INTRODUCTION TO ENGINEERING MATERIALS - ENGINEERING MATERIAL SELECTION

Audio: Introduction to engineering materials – engineering material selection

In the engineering material introduction series, we discussed a wide range of materials and why they are important. Material selection for your specific application and environment is not a straightforward process, and there is no general rule on choosing it.

Mechanical design engineering needs to consider a great amount of information to make the right decision. In this article, we will discuss what information we have to keep in mind when choosing a material, and we will share a material selection checklist that you can download for free.

Introduction to material selection

Today, you can find thousands of materials from different suppliers on the market. Let us look into the example of PC resins. If you search for PC resin in the google search engine, you can find different material suppliers with various trademarked resins.

I will show you only two suppliers I found:

Company Covestro:

Makrolon® PC:
- Makrolon® 6265 X
- Makrolon® 6555
- Makrolon® 6717
- Makrolon® 9125
Apec®:
- Apec® 1695
- Apec® 1745
- Apec® 1795
- Apec® 1895
- Apec® 2095
- Apec® DP1-9354

Company Trinseo:

CALIBRE™ PC:
- CALIBRE™ 1602 LTD
- CALIBRE™ 200-14
- CALIBRE™ 200-23
- CALIBRE™ 200-3
- CALIBRE™ 201-22
- CALIBRE™ 2060-3
- CALIBRE™ 2060-6
- CALIBRE™ 2061-10
- CALIBRE™ 2061-15
- CALIBRE™ 2061-22
EMERGE™:
- EMERGE™ PC 6900
- EMERGE™ PC 8070-15
- EMERGE™ PC 8110-15
- EMERGE™ PC 8130-10
- EMERGE™ PC 8130-15
- EMERGE™ PC 8130-6

In addition to these two suppliers, a quick search on google revealed other suppliers: Sabic, Mitsubishi gas chemical, Teijin, GE Plastics, Avient, RTP Company, and many others. Each of these suppliers has its own trademarked resin family with different resin subtypes. Each of these resins has different properties. Furthermore, each resin has a different price and, depending on the market situation, has different availability.

We did not even look into the product itself, and we can see that there are many things that we need to consider. However, before we go into more detail, I would emphasize that you should always involve a material supplier in the decision-making process. The material supplier will consider all your requirements and suggest a suitable material for your application and environment. But in order for the material supplier to answer your request, you need to know how to ask the right questions.

Material selection

As I said before, choosing a suitable material is not a straightforward process with a one-fit-it-all solution. However, we can define a general guideline for approaching the material selection process. Furthermore, with adequately structured documentation, we can ensure or at least reduce the possibility that some of the requirements were not addressed.

The following text will explain different requirements that we need to keep in mind during the material selection process.

Part function

Before we start talking about the device’s different requirements, I suggest you start by writing down the part function. The design engineer is usually aware of its function, but I find it extremely helpful to write it down on paper. This will force you to structure and verbalize your thoughts in clear and plain language.

More importantly, this helps material suppliers or anyone else involved in a material selection process to clearly understand what they are working with and for what part will be used.

When writing the part function, a part function is not a “filter holder.”

The part function is to position and stabilize the filter fabric.

In a nutshell, you are not stating the part name but the part’s intended use.

Service life

Each part has a service life and will reach the end of life sooner or later. We can define the life expectancy of part in numerous different ways:

time duration: for example, three years of indoor use,
time duration under the load: for example, three years withstanding three kilograms of weight,
time duration under specific conditions: for example, three hours in high temperature (< 120 C°),
load cycles – for example, two hundred opening cycles,
usage: for example, one-time use.

For example, if you are designing a kettle with a small plastic switch that turns the kettle on and off. If the planned service life of the kettle is two years and, on average, it will be turned on and off six times a day, your plastic switch should withstand a minimum of 4380 cycles in a period of two years.

Mechanical requirements

The product end-use conditions and loads should be carefully examined. Also, the loads that could occur during the manufacturing, assembly, and shipping process should be taken into account. Furthermore, short- and long-term loads, impacts, vibrations, and temperature loads should be considered.

Based on mechanical load analysis, a material with corresponding properties like flexural strength, flexural modulus, tensile elongation, impact strength, creep resistance, etc., will be selected. Furthermore, other mechanical requirements could be considered, like the factor of safety, etc.

Economic requirements

When working on developing products, one of the common themes that will follow you around is cost. Your customer will not overpay for your products if they have an alternative to buying a less expensive product with the same quality as your competitors.

There are definitely some products that we are overpaying, but the amount of money these companies paid to build their brand and be recognized as industry leaders is on an entirely different level. In these cases, the cost is lowered to maximize the profit. In any case, the cost has to be controlled.

Manufacturing technique

There are multiple different processing techniques that you can use to produce parts from different materials. Not every material is suitable for every processing technique.

For example, plastic parts can be manufactured by:

extrusion,
injection molding,
thermoforming,
blow molding,
foam molding,
compression molding,
calendering,
3D printing, etc.

Based on the manufacturing technique, the material supplier will recommend which material can be processed with a defined technique.

Production Quantities

Production quantities or, in other words, the number of parts that you want to manufacture. Production quantity will influence which manufacturing technique to select, tooling cost, and overall part cost. For example, higher product injection molded quantities will result in higher tooling costs, but the individual part cost will be lower.

Price

Let us look now at how we can quickly calculate the price of the material you need to buy for manufacturing the required quantities.

Weight of raw material for required quantities:

Required weight of material [kg] = Volume of part [m³] * Material density [kg/m³] * Quantity * Scrap [%]

Overall price for the whole series:

Overall material cost [€] = Required weight of material [kg] * Price of material [€/kg]

We can calculate price of the material for each part:

Material cost / piece [€ / piece] = Overall material cost [€] / Quantity

Example for injection molded part:

Volume of part: 1.617 x 10^-5m³

Density of material: 1060 kg/m³

Price of material: 3 € / kg

Scrap: 15%

Quantity = 20 000

Required weight of material [kg] = 1060 * 1.617 x 10^-5* 20 000 * 1,15 = 391 kg

Overall material cost [€] = 391 * 3 = 1 173 €

Material cost / piece [€ / piece] = 2 070 / 20 000 = 0,0586 € / piece.

We will approximate the material density for our calculation, but when we send the request to the material supplier, we will provide him with the part volume. Then, based on the selected material and the density of that material, they will calculate the weight. Therefore, it is essential to communicate with your material supplier at least the required quantities and part volume, and they will calculate how much material you would need. Usually, the higher the amount of material you would need, the lower the price is.

Environmental requirements

The environment in the context of the product (in this case, part) is defined as the surroundings or conditions in which a product operates. It is not the same if a part is intended to be used indoors or outdoors, in low or high temperatures, or maybe both. Let us now investigate what to consider when defining the environmental requirements.

Weather conditions

The material properties and aesthetics can change due to temperature, moisture, and UV light exposure. Therefore, carefully consider the targeted geographical area in which the part is intended to be used. For example, suppose the part is intended to be used in dry, hot, and windy areas. In that case, you should consider the material with suitable properties under high temperatures, continuous exposure to the sun, and wind with the sand particles that could cause the material abrasion.

Chemical exposure

The indented part’s environment could be highly aggressive, and understanding to which type of chemicals the part is exposed is crucial for the successful use of the part. For the chemicals to which part is exposed, we need to define the exposure time, on which temperature it was exposed, the concentration of the chemical, and what type of loads are acting on the part. Keep in mind that not all commercially available materials have been tested for chemicals and that if there is a special environment, further testing should be carried.

Radiation

Exposure to radiation sources can degrade certain materials and ruin the part’s aesthetics. If your part could possibly be exposed to a radiation source, additional additives or coatings should be considered.

Physical requirements

Physical appearance plays a significant role in product development and material selection. Many companies have their own set of branding rules that your product must be complied with. Depending on the product you are developing, the quality of the part surface, color, and fit between the parts could be the main differentiator for your customer to decide to buy or not buy your product.

I heard many times from different people that they did not buy the product because it was not aesthetically pleasing. I even heard a guy not buying a car because the plastic feels cheap under the fingers. Even thou we can not quantify “feels cheap under the fingers,” we should pay extra attention when selecting the suitable material and its physical appearance.

Appearance

The part aesthetic is an important segment of the material selection. We need to define if there will be any special surface finishes, engraved finishes, colors, coatings, engraved part information, labels, prints, etc. All this information can influence which type of material we can use.

Dimensional tolerances

Your products could have multiple parts that had to fit together. If you have not completed the engineering drawing, state the standard that you will use for general tolerances to give a supplier a general sense of dimension accuracy. Also, consider the loads, temperatures, and other effects that could influence your part’s dimensions.

Assembly requirements

If your product is built from several parts, that means that these parts are connected to each other and that they are “influencing” each other somehow.

You should state how these parts are interconnected, how you plan to assemble them, and from which materials they are made. This goes for all the parts connected to your part, plastic and non-plastic.

Agency Approval and Testing requirements

Before submitting the material checklist, you should investigate what type of standards and regulations your product should comply with. This is mainly linked to the industry that you are in. Of course, your company should have different industry standards available, and you should read them and mark all of the regulatory requirements that your product should fulfill.

This could expand the list of requirements for your material, but it will ensure that you are preparing your product for success from the start. Furthermore, the testing requirement should be defined from industry standards and other sources like customer requirements.

Similar commercial application parts

If your product is part of the family of products, you should check what type of material was used to build legacy products. Also, if a similar product is on the market, it is worth investigating the materials used to create those products. Learning from previous experience could drastically reduce the time needed to find the material type for the new product. Even if the material from the older products is no longer available, there is a possibility of finding new similar material available on the material market.

Closing words

As shown in this article, selecting suitable materials for the intended application and environment is not easy. A design engineer needs to keep in mind numerous different requirements in his mind. For this reason, we have created a material selection checklist that you can download (if you have not already).

Keep in mind that maybe the checklist will not cover all the requirements for your company. In that case, use our checklist as a starting point for creating your own checklist. And if you think that something should be included in our checklist, please let us know!

Now you have great guidelines on the material selection and a checklist that you can use to make sure you are not missing something. However, I suggest you go through the text once more and identify areas you think need more understanding and clarity. Then, once you have identified those areas, start building up your knowledge in those areas.

To make it easier for you to find related posts, check the “Further reading” chapter below. Do you have any questions or need something to be clarified better? Leave a comment below, and I will give my best to adjust the post accordingly.

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