When designing a plastic product, it is essential to consider the cold conditions in which it may be used, including the temperatures at which it may be stored.
In cold conditions, the strength and modulus (stiffness) of thermoplastics naturally increase, which might suggest that the product becomes more durable compared to, for example, at room temperature. However, it's not that simple. The increase in stiffness reduces the product's toughness and ductility.
Think of the difference between a dry branch and a green branch. A dry branch is stiffer but breaks easily if stepped on, while a green branch is much harder to break. Cold conditions can affect your plastic product in a similar way, changing its behavior from that of a green branch to a dry branch — i.e., from ductile to brittle.
Another aspect to consider is the product's dimensions. Due to thermal expansion (or contraction in cold temperatures), components of your product become slightly smaller when exposed to the cold. For example, a POM wheel on a metal shaft may rotate smoothly indoors, but when taken outside into the cold, the wheel may jam.
Here are some tips for avoiding unpleasant surprises when operating in low service temperatures:
Determine the temperature range in which your customer will use the product. For instance, if your product is sold in Scandinavia, the conditions may differ significantly from those in your home country.
Also, consider how the product is stored. While nobody plays padel in -20°C, the racket might have spent the night in a cold car.
Obtain a reference product or a sample of the material you're planning to use. Place it in the freezer and observe how its behavior and dimensions change. Practical experience is critical to understanding these effects.
Pay special attention to the product's impact strength. When using semi-crystalline polymers, check the material's Glass Transition temperature (Tg). Below this temperature, the material may become significantly more brittle. Overlooking the Tg is a common mistake, particularly with PP homopolymer (Tg around -10°C).
Also, try bending the cold product until it breaks. If you can seamlessly join the halves afterwards, you can conclude that the material has lost its plastic behaviour, indicating it is likely brittle.
If your product has an integral hinge, latch, or snap fit, be aware that these features may perform differently in cold conditions.
Materials with good impact resistance at sub-zero temperatures include ABS, POM, and PE. Modified grades for enhanced cold resistance are available for most materials, so ask your material supplier about these options.
Remember the dimensional changes caused by thermal expansion when defining the clearances between components.
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