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 (POMS-Funnel-Method)
Enabling a systematic way
for polymer material selection, I created the “Polymer Selection Funnel” framework (POMS-Funnel-Method).
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.
Following questions can help us with this assessment:
-What are the performance requirements (structural, etc.)?
-Do you want to combine multiple parts or functions?
-What will be the structural load of the part (static, dynamic, cycling, impact, etc.)?
-What will be the environmental impact on the part (chemical, temperature, time)?
-What is the expected lifetime of the product?
After answering the functionality questions, we continue with the, in my point of view, six essential questions on material selection factors (6 What's):
1. What is the service environment of your part?
2. What are the regulatory requirements?
3. What types of load at which service temperature and time need to be fulfilled?
4. What other things such as wear and friction, electrical properties such as CTI, electrical breakdown strength, aesthetics and colour (relevant for application with food contact, and toys), and more, need to be considered?
5. What is the processing and fabrication method?
6. What are the economic and commercial considerations?
A more detailed list can be found here (incl. download): Material Selection Requirements Checklist
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.
 |
| Example of a requirements overview worksheet. |
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 new book "Polymer Material Selection"
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.
Thanks for reading and #findoutaboutplastics
Greetings
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
