Permanent fixings include adhesives and welding. These types of join can provide a strong bond and are not designed to removed. Temporary fixings use mechanical fastening methods, such as screws or knock-down fittings, although nails can also provide a temporary solution for joining wood. These types of fixing are easy to put together and can be removed as required using basic tools such as a screwdriver.
Adhesives bond surfaces together permanently and differ in their strengths and types according to the materials they are joining together. Common types of adhesive used to join wood include polyvinyl acetate (PVA), epoxy resin (ER) and contact adhesives.
- PVA is a common wood-working glue which can also join boards and paper, as well as being used for making papier mâché. Some PVA adhesives are water-resistant
- ER is commonly used to join wood to other materials, including metals and plastics. ER is a waterproof two-part adhesive that needs to be mixed just before use
- Contact adhesives can also be used to join wood to other materials, providing a strong bond. These adhesives are applied to both materials and then left to dry
- Other glues that can join wood include super glue and hot glue
Bolts, nails and screws come in a wide variety of types and sizes. Screw heads can be round or counter-sunk and can have slot, crosses, or square holes for different screwdrivers. Screw types include machine screws and self-tapping screws. Bolts are secured with a nut to create a removable fixing. Bolt heads come in a range of types including those with slots or crosses, heads that can be used with a spanner, and hex or Allen key heads.
These types of fitting are easy to put together, usually with the use of basic tools like a screwdriver, drill or hammer. They often consist of corner blocks or bloc-joint fittings that are made from plastic or nylon along with screws to fasten through the wood and into the fitting. Although they are a temporary fixing, knock-down fittings are often used in flat pack furnishings that are designed to be held together permanently or at least semi-permanently.
TWI has been testing the feasibility of using a welding process to join wood without the use of mechanical fastenings or adhesives. Using linear friction welding, we have shown that it is possible to create bonds between two pieces of wood in ten seconds or less. The potential benefits of this innovative process include environmental ones as there are no additional chemicals or materials used to form the bond, making end of life recycling more environmentally friendly.
Many of the methods used for joining wood have been in use for hundreds, if not thousands, of years. While modern adhesives offer a permanent joining solution for wood with both similar and dissimilar materials, mechanical fastening techniques such as bolts, nails, screws and knock-down fittings are widely used for temporary or semi-permanent joining applications.
Welding is an emerging method for quickly joining two pieces of wood together without the need for additional fasteners or chemical adhesives, which could offer potential benefits with regards to manufacturing cost, time, and environmental impact.