inspect materials, processes, and molds.

  1. Qualification Criteria

  • Short-term change rate ≤0.1%, long-term (30 days) change rate ≤0.3%

  • Environmental testing change rate ≤0.5% without defects

  • Mechanical testing deviation within design tolerance

  1. Technical Improvement

Technical optimization can adopt online monitoring systems for real-time early warning, digital traceability of full-life cycle data, and machine learning models to predict dimensional changes and optimize processes, improving quality consistency.

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