Anealing Polypropylene

[Sceadwian]

This isn't exactly a robotics question, but it's related to a common material used in it. I have some Sterilite (brand) containers that are made from clear (foggy white) plastic with a #5 (Polypropylene) recycling mark on the bottom.

The problem is some of the drawers are warped. A couple of the drawers were warped juuust enough to keep them from closing properly and completely. Random heat gun tests made the warping worse not better as I wasn't able to figure out the various states the plastic was in during heating and cooling.

Any experts here on this type of plastic? I want to basicaly after I obtain the position they need to be in heat them to the point where it aneals (relaxes all internal stressing) but not melts, and then cool it so that it keeps the form it was in when it was annealed.

I think this might require heating to a specific temperature and then cooling gradually but I don't know to what temperature and and what rate the cooling needs to be done. Can anyone provide reference? As best as I can tell it's not cross linked Polypro, it zippers just fine if torn.

 

[upand_at_them]

If you don't get any luck here, you might try some of the machining forums, like this one: 7x12minilathe : 7x12minilathe - mini lathe users group A lot of the guys on there have experience with many different materials besides metal.

 

[Sceadwian]

I broke my own rule =) I should have Googled it first! Then again I didn't think of using the word aneal until I wrote this post.
https://www.plasticsintl.com/documents/Polypropylene%20Annealing.pdf

Now just to figure out how to adapt those instructions to an electric oven at home =O I'm worried about the infrared radiation from the heating coil spot heating the polypro. I've torched silicon sheeting in an electric oven before because of that.

 

[upand_at_them]

Heat it by conduction? Heat a stone and then use the stone to heat the plastic?

 

[Sceadwian]

Sorry upand, not sure you understand my intent or the process involved, heating a stone and directly applying it to polypro is about the worst possible thing you could do, even worse than the hot air gun I was using.

I successfully annealed the box in my home oven. I preheated the oven to 170 to avoid setting the cardboard on fire from direct IR from the primary heating coil (the cardboard will be clear later). The PDF file I linked recommend you gently increase the temperature and use fan forced air but I had no problem with passive convection. Once it was preheated I put a couple sheets of cardboard in to avoid direct IR exposure to the plastic box and placed the box on top of the cardboard on the center rack. After an hour the temp was only 180 (as read by an IR non contact thermometer) so I set the oven set point up to 180. The peak temperature measured was about 190 degrees. I let it sit at the 180 oven set point (which resulted in the 190 temperature reading from my IR gun) for about 30 minutes and then shut the oven off, about two hours later it was down to near 110 degrees so I popped it out and set it aside.

The side only had the slightest hint of a wave left to it when I took it out, and after it open air cooled it was straight as an arrow, I can't even tell which drawer it was anymore.

Please note, I'm not entirely sure this process will work with crosslinked polypro but most consumer devices don't use crosslinked polypro.