Lexan fixture with flexible tabs breaking. Is there a better material?

[braclark]

I just inherited a project to design fixtures for holding PCB’s in a De-ionized water wash machine. All current fixtures were made out of machined Lexan sheets and glued together.

The part of these fixtures that has failed the most is a spring retaining feature. It is machined into a row of tabs, heated, and the tabs are bent to form the part that retains one edge of the PCB’s. I’m told they are heated in a fixture that applies pressure while they are baked in the oven, and then they are cooled slowly while still under pressure.

All of the tabs show cracking at the location where they bend, and many have broken off. Since this fixture is glued together, replacing just the tabs is not an option.

Is Lexan a material that can be used as a spring?
What would a better material be for this?
I can easily change the design to remove heating and bending of the tabs. Would that help the durability of the tabs?

 

[btrueblood]

Yes, heating and bending the Lexan (polycarbonate) induces residual stresses in the parts. Lexan is also subject to "crazing" or cracking due to combined stress and exposure to certain chemicals (like detergents and salts), or to water above a certain temperature. Depending on the temperature and level of contaminants in the washing machine, this could play a role as well.

Could you replace the broken tabs with a metal (e.g. stainless steel) piece that then attaches to the remaining fixture with screws (or glue)? Or re-make the fixture using polyethylene?

 

[patprimmer]

Like btrueblood said, plus Polycarbonate has quite poor fatigue resistance.

As stated above, it is susceptible to attack from hot water, but especially if it is already stressed and also if the water is alkaline.

Many common solvents and chemicals will also stress crack it.

IF, and only if, it is dried, heated to the CORRECT temperature, then slowly cooled, there should be little residual stress after bending, but IF a spring seats on it, the spring will be applying some stress, quite probably enough to make it quite sensitive to solvent stress cracking.

Possible alternatives are many. It really depends on a lot of details you have not provided to date.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[braclark]

Here is a view of the 3D model:

Could you replace the broken tabs with a metal (e.g. stainless steel) piece that then attaches to the remaining fixture with screws (or glue)? Or re-make the fixture using polyethylene?

Yes, I can change the material. Due to a reorg, it's now my project. They told me that they need 12 more fixtures. I found out that they need them since all of the current fixtures have broken. So I'm trying to re-design them before I order them so they don't break as quickly.

I am looking into the chemicals and temperatures that the fixture sees, and I'll post that info later.

Thank you very much.

 

[dwightdixon]

Try nylon if you really need it to be plastic. Springs made from stainless steel are more stable, reliable than any plastic and do not creep and lose retaining power like plastic.

 

[patprimmer]

At room temperature, acetal makes a better spring than nylon. That is why it is used extensively in keyboards to keep the keys in the normal position.

The best material for the job depends on a combination of factors, most of which are undisclosed to date.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.