Understanding Tolerances in Injection Moulding: Why They Matter

When it comes to plastic injection moulding, precision isn’t just important — it’s essential. Whether you're producing complex components for medical devices, automotive parts or everyday consumer goods, tolerances play a critical role in ensuring that parts fit, function and perform as intended.

At Amey Plastics, we understand how crucial getting the details right is. That is why the team have put together this blog to explain what tolerances are, why they matter and how maintaining them supports high-quality, reliable manufacturing.

What Are Tolerances in Injection Moulding?

In manufacturing, a tolerance is the permitted amount of variation in a physical dimension. It defines how much a measurement can deviate from its specified value and still function correctly within the final application.

For example, a plastic gear may need a shaft hole diameter of 10mm ± 0.05mm — meaning the hole can be anywhere between 9.95mm and 10.05mm and still work properly.

In plastic injection moulding, tolerances apply to:

• Dimensions (length, width, thickness)

• Angles

• Straightness and flatness

• Roundness

• Surface finishes

Why Are Tolerances So Important?

1. Functionality

Precise tolerances ensure that parts fit and function as they should. For example, a lid must snap securely onto its container or a mechanical part must fit perfectly with surrounding components. Small deviations can lead to products not working, jamming or wearing out prematurely.

2. Product Quality and Reliability

Maintaining strict tolerances is vital for the durability and performance of a product. Even minor imperfections can lead to issues like leaks, weak joints or mechanical failure — particularly in industries like automotive, aerospace or medical devices where reliability is critical.

3. Interchangeability

When manufacturing high volumes of parts, tolerances ensure consistency across batches. This interchangeability means one component can be swapped for another without any issues — essential for efficient assembly and maintenance.

4. Aesthetic Appeal

In many products, particularly consumer goods, visual uniformity is important. Tight tolerances help maintain consistent surface finishes, edges and overall product appearance.

5. Cost Efficiency

While tighter tolerances may sometimes require additional design considerations or tooling precision, maintaining appropriate tolerances avoids costly errors, scrap, rework and delays later in the production process.

Factors That Influence Tolerances in Plastic Injection Moulding

Plastic injection moulding is a highly precise process, but several factors can affect final tolerances:

• Material Shrinkage: Plastics shrink as they cool. Different materials (like ABS, polypropylene, or nylon) shrink at different rates.

• Tooling Design: The design and maintenance of the mould itself directly impact the part’s accuracy.

• Process Conditions: Variations in temperature, pressure and cooling times can cause subtle dimensional changes.

• Part Geometry: Complex or large parts are generally harder to maintain tight tolerances on compared to smaller, simpler shapes.

At Amey Plastics, we take all these variables into account during the design and production phases to ensure the most accurate and consistent results possible.

How Amey Plastics Achieves Precision

• Material Expertise: We select and work with the right material grades to minimise shrinkage and warping.

• Advanced Tooling: We invest in high-quality moulds and maintain them rigorously.

• Process Control: Our manufacturing process is tightly controlled with regular calibration and real-time monitoring.

• Design for Manufacturability: We work closely with our clients during the design phase to ensure parts are optimised for production.

Tolerances are much more than just numbers on a technical drawing — they are the foundation of a successful plastic injection moulded part. Precision ensures functionality, aesthetics, reliability and cost-effectiveness.

At Amey Plastics, we are committed to delivering high-quality products where attention to tolerances is a top priority.

Contact Amey Plastics today and find out how we can help you bring your designs to life — accurately, consistently, and cost-effectively. Call us 01730 266 525 or email sales@ameyplasticsltd.co.uk.