Wednesday, March 26, 2014

[Tech] The Devcon Tenets: Methacrylate plastic welding adhesives

There are very few tools and materials that have reached the level of "indispensable" for my work in this hobby, but one of the few on that list would have to be the methacrylate 2-part structural adhesives often known as "plastic welders", "plastic bonders" or "Devcon". These are a tool for the practical nerfsmith or structural plastics fabricator that I absolutely cannot overstate the utility of.

A general overview

Adhesives have a bad name in nerf. There have been far too many failures and flaky results over the years, but these are almost always caused by inept methods and simple use of the wrong tool for the job, not complete unsuitability of the entire class of adhesive assembly methods, which is extremely broad. Adhesive bonding and solvent welding, particularly of plastics, should never be counted out - and in some cases is indisputably superior, provided the correct methods and materials are involved.

Frequently "the job" in this hobby is:
  • To create an effective single unit from two or more pieces of a bondable plastic*
  • To bond a plastic to another material, such as a metal or a composite
* Bondable plastics include ABS, from which a great number of stock nerf gun parts are made; other styrene-based plastics such as PS, SAN, HIPS, etc. which are similar to ABS; PVC, which is frequently used as a cheap and mechanically high-performance material by hobbyists; polycarbonate; acrylic, and many others.

For these uses, methacrylates like "Devcon" should always be a consideration! They excel where others often fail.



A note on Devcon

Let's get this straight: Devcon is a company (ITW Devcon, Inc.) and a brand of adhesives and similar products. You can go here and check them out.

The company has a full line of hardware store 2-part adhesives branded "Devcon" that you may have seen, including many epoxies. But the term "Devcon" in the nerf hobby is almost ALWAYS referring to Devcon Plastic Welder and its relatives.



Shown here is its current retail packaging on the left and the older on the right, which you may have seen at some point. This product is available both at hardware stores and from more specialist suppliers (in which case the familiar old Devcon logo will still be used).

When Devcon... isn't Devcon

There are a number of competing and other brand products of this type in the hardware store market, including Permatex and other rebadges of the Devcon product (recognizable by the Devcon tube), but the most notable is Loctite's Plastic Bonder:

Note the presence of the term "epoxy" - long story short, it's wrong and is there for product recognition. We will get to that later. However, of special note is that the Loctite packaging for their Devcon PW knockoff is labeled "PLASTIC BONDER" and "Fuses Plastics". Beware a similar product without these markings, because that stuff is not methacrylate.

You may also buy MMA adhesives from industrial/specialty sources, in which case, do your homework on the manufacturer's website and use something appropriate. Devcon has a "Plastic Welder II" that may come in handy for applications involving nylon and other difficult plastics.


Performance characteristics, and comparison to epoxy adhesives

The main reason to use MMA adhesives rather than alternatives in nerf is inherent to the chemical constituents. Methyl methacrylate itself is a solvent for most bondable plastics. Thus, the terminology "weld" or "fuse" is not incorrect; significant fusion occurs when a MMA adhesive contacts a material like ABS or PVC. If you spill a little Devcon and try to scrape it off, the surface of the plastic will be softened, just like you may have noticed while using solvent cement to weld PVC piping systems together. The result of a Devcon joint is a weld, with a seamless transition from i.e. ABS into the cured acrylic resin of the adhesive.

This is in contrast to epoxies, which have no solvent property, create absolutely no fusion and function only as a glue producing a much weaker mechanical bond.

MMA's solvent properties also extend to surface contaminants. Combined with the lack of dependence on surface roughness to achieve "keying" (often emphasized by epoxy users as of prime importance) due to the solvent-welding action, MMA adhesives are very carefree to work with. I have made bonds between pieces of completely unprepared, 15 year old, oxidized PVC that had been sitting outside in the sun, and was unable to break them without failure of the material itself. That said, sanding is always cheap insurance and is necessary on metals and other materials that cannot be dissolved.

Epoxy, however, is very sensitive to prep and cleanliness. Oil, silicone residue from injection molding release agent, oxidation or even just a glossy ABS surface can make epoxy fall right off.


The second main advantage to the MMA "plastic welders" is the plastic-optimized mechanical properties. The cured resin, unlike most consumer epoxies, is a very tough and impact-resistant material and has similar characteristics to i.e. ABS and PVC; thus there is not a major modulus mismatch to create stresses in the bond, as occurs when you have rigid JB Weld (epoxy) or 5-minute epoxy in contact with ABS.


Wait. Plastic epoxy?

Confusion of 2-part methacrylates with epoxy is very common. Consumers will call any 2-part glue epoxy. Many nerfers have unwittingly called a methacrylate adhesive epoxy at some point, including noob me. Henkel/Loctite isn't helping. In fact, "epoxy" is used as a "product category" for 2-part adhesives by Loctite.

This is unfortunate, because it creates much confusion about the available adhesives and their performance.

So be aware of the difference. Epoxy and methacrylate resins are not chemically related in the slightest. Read the warning on the package - if it includes Contains methyl methacrylate and methacrylic acid or the like, this is the stuff you want. This is a surefire way to make sure you get the right stuff no matter what brand you use. Conversely, epoxies are always labeled with epoxy resin. That "non-Bonder" Loctite stuff, sure enough, contains epoxy resin, and not methyl methacrylate. Be a savvy buyer.


But there ARE gremlins!

What, you thought it really was a wonder material? Was I sounding like Billy Mays? Too good to be true?

Actually, there are only a few "gotchas" about MMA adhesives.

One is shelf life and environmental sensitivity. Epoxies store very well. You can use decades old epoxy. This is NOT true of methacrylates. Unlike epoxies, the resin is not a simple system of 2 monomers that react automatically on mixing, it involves a small amount of "activator" which triggers polymerization. This activator is sensitive to heat and ages rapidly, and as it degrades, the resin may cure slowly, not at all, or may not achieve its full properties.

As a result:
  • Devcon has a shelf life. Only buy as much Devcon as you think you may use soon. For most of you including me, that is ONE tube. Do NOT stock up. Do NOT buy from a store that never moves any product. Go to the busiest supplier you can find.
  • "Store in a cool dry place" is not a generic suggestion. Store Devcon AWAY from heat sources. A hot car is NOT the place for Devcon; I have had multiple tubes go bad from my toolbox being left in a car! Sometimes I put Devcon in the fridge.
  • ALWAYS test an unknown (stored, newly bought, etc.) tube of Devcon by mixing up a small batch BEFORE using it on a critical application! If it doesn't cure HARD within the manufacturer's specified cure time, be suspicious! If it doesn't cure hard within a few hours, it's a total loss, go get a new tube!
The second big "gotcha" is that some metals, such as copper, inhibit MMA curing. Do not use Devcon to glue brass tubing, as the bond will never achieve strength. This is an application where epoxy is a better choice. Steel and aluminum, however, are perfectly fine.

Also, MMA adhesives have a distinctive sharp acrylic odor (the smell of MMA vapor). I am used to it; the smell of Devcon is the smell of work getting done and impending zombie annihilation! However, ventilation is probably desired. You can really stink a place up with the stuff.


Miscellaneous usage considerations

"Plastic welder" is a 2-part adhesive - most nerfers are familiar with epoxy usage. Dispense the correct quantities as specified (usually equal volumes in consumer products) and mix very thoroughly, then apply to part to be bonded, assemble and let cure.

Some "dual syringe" tubes fail to dispense evenly. I usually cut the 2 plunger rods apart and operate each independently. MMA is not very sensitive to mix ratio variations; and these tubes usually don't measure any more accurately than "eyeballing" the quantities (in fact most end up way off), so you can save on wasted product from a malfunctioning tube by separating the plungers and estimating the correct ratio.

Most "plastic welders" have excellent gap fill characteristics.

As an alternative to solvent welding and thermal welding

Consider "plastic welder" if you were thinking of using either aforementioned processes.

Solvent welds are typically weaker than the base material because of voids/poor structure. True solvent welding involves a solvent that evaporates or diffuses away, so the joint shrinks. Pure solvents and almost-pure solvents like MEK, Weld-On 3, etc. have absolutely zero gap fill and require perfect fit and interference to bond parts. Even filled solvent cements (like PVC and ABS pipe cements) that contain added plastic resin will still shrink, create voids and fill gaps poorly as the solvent is present in the initial adhesive but not the final bond.

By contrast, MMA still achieves fusion but is 100% solids. It cures by polymerizing, not by evaporation of solvent, and doesn't change volume in the process.

Thermal welding without specialized equipment similarly creates a weak joint. Save your soldering iron tips for soldering and quit breathing ABS fumes. MMA adhesive will make a stronger result and is easier to use and add material to a joint with.

As an alternative to mechanical fasteners

"Mechanical fastening" is a buzzword, particularly in the Proper NIC, and is frequently listed as a feature: "No adhesives - 100% mechanically fastened" (specifically referring to cases where mechanical fasteners such as screws or bolts are used to replace an adhesive or weld rather than where removability is required). This is due to that previously discussed mistrust for adhesives and history of failures. Since epoxies are often regarded as the go-to structural adhesive in nerf and alternatives are infrequently encountered for applications like installing grips and integrations and doing structural mods and repairs, it is easy to see where the idea originates.

However, the idea that mechanical is superior is flawed. Often, a mechanical joining of two parts creates stress concentrations and the seamless bonded or welded part would be stronger, cheaper and cleaner - of course, given that a weak or incorrect adhesive or bad workmanship is not there to throw a wrench into the matter.

MMA "plastic welder" on ABS, PVC et al. is certainly an example of a proper adhesive selection, and one that can actually perform better than any mechanical fastening in some cases since a uniform area (rather than individual fasteners) applies forces to the material.

Example usage of MMA adhesives in nerf

Um, Read this blog? Everything I do... 90% chance the stuff is there! Some notable and some mundane examples below:

 "Lockheed Bot": Devcon was used to bond an aluminum bracket and a steel hinge half to an ABS nerf gun receiver after continual problems with failure of screws and bolts. Introduced me to the use of this material.


Stryfe Cyclone: Devcon was used to fabricate motor mounts and the custom stock.






Numerous underslung integrations: Devcon was used to bond ABS clamshells to produce barrel shroud/handguard sets with integrally mounted Swarmfires, Rapidstrikes and Roughcuts. All of these have minimal bond areas and are subjected to considerable mechanical abuse from gameplay.






Oblivion Rifle: A Stryfe was structurally modified and fitted with scratchbuilt shrouds and other parts to create a prop replica. The materials were mainly PVC and ABS. All structural bonding, with very few exceptions, was with Devcon and Loctite Plastic Bonder.



RP Alpha Trooper: Devcon was used to bond a steel reinforcement to an ABS plunger tube. This application is subject to heavy shock and vibration that could crack or delaminate epoxy from the ABS. The part has been in service since mid 2011 without failure. Additionally, Devcon was used to repair and bond a reinforcing gusset to a cracked sear lug, which was initially stock, on this same PT at a later date.


Grips and trigger packs: Devcon has been used in many installations of integral pistol grips, trigger units and the like on clamshell host parts. This is a very failure prone application in many cases, since the bond area is small and the protruding grip offers much mechanical advantage when a gun is hit, fallen on, etc. This includes use on a circa 20lb (equipped and loaded) Vulcan.


Stampede BCG: Devcon was used to repair a busted-up bolt group and fabricate the destroyed endplate.
Countless others have been left out, but rest assured, if I did it, Devcon is holding it together. What is surprising even to myself is that despite all the abuse HvZ gear gets from me and others who use my work, not one of my Devcon joints has ever failed. Generally, if a structural failure is to occur, adjacent parent material will break long before anything happens to the Devcon bond line.

So if you don't use this stuff already, give it a shot. I guarantee it will change the way you build.

5 comments:

  1. Another hopefully habit changing blog post! Just waiting on the epoxy user backlash......
    Thank you for posting another proper and nicely illustrated technical discussion.

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  2. Well that was eye opening. I shall be investing in some devcon next time I am planning some body work.

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  3. Devcon employee here. Awesome post!! I've been selling this stuff for years and use it for several projects around the house. We store this in refigeration units in our factor until it is packaged and ready to ship. Also, notice that we only use black syringes for Plastic Welder to help protect it from UV life and extend shelf life. We sell the same product under the Devcon Home and Permatex brands - the adhesive is the same.

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    Replies
    1. AWESOME! That is so cool to see someone from the company show up in our hobby and comment on a blog post!

      That is interesting to know about the refrigeration. I guess I wasn't far off to refrigerate Devcon.

      You make an awesome product much appreciated in the community. I have done some serious things with Devcon Plastic Welder and I have still never had a failure. Also, I do prefer and recommend your product over competitors. Loctite's product is a bit brittle and I don't quite trust it.

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    2. Any idea if I can get the same product in Singapore?

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