PTFE parts expanding after transporation :(

aerobharath · Jun 4, 2015

Hi all,

I would really appreciate if anyone can help me with this issue.

1mm thick PTFE parts made in the UK is found to expand after the customer receives it in ASIA.

All parts successfully passed inspection to drawing in th UK but found to have expanded beyond tolerance after it has reached the customer.

Im aware that the thermal expansion coeff is really high for PTFE but would it make much difference from UK to ASIA?

Do you think the temperature fluctuation when transporting made it or PTFE requires a SPECIAL PACKING?

Kindly let me know your thoughts on this.

thanks.

gustafson · Jun 4, 2015

aerobharath said:
Hi all,

I would really appreciate if anyone can help me with this issue.

1mm thick PTFE parts made in the UK is found to expand after the customer receives it in ASIA. All parts successfully passed inspection to drawing in th UK but found to have expanded beyond tolerance after it has reached the customer.

Im aware that the thermal expansion coeff is really high for PTFE but would it make much difference from UK to ASIA?

Do you think the temperature fluctuation when transporting made it or PTFE requires a SPECIAL PACKING?

Kindly let me know your thoughts on this.

thanks.

I remember an engineer going on about how teflon had a bump in its thermal expansion in the vicinity of room temperature when I was doing some fussy parts many years ago.

perhaps you and your customer need to come to an agreement on what temperature the parts are measured at.

or they are just trying to avoid paying............

Davis In SC · Jun 4, 2015

Not really familiar with machining much PTFE, but size changes are often an issue with plastics. Temperature is a factor, but humidity is usually the biggest problem. I have seen lots of parts, right on spec, grow or shrink out of tolerance, depending on the destination..

18my · Jun 4, 2015

by any chance were the parts machined from an area that was previously compressed from clamping? e.g. vice or lathe chuck? from my experience machining similar material once the material is compressed it takes a long time for it to return to its previous state, meaning it retains compression whilst it gets machined and after a period of time once the part is finished it often relaxes and goes out of tolerance. Personally i NEVER machine a finished part out of an area that has been subjected to clamping stresses unless its absolutely necessary

18my · Jun 4, 2015

ive machined shitloads of carbon filled teflon and using the above method had great success at achieving 0.02-0.03mm tolerances (which have been calculated to be measured at 21 degrees c. also helps to work in a air conditioned workshop lol)

upnorth5axis · Jun 4, 2015

The other shop is most likly hotter than yours. Parts should be inspected in the same temperature. you don't say what your tolerance is PTFE moves around alot.

18my · Jun 4, 2015

from my experience a 8-10 degree c drop in temperature is enough the shrink the part anywhere around 0.07mm or so. Had this argument with customers before. the sizes should have thermal expansion calculated into them and parts should be measured and manufactured at a set temperature. PTFE isnt too bad, dont get me started on nylon lol

aerobharath · Jun 4, 2015

these parts are shipped to India and Malaysia. Temp are definitely higher than the UK. From the feedback form the customer the holes in the parts have moved away almost 2mm wider

aerobharath · Jun 4, 2015

Davis In SC said:
Not really familiar with machining much PTFE, but size changes are often an issue with plastics. Temperature is a factor, but humidity is usually the biggest problem. I have seen lots of parts, right on spec, grow or shrink out of tolerance, depending on the destination..

thanks. Do you know any handbook which specifies working humidity range?

aerobharath · Jun 4, 2015

18my said:
from my experience a 8-10 degree c drop in temperature is enough the shrink the part anywhere around 0.07mm or so. Had this argument with customers before. the sizes should have thermal expansion calculated into them and parts should be measured and manufactured at a set temperature. PTFE isnt too bad, dont get me started on nylon lol

Haha... Do you know at what temperature PTFE will go into non-elastic deformation please?

aerobharath · Jun 4, 2015

upnorth5axis said:
The other shop is most likly hotter than yours. Parts should be inspected in the same temperature. you don't say what your tolerance is PTFE moves around alot.

The issue is that our customer hasnt given us any quantitative data. All they said was that the holes have moved away almost 2mm. And we dont know their inspecting room temp.

aerobharath · Jun 4, 2015

18my said:
by any chance were the parts machined from an area that was previously compressed from clamping? e.g. vice or lathe chuck? from my experience machining similar material once the material is compressed it takes a long time for it to return to its previous state, meaning it retains compression whilst it gets machined and after a period of time once the part is finished it often relaxes and goes out of tolerance. Personally i NEVER machine a finished part out of an area that has been subjected to clamping stresses unless its absolutely necessary

We machined these on CNC Mill by stacking them in jigs. We didnt measure any compression and we finished the part complete in one go. Recently I read online that they have to be rough cut first,left untouched over night to let stresses out and to finish it next day. Is that right?

specfab · Jun 4, 2015

aerobharath said:
The issue is that our customer hasnt given us any quantitative data. All they said was that the holes have moved away almost 2mm. And we dont know their inspecting room temp.

You must get the specific data from your customer. You need to then fully understand the circumstantial differences in your facility vs. their facility. Then get a data sheet for the material, and look at the thermal properties and mechanical properties, and do some calculations to see what the materials physics tells you.

I had experience years ago with making and shipping instrument systems from the southwest USA to southeast Asia. These systems were used for measuring hard disk platters for flatness and shape to nanometer-level accuracy. The disks were held on a plastic hub in the center hole, and worked fine at the factory with the +0-.0005" tolerance applied. When they were installed in the customer site, the disk hubs were +.001" or so, due to humidity absorption. In your case, the PTFE material has practically no water absorption (<0.01%), so this is likely not the issue.

http://catalog.wshampshire.com/Asset/psg_teflon_ptfe.pdf

18my · Jun 4, 2015

Haha, No sorry im not claiming to be a engineer in polymers. Im just speaking from my personal experience with particular jobs i have done in the past, im a cnc machinist not a engineer.. Sorry if my previous post bothered you in some way

specfab · Jun 4, 2015

18my said:
Haha, No sorry im not claiming to be a engineer in polymers. Im just speaking from my personal experience with particular jobs i have done in the past, im a cnc machinist not a engineer.. Sorry if my previous post bothered you in some way

I don't know to whom you are addressing the comment. I can say that your experiences are perfectly in line with the way the material can behave. My previous comments were addressed to the OP.

To the OP: A brief example of thermal effects for PTFE as follows --
Coeff of thermal expansion (CTE) = 6.5x10^-5 in/in/degree F

For a part 10" long, a change of 1.0 degree F will result in length change of 65 x 10^-5 inches. (.000650")
For this same part, a change of temperature of 10 degrees will change the part .0065" in length, or about 0.16mm.

For a part 1" long, the overall size change indicated would be 1/10th of the example above.

If your parts are 50" long, and go from 68 F to 80 F environment, the length change will be 6.5 x 10^-5 x 50 x 12, or .039" (about 1mm overall expansion). As previously noted, you really need to get the facts from your customer, and then start figuring out the problem.

SVE Performance · Jun 4, 2015

We used to manufacture parts out of PTFE .They were bottle caps about 3" in dia about 1" thick when we first started making them some would literally turn inside out over night . We finally started cutting off blanks from bar stock and stress relieve them by putting them in hot water and then cold water for a half dozen cycles .After that we had far fewer problems with the parts changing dimension. Bill

Davis In SC · Jun 4, 2015

As someone mentioned, is there a chance that they are trying to get out of paying? After all, Asia is not a role model for business ethics.

18my · Jun 5, 2015

specfab said:
I don't know to whom you are addressing the comment. I can say that your experiences are perfectly in line with the way the material can behave. My previous comments were addressed to the OP.

sorry it was directed at aerobharath

RTM · Jun 5, 2015

aerobharath said:
We machined these on CNC Mill by stacking them in jigs. We didnt measure any compression and we finished the part complete in one go. Recently I read online that they have to be rough cut first,left untouched over night to let stresses out and to finish it next day. Is that right?

It varies depending on how hard you've machined it and how much of the material you are removing but creep is a significant problem in these fancy plastics.

I don't know if overnight is necessary but I do plan some jobs to finish in the evening so they can have all night to "relax". Even if you aren't trying to hold +-0.003" it's always a good idea to rough the entire part first (and releasing any heavy clamping forces) before finishing.

PTFE is the only material that has lost us money. We used to make a product called a needle guide (does just what it says). We supplied them for an OEM but the only way we could make money on them was by making them in the 10's of thousands. Unfortunately we threw away half of the parts after finding that after 4 months they had "creeped" right out of spec. They were shattering on installation which was the best case scenario. Worst case is that they only split during installation but would fail catastrophically when the machine was running. That job hurt.

I machine 12 different plastics on a daily basis and I've learned to deal with all the little nuances. PTFE (unfilled) is something I just don't touch, I hate it.

birsmachine · Nov 20, 2015

PTFE class A has a max thermal expansion .005". The PTFE class A product is stress-relieved during production. Engineered Plastic sheet, rod, tube & film

PTFE parts expanding after transporation :(

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