Injection Molding Tolerances: Effective Control Solutions
In plastic part manufacturing, injection molding is one of the most common and efficient processes. However, like any manufacturing method, dimensional deviations are inevitable. Especially in large-scale production, parts may show variations due to material properties, mold design, and processing conditions.
To allow exact assembly and reliable performance, the manufacturers must pay special attention to injection molding tolerances. In this article, we will discuss injection molding tolerances in more depth. Let’s begin!
Table of Contents
Part 1. What Are Injection Molding Tolerances?
Injection molding tolerances are allowable limits of variation from the intended design sizes of plastic parts in actual manufacturing. Simply put, it is the difference between “theoretical dimension” and “final product dimension.”
Parts would best match the design requirements. However, because of factors such as material shrinkage, process conditions, and equipment accuracy, small tolerances cannot be eliminated. Through reasonable tolerances, manufacturers can ensure that parts continue to meet functional and assembly requirements.
Part 2. The Importance of Injection Molding Tolerances
Tight tolerances aren’t always better in manufacturing industry. Instead, manufacturers need to balance cost, manufacturing capability, and functionality. Extremely tight tolerances will provide greater dimensional accuracy but increased tooling cost, reduced manufacturing efficiency, and higher scrap rates. Very loose tolerances, on the other hand, will compromise part function and assembly integrity.
1. Preserving Assembly Precision
Plastic parts fit or interface with most other parts. If tolerances are excessive, parts will be loose, misaligned, or even unassembled, eventually sacrificing the finished product’s structural integrity and performance.
2. Improving Product Reliability
Properly defined tolerances ensure dimensional stability throughout the lifecycle of any component, helping prevent issues like higher wear, decreased performance or component failure. When it comes to medical devices, tolerances directly impact product effectiveness and safety.
3. Reducing Rework and Scrap Rates
In high-volume production, minor tolerance issues can lead to hundreds or thousands of defective products. Manufacturers can significantly reduce defects in production and expensive reworking and avoid waste by establishing realistic tolerances.
4.Maintenance of Industry Standards and Customer Specifications
Different industries will have different tolerance needs:
Automotive parts are often needed to be highly precise, e.g., ±0.05 mm, to give reliability in engines, gearboxes, and safety equipment.
Consumer electronics usually function optimally within ±0.1 mm tolerance, compromising cost-effectiveness and functional specifications.
Medical devices and aerospace components have long been expected to have even higher tolerances to international standards to guarantee safety, longevity, and reliability.
Tolerance control of injection molding is not only a technical necessity but also the key factor in controlling quality, reducing costs, and enhancing competitiveness.
Part 3. Factors That Affect Injection Molding Tolerances
Injection molding shrinkage tolerances are established by a variety of parameters. The most significant ones are:
- Material Shrinkage
Different injection molding plastics all exhibit different cooling shrinkage behaviors, so this is the most significant factor of final dimension:
- ABS (Acrylonitrile Butadiene Styrene): Low shrinkage figure (approximately 0.4%–0.7%), with very high dimensional stability, well-suited for precision components.
- PP (Polypropylene): Larger shrinkage figure (approximately 1.0%–2.5%), which can result in warpage and distortion, with the requirement for critical mold compensation.
- Nylon (PA): Slight to moderate shrinkage (approximately 0.7%–1.5%) with high moisture absorption. Parts will continue to change dimensions further during storage or use as they absorb water and become swollen.
2. Mold Design and Accuracy
Mold is the most important factor in tolerance determination. Of prime concern are:
- Mold steel and machining quality: Highly hard steel with wear resistance provides dimensional accuracy on long production runs.
- Gate design: Gate location and geometry determine the flow of the melt, causing differential shrinkage.
- Cooling system arrangement: Non-uniform cooling creates differential shrinkage and increased chances of warpage and dimensions deviation.
3. Processing Parameters
Molding conditions directly affect tolerances:
- Injection pressure: Too low will cause incomplete filling, while too high will cause internal stresses.
- Melt and mold temperature: Excess heat encourages more shrinkage, and poor flow with incomplete fill on account of low temperatures.
- Cooling time: Insufficient cooling allows parts to shrink after ejection, impacting final dimensions.
4. Part Design Complexity
Part geometry plays a major role in dimensional accuracy:
- Non-uniform wall thickness: Thick sections cool more slowly, leading to greater shrinkage and potential warpage.
- Ribs and support: Increase strength but, if poorly designed, may create localized shrinkage or distortion.
- Hollow or involved details: May result in non-uniform melt flow, reducing dimensional stability.
5. Environmental Factors
Parts may be affected by external influences even after molding:
- Temperature fluctuations: Thermal expansion and contraction may alter dimensions in storage or in use.
- Humidity: Some materials like nylon are hygroscopic in nature, i.e., they absorb moisture and swell, reducing dimensional accuracy.
Part 4. Standard Tolerances for Injection Molding
Practice and standards in the plastics industry include tolerances of dimensions in injection-molded parts as a guide to help designers and producers control cost and manufacturing complexity while preserving functional parts intact.
1. ISO 20457
ISO 20457, published by the International Organization for Standardization (ISO), defines acceptable tolerances on plastic materials, part size and wall thickness tolerances for worldwide design and manufacturing; particularly important when international production or interchangeability of components is involved. It serves as a universal benchmark.
Key features are:
Dimension grades: Parts are divided into several levels of accuracy (e.g., Grade A, B, C) based on functional and cosmetic requirements.
Tolerance ranges: Suggested tolerances are given based on part length, width, or wall thickness.
Example Table (Reference from ISO 20457):
| Dimension Range (mm) | Grade A (High Precision) | Grade B (Medium Precision) | Grade C (General Precision) |
| 0 – 50 | ±0.02 | ±0.05 | ±0.1 |
| 50 – 100 | ±0.03 | ±0.07 | ±0.15 |
| 100 – 200 | ±0.04 | ±0.1 | ±0.2 |
| 200 – 300 | ±0.05 | ±0.12 | ±0.25 |
Note: Actual values should be adjusted depending on material type, part geometry, and functional requirements.
2. SPI Tolerance Guidelines
The Society of the Plastics Industry (SPI) Tolerance Guidelines are widely utilized within North American markets. They provide recommended tolerance ranges based on part size, wall thickness and material for quick reference when designing injection-molded parts.
Example Table (Reference from SPI Guidelines):
| Part Size (mm) | Material | Tolerance (General Part) | Tolerance (Precision Part) |
| 0 – 50 | ABS | ±0.05 | ±0.02 |
| 0 – 50 | PP | ±0.08 | ±0.03 |
| 50 – 150 | ABS | ±0.08 | ±0.03 |
| 50 – 150 | PP | ±0.12 | ±0.05 |
| 150 – 300 | ABS | ±0.12 | ±0.05 |
| 150 – 300 | PP | ±0.15 | ±0.07 |
Note: Recommendations of SPI also take into consideration wall thickness and part complexity. In reality, tolerances may further be modified based on mold design and process parameters.
3. General Reference Values
Standard plastic parts: Tolerances may vary between ±0.05 mm and ±0.1 mm, sufficient for most non-critical components.
Precision parts: For high assembly accuracy or demanding performance criteria, tolerances can as low as ±0.01 mm.
4. Industry-Specific Tolerance Requirements
The majority of industries have varying levels of dimensional accuracy requirements:
Medical devices: Relying on tight tolerances that often fall below +-0.02 mm for safety and dependability, medical devices require tight control in terms of their design.
Automotive Components: Most automotive components meet functional and assembly specifications within tolerances of +-0.05 mm.
Industrial components or consumer electronics: +-0.1 mm is considered standard, trading some cost for improved performance.
In general, the appropriate injection molding tolerance standard to employ is based on part function, material properties, capability of manufacturing, and industry requirements. The application of these standards in a proper manner not only delivers assembly accuracy and part performance but also best constrains the cost of manufacturing and rework quantities.
Part 5. How to Control and Optimize Injection Molding Tolerances
In order to ensure tolerances in dimensions and assembly reliability of injection-molded parts, manufacturers can choose the sequential path from process control, material selection, design, and quality control to tolerance optimization.
1.Optimize Mold Design
The mold plays a leading role in tolerances of parts. Scientific design and precise machining reduce dimensional deviations significantly:
High-precision machining: Ensures cavity dimensions with minimum part deviations.
Gate design: Gate location and type influence melt flow and cooling distribution, and consequently shrinkage and warpage.
Optimized cooling system: Even cooling reduces differential shrinkage in the part, lowering the likelihood of deformation.
2.Select Appropriate Materials
Material properties directly impact shrinkage and distortion:
Factor in material shrinkage in the design and include appropriate compensation for part dimensions.
Take into consideration the conditions of operation (mechanical load, temperature, humidity) and choose materials with higher dimensional stability to reduce post-manufacturing deformation.
3.Strictly Control Process Parameters
Process stability is critical in maintaining tolerances:
Injection speed: Too high can lead to non-uniform filling, and too slow can affect surface quality.
Injection pressure: Pressure oscillations lead to variations in internal stresses, affecting shrinkage and warpage.
Control of temperature: Consistent melt and mold temperatures reduce dimensional fluctuations.
Cooling time: Fully cool parts prior to ejection to avoid secondary shrinkage.
4. Quality Inspection and Feedback
Scientific measurement and monitoring are used to detect tolerance deviations and make timely adjustments:
Coordinate Measuring Machine (CMM): Precise measurement of part dimensions ensures mass production consistency.
Optical inspection systems: Rapidly inspect part surface dimensions and forms for potential problems.
Data feedback: Feedback from inspection into mold design and process parameters for refinement.
5. Work with Experienced Manufacturers
Upper-end injection molding companies are able to provide accurate machinery and provide tolerance guidance at the design level:
- Provide recommendations for complexity of part and functional necessity-driven optimisation, without being costly.
- Provide realistic experience and case reference to allow designers not to fall victim to tolerance issues and improve part uniformity and reliability.
Part 6. How KENENG Ensure the Reliability of Injection Molding Tolerances
KENENG understands the significance of part quality and assembly correctness through injection molding tolerances, providing highly reliable solutions to our customers with advanced injection molding equipment, precise mold-making technology and stringent quality control procedures.
Conformance to International Standards: Our components are manufactured strictly to ISO and other international tolerance standards for dimensional accuracy and interchangeability.
Material and Process Matching: We customize molding process parameters based on the specific properties of different plastics in order to obtain dimensional stability both in production and service.
Design Stage Optimization: We render expert tolerance optimization guidance at the design stage of components to allow clients to reduce production risk and cost.
Industry Versatility: Our product solutions meet the precision requirements of different industries, from consumer electronics to industrial components, to high-precision small-batch customization or large-scale constant manufacturing.
With experience and technology combined, KENENG offers consistent, reliable, and quality injection-molded parts so that each batch experiences strict tolerance levels achieved.
In Summary
Tolerance requirements in injection molding are key components in assuring part quality, assembly precision and overall product reliability. By considering tolerance requirements early in design and manufacturing processes as well as employing technical qualified injection molding providers with extensive experience, companies can significantly decrease risks and costs associated with their processes.
KENENG is a high-precision injection molding specialist. With advanced equipment, strict process control, and extensive experience, we help customers to attain stable part size and guaranteed quality. From high-precision industrial components to large-sized consumer electronics pieces, we provide personalized solutions to meet your needs.
Contact KENENG today and realize professional injection molding tolerance control solutions and create your products fit perfectly and work reliably.
