Tech Thursday: Stuck on You
Adhesive fasteners may be the answer to your problems. Most design programs cover only Elmers, 5 minute Epoxy, and maybe some Super77 spray adhesive. But recently, manufacturers have answered the challenges set by increasingly complex, tiny assemblies by developing a staggering variety of industrially viable adhesives. Lets check them out.
So what's in it for me?
These glues ain't what you stuck your fingers together with in elementary school. Industrial adhesives can be sprayed, robotically applied, gun extruded, or even transferred from pre-metered tapes. They can offer varying degrees of bond, from basically fused, to flexible to allow for vibration, to repositionable or removable. And, wonder of wonders, they allow bonding of totally dissimilar materials, like glass and metal, or metal and wood, or metal and plastic. And, since they can be applied flat, they allow reduced size of fastening components, and very good spreading of loads over plastic parts. Even loading is important in avoiding those ugly light stress spots where screws are over tightened.
Lets look at some of the major adhesive types used in product design. For a more extensive list, AZoM has these in-depth descriptions
These glues cure when exposed to moisture in the surfaces of the bonding materials. Because of this, hydrophobic materials like metals and glass do not bond well with these adhesives. CA glues work best with tight fitting joints, and with plastics, ceramics or rubbers. The liquid glue is very low viscosity, so porous woods and foams do not glue well with it. QuickTite Super Glue Gel is a consumer example of a CA glue, while Loctite produces a good range of industrial grade adhesives. Brush up on your superglue skills here.
By far the most used form of industrial glue. Also the most differentiated. Usually two part, or heat/UV cure, they can bond just about everything, to varying degrees of effectiveness. Their major downside is a lack of flexibility in the final joint, and the care needed to ensure a complete cure. Epoxies cannot cure at low temperatures, or if their mix ratio is incorrect. Epoxies are made by everyone and their uncle, but West is a good place to start.
UV Curable Glues
These guys are real heavyweights. Available in many component formulations, from epoxy to polyurethane, their real selling point is almost infinite "mess with it time" or pot life. This means that even if your assembly is contingent on the placement of the first assembled part, you can wait to cure the assembly until all parts are in place. One downside of this glue is that all joints must be reachable by UV light, which makes some applications impractical. Their best strength is in high strength, rigid joints in clear material(like glass), and in field repairs, where instant, heatless curing is a necessity. Loctite and Holdtite make competing brands of UV cure (and CA for that matter). Holdtite has a neat description of the curing process.
Heat Curable Glues
These have basically the same qualities as UV cure - fast cure, long pot life, and variable strength and bonding substrates. Parts must be brought to temperature, often higher than the softening temperature of plastics, so they are not used in plastic applications. Metals and ceramics are a prime use, and interestingly enough, laminated wood products, where it is cured with microwaves. Loctite and Holdtite both make heat curable adhesives.
While they aren't too tough or strong, they are some of the best at bonding glass, and they can also be very flexible. If you have applications that involve very smooth parts, like metal/glass bonds, or that will endure high frequency vibration or thermal expansion (like motor mounting or window glass) silicone adhesives are great. Silicones are also extremely high temperature, hence their use in baking products. They are available in two part or one part air curing, and are usually applied from a gun. Xiameter is one supplier.
Great for assembly lines, pre packaged glues come as little dots or strips on disposable backing tape, and can be applied consistently with machinery. You've probably seen them as the little boogery things holding magazine inserts in, or sticking multi-packs of product together in the store. They're not very strong at all, but their convenient application and zero cure time makes them coveted for packaging design and part securing in electronics. Glue Dots is one manufacturer of consumer and industrial grade material.
Tape adhesives or PSAs
Pressure sensitive adhesives, or PSAs are the adhesive of last resort. They aren't high strength, but they allow you to stick anything together - and we mean anything. PSAs can bond glass, metal, plastic, even acetal homopolymer, a plastic who's surface energy or "bondability" is close to Teflon. PSAs ship on a tape backing, and like pre-packaged glues can also be easily applied in automated assembly lines. 3M adhesives specializes in these tapes.
The Wrap Up
Product assembly is a complex process, and a solid knowledge of adhesives will only make it easier. Plus, now you know that if you want that superglue on your boss's phone to cure faster, you should sprinkle baking soda on it. Scientifically speaking of course. Also remember that most of these companies will offer product samples to test, as long as you seem professional. So don't be a stranger, just ask.
If all that wasn't enough for you, and you want to book it up a little, check out:
The Handbook of Adhesives & Sealants - pretty dry and a half, but definitely worth the pain if you really want to learn this stuff.
The Science of Adhesive Joints - If you can believe it, even drier, but covers more of the mechanical aspects that make these adhesives work. Often, a well designed joint means the difference between holding and failure.
Fueler Wilcifer recommends:
The Prop Builder's Molding & Casting Handbook - by Thurston James is a great reference for using adhesives like hot melt, silicone, and epoxy glues for all kinds of tricky applications like molding, and modelmaking.
This to That - a cute little scripted webpage that makes recommendations for adhesive based on your choosen pair of materials to glue. Good if you need fast advice when you're making a model, but not really well suited for specing out parts, since it'a pretty simplistic. (Thanks Gizmodo for the link)
Copyright 2004-2006 Dominic Muren and IDFuel Team