Friday, 27 November 2020

Plastic Part Failure – Part 2: The Antidote

 


Welcome back to the second part of the blog post series “why plastic parts fail”.

Here you can jump to the first part and here to theYouTube video.

In part 1 of this series we discussed the types of plastic part failure such as environmental, thermal, chemical, mechanical failures, as well as time overarching all previous four types. Among the causes of failure we discussed the phenomenological and human related viewpoints [1].  We learned that among the phenomenological viewpoints, environmental stress cracking (ESC) is the major reason for plastic part failure. Among the human related failures, material misselection and poor specification are the main drives for failure.

Here, we focus on one of the most effective antidotes for battling plastic part failure.

Polymer Material Selection (PoMS) as a way to reduce failure

There are almost 100 generic “families” of plastics. Blending, alloying, and modifying with additives results in 1,000 sub-generic plastic types [3, 4]. The total number of grades is not known, however estimates range between 20,000 and 30,000 named grades covered by 500 suppliers [3]. When you add up the two factors “material misselection (45%)” and the diversity of plastics, you can see that it is a challenge, even for experienced people, to find the optimal polymer material for the application.

Successful plastic material selection includes the understanding of plastic material characteristics (amorphous vs. semi-crystalline thermoplastics), the specific material limitations, and failure modes. Furthermore, the application requirements such as mechanical, thermal, environmental, chemical, electrical and optical requirements need to be taken into account.

Furthermore, production factors (selection of the most efficient method of manufacture in relation to part size and geometry), together with economics (material cost vs. density, cycle times and part price) need to be considered too.

The Polymer Selection Funnel - A Systematic Approach

Enabling a systematic way for polymer material selection, I created the “Polymer Selection Funnel” framework.

The framework consists out of four funnel steps (Figure 1).


Figure 1: the four funnel steps for systemic polymer material selection and testing 

Let us discuss each phase briefly.

The information gathering phase:

In material selection, preparation is the be-all and end-all. In this phase it is important to collect as much information on environmental conditions (consider the trinity of thermal, chemicals and time, Figure 2), part cost estimations, agency approvals, industry specifications, just to name a few.


Figure 2: the environmental trinity

Funnel stage 1: Material selection factors

In this first stage we map out the true part functions and material requirements. After this we translate the requirements into material selection factors.

This can be done with the support of questions such as what load does the plastic part need to carry? Or/and will the part be exposed to chemicals?

At the end of stage 1 you have a clear picture on the material requirements which can be summarized in a work sheet as material selection factors.

Funnel stage 2: Thermoplastic vs. Thermoset

After translating the requirements into material selection factors, the first decision is made:

Which is the most suitable polymer chemistry to fulfill the listed requirements and selection factors?

In the funnel methodology, this stage is supported by a decision tree (Figure 2). It has two main paths: the thermoplastic and the thermoset path. The thermoplastic path is further split into making a decision about selection of amorphous or semi-crystalline polymers. In the end of this funnel stage, between two and three materials are obtained as input for the third funnel stage.


Figure 2: decision tree of funnel stage 2

Funnel Stage 3: selection discussion with worksheet

The third funnel stage represents a core element in the whole material selection funnel. It is a detailed selection discussion with a worksheet. I call it the decision matrix analysis and it ranks all of the pre-selected polymers. The decision matrix analysis consists of five steps. The base calculation principle is a scoring of each of the pre-selected materials for each of the material selection factors. In the end we add up all weighted scores for each material. The material with the highest score is most suitable for selection and further investigation in the fourth stage.

Figure 3: decision matrix analysis of funnel stage 3

Funnel stage 4: Testing, Material and Vendor Selection

In the last funnel stage, we would like to know in detail how the materials with the highest scores perform as a final part in a system of plastic parts or as a single plastic part alone.

For this, we can set up testing in the real (=laboratory and environmental) domain as well as in the virtual domain (processing simulation and mechanical analysis). In case of the real domain, prototyping needs to be done. There are several companies specializing in providing fast tooling and parts out of the selected materials for further tests.

In this phase, the material suppliers can be already involved. The costs of system validation are high and therefore the material from one supplier, maximal two may be evaluated.

After all the tests are done and the material has passed all tests, commercial conditions with the material supplier can be finalized and first small serial production can start.

Conclusions and Lessons for Polymer Material Selection

There are several different approaches on how to select polymeric materials for applications. On the basis of all material selection processes is a fundamental understanding between the nature of polymeric materials and traditional engineering materials such as metals.

Having a systematic approach for the polymer material selection reduces part failure. Furthermore, the time and effort of engineers and designers is limited and therefore knowledge transfer needs to be as efficient as possible. Here again, a systematic approach is useful for efficient communication within the project team and external partners such as material suppliers.

Another key element for efficient knowledge transfer is the, what I call the Polymer Product Pentagram (Figure 4): the optimal outcome occurs when part designer, material supplier, mould maker and injection moulder work together in a collaborative way.


Figure 4: overview of the Polymer Product Pentagram 


Learn more about PoMS and the Polymer Selection Funnel

If you have interest in learning the detailed execution of all the funnel steps for systematic polymer material selection, then have a look at my online course “Polymer Material Selection” on the platformThinkific.

In a few hours you will learn everything you need to select the optimal polymer material for your project, will save thousands of dollars by preventing part failure, and will have fun in the process.

There are also free chapters available to start the training immediately. This allows you to get a feeling if this course is interesting for you. 


Greetings and #findoutaboutplastics

Herwig Juster

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Literature:

[1] David Wright: Failure of Plastics and Rubber Products Causes Effects and Case Studies Involving Degradation, 2001, Rapra Technology Ltd.

[2] https://www.space.com/31732-space-shuttle-challenger-disaster-explained-infographic.html

[3] Jenny Cooper et.al. : Why Plastic Products Fail, Smithers Rapra Technology Ltd. 2010

[4] Ezrin Myer: Plastics Failure Guide - Cause and Prevention, 1996, Hanser

[5] https://www.findoutaboutplastics.com/2018/03/polymeric-material-selection-critical.html



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