Why are plastic component tolerances important?

If you’re selecting plastic components to include in your product, then you want them to be as high-quality and accurate as possible.

Depending on the product you’re creating, there might be legislation or quality standards you need to meet as part of the design and development process, for example, if you’re creating a car interior.

The tight controls and parameters of the injection molding process mean it has the ability to manufacture a high volume of highly accurate plastic components within just a single production cycle. If one of these controls or parameters isn’t monitored carefully, then the overall accuracy of the manufactured components can diminish.

Measuring and monitoring manufacturing tolerances is a standardised way of ensuring the quality and accuracy of plastic components. Yet it can still be a challenge to find a manufacturer that offers the part tolerance measurements you need from your plastic components. This article will give you the tolerances knowledge you need to find a high-quality components manufacturer.

What is the definition of tolerances in injection molding?

There are many different types of tolerances in manufacturing. However, the main manufacturing tolerances definition is a range of specified dimensions within which particular measurements can sit in order to be considered accurate and high quality. Within injection molding, there are two types of tolerances that need to be considered: part tolerances and process tolerances.

What are injection molding part tolerances?

Part tolerances refer to the dimensions of a component. For example, the tolerances for a hole within a component might sit within a range of 0.5mm.

This means the final dimension of the hole can deviate by + or -2.5mm less than the set dimension. Also known as a base point or nominal value, it won't significantly impact function or aesthetics. Different types of parts need to meet different tolerance standards depending on the level of accuracy required for their end-use.

Each manufacturer usually has its own standard reference charts for limits and fits to ensure it maintains its part quality. When considering a new manufacturer, it may be worth comparing their manufacturing tolerances charts to ensure you get the level of accuracy you need.

What are injection molding process tolerances?

Process tolerances refer to the controls put in place on the injection molding process. These parameters include:

• injection pressure
• injection speed
• thermoplastic temperature.

The tolerances for the above are measured using a variety of units. For example, the tolerance of injection speed may be set at plus or minus 0.1mm/s. The smaller the tolerance on a parameter, the tighter the control and the higher the accuracy of parts produced.

Due to their more consistent performance, electric injection molding machines can generally produce components with more accurate tolerances than hydraulic machines. As part of our commitment to sustainable manufacturing, Essentra Components is investing in upgrading to electric machines. This will show significant benefits to both the stability of our processes and the final products would sound better.

Which factors can affect the tolerances of injection molded components?

Although injection molding is a highly accurate process, there are many parameters that need to be tightly controlled to ensure the components produced are within their set tolerances. The repeatability of these tight parameter tolerances is also important. This will ensure that each of the thousands of components produced is of consistent quality.

Simply, the injection molding manufacturing process involves injecting molten plastic under high pressure into a mold tool. This is then packed and cooled before being ejected as the final component. As the plastic cools, the part shrinks in size. The rate at which the part shrinks will depend on the type of plastic that's been chosen, as each plastic has a different shrinkage rate.

This is a natural and expected part of the injection molding process. When developing the component, the level of shrinkage needs to be kept in mind to ensure it doesn’t cause the component to fall out of its tolerance range.

Putting the thermoplastic under excessive temperatures or pressures can also cause the part to shrink too much or cause the part to warp and lose structure altogether. It can also cause some thermoplastic to escape the mold and cause excess material to form a burr around the edge of the piece.

A component will meet its tolerance levels by controlling:

• temperature
• mold design
• thermoplastic selection
• injection pressures
• holding pressures
• time

How do manufacturers ensure tolerance levels are met?

There are a few controls injection molding manufacturers can use to ensure their components meet their set tolerance levels. By thinking about the tolerances that a component needs to meet from the beginning of the design and development process, manufacturers can ensure no time is lost during manufacturing.

The molding tool design plays an essential part in ensuring the final components meet its tolerance ranges. Depending on the type of thermoplastic, the component will shrink at a different rate as it cools. This needs to be considered when setting the machine parameters.

For example, PA66 (polyamide or nylon 66) plastic has a shrinkage rate of between 0.7% and 2% on average. In comparison, P-Pro (polypropylene) typically shrinks between 1.2% and 2.2% as it cools. This means that even if you’re looking to create the same component from these two different plastics, manufacturers will need to develop two different tools to account for the different shrinkage rates. Tools also need to be made within the same range of tight tolerances to ensure complete accuracy.

It's also important that the right type of material is chosen for the component and mold shape. Thermoplastics are split into two main types, amorphous and semi-crystalline, each of which has its own particular material characteristics. While semi-crystalline is stronger and more durable, it is also less flexible than amorphous thermoplastics. They also offer different levels of heat resistance.

Choosing the right type of thermoplastic for the component and mold type will help to ensure the shape of the final component is accurate and within tolerance.

Experienced manufacturers will also know exactly what temperatures and pressures to put these thermoplastics under to create high-quality components. They’ll also be able to monitor these parameters under tight process tolerances thanks to their high-quality machinery. This level of injection molding knowledge and quality machinery are key factors to look for in your components manufacturer.

When choosing the correct materials, Essentra Components considers the long-term effects of stainability and recyclability of the parts down the waste stream.

Why is ensuring the right tolerances important?

The main reason that tolerances need to be correct is to ensure components function correctly.

Parts need to be within their set tolerances to work accurately. This is whether they’re part of a system, join to other components or need to have specific aesthetic qualities. For example, if a cap needs to fit a particular pipe, then the components tolerances for the component need to be tight to ensure the cap fits correctly without being too tight or too loose.

If an injection molding process makes accurate components, this minimizes the amount of waste created in the production process. For example, an inaccurate or badly designed component may have some excess flash that needs to be removed by hand. This not only takes extra time to be processed and finished but means there’s lots of waste material left over. Faulty parts that have to recycled and re-used also make an environmental impact, as that requires energy.

Getting the part tolerances right at design and planning stages can minimize money and materials later on in the production cycle. This is achieved by creating an accurate tool design that fits the specific thermoplastic required. For customers, this means they can be sure their components are of consistently high quality and will be delivered on time.

How does Essentra Components maintain accurate tolerances?

At Essentra Components, part tolerances are considered from the very start of the process, and tolerance checks and controls are also put in place throughout the production cycle.

All processes meet ISO 2002 standards of repeatability and traceability. There is a well-trained team, and the company constantly invests in the latest injection molding technologies.

At the Kidlington site in Oxfordshire for example, 4.2 million high-quality and accurate components are produced every day and, in the US, the optimum output is 6 million. This is thanks to the team’s knowledge of the impact on part tolerances of:

• tool design
• thermoplastic selection
• process tolerances

This scalability of quality component production is how it is made it easier for customers and ensure they receive parts with the accurate tolerances they need every time.

Tolerances contribute to delivering quality components

Plastic components tolerances are an important marker of part quality. It’s key that parts meet accurate tolerances to ensure they fit, function and operate correctly. To meet these accurate tolerances, experienced manufacturers use tight process tolerances, high-quality machinery and thorough injection molding knowledge.

These controls mean manufacturers can produce thousands of components that meet the tight tolerances you need, so you can focus on delivering the best product for your customers.