Design Tip - 7

The resin selection is a really important part of any plastic part design. There are some selection criteria easy to find on Internet but as usually there is not a formula to apply.

In my opinion the thing to check is the experience. It doesn't mater if you don't have enough experience on the part you are designing. It should be people around you with experience on it. So ask the people! Look for the more experienced people in the company, it doesn't matter if they don't work on the design department. Quality, injection, purchasing, process are departments with an important knowledge of resin problems and solutions (maybe they don't know why, but the 'why' is our job).

Sometimes you have to design a part for a completely new application for your company, so you are not able to find information about the right material to be used. Then you have to use other ways, ask Google is one of the easiest ways, look into plastic design forums, look for blogs (why not alti-depth), and of course take a look to your competitors. If you don't have competitors you are so lucky... unfortunately I've always had competitors :(.

In order to compare material properties you need the material data sheet. Sometimes it is difficult to find the data sheets from the resin makers (why? I've never understood why! they should be the most interested on we choose their resins! Well... maybe they focus their efforts on the Purchase department :( ). You can use Campus, a good tool but you also have to download the information from the resin makers (not all resin makers have the Campus ata to be donloades). I know they have now an online version of campus, but I've never use it so I could not talk about it. I use Ides (The PlasticWeb), it is a web site with a large amount resin data sheets ready to be downloaded. Easy to search, a lot of information about UE and US resins (important if you are designing parts for both markets), free registration, etc. There are other sites with good database such as MatWeb, but I'm happy with Ides.

I use to design/review parts that have movement, so friction is important. The use of POM for these applications is a good start, the main issue of POM on my applications is the bad aesthetics properties of this material. Then sometimes we are not able to use POM, but we need to move the parts. It is not a good idea to use the same material on both parts so, if there are aestheticas requirements the use of ABS and PC could be a solution. You'll never get the smoothness of a POM part, so take care on the design of the friction areas and part shapes in order to improve the movement. In summary, if there is friction: POM if possible, do not use the same material in both parts and take care on your design. There are better materials than POM for friction but usually more expensive, PTFE (Teflon) is a very nice material for friction applications, you can also find PAs (nylon) filled with PTFE, a very good but expensive blend. On my current job PTFE is never used for cost reasons :(

Other applications such as chrome plating, limits a lot the possible resins to be used. I've never used other resins than ABS or ABS/PC for chrome plated parts. And not any grade of that materials, most resin makers have special grades for chrome plating.

On other applications I worked with, the cost was one of the most important requirement, so PP and PE were widely used.

As you can see, every application, finish, requirement is limiting the possible resin choose, so list your requirements and choose the resin that could fulfill most of them or the most important ones. 

Any questions on resin selection? Post a comment. I have experience on few applications but maybe another blogger could help you.