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

If you liked this post, please share and like!

Interested in my monthly blog posts – then subscribe here and receive my high performance polymers knowledge matrix.
New to my Find Out About Plastics Blog – check out the start here section
Polymer Material Selection (PoMS) - check out my new online course

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



Tuesday, 24 November 2020

Plastic Part Failure – Part 1: Reasons

 


In this two part blog series we will shade some light into a very interesting and important topic: plastic part failure.

Parallel to this two part blog post series I made a presentation which can be watched here on YouTube:



The field of plastic part failure analysis is wide and we will focus first on the “why plastic parts fail”, together with showing the main reasons. After this we focus on the antidote – what can we do to prevent failure?

Why Plastic Parts Fail

In general, product failure is a costly business. There may be several consequences of part failure such as product liability which can result in significant settlements and penalties [3]. For example a manufacturer may be held liable if the product is defective. Furthermore, if the product is manufactured in a defective way and proper testing as well as inspection was not conducted, then product liability may be enforced too. There are several more reasons (missing of adequate labeling; missing instructions and warnings of the product).

“Nobody Wants to Air Their Dirty Laundry in Public”

In the past it was difficult to guide designers on plastic part failure. Plastic part failure was kept secret, since nobody wanted to air their dirty laundry in the public.

However, there was a study published by David Wright [1] which classified the causes of failure for  over 5,000 failed plastics parts. One of the amazing key findings was that the vast majority of failures were avoidable.

What was the big problem? The know-how on how to prevent failure was publicly known , however it was inadequately communicated along the plastic part manufacturing chain. This chain usually consists of specifiers, designers, processing companies, purchasing, and material suppliers. Designers might be aware of certain material differences and their impact on the overall part performance. Contrary, material purchasers might choose a cheaper material without knowing the aforementioned impact on the part performance.

Causes of Failure

Mr. Wright shows in his study two viewpoints on the causes of failure:

1.     Phenomenological causes of failure: in this viewpoint failures are attributed to a physical mechanism (Figure 1).

2.     Human viewpoint: in this viewpoint failures are attributed to human related decision making and execution (Figure 2).

Figure 1: Overview phenomenological causes of failure [1].

Figure 2: Overview causes of failure from the human viewpoint [1].

Figure 1 shows that environmental stress cracking (ESC) is the biggest cause of failure in plastic parts (30%), followed by static notch fracture (20%), and dynamic fatigue (19%). 

Interesting to see are the human caused failures in Figure 2. Here, material misselection and poor specification are with 45% by far the biggest reason for plastic part failure. The other reasons are fairly equally distributed.

There are several known cases where misselection and poor specification lead to catastrophes.

One of them was the space shuttle Challenger disaster from 1986 (Figure 3). 


Figure 3: Space Shuttle Challenger Disaster [2].

The space shuttle broke apart 73 seconds into its flight. All seven crew members were killed. The so-called “Roger Commission” was initiated, where Dr. Richard Feynman was part of the investigation. They found that the accident was caused by a failure of the O-ring sealing joint on the right solid rocket booster. The selected O-rings showed less resilience at 10°C. On the flight day it had 2°C and the seals were never tested at 10°C and below temperatures. Dr. Feynman presented the low resilience by putting the O-ring in ice water. He took it out and stretched them. The rings did not return to their original position.

This example highlights that material specification, selection, and testing are crucial points of having a proper function plastic part.

Polymer material selection as the antidote of plastic part failure will be discussed in the second part of this blog series.

Thank you for reading and #findoutaboutplastics

Greetings,

Herwig Juster

If you liked this post, please share and like!

Interested in my monthly blog posts – then subscribe here and receive my high performance polymers knowledge matrix.
New to my Find Out About Plastics Blog – check out the start here section
Polymer Material Selection (PoMS) - check out my new online course

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


Tuesday, 17 November 2020

Rule of Thumb for Plastics Injection Moulding: Usage of Regrind

 



In this rule of thumb post, we discuss the motivation for using regrind, what to be aware of when using regrind and which levels of regrind can be applied.

Motivation to use regrind

Regrind is used to mix it with virgin resin or completely use it for new parts to decrease the thermoplastic resin costs. One common source for obtaining regrind are sprues from moulded parts. They will be collected after an injection moulding project is set properly and parts are of good quality. However, also rejected parts can be transformed to regrind.

Thermal history

By adding regrind, thermal history is important. High melt temperatures in combination with too long residence times in the plasticizing unit can lead to thermal degradation of the thermoplastic. Adding the first time i.g. 20 % of regrind to the virgin material is fine. However, when using the sprues or parts of this 20%/80% compound and adding new regrind, decrease of mechanical properties due to thermal degradation may occur. Therefore, contacting your resin supplier is useful and check how often a chosen thermoplastic can be moulded without losing its mechanical properties by more than 10%.

Ratio of regrind

The amount of regrind added to virgin resin is between 20-25%. This ratio is valid for most plastics. In moulding operations, some parts allow using even 100% of regrind and other parts allow only for 100% virgin materials.

Thanks and #findoutaboutplastics

Greetings

Herwig

If you liked this post, please share and like!

Check out my other rule of thumb posts: 



Interested in my monthly blog posts – then subscribe here and receive my high performance polymers knowledge matrix.
New to my Find Out About Plastics Blog – check out the start here section
Polymer Material Selection (PoMS) - check out my new online course


Literature:

[1] https://knowledge.ulprospector.com/8055/pe-regrind-resin-qa/#:~:text=Generally%2C%20the%20molding%20community%20targets,100%20percent%20regrind%20during%20production.


Friday, 13 November 2020

Design Properties for Engineers - Compression Stress of High Performance Polymers

In this blog post, we discuss the compression stress properties of high performance polymers. 

Compression stress is estimated according to ISO 604. In this test, an axial load is put on a cylindrical test specimen. As a result, stress value at a defined compression level is obtained (1%, 2% or 10% in most cases). The higher the compression border, the more stress the tested plastic is able to handle.  

Among high performance polymers, PEEK and PBI have the highest compression values. Also, PAI and PEI show high pressure resistance. PTFE shows compared to the other polymers low compression values. This can be turned into an advantage, i.e. if you need a material which deforms under pressure to obtain a sealing function toward a part then PTFE is most suitable. 

Fiber-reinforced high performance polymers are able to take up a higher compression load at low deformation compared to unreinforced high performance polymers. 

Fiber-reinforced PAI and PEEK are able to handle 50 MPa compression stress and deform only 1%. If you add fiber reinforcement to PTFE, compression stress value will still remain below the unreinforced PI and PAEK. 

All the published design properties of high performance polymers can be found in the start here section of my blog. 

Thank you and #findoutaboutplastics

Greetings, 
Herwig Juster

If you liked this post, please share and like!

Interested in my monthly blog posts – then subscribe here and receive my high performance polymers knowledge matrix.
New to my Find Out About Plastics Blog – check out the start here section
Polymer Material Selection (PoMS) - check out my new online course

Literature: 
[1] Erwin Bauer: Saechtling Kunststoff Taschenbuch
[2] https://www.polytron-gmbh.de/

Friday, 6 November 2020

Rule of Thumb for Plastics Injection Moulding: 20% Material Viscosity Variation over Time

 


In this blog post, I present to you another helpful rule of thumb for plastics injection moulding.
 
Viscosity is one of the important parameters for running consistent injection moulding operation. In general, polymer viscosity is a function of temperature, pressure, time, and shear rate. Polymer melts have a non-Newtonian flow behavior (shear thinning).
 
Moulders specify the viscosity range of the polymer compounds received by the material supplier. Most often the so called Melt Flow Index (MFI) is used. Based on the MFI results, upper and lower viscosity borders are defined. In general, 20% viscosity variation can be expected by the material itself [1]. This variation is already there, although the moulder did not yet start processing or damage the material during processing.
 
Cavity pressure sensors in injection moulds are a crucial part for monitoring viscosity changes over time. It could be shown that viscosity correlates with cavity pressure. If the viscosity increases, cavity pressure decreases. This in turn may produce short shots, smaller parts or even sink marks [1].
 
Dimensional part variations, flash, warp, sink marks, and short shots are all linked to viscosity changes. If such moulding problems occur and you have a viscosity tracking in place, then it is easy to pin-point viscosity changes and you can take immediately counter measures.
 
Thank you and #findoutaboutplastics
Greetings, 
Herwig Juster

If you liked this post, please share and like!

Check out my other rule of thumb posts: 

Interested in my monthly blog posts – then subscribe here and receive my high performance polymers knowledge matrix.
New to my Find Out About Plastics Blog – check out the start here section
Polymer Material Selection (PoMS) - check out my new online course

Literature: 
[1] https://rjginc.com/have-a-molding-problem-the-answer-might-be-viscosity/