Common Defects in Injection Molding Products And Solutions

Table of Contents

I. Appearance Defects 1. Shrinkage 2. Weld Lines 3. Short Shot

II. Structural Defects 1. Warping 2. Internal Bubbles

III. Dimensional Defects 1. Dimensional Tolerance Issues

IV. Preventive Measures 1. Mold Design Phase 2. Production Preparation Phase 3. Production Process Control

V. Quality Control Key Points

Introduction

Plastic injection molding is one of the core processes in modern manufacturing, and its quality defects directly impact product performance, production costs, and brand reputation. According to statistics, the global injection molding industry incurs waste losses of up to $12 billion annually due to defects. Based on cutting-edge industry research and Alpine Mold's 23 years of practical experience, this article systematically analyzes the causes and solutions for common defects in injection molding products.

I. Appearance Defects

1. Shrinkage

Shrinkage is one of the most common appearance defects in injection molding products, with approximately 30% of quality issues related to it.

Phenomenon: Shrinkage is manifested as visible depressions on the product surface or actual dimensions smaller than the design specifications. This phenomenon often occurs in thick-walled areas, near ribs, or other structurally thicker sections, with deviations typically ranging from 0.5 to 2.0 mm.

Causes:

• Unreasonable cooling system design, leading to cooling rate differences greater than 15%.

• Improper holding pressure settings, with holding pressure below 80% of the recommended value.

• Material temperature exceeding recommended limits, with common PP temperatures exceeding 230°C.

Solutions:
Alpine Mold recommends the following measures:

• Optimize mold cooling system design to ensure uniform cooling exceeds 90%.

• Adjust holding pressure parameters: recommended holding time is wall thickness (mm) × 3 seconds, with holding pressure maintained at 80-90% of injection pressure.

• Control material temperature within the supplier's recommended range, e.g., for PP, between 190-220°C.

2. Weld Lines

Weld lines not only affect product appearance but can also reduce product strength by 20-30%.

Phenomenon: Weld lines are common defects on the surfaces of injection molded products, appearing as visible joining marks where two or more melt streams converge without fully integrating. These marks can compromise both the appearance and mechanical performance of the product, particularly under tensile or impact loads, making them weak points.

Causes and Solutions:

Cause	Specific Parameters	Improvement Measures
Unreasonable runner design	Convergence angle > 90°	Optimize to a 45-60° angle
Low mold temperature	Below recommended by 30%	Raise to standard range of 80-100°C
Improper injection speed	Speed variation > 15%	Stabilize at 40-60 mm/s

3. Short Shot

The short shot issue directly leads to product scrap and needs to be emphasized in prevention. Alpine Mold's data shows that precise mold design can control the short shot rate to below 0.1%.

Phenomenon: Short shot is a common defect in injection molding products, characterized by partial or complete failure to fill the product, resulting in material shortages. This issue not only affects the integrity and appearance of the product but may also lead to functional deficiencies, making the product unusable in severe cases.

Key Control Parameters:

• Injection pressure: Recommended to maintain at 80-90% of material viscosity requirements.

• Material temperature: Ensure it reaches the optimal flow temperature, typically 20-30°C higher than the recommended minimum temperature.

• Mold temperature: Adjust according to material characteristics, generally controlled at the midpoint of the supplier's suggested range.

II. Structural Defects

1. Warping

Phenomenon: Warping is manifested as bending or twisting of the product, deviating from the designed shape. This defect severely affects the dimensional accuracy and appearance of the product, potentially leading to sealing issues or jamming during assembly.

According to Alpine Mold's experience, the following parameters are crucial for controlling warping:

Control Item	Standard Range	Actual Control Recommendations
Wall thickness uniformity	±10%	Control critical areas to ±5%
Cooling temperature difference	≤15°C	Recommended control at 8-12°C
Internal stress control	-	Use stepwise cooling

2. Internal Bubbles

Phenomenon: Internal bubbles are common structural defects in injection molding products, appearing as bubbles or cavities within the product, especially noticeable in transparent or high-precision items. This defect can lead to decreased mechanical performance, poor appearance, or functional failures.

Alpine Mold recommends the following measures:

• Material pretreatment: Control moisture content below 0.02%

• Venting system: Set vent grooves of 0.02-0.03 mm deep at the mold joining surfaces

• Injection speed: First stage 20-30%, second stage 60-80%, last stage 40-50%

III. Dimensional Defects

According to ISO standards, the dimensional tolerance ranges for different precision levels are:

• Precision level: ±0.05 mm

• General level: ±0.1 mm

• Rough level: ±0.2 mm

Phenomenon: Dimensional tolerance issues refer to actual dimensions deviating from design requirements, exceeding tolerance limits. This may manifest as oversized, undersized, or uneven dimensions, directly impacting product assembly and usability.

Alpine Mold strictly enforces the following standards during injection mold manufacturing:

• Mold accuracy: Control cavity accuracy within ±0.02 mm

• Shrinkage rate: Ensure prediction accuracy of shrinkage rates reaches over 95% through scientific calculations and experience databases

• Process parameters: Establish a digital parameter management system to ensure parameter fluctuations are controlled within ±2%

IV. Preventive Measures

In Alpine Mold's quality management system, the following preventive measures are particularly emphasized:

Mold Design Phase

• Use CAE flow analysis to optimize the runner system, ensuring filling uniformity ≥95%

• Cooling system layout must ensure temperature differences are controlled within ±5°C

• Product structure DFM analysis to ensure moldability scores ≥85 points

Production Preparation Phase

• Raw material inspection: 100% detection of key indicators such as moisture, melt flow index, and impurity content

• Equipment maintenance: Precision inspection cycle for key components not exceeding 500 hours

• Standard process documents: Include over 30 critical control points

Production Process Control

Alpine Mold employs a digital production management system to achieve:

• Real-time monitoring of process parameters, with deviations over ±3% triggering automatic alerts

• Hourly sampling of product quality, establishing a quality tracking database

• Response time for abnormal situations controlled within 15 minutes

V. Quality Control Key Points

Key Parameter Monitoring Standards:

• Temperature: Material temperature fluctuation ±5°C, mold temperature fluctuation ±2°C

• Pressure: Injection pressure fluctuation ±3%, holding pressure fluctuation ±2%

• Time: Injection time fluctuation ±0.1 s, holding time fluctuation ±0.2 s, cooling time fluctuation ±1 s

Testing Methods and Standards:

• Appearance inspection: Use standard light sources to ensure illumination of 800-1000 lux

• Dimensional measurement: 100% detection of critical dimensions, general dimensions AQL 2.5 standard sampling

• Performance testing: Execute mechanical, weather resistance, and other tests according to product requirements

By strictly implementing the above control measures, Alpine Mold helps clients keep product defect rates below 0.1%, ensuring product quality remains at the industry's leading level.

About Alpine Mold

With 23 years of experience in the injection molding industry, Alpine Mold serves industries including automotive, medical, and consumer electronics, holding 50 core patents. The core team has over 15 years of experience, providing clients with one-stop solutions, from initial DFM reports and mold flow analysis to mold design proposals and high-precision mold manufacturing. Injection molding technicians are skilled in solving various product defect issues, adhering strictly to ISO quality standards and IATF16949 standards, ensuring quality assurance for the final products.