Using localized heat to fuse UHMWPE and others

[TurboLS]

Hey guys. I am a newcomer to this forum. I am trying to get fully cured or even B-stage ultra high molecular weight polyethylene to fuse to itself with the use of localized heat. I'm just wondering if there is a resource out there that goes over different thermal profiles used in post-process joining. I read over a few of the threads on plastic welding and other adhesive methods, but I haven't found anything definitive.

 

[dgowans]

http://www.bransonultrasonics.com

Don't let the name fool you, these guys do much more than ultrasonic plastic joining. We use a hot plate welding operation on our products and have a Branson machine.

 

[TurboLS]

I'll check them out. thanks.

 

[Demon3]

I suspect that you will need both heat and pressure to get that to work, UHMWPE does't really melt and flow. You should be able to find conditions for pressing parts from it because that's how they make hip replacement joints commercially. It may be possible to use a lower molecular weight PE to weld the UHMWPE together, worth a try.

 

[TurboLS]

yeah, I can apply pressure if I need to. I have already burned a bunch of samples trying to get this stuff to flow. I did, however, get one case where I successfully got it to work, but the joining area was too small. I got a shear test of about 6 MPa on a 9mm x 2mm lap joint. I'm also trying to do similar things with PC-ABS plastic.

 

[patprimmer]

UHMWPE will need a lot, and I mean a lot of pressure.

If the joint geometry allows it, ultrasonic, vibration or spin welding (ie friction) will be the best ways to generate the heat.

PC-ABS will be easy. It can be solvent or friction welded. Respectable raw material suppliers will be able to provide volumes of information.

Regards

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.

 

[TurboLS]

let's say i load it down (.5mm thick lap joint of UHMWPE) on a load cell with 100lbs or so over a 2.5cm x 2.5cm area. Any idea what kind of heat profile I would need for that?? I'm trying to move away from the ultrasound and other specialized devices, if I can.

 

[patprimmer]

You really need to do it by trial and error, but a substantial time with both surfaces in contact with a hot surface at say 100 deg C. It might work as cold as 90 deg C or it might require over 130 deg C, depending on time, heat transfer efficiency and time between heating and bringing the pieces together and applying pressure.

If I understand you last post, I read an applied pressure at the welding interface of 100 psi. once again, trial and error, but I would guess more like 500 psi or more might be required. Like I say, I GUESS. I have no first hand experience at this specific material.

Regards

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.

 

[Compositepro]

From my limited experience in this area I would guess that a lot more than 100C will be required to weld UHMW with a hotplate method - more like 200C.

 

[Trlby]

Depending on your application this may be done using impulse sealing. We have done this succesfully for many customers in the past, and would be happy to advise you.

Bob Mileti

 

[TurboLS]

i'm not looking to buy equipment or other products, i'm just looking for profiles of how other people/methods get this done. you say you use impulse sealing. could you describe the process in greater detail?? for example, if i wanted to fuse a 1cm x 1cm joint of cured UHMWPE, what pressure, heat, time at temp, etc. would i need to get that done.

thanks for the input BTW.

 

[Trlby]

First off I wasn't trying to sell you equipment or other products, since I still don't know what your trying to do. I only offered to advise you.

Like any process the materials define what can and cannot be impulse welded. Impulse welding can be very efficient in thin ( .7mm thick or less )gauges or butt welding, but this may not fit your application. So my first question would be how thick are the 1cm X 1cm parts your looking to seal? There is also a bit of "black magic" in welding, since there are hundreds of formulations of UHMWPE, due to additives, such as colorants and processing agents, etc.

 

[TurboLS]

i see. well one batch i have is roughly 40um thick. i think that would qualify for the "thin" applications to which you refer.

 

[Trlby]

Yes that make it a lot easier.

So here's the answer to your question as to how "impulse welding" works.

Impulse Welding uses a thin resistant wire or band, usually called a heating element, that heats up in a few hundreds of a a second to operating temperature. Once it reaches the desired temperature it holds it for , once again a desired time, then the energy is removed from the element to allow the sealed components to cool to a state that give you your weld. This is different that a heated die type tool, since the advantage is very quick seal times, and the ability to cool the process down before taking the pressure away. This ability to hold pressure is very important when sealing hard to seal materials, since you need to insure the two parts are properly knitted together before removing the pressure. Constant heat dies tend to pull the seal apart prior to the knitting of the 2 parts.

Pressure is important, but too much can cause "extrusion" of the material, therebye reducing the strenght (cross section) of the seal. So the "black magic" part is balancing the amount of pressure with the correct profile of temperature.

Something in the 40um thickness could be sealed in about 1 to 2 seconds and if all goes well can end up to be stronger than the material around it.

Now the next question is shape. Straight seals are the easiest but many shapes are also very doable. We've made sealing elements as small as a 12mm dia. ring seal, with and element width of .8mm wide to 10mm wide bands 4 meters long.

Hope that helps.

Bob

 

[Demon3]

Your question "I am trying to get fully cured or even B-stage ultra high molecular weight polyethylene to fuse to itself with the use of localized heat" confuses me. As curing refers to thermoset materials like epoxies that harden on hearting. UHMWPE is not that type of material, it does not cure.

PE is a thermoplastic where adding some heat and maybe pressure will allow parts to fuse (actually due to interdiffusion and then entanglement of the long polymer chains). UHMWPE, although chemically similar to PE, has chains that are so long that it's impossible to make it flow / melt. That means a lot of heating / pressure and waiting will be needed to allow the extremely long chains to interdiffuse giving adhesion.

Therefore as mentioned you might use a thin piece of normal PE inbetween the UHMWPE to help or just look for a glue. Polyolefin glues exist I believe, for example from Loctite.

 

[patprimmer]

X linkable PE cures. Maybe there is a terminology problem here. X linkable is a thermoset and will be very difficult to cure.

I am a bit perplexed about the changing dimensions of the part.

If it is UHMWPE, it will take a lot of pressure to get any real extrusion as the melt viscosity is very high.

The temperatures required will depend partly on molecular weight and thickness, but more on degree of crystallinity. I have welded metalocine LDPE at 90 deg C but needed 130 for MDPE. Even higher temperatures will be required for HDPE.

Molecular weight has nothing to do with density.

To my knowledge, UHMW and X linked PE are both normally LDPE.

If it is X linked, if you can weld it together then cure you should get an excellent bond.

Regards

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.

 

[Demon3]

There are silane derivatised PE types used in the cable industry. They cross-link themselves when exposed to moisture. That's the only curing type I know about.

 

[TurboLS]

so just to confirm, we are talking the ballpark of hundreds of psi to apply sufficient pressure assuming I can control heat delivery.

 

[patprimmer]

I just checked. On a welder I built quite a few years ago, to weld metalocine LDPE film and MDPE film I used up to 1000# force over 20 square inches, so that was 50 psi. UHMWPE will need higher pressure than that. how much more is a wild guess.

I determined the pressure by building a small prototype and testing before I built the full sized machine. The prototype had 2 square inches and a single air cylinder and a pressure regulator.

Metalocine LDPE will flow a lot easier than UHMWPE.

As I said earlier, a lot of guesswork involved, so some trial and error will be required.

Some sort of vibration or friction to generate the heat at the welding interface will be a lot better as the flow generated will mix the melted plastic where required.

If you use conducted heat, the welding interface will be the coolest spot and the material will flow everywhere but where required.

Regards

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.

 

[Compositepro]

Another interesting method is hot-wire welding where the wire is electrically heated and left in the joint. For larger area bonds you can use a woven wire screen, like aluminum window screen. In any type of plastic welding you do need to be aware that at high temperatures the plastic will oxidize when exposed to air and that will interfere with fusion. So you want to minimize time at temperature and keep air away by nitrogen purge or intimate contact at the bond line. Its not so important with the low melting polymers but is important with UHMW.