Tolerances in Plastics: What You Need to Know
When working with plastics, like sheets, rods, and tubes, understanding tolerances is important. Tolerances specify how much a product's dimensions can deviate from the nominal value. But what does that really mean, and what can you expect?
Extruded and Molded Plastic Products
Extruded plastic products are made by pushing molten plastic through a die. This process generally provides better tolerances compared to molded plastic products. Extrusion is a more controlled process, allowing for greater accuracy in shaping. Molded plastic products, on the other hand, are made by pouring molten plastic into a mold. This process can lead to greater variations and therefore larger tolerances. According to standard DIN 16941, for example, extruded HDPE sheets have a thickness tolerance of +/- 1.5 mm, while molded HDPE sheets can have a tolerance of +/- 3 mm. Therefore, it’s important to investigate what type of plastic product you need and what tolerances are achievable.
Tolerances in Machining
When we machine plastic products, for example, by cutting or drilling, the tolerances can be affected. Machining can cause changes in the material's shape and size. How much tolerance you can expect depends on several factors, including the material's hardness and the machining method used. For example, if you cut a UHMWPE sheet with a thickness of 10 mm, you can expect a tolerance of +/- 0.5 mm for the cut surface, assuming you use a high-precision cutting machine. If you drill into the same sheet, the tolerance can increase to +/- 1 mm due to the material's elasticity and the drill's vibrations.
The material's Shore hardness is also an important factor. A UHMWPE sheet with a Shore hardness of 65° Shore D can have better tolerances during machining than a PA6 sheet with a Shore hardness of 80° Shore D, thanks to UHMWPE's lower elasticity.
Thermal Expansion and Moisture Absorption
Plastic products can expand or shrink when exposed to temperature changes or moisture. This can affect the material's shape and size, and thus also the tolerances. For example, a PA6 sheet can expand by up to 2% with a temperature change of 100 degrees Celsius.
To manage thermal expansion and moisture absorption in your design, you can use a material property database. These databases contain information about the thermal and moisture properties of various plastic materials. This way, you can predict how the material will behave under different conditions and design the product to minimize the effects. Should you use a material with low thermal expansion and moisture absorption, such as UHMWPE? Or should you design the product to take these effects into account by using expansion joints or moisture-proof constructions?
Practical Advice for Designers
When designing a plastic product, it's important to consider tolerances and material properties. Here's some practical advice:
- 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 joints or moisture-proof constructions.
Do you have questions about how you can optimize your plastic products? Feel free to contact us; we're here to help.
Tolerances in Plastics: What You Need to Know