How Moisture Makes Engineering Plastics Expand

A day in the life of an engineer
You’ve got yourself a regular order from your customer, he orders fifty components from you every six months regular as clockwork, you have a thought, “why don’t i make a hundred, that way i’ll save on the set-up time, i’ll probably get a bigger discount on the plastic, everyone’s a winner”. You deliver your first batch of fifty and put the second batch in your cupboard ready and waiting.

Sure enough, six months later, the order arrives, your customer thinks you are amazing, they are on his doorstep the very next morning, things couldn’t be better, you’ve cracked it. Around lunch time the phone rings, its your customer, he’s obviously going to place a big order you ponder, but what he does tell  you puts you off your dinner, “your recent batch of components have failed inspection”, not convinced you arrange to pick them up in the morning and promise to get to the bottom of it.

Back at your bench, you dig the drawing out and your vernier and you start to measure, they’re not 80mm as per the drawing, they’re 80.4mm, you re-set the vernier and try again, they’re still measuring 80.4mm, what’s happened? Am i losing my mind? How come the first batch was fine and this batch is completely different? You look round for the camera’s surely you’ve been setup.

So what could have happened?
Moisture in the environment is absorbed by plastics and as the water is absorbed your component begins to swell in all directions, common sense tells us that the more moisture, the greater the expansion but it also matters which plastic

Who are the bad guys?
Who are the main offenders? The ones to watch out for are actually the most common engineering plastics out there, Nylons and actually the biggest offender of them all is good old Nylon 6. If we’re looking at numbers, Nylon 6 can absorb upto 0.6% in the wrong environment

Who are the good guys?
No one gets off scot free as they are all guilty of the offence in some way but plastics such as Acetal and Delrin are pretty good as are HDPE and UHMWPE and again if we are looking at numbers Acetal for instance will only absorb a maximum of 0.1% which is 6 times less than Nylon 6.

Where it matters
Actually this only matters in components that are going to live their life in a wet or damp environment and also where the component needs to hold its very tight tolerance, we’ve heard of examples where a freely moving bush one day ends up locking solid the next. If the component isn’t on a tight tolerance then its irrelevant.

On another note, this potentially critical component now has H20 as a percentage of its make up which alters its performance in other area’s such as its rigidity and other strengths.

To be honest, if its a tight tolerance component and its used in maybe a marine environment then its pretty safe to discount the use of nylon 6 or any of the nylons, its probably a time to look at other engineering plastics such as Acetal or Delrin where they dont prefer a drink. If tolerances are not so tight then nylon is back on the menu.