Tolerances in Plastics: What You Need to Know
When working with plastics, such as sheets, rods and tubes, it is important to understand tolerances. Tolerances specify how much a product’s dimensions may deviate from the nominal value. But what does this actually mean, and what can you expect?
Extruded and Molded Plastic Products
Extruded plastic products are manufactured by forcing molten plastic through a die. This process generally results in better tolerances compared with moulded plastic products. Extrusion is a more controlled process, which allows for greater precision in shaping. Moulded plastic products, on the other hand, are manufactured by pouring molten plastic into a mould. This process can lead to greater variations and, consequently, wider tolerances.
According to the DIN 16941 standard, for example, extruded PE-HD sheets have a thickness tolerance of +/- 1.5 mm, whilst moulded PE-HD sheets may have a tolerance of +/- 3 mm. It is therefore important to investigate what type of plastic product you require and what tolerances are achievable.
Tolerances during machining
When we machine plastic products, for example by cutting or drilling, the tolerances may be affected. Machining can cause changes to the shape and size of the material. The extent of the tolerances you can expect depends on several factors, including the hardness of the material and the machining method used.
For example, if you are cutting a 10 mm thick PE-1000 sheet, you can expect a tolerance of +/- 0.5 mm for the cut surface, provided you are using a high-precision cutting machine. If you are drilling into the same sheet, the tolerance may increase to +/- 1 mm due to the material’s elasticity and the vibrations of the drill bit.
The material’s Shore hardness is also an important factor. A PE-1000 sheet with a Shore hardness of 65° Shore D may have better machining tolerances than a PA6G sheet with a Shore hardness of 80° Shore D, thanks to the lower elasticity of PE-1000.
Thermal Expansion and Moisture Absorption
Plastic products can expand or contract when exposed to temperature changes or moisture. This can affect the material’s shape and size, and consequently its tolerances. For example, a PA6G sheet can expand by up to 2 per cent with a temperature change of 100 degrees Celsius.
To manage thermal expansion and moisture absorption in your design, you can use a materials properties database. These databases contain information on the thermal and moisture properties of various plastic materials. This enables you to predict how the material will behave under different conditions and design the product to minimise these effects.
Should you use a material with low thermal expansion and moisture absorption, such as PE-1000? Or should you design the product to account for these effects by using expansion gaps or moisture-proof constructions?
Practical Advice for Designers
When designing a plastic product, it is important to take tolerances and material properties into account. Here are some practical tips:
- Use a material property database to predict the material's behavior.
- Consider materials with low thermal expansion and moisture absorption.
- Design the product to minimize the effects of these factors.
- Use expansion gaps or moisture-proof designs.
Do you have any questions about how to optimise your plastic products? Please feel free to contact us – we’ll be happy to help.
Toleranser i plast: Vad du behöver veta