Using UHMWPE or HDPE/ cutting board plastic as kydex alternative.

[Huntsman Knife Co. LLC.]

Since I've become a full time maker I'm alot more cautious about exposure to chemicals and vapors. I'm taking steps to reduce the amount of dust in my shop and have continual clean airflow through a positive pressure system but the one thing that I am really worried about in terms of longterm exposure is kydex.

Kydex is great but it's vapors and dust are carcinogenic and I am trying to get away from using it due to the health risks. Even with a massive high volume fan blowing on the oven and the door open, the fumes still permeate the the shop. Kydex dust is also extremely difficult to control and clean up and seems to get everywhere.

I'm looking for an alternative to kydex that is safer and possibly better for sheaths. UHMWPE or possibly HDPE/ cuttinboard seems like it may be the answer. Its tougher than kydex, self lubricating so blades wont stick, its inexpensive, is much more temperature resistant than kydex, and most of all is completely harmless to humans. It also has antibacterial properties which make it a great choice for hunting knife sheaths ( getting blood in your leather or kydex sheath can't be healthy).

I've toyed with it some thick sheets and it definitely can be thermoformed but Im not sure how much. Does anyone have any experience using this stuff? Has anyone made a sheath out of it? If so how did it go? I plan on buying some in .060, .093, and .125 and testing it for sheaths.

 

[Nathan the Machinist]

UHMW PE does not thermoform very well. It is also quite soft and mushy.

HDPE does thermoform well, but not as well as Kydex because, being crystalline rather than amorphous, it has a narrow processing window. In my opinion it would work okay, but not as well as kydex.

No polyolefin is as snappy as kydex. They also tend to creep over time.

Kydex ages well in various environments, whereas something like polycarbonate (which forms well at higher temps and is quite snappy) tends to crack over time, particularly when exposed to oils.

A thermoformable grade of nylon would have good potential, and I have seen polyphenylene oxide available in formable sheet that would be pretty cool too, though more expensive than kydex.

 

[Don Nguyen]

UHMW PE does not thermoform very well. It is also quite soft and mushy.

HDPE does thermoform well, but not as well as Kydex because, being crystalline rather than amorphous, it has a narrow processing window. In my opinion it would work okay, but not as well as kydex.

No polyolefin is as snappy as kydex. They also tend to creep over time.

Kydex ages well in various environments, whereas something like polycarbonate (which forms well at higher temps and is quite snappy) tends to crack over time, particularly when exposed to oils.

A thermoformable grade of nylon would have good potential, and I have seen polyphenylene oxide available in formable sheet that would be pretty cool too, though more expensive than kydex.

Man, where did attain such knowledge? It would be great for me to learn this stuff. I'm in Materials Engineering but we haven't touched on anything specific like this yet.

 

[Huntsman Knife Co. LLC.]

Nathan,

Thanks so much for the advice. I plan on making most of my sheaths a plastic/ leather hybrid~martin swinkles style. I think I will buy some .060 or .093 HDPE and see how it does when covered in leather. The leather should ease the environmental and UV stress on the plastic and maybe help a little bit in preventing creeping since it will give some support to the plastic. The big questions will be if I can form it tight enough and how it holds up over the years in various environments. I'll see if I can design the sheaths in such a way that the snappy-ness is not as critical. Im thinking something like the adjustable tension screw system that ESEE uses on their Jungals sheath. Im guessing that most of the knives will more or less slide in as opposed to snapping into place. This could be an advantage for wooden handles where the extreme rigidity of kydex causes wear on the finish.

I'd really love to find something that works as well or nearly as well as kydex and is non toxic and non carcinogenic. HDPE seems to be the best option that is readily available in thermoforming sized sheets. I'm going to give it a try and report back the results.

 

[Nathan the Machinist]

Man, where did attain such knowledge? It would be great for me to learn this stuff. I'm in Materials Engineering but we haven't touched on anything specific like this yet.

Oh I love materials engineering, it is a fascinating subject. I could probably listen to you talk for hours. To answer your question, I was a plastic product design consultant for a number of years before I opened shop so I was once deeply involved in plastics. I was educated at NCSU, but really didn't learn much specifics about plastics until I was immersed in it after I graduated. The SME have some very good books on the subject I can recommend if you're interested. Plastic Part Manufacturing Edited by Philip Michell is just fantastic. Absorb that book and you can form a career on it. ---> Seriously <---

While there are a daunting number of different polymers, for the most part they break down into a few families of commodity plastics used in various plastic manufacturing processes. For this application you can look at the materials commonly used in thermoforming and blow molding and to a lesser extent extrusion because they have the same melt strength issues and materials available. After a while you'll go, "oh, snappy creep and chemical resistant formable plastic, that's gonna need some acrylic and PVC in there, so yeah, kydex."

To my knowledge nobody is using nylon for this, but I know it can be blow molded, so I'm sure there is a grade out there somewhere that would work from a processing point of view, it is has great stiffness, strength, lubricity and chemical resistance and it would gall less than kydex. And while it is often thought of as hard and somewhat brittle, there are so many different flavors of it that you can actually find some that are rubbery even without the use of plasticizers. This is where you materials engineers come in with your fancy block co-polymer elastomeric PA or whatever that guys like me see on the sheet and point at and say "oh, I like that one"...


A few years ago there was some excitement surrounding cyclic butylene terephthalate for stuff like this. It's a polyester that when heated it becomes water thin and can be used to wet a fiber such as fiber glass or carbon fiber. A reinforced sheet of it can be processed in shallow parts like this just like kydex, but once processed I believe it either crosslinks or otherwise further polymerizes and you get a hard smooth tough snappy part that would be great on a sheath. I wonder if they ever finished developing it?

 

[Nathan the Machinist]

Nathan,

Thanks so much for the advice. I plan on making most of my sheaths a plastic/ leather hybrid~martin swinkles style. I think I will buy some .060 or .093 HDPE and see how it does when covered in leather. The leather should ease the environmental and UV stress on the plastic and maybe help a little bit in preventing creeping since it will give some support to the plastic. The big questions will be if I can form it tight enough and how it holds up over the years in various environments. I'll see if I can design the sheaths in such a way that the snappy-ness is not as critical. Im thinking something like the adjustable tension screw system that ESEE uses on their Jungals sheath. Im guessing that most of the knives will more or less slide in as opposed to snapping into place. This could be an advantage for wooden handles where the extreme rigidity of kydex causes wear on the finish.

I'd really love to find something that works as well or nearly as well as kydex and is non toxic and non carcinogenic. HDPE seems to be the best option that is readily available in thermoforming sized sheets. I'm going to give it a try and report back the results.

HDPE does just fine with environmental exposure. The only problem compared to kydex is that it is comparatively soft and has less creep resistance.

 

[Don Nguyen]

Oh I love materials engineering, it is a fascinating subject. I could probably listen to you talk for hours. To answer your question, I was a plastic product design consultant for a number of years before I opened shop so I was once deeply involved in plastics. I was educated at NCSU, but really didn't learn much specifics about plastics until I was immersed in it after I graduated. The SME have some very good books on the subject I can recommend if you're interested. Plastic Part Manufacturing Edited by Philip Michell is just fantastic. Absorb that book and you can form a career on it. ---> Seriously <---

While there are a daunting number of different polymers, for the most part they break down into a few families of commodity plastics used in various plastic manufacturing processes. For this application you can look at the materials commonly used in thermoforming and blow molding and to a lesser extent extrusion because they have the same melt strength issues and materials available. After a while you'll go, "oh, snappy creep and chemical resistant formable plastic, that's gonna need some acrylic and PVC in there, so yeah, kydex."

To my knowledge nobody is using nylon for this, but I know it can be blow molded, so I'm sure there is a grade out there somewhere that would work from a processing point of view, it is has great stiffness, strength, lubricity and chemical resistance and it would gall less than kydex. And while it is often thought of as hard and somewhat brittle, there are so many different flavors of it that you can actually find some that are rubbery even without the use of plasticizers. This is where you materials engineers come in with your fancy block co-polymer elastomeric PA or whatever that guys like me see on the sheet and point at and say "oh, I like that one"...


A few years ago there was some excitement surrounding cyclic butylene terephthalate for stuff like this. It's a polyester that when heated it becomes water thin and can be used to wet a fiber such as fiber glass or carbon fiber. A reinforced sheet of it can be processed in shallow parts like this just like kydex, but once processed I believe it either crosslinks or otherwise further polymerizes and you get a hard smooth tough snappy part that would be great on a sheath. I wonder if they ever finished developing it?

Thanks for such a detailed response! I put a bookmark on Plastic Part Manufacturing Edited by Philip Michell; will probably read it during the winter or during one of my breaks.

Really neat stuff for sure. I can't even imagine what kind of stuff will be developed in the future.

 

[Nathan the Machinist]

Nathan,
HDPE seems to be the best option that is readily available in thermoforming sized sheets. I'm going to give it a try and report back the results.

Here's another thought. I don't know if it makes any difference in your application, but the shrinkage for HDPE is probably about 3X that of kydex. You can probably expect about 1/8" of shrinkage across the length of a typical length sheath as it cools in HDPE.

 

[grizzled gizzard]

I've never heard or seen warnings that Kydex is bad for you.

Would a charcoal filtered respirator take care of the fumes?

 

[Hankins]

This is a link to my local supplier. Very knowledgeble or will point you in the right direction!
http://www.professionalplastics.com/

 

[1066vik]

Nathan,

Thanks so much for the advice. I plan on making most of my sheaths a plastic/ leather hybrid~martin swinkles style. I think I will buy some .060 or .093 HDPE and see how it does when covered in leather. The leather should ease the environmental and UV stress on the plastic and maybe help a little bit in preventing creeping since it will give some support to the plastic. The big questions will be if I can form it tight enough and how it holds up over the years in various environments. I'll see if I can design the sheaths in such a way that the snappy-ness is not as critical. Im thinking something like the adjustable tension screw system that ESEE uses on their Jungals sheath. Im guessing that most of the knives will more or less slide in as opposed to snapping into place. This could be an advantage for wooden handles where the extreme rigidity of kydex causes wear on the finish.

I'd really love to find something that works as well or nearly as well as kydex and is non toxic and non carcinogenic. HDPE seems to be the best option that is readily available in thermoforming sized sheets. I'm going to give it a try and report back the results.

If you're planning on gluing the leather to the HDPE as part of the sheath construction, you'll need to flame the surface to be glued or wipe it down with acetone to strip the surface oils -- or use a contact cement with a scary high volatile count, which somewhat defeats the purpose of using that material. If you don't do something for surface prep, most adhesives compatible with leather won't adhere well to the HDPE.

 

[Shawn Hatcher]

Man, where did attain such knowledge?

I have it on good authority that a deal with the devil can get you such knowledge. But I hear tell being an Evil Genius isn't all it's cracked up to be, regarding benefits and such. But all the chicks make it worth it.

 

[Huntsman Knife Co. LLC.]

If you're planning on gluing the leather to the HDPE as part of the sheath construction, you'll need to flame the surface to be glued or wipe it down with acetone to strip the surface oils -- or use a contact cement with a scary high volatile count, which somewhat defeats the purpose of using that material. If you don't do something for surface prep, most adhesives compatible with leather won't adhere well to the HDPE.

I was planning on trying to use G flex to bond the plastic and leather. I'm not sure if it will work though.

 

[timos-]

You may have luck with a pressure sensitive adhesive tape. I have seen that used with uhmw/hdpe wear strip.

 

[Nathan the Machinist]

PE is not very good to bond to. There are some surface insensitive CA glues and primers that work okay on polyethylene and some of the acrylate, methacrylate type pressure sensitive adhesives. Not great, but okay. Epoxy not so much. In fact, if you'll use a PE mixing container for mixing your epoxies you can reuse them because the epoxy peals right off after it dries. I wouldn't use PE if you want a good long term reliable bond.