Friday, 7 February 2025

Polymer Selection Funnel Example - Base plate of a filter coffee machine (POMS-Funnel Method)

Hello and welcome to another polymer material selection example for which we use the POMS-Funnel Method (in detail explained here and in this video). Today’s mission is to select the optimal polymer for a base plate used in traditional filter coffee machines (Figure 1). 

Figure 1: Polymer Selection Funnel Example - Base plate of a filter coffee machine using the POMS-Funnel Method.

You want to get quickly started with plastics selection -  I created a practical guide which leads you fast in 5 steps through a selection journey (check it out here). 

Also, my post on 10+ key considerations for selecting the optimal polymer material can help to kickstart your material selection journey. 



Now, back to the selection example. 

For a more detailed selection approach, as mentioned before,  I created the Polymer Selection Funnel methodology (POMS-Funnel) which you can check out here and practical examples in the start here section.

Figure 2 presents the four different stages of the material selection funnel as well as the tools we can use to facilitate the selection. We can use this as a guideline throughout the selection journey.

Figure 2: Polymer Selection Funnel - overview of the four different funnel stages and tools.

Funnel stage 1: Material selection factors

In the first Funnel stage we focus on gathering and understanding all the requirements of the base plate for the  filter coffee machine. 

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

I split the requirements in three different sections: 
  1. Functional requirements related to the part: the base plate serves as a housing for electrical components and needs to protect the electrical equipment against external mechanical impacts according to the IEC 62262 (needs to pass the “spring hammer test”). Therefore, design weaknesses need to be avoided and materials with high impact resistance are needed. To stand properly on the floor, good mechanical stability needs to be ensured. Achieving this stiffness can be done over ribs, however ribs may increase surface defects and increase tooling costs. Therefore, a polymer with high stiffness  is preferred  (high elastic modulus; engineering polymers). Keeping the coffee warm for a longer time, temperatures up to 110°C can be reached. Polymers with high continuous use temperatures, as well as long heat aging durability  are preferred. Being used in a kitchen, the coffee machine is exposed to all kinds of fats, oils, soaps, cleaning agents, alcohols, fruit and vegetable acids. Additionally, water vapor and hot coffee will touch the base plate too and therefore hydrolytic resistance is needed. Surface finish and optical quality are a key requirement too. High gloss and scratch resistance surface are required, together with customer specific coloring of the parts. 
  2. Production/processing-related requirements: Since long screw bosses need to be used, polymers with good flow properties need to be used (including good ejection). In addition, there should be no shrinkage and warpage problems after the part was moulded. 
  3. Assembly-related requirement: In order to save costs in the assembling, snap fits should be used to easily connect parts with each other. Therefore a material is needed with high elongation at break and flexural strength. Additionally, some self-tapping screws need to be used and therefore more viscoelastic material is needed. 
After we collected all requirements, we turned them into quantifiable data and added them to our requirement worksheet.  The main criteria will be continuous use temperature (CUT), heat resistance and ageing stability, and optical surface quality. Heat ageing stability can be achieved by additives and therefore, CUT and optical properties are the main driver for the selection. High filled polymers achieve not the desired outcome (with expectation such as PARA) and only neat polymers will be considered. 

Table 1: Overview of requirements for the coffee machine base plate using the requirement worksheet.


Funnel stage 2: Decision on thermoplastic or thermoset

Funnel step 2 leads us through a decision tree, starting with the first branch of taking the thermoplastic route or thermoset route. Reviewing the application requirements (impact, optical, processing), thermoplastics can fulfil the requirements in a better way. Next decision branch in the thermoplastic route is to select whether amorphous or semi-crystalline polymers are feasible. Both amorphous and semi-crystalline materials are feasible to fulfil the coffee base plate requirements. 

Now we can screen the databases and material suppliers for suitable material candidates.  There are reliable database such as Campus and Omnexus and I created dashboards to support this step too: 
  • Dynamic Mechanical Analysis (DMA) Data of Engineering and High Performance Polymers
  • Plastics Pricing Dashboard 
  • Chemical Resistance of Polymers 
  • Plastics Environmental Sustainability Dashboard
All dashboards can be found here.

Among the amorphous polymers, unfilled Polycarbonate (PC) can match the requirements. However, due to long flow length combined with thin ribs and high viscosity, filling of the part is harder. Styrene acrylonitrile resin (SAN) can be considered too, however will not fulfil the high continuous use temperature of 110°C (SAN: 85°C). Polyamide 6.6 will fulfil the 110°C continuous use temperature requirement (PA 6.6: 120°C), however there are limitations of use due to easy coloring when in contact with food. For example, parts moulded in PA 6.6 easily take up the red spots when touched by red beet. 

For our detailed discussion in Funnel step 3, Polybutylene terephthalate (PBT), Polypropylene (PP), and Polyketone (POK) are considered (Table 2):
Table 2: Pre-selected materials for Funnel step 3. 

Funnel stage 3: Selection discussion with worksheet (qualitative matrix analysis)

Now we reached the core element in the whole material selection funnel: the  qualitative matrix analysis. It is a detailed selection discussion with a worksheet. It ranks all of the pre-selected polymers on how good each fulfills the requirements. Additionally, the requirements are ranked according to their importance too. The decision matrix analysis consists of five steps (explained in detail here) and the calculation principle is a scoring of each of the pre-selected materials for each of the material selection factors. As a result, we obtain scores for each material. The materials with the highest score are most suitable for selection and will be further investigated in the fourth stage.

How to start the qualitative matrix analysis?

We can use my online tool in order to facilitate this step (Polymer Material Selector V1.1). As an alternative, you can reach out to me and I will provide you with an excel version of it. I only considered the must-have requirements and ISO test standards; Long-term and multi-point test data needs to be checked in Funnel step 4 too. 

In Table 3 the outcome of the qualitative matrix analysis is presented. PBT scored the highest number of points (score: 182 points), followed by PP (score:180 points) and POK (score: 180 points). All three materials should be validated in the Funnel stage 4 since there are important tests. Also, assessing the commercial and processing situation of the materials is important. 

Table 3: Outcome of the qualitative matrix analysis - PBT scored the highest points, followed by PP and POK. 

Funnel stage 4: Testing, selection of material and vendor

In the previous steps, we exclude PA 6.6. due to the potential of being colored by kitchen foods and liquids. Also, Polycarbonate was excluded due to low flow and difficult part filling. Now, in Funnel step 4, PBT, PP, and POK will be tested. In this test series, simple ISO based tests are one part, however the major knowledge on the material performance comes by making prototype parts and assembling them into a part group. 

Major tests include safety tests according to the IEC, and CEE, as well as long-term testing. Long term endurance testing is done in two phases and represents 10 years of life time. In phase one, prototypes of all parts with all three materials are done, including the base plate. Then, water in the tank is overheated and kept for 20 minutes at the high temperature. Then, in phase two, the device is allowed to cool down for 5 minutes and the tank is refilled. This is repeated 7.000 times. This long-term test allows access to things such as deformation, warpage, coloring, material change, and embrittlement of the material. As a result, PBT performed well. PP and POK performed well too, however the design of the part needs to be adapted to compensate for higher shrinkage and warpage. The design of the coffee machine gets more complex by using PP and POK, in order to have the same part quality as with PBT. It is challenging to achieve the same part performance with a material which has a lower overall performance profile. However, with good training in part design and part optimization, materials such as PP and POK are replacing more and more expensive engineering polymers. 

Conclusion

In conclusion, PBT can be used for the premium segment of filter coffee machines, and PP, as well as POK can play an important role in the more mass market application with more price sensitivity. 

More polymer material selection examples using the funnel approach can be found here and below:

I offer to select the optimal polymer for your project, doing the polymer material selection together with you, and also teaching polymer material selection as a training in a group - let me know how I can help you here.

Thanks for reading and #findoutaboutplastics

Greetings

Literature: 
[1] https://kdfeddersen.com/de/produkte/polymere/polyketon#Eigenschaften
[2] https://www.campusplastics.com/campus/en/datasheet/Ultradur%C2%AE+B+4450+G5/BASF/20/401a0566
[3] https://www.borealisgroup.com/products/product-catalogue/bh348mo
[4] Bednarz J: Kunststoffe in der Elektrotechnik & Elektronik



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