How to avoid stress cracking in containers

Within the field of plastic products, stress cracking is a common – if not the most common – quality problem. Its consequences are often severe, since problems tend to occur only after the products have already hit the market. In most cases, however, customer dissatisfaction could be avoided in the design phase of your product.

So what is stress cracking? Stress cracking occurs when a plastic component is subjected to stress, or a force is transmitted along the polymer chains. Under stress, the polymer chains start to orientate towards the direction of the pull and some soon you find cracking. This is called “creeping”. Depending on the level of the load, at some point you may notice cracking on the component as small cracks surface. Chemical substances and elevated temperature – separately but especially as a combination – catalyze the process. In such cases the phenomenon is known as environmental stress cracking (ESC).

You can prevent stress cracking in you product design.

Stress cracking is affected by time, load, temperature and chemicals. You should define the expected lifetime for the product and recognize the occurring forces, surrounding temperature and the chemical environment. Occasional peaks on load or temperature are not dangerous – constant, long term conditions are the determining factors behind stress cracking. Chemical exposure also easily triggers the effect. In particular, essential oils or their by-products may cause it to occur almost immediately. Amorphous materials like ABS, PS, PMMA or PC are more sensitive to continuous load, as well as to chemicals, than semi-crystalline plastics/polymers.

Internal stress can also break the component. Caps, metal inserts and snap-fits are all features that typically fail under stress cracking. Considerable changes in material thickness may also, in time, cause internal stress and cracking.

Testing against stress can be difficult since it may take hundreds of hours before the results appear in actual conditions, and you most likely do not have the time at your disposal. It may be worth your time to first test your product under higher stress than normal, so that you can catalyze the process by adjusting both stress and temperature slightly above the your expected values. You will have have at least some indication of the sensitivity of the component to stress cracking. If you are using essential oil in your product at 1%, then run a test with 2%.

In this article there is a list of chemicals and how they may affect your plastic container. Are any of your ingredients on the list?

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