Monday 28 March 2022

Dimensional Stability of Polymer Based Parts after Processing: 3 Considerations

 Hello and welcome back to a new blog post. Today we discuss three considerations for optimal dimensional stability of plastics parts after processing.

Polymer based parts have a dimensional stability which is not equal to that of metals. It can vary with several factors which we discuss in the following in more detail. If it is a critical part, this needs to be considered during the polymer material selection.

Definition dimensional stability

In short, dimensional stability means that the required dimensions are kept after processing and when the application is in use. Three considerations help to keep the dimensional stability of your part: moisture, mechanics, and thermal stability (Figure 1). 

Figure 1: Plastic part design - three considerations help to keep the dimensional stability of your part.

Consideration 1: Residual moisture and moisture uptake during use

General rule of thumb is that when materials are exposed to moisture, dimensional changes are likely to occur. In case your application has tight tolerance requirements, polymers with low moisture absorption should be taken. For example, an aliphatic Polyamide was specified for an application with tight tolerances. Due to the moisture uptake, part performance decreased and a replacement material is needed. In such a case, a semi-aromatic Polyarylamide (PARA) can be an alternative, since it has the lowest moisture uptake of Polyamides. There are also other polymers such as PEI, PPS, and PEEK, which have excellent mechanical, and moisture performance. PPS, PPA, and PEI can be used for applications, which are exposed to high temperature and moisture during the use of the application (water pumps in cars for example). Also during processing, keeping a maximum allowed moisture level is essential to not harm the polymer during processing. In this post, different maximum moisture levels after resin drying to ensure proper processing are shown.

Consideration 2: Mechanical strength

In case of structural applications, loading strength of the selected polymer is important and can influence the dimensional stability. For complete evaluation, short-term property data such as tensile and compression strength, together with long-term data such as tensile creep should be considered. Examples of high performance polymers which show high dimensional stability are PPS, PAI, and PEEK.

Consideration 3: Thermal stability

Temperature load can have a severe impact on the plastic part dimensions. Therefore, it is critical to evaluate the maximum use temperature and the continuous use temperature, together with the environment (air, water-glycol) of your application. For evaluation of the temperature impact, dynamic mechanical analysis (DMA) data, as well as head deflection data (HDT) of the selected polymers are helpful.

Overall, there are some factors, which influence the polymer part performance. In this post, I show you additional factors to consider for your plastic part design.

Thanks for reading and #findoutaboutplastics



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