Popular Woodworking 2009-10 № 178, страница 41
more brittle. They are expensive, have a short shelf life but set in a matter of seconds. They have great pull strength but not very good shear strength. Super glues come in different viscosities and the thicker formulations have good gap-filling properties. They resist moisture but can't handle heat. They sand fairly well but don't take stains. They are great in woodworking for quick fixes and repairs, and are often used as wood filler with veneers and woodturnings. Be careful using accelerator on woods that are finished. CA glues are not suited for most wooden j oinery but are handy for craft items. It is now believed that CA glue bonds get brittle in a very short time period which can cause varying degrees of failure. Contact Adh esives Contact adhesives are made from synthetic rubber compounds. They offer an initial tack better than any other glue, and are typically used to bond laminate or veneer to a substrate, or to bond any other impervious surfaces. There are two primary types: solvent-based and water-based but both offer the same characteristics when cured. Do not use water-based contact to bond anything to steel or iron. In the long run, contact adhesives have a low resistance to heat, cold and solvents. Contact adhesives are not structural and have no strength. They do not restrict the movement between the parts, so the core or substrate moves independently to whatever is bonded to it. Contact adhesives are applied to both surfaces by brushing, rolling, spraying or using a trowel. As the solvents evaporate, water can condense on the surface of the rubber. This water must be removed through heat or evaporation before the bond is made. Contrary to most articles that say that pressure is not necessary, it takes 45 pounds of pressure per lineal inch to make a bond. This is typically accomplished by rolling the surface with a "J" roller. The Gluing Process When properly used, most modern glues are stronger than the wood. But the method To download a list of Marc's tips for successful gluing, visit: popularwoodworking.com/oct09 54 ■ Popular Woodworking October 2009 Quick fix. Cyanoacrylates (super glues) don't have much strength, but are ideal for quick repairs. Under pressure. Contact cements grab immediately, but need pressure applied to form a complete bond. you use can dramatically affect the results. If you are using a specific glue for the first time read the instructions, do some research, ask questions and consider doing a test glue-up. Each step in gluing, from selecting the best glue to the correct ways to apply it, to the final cure is a factor in attaining the full potential from the glue. Do a dry run and consider the effect of air temperature and humidity on the entire process. Glue doesn't just go bad from one day to the next. But it loses some of its effectiveness over time - especially if incorrectly handled, stored or misused. For the most part, glues are usable as long as they are spreadable according to the instructions from the manufacturer. A good rule when buying glue is to only purchase what you think you will use within the next year. I write the date on my glue bottle with a marker when I open it. If your glue is unusually thick, has changed color, smells funny or is lumpy it's probably time to dispose of it. Pot life is similar to shelf life, but is much more immediate. Pot life applies to glues that get mixed and the proportions of that mix. Once mixed the glue will start to coagulate and set. There is a definite time between being spreadable and becoming hard. Surface Prep and Spreading Make sure the wood to be bonded is dry, clean and free from dust and debris. Ideally, wood should be freshly cut with smooth surfaces. The joint should have intimate fiber contact between pieces. I consider the best j oint to have slight friction between the mating parts - not so tight that they have to be hammered together and not so loose that they have no fiber contact. Properly cut surfaces help ensure the glue can be spread and absorbed uniformly over the mating fibers of each piece of wood. A uniform spread of glue that covers the surface of both mating parts is important. Glue can be applied to one surface (a single spread) or to both surfaces (a double spread) and can be applied with a brush, roller, stick or right from the bottle. Some people use their index finger to spread the glue. Luthiers use their little finger to keep the index finger clean for handling the wood and clamps. Regardless of the method, the purpose in spreading glue is to distribute an adequate amount with uniform thickness over the entire area to be bonded. When pressure is applied the glue will flow into a uniform thin layer. A thin glue line is the best glue line. The amount of glue on the surfaces to be joined depends on the wood species, moisture or resin content, type of adhesive, temperature and humidity, and whether you use a single or double spread. Getting the right amount of glue is hard to control. Too much is wasteful, hard to clean up and can add excess water to the joint. Too little can reduce needed working time or result in a starved joint and poor bond. Open and Closed Assembly Time Assembly time is the total time between spreading the glue and applying the clamps. Open time is the time between when the glue is applied to the time the two surfaces |
