What is a tight tolerance in plastic manufacturing?

July 24, 2018

design measurements

OK, so let’s say you’ve got a plastic product in mind. Before you get to the stage of actual plastic manufacturing with injection or roto-moulding, you will at some point have to consider its tolerances. The issue of tolerances concerns designers, engineers and manufacturers alike, and it’s an essential step in the overall process. And because of this overlap, it can sometimes be hard to get a simple answer to the question: what is product tolerance? Well – read on to find out!

What is product tolerance (and why is it important?)

If you want a technical answer, a product’s tolerance is defined by the maximum acceptable level of deviation from its specification, measurements or standards. Basically, this means that products with tight tolerances have extremely strict design parameters. If those parameters are deviated from even slightly, the product becomes useless and it has to be manufactured again from scratch.

‘What’s defined as a tight tolerance?’ you may be wondering. The typical tight tolerance is about 0.02 inches. The vast majority of designs are far more forgiving than that, so unless your plastic product is being designed for very exacting purposes, it’s unlikely to have a tight tolerance.

You’ve probably worked out at least partially why tolerances are so important. Essentially, when it comes right down to it, as a designer you can’t expect the machinist or manufacturer to know all the ins and outs of your product. Unless you specify a tolerance at the design stage, nobody knows how important (or unimportant) a particular dimension is. If you leave this up to someone else’s best guess, you risk a lot of potential delays and extra costs.

Why not simply always use exact tolerances?

OK, it’s true – a tight tolerance sounds brilliant from a designer’s perspective. After all, you’ve spent time working out the exact measurements of your product. Why would anyone think you’d be satisfied with changing them? Well, unfortunately it’s not quite that simple. There are all sorts of reasons why tight tolerances aren’t viable for every single product.

For starters, tight tolerances can add unnecessary costs to your project. It could need more than one service to ensure these tolerances are being adhered to, as well as multiple inspections along the way. This takes time, and more time means more money! For the same reasons, tight tolerances mean much greater lead times on a product, which can quickly add up if you’re producing a significant number of them.

There’s also the fact that even if you’re using highly advanced manufacturing processes, there are always a huge number of factors in play that affect the dimensions of your finished product, and not all of these are always controllable. In short, it’s not necessarily realistic to expect machinery to be able to produce every single part with the same incredibly-detailed 100% accuracy. Due to miniscule deviations in otherwise perfect specifications, you might end up discarding a significant number of plastic products that are otherwise perfectly fit for purpose.

Just to be clear, we’re certainly not saying that tight tolerances are a universally a waste of time or resources. What we’re saying is that they’re so time-consuming and resource-intensive that they’re only really worth specifying if you’re absolutely certain the product will be useless otherwise.

Are tight tolerances viable with rotomoulding?

In a word: no! Our roto-moulding plastic manufacturing process relies largely on centrifugal force to create plastic products. Once raw plastic material has been placed inside a specialised mould, that mould is spun rapidly on its axis as it’s heated. This generates this centrifugal force, which throws the plastic material outward from the centre until it’s completely covered the inside walls of the mould. Then, the plastic powder cools to create the final plastic product.

Now, while it’s our job to ensure that the plastic powder settles relatively evenly on the inside of the mould, we can’t guarantee that its accuracy will be within 0.02 of an inch! Luckily, as we’ve touched on above, the vast majority of plastic products can take a reasonable amount of variation without becoming unfit for purpose. If in doubt about the tolerances of your product, feel free to ask us, and we’ll be happy to help any way we can!

Here at Excelsior, we have a fantastic record of producing reliable plastic products for businesses just like yours. Feel free to check out our case studies to see examples of our previous work, or give us a call on 0161 765 2010 to find out what we can do for you!