Why does plastic crack and break

When workpieces break ...

In plastic injection molding, the main focus is of course on ensuring that the workpieces are sufficiently stable. With the many years of experience of Stocker Kunststoff GmbH and all of our specialists, it is almost impossible that this will occur in the production of our products dreaded material failure comes.

Nevertheless, it can never be completely ruled out that a plastic used does not meet the requirements and breaks, melts or cracks. In the previous news we reported extensively on the selection of suitable materials and the respective properties. Today let's look at the reasons why it's too Material failure in workpieces from plastic injection molding can come.

Material properties and time

Just like any other material plastic has its load limits. That is why it is very important to carry out a precise analysis of the requirements that can arise in use before selecting any material. The temperatures, e.g. in a permanently installed flash unit, can rise considerably higher in the sun than one could imagine. This can then simply be predicted with precise measurements. But that's just the most obvious example. How much pressure is exerted on the workpiece, how strong are the tensile forces that it has to withstand? Every material has a yield point and a certain tensile strength. In addition, there is the stress limit, which plays the main role, especially with brittle plastics. The calculation of the so-called creep resistance is downright sneaky. Relatively gentle stress, however, constantly applied to the workpiece, can lead to long-term failure, even though the material has a much higher stress limit for a limited period of time.

The relationship between tension and elongation is known as the modulus of elasticity. A calculated stress-strain curve is used to determine the point in time at which the ratio of the two forces leaves the linear line and the material becomes unreliable. We have already written about the effects of large temperature differences over a longer period of time. Many plastics will change over time brittle and break. For this reason, the plastic used must always be provided with an expiration date, similar to that for food. Possible chemicals in the area

It is even more difficult to assess all chemical effects to which a substance is exposed. The most common reason for the failure of a plastic part lies in the stress cracks. It is almost impossible to predict the behavior of a plastic for all kinds of chemicals. This is even more true if one cannot precisely determine the chemicals at the point of use. What is certain, however, is that some chemicals reduce the maximum working voltage for materials. Of course there is also a technical term for this. One speaks of environmental stress corrosion cracking when the occurrence of chemicals mechanical resilience of a plastic belittles.

The speed of influence

Another factor that can rarely be recorded 100% is the so-called stretching speed. Their effects can easily be explained using the well-known game of tug-of-war. Two teams pulling the rope at full speed will reach a great deal of tension before either of them has to give up exhausted and is pulled over the line. This means that their “yield point” or “tensile strength” is reached. However, if one of the two teams manages to suddenly achieve the high level of traction, the other team cannot sort itself out and effectively distribute its performance among each individual participant. The result is clear, they are drawn across the line in an uncoordinated manner. The situation is very similar with the molecules in plastic.

Multi-component injection molded parts

If you consider these time and environmental factors influencing the materials, it becomes clear why the two- or multi-component workpieces in particular need so much preparation time before they can be successfully produced and used. The complexity of the individual material properties in connection with the time factors and also the interactions of the various materials make this undertaking a major task. Nowadays most of the possibilities can be calculated by computer. Nevertheless, long series of tests are necessary in order to compare the calculated effects with reality. Otherwise it just comes to a break.

Tags: plastic, injection molding, injection molding process
Carsten Schumann 14:10