Accuracy and quality control

Most polymer products that are injection moulded are made by machines with very little human intervention. This ensures that each product is made to a fine tolerance if the mould has been made correctly - a formed product is only as good as the mould or injection-moulding tool.

If machines are maintained and stocked with the required material, the product will be near perfect as machines follow the same motion every time. On-screen simulation reduces the need to produce trial runs and can eliminate errors before production starts.

Digital micrometers can be used to measure the width of a material and digital vernier calipers can be used to measure the outside width, inside dimensions and depth of holes. Both tools measure to 1/100th of a millimetre (mm) and can be read quickly because of the digital screen.

Someone holding a digital micrometre with a lathe in the background.

Digital micrometer

Quality control takes place during the manufacture of any product, but, since polymer parts are engineered to a fine tolerance, there are specific quality control tools to ensure that polymer parts have been made correctly - one such tool is called a ‘go-no-go gauge’. The ‘go-no-go gauge’ has a ‘go’ side and a ‘no-go’ side - when testing the product one side must pass and one side must fail.

A go-no-go gauge with a green 'go' side and a red 'nogo' side, used for quality control.

Example

It is common to hear engineers say they can work to a tolerance of ‘one thou’, meaning 1/1,000th of an inch.

1 inch = 25.4 mm

25.4 ÷ 1,000 = 0.0254 mm, so:

‘one thou’ = 0.03 mm (to 2 decimal places)

If an engineer was asked to mill a 30 mm slot in a block of acrylic, it would be possible to check whether the slot was correct by using a ‘go-no-go gauge’:

30 mm - 0.03 mm = 29.97 mm

This side of the gauge must be able to slide into the milled slot.

30 mm + 0.03 mm = 30.03 mm

This side of the gauge must not be able to slide into the milled slot.

Question

If 0.5 m lengths of acrylic tube were cut +/- 5%, what would the range of tolerance be?

0.5 m = 500 mm

1% = 500 ÷ 100

= 5 mm

5% = 5 mm × 5

= 25 mm

Range of tolerance = 25 mm either side of the 0.5 m mark

= 25 mm × 2

= 50 mm