Joining Techniques

Joining Techniques

1. 2.4c Welding

   • -Uses heat to join metal
     • -A weld is a fusion joint.
   • Two Types of Welding
     • OXYACETYLENE WELDING
       • ARC WELDING
         • MIG Welding is a form of electric arc welding
           • = process where using an electric current, an arc is struck between the work and the electrode, generating heat
             • FILLER = thin wire and as the welding progresses the wire is gradually fed into the joint
             • An INSERT GAS is used (usually Argon) - made to flow over the area being joined, this keeps oxygen away from the joint, preventing oxidation therefore creates a better joint.
             • Process is ideal for metals that cannot be gas welded, e.g. Aluminium
       • Two gases, oxygen and acetylene are mixed in a gas torch, ignited and used as the heat source
         • A FILLER ROD (same material) is often used to complete the joint
   • Advantages of Welding
     • Weld is as strong as parent metal
   • Disadvantages of Welding
     • Can be safety issues with both gas welding and MIG welding
   • Applications= General and automotive engineering

2. 2.4d Brazing

   • Method of joining MILD STEEL using heat and a second material called SPELTER
     • (Spelter = alloy of Copper (65%) and Zinc (35%) and has a melting point of 875C)
   • 1. The join area is cleaned with emery cloth. FLUX is applied to the join area.
     • 2. The work is carefully arranged on REFRACTORY BRICKS and the heated with a BRAZING TORCH. When the work reaches 875C the spleter will melt and flow into the joint.
       • 3. Once completed, the work is allowed to cool and is removed from the BRAZING HEATH.
         • 4. The work is cleaned up and all traces of FLUX removed.
     • Flus serves two purposes:
       • 1. Prevents OXIDATION, forming a barrier between the work and the atmosphere.
       • 2. It breaks down the SURFACE TENSION on the molten spelter, allowing it to flow between the two pieces to be joined.

3. 2.4e Hard Soldering

   • Very similar to brazing also referred to as SILVER SOLDERING
     • Most commonly used to join materials such as COPPER (jewellery making)
   • It requires a FLUX to allow the solder to flow and to prevent oxidation
     • Main difference between BRAZING & HARD SOLDERING is the temperature at which the solder melts. The melting temperature of hard solder ranges from 625C to 800C. Different grades of hard solder have different melting points can prove useful when attempting to join a number of components to the same piece of work.
   • Advantages
     • Relatively low temperature required
       • Comes with different melting points to enable complex decorative pieces to be produced
     • Disadvantages
       • Weak joint
     • Applications: Decorative metalworking

4. 2.4f Chemical Joints

   • Tensol Cement (dichloromethane)
     • Used to join THERMOPLASTICS, could be described as CHEMICAL WELD
       • Tensol is a solvent, dichloromethane and when it is applied to a thermoplastics it will melt the material
     • The Tensol Cement should be applied to both surfaces to be joined and the held togther
       • PERMANENT BOND is made very quickly although for full strength, the joint should be CLAMPED together for 24 hours.
         • Application: Joining acrylic
     • Advantages
       • Gives excellent bond when joining acrylic
       • Disadvantages
         • It must be applied with care to avoid damage to the surface of the acrylic
           • Gives off strong fumes as it is a solvent
         • Only works with THERMOPLASTICS
   • Polystyrene Cement
     • Contains a solvent that melts the surface of the plastics being joined. As the solvent evaporates the strength returns to the plastic
       • Parts need to be held together until set
       • Advantages
         • Gives excellent bond when joining polystryene
         • Disadvantages
           • It must be applied with care to avoid melting the polstryene
           • Applications: Joining polystyrene (Airfix Kits)
   • Contact Adhesives
     • Made up of NATURAL RUBBER and POLYSCHLOROPRENE (NEOPRENE)
       • Useful in situations where materials like LAMINATES must be glued to flat surfaces. E.g. when they are used to glue plastic laminates such as Formica onto kitchen worktops.
         • 1. Surfaces being glued must be clean.
           • 2. Contact adhesives are applied to both surfaces. The adhesives must be applied THINLY on those surfaces and left to dry for a few minutes.
             • 3. Once adhesive is dry, the surfaces are pushed together. When the two elements come into contact the bonding occurs almost immediately.
     • Advantages
       • Instant bonding
       • Disadvantages
         • Cannot be adjusted during joining process
         • Applications: Applying Formica and veneers
   • Hot Melt Glue
     • Used in glue guns and the glue itself is a THERMOPLASTIC ADHESIVE and melts when heated.
     • Glue gun contains an electric heating element that melts the glue. When the trigger is pulled, the melted glue is forced through the nozzle. As it cools it solidifies.
     • Advantages
       • Fairly good and convenient bond
       • Disadvantage
         • Safety issues - can burn skin

5. 2.4g Adhesives

   • Polyvinyl Acetate
     • PVA is probably the most widely used wood glue. It is thick, white, viscous liquid and is very useful for gluing together different types of wood. PERMANENT JOINT.
     • It is water SOLUBLE giving it certain advantages.
       • Wood surfaces to be joined must be clean and must watch with no gaps. It takes about 24 hours to harden, so CLAMPS necessary.
     • Advantages
       • Produces a strong bond when joining wood
         • Can be used on cards and papers
       • Disadvantages
         • Can take up to 24 hours to dry
         • Requires clamps to hold in position while hardening
       • Applications
         • Wood joints
   • Epoxy Resin
     • = Thermosetting plastic that hardens when mixed with a CATALYST/HARDENER
     • Used with most materials plus creates a strong bond
     • Common brand of epoxy resin is ARALDIDE
       • 1. Equal quantities of resin and catalyst are squeezed out of their tubes onto a piece of scrap material and then mixed together thoroughly.
         • 2. The paste is then applied to the surfaces being joined. It is advisable to wear latex gloves to avoid contact with the skin.
           • Ideally, it should be left a couple of hours to dry with pressure from clamps.
         • Advantages
           • Very strong
           • Waterproof
           • Heat and chemical resistant
         • Disadvantages
           • Takes time to cure and harden
           • Requires two elements; resin and catalyst
         • Applications: Aircraft, boats, golf clubs, skis

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