shop talk / update

 I have several short subjects to cover today, and they are all in this image. There are three or four little New Things I've Learned about hot melt adhesives since I wrote up my short summary, and one handy shop trick.

 

Item 1

 The shop trick is the two gray-covered paint-stirring sticks.  I didn't quite include the handle of the second stick in the shot, which has '1500' written on it.

 

 These are just paint stirring sticks (free for the asking from many hardware and paint stores) to which I have spray-glued silicon carbide "wet-or-dry"... uh, abrasive paper.

 

 aside: (Americans at least) throw the term "sandpaper" around a lot, but strictly speaking, sand is silicon dioxide, while most sandpaper is made from aluminum oxide.  But by the same token, this is not sandpaper. ^_^  Okay, I'll stop.

 

 Simply put, the idea is to have a file - something flat and rigid which will remove only the highest peaks of a surface to which it is applied - with a much finer "pitch" than any conventional metal file.  This gets us much finer surface finishes at a much slower material removal rate.

 

 They are best used wet on most materials - I use soapy water - which helps prevent them from clogging and helps premature dulling of the abrasive.  Abrasives should be thought of as tools - they get dull with use as the sharp grains of grit which cut surfaces slowly - or quickly - become rounded lumps of grit which can only rub the surface, and don't cut well.  Once that happens there's nothing for it but to replace the abrasive paper.  The dulling process happens much faster as applied pressure goes up.  So, much as with many other tools in the craftsman's repertoir, the wise rule of thumb is once again: don't apply very much pressure, let the tool do the work.  Those two sticks look newish, but I have been using them extensively, I just keep washing them with a soft bristle brush and soapy water, and sometime I use a gum rubber "sandpaper eraser" - intended for belt sanders - to unclog the grit when it gets clogged with something soft.

 Which brings me to the hot melt topics.

 

Item 2

 The cylinder and plate in the lower right corner of the picture above are glued to each other with a high-temp, high-performance (claimed all-material, but is not optimized for plastics) hot melt adhesive.  The tube is "foam-core" ABS plumbing pipe, and the plate is a random scrap of aluminum which I scratched up (aha! cheating!) with sandpaper and cleaned off with acetone, after my first attempt stuck quite firmly to the filthy dirty aluminum... but I was able to pull it apart.

 

 Once I cleaned it as above I was not able to get these two things apart with main strength.  It's going to take tools / leverage / a hammer.  Anyone who has worked with normal low-temp consumer hot glue knows, it does not stick to metal worth a damn.  There are multiple reasons for that.  This adhesive doesn't suffer from those reasons, so it bloody well does stick to just about anything.  This particular adhesive is known to stick to GLASS.  Use with caution.  It removed silver oxide from the silver of an antique silver dollar I got it on by being clumsy, and while I was able to remove it eventually by temperature cycling (and acetone), it was a wakeup call.  This is the kind of adhesive that makes one a believer in high-temp hot melts.

 

  It will also burn the everlovin' FUCK out of you, with a deep-tissue burn that takes weeks or even months to heal because all the capillaries way down below the surface are all shriveled and dead, so nutrients can't reach the burned skin to heal it... do you feel my pain yet?  Wear gloves, ferpetesakes.

Item 3

 The two smaller bits of aluminum were another hot glue experiment.  The two pieces were faced off and left with as-machined surfaces.  I heated them both up with a hot-air gun to slow crystalization of the adhesive (increases strength).  I glued the two faces together.  After patiently waiting for the assembly to cool to room temperature, I stuck one piece in the lathe and machine the other piece.

 

  The glue held just fine until I took an over-aggressive (and also eccentric / interrupted) cut, but even then, it held for a surprisingly long time.  When I picked up the work after it finally came loose, I realized the adhesive probably would have held but I had been cutting so aggressively I had heated up the work, and softened the glue.

 To the left is the cut, the work piece is about .75" / 2cm in diameter, and the off-center cut, as well as the spot where the tool dug in, are visible.

 

 This has implications for hard-to-fixture / hard-to-chuck work pieces, wouldn't you agree?

 

 

 

Item 4

 Semi-finally, and referring back to the top image in the upper left corner, we can see that the alleged "anti-drip valve" in my fancy, industrial 3M brand "Hot Melt Adhesive Applicator" is not preventing dripping!  What is going on?  Don't ANY of those valve gimmicks* work? 

 

 Well, yeah, actually, they do.  I have found with this applicator as with cheaper ones, most of the time, it doesn't drip, but with some glue sticks, it does.  In this case, it hasn't dripped much with most of the different glue sticks I bought (general purpose, wood, plastics) except one: the multi-material "high strength" sticks (Infinibond Tough Guy™).

 

 Interestingly, the sell sheet for this adhesive specifically states that it requires the full temperature - and a large quantity of heat at that temperature - and recommends an applicator (gun) of not less than 150 watts.  The one I am using is 200 watts. The problem is simple: this adhesive expands when it melts - much more than other adhesives, and this expansion either pushes the nozzle valve open, or it pushes the glue stick backward out of the gun. There doesn't seem to be a way around this, but the nozzle doesn't drip much with the other adhesives, so I'd say the fault lies with the adhesive, not the anti-drip valve.

Item 5

 Before I close this, I have found one more interesting hot-melt adhesive worth mentioning: "reactive polyurethane". This stuff addresses a common complain about hot-melts: they are soft and they will re-melt if exposed to heat.  Not this stuff.  This stuff reacts to moisture in the air or in or on the glued substrates (you can wet them if necessary to speed curing) and re-polymerizes into a hard resin which will not melt again - it is a one-way trip.

 

  The cured resin usually has higher strength bonds (peel, shear, pull-apart) and better material properties (tensile strength, modulus) than any other hot-melt too.

 

 And now the bad news: I have not found any of these that work in a conventional hot-melt gun, low or high-temp.  They all appear to require a special dispenser and possibly even adhesive pre-heaters.  So for now, that option isn't really available to you or I.  I'm sorry; I'm such a tease.

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*I looked for the patent covering the ball-and-spring valve which everybody uses and
I could not find it, but I am sure it's out there somewhere... since various competing
companies use the same valve, I assume that patent is expired.