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This lesson covers the methods used to join materials together in product manufacture, as required by AQA GCSE D&T (8552), Section 3.2.5. Joining is essential because most products are assembled from multiple components. The choice of joining method depends on the materials being joined, the strength required, whether the joint needs to be permanent or temporary, and the production volume.
Joints are classified as either permanent (cannot be taken apart without damaging the components) or temporary/semi-permanent (can be taken apart for repair, maintenance, or disassembly).
| Classification | Examples |
|---|---|
| Permanent | Welding, soldering, brazing, adhesives (most), rivets |
| Temporary / Semi-permanent | Screws, bolts and nuts, clips, press-fits, knock-down (KD) fittings |
Adhesives are substances that bond two surfaces together. Different adhesives are suitable for different materials and applications.
| Adhesive | Type | Materials | Setting Time | Strength | Common Use |
|---|---|---|---|---|---|
| PVA (polyvinyl acetate) | Water-based | Wood, paper, card | 30 min to 24 hrs | Moderate | Timber joinery, paper crafts, school use |
| Epoxy resin (e.g. Araldite) | Two-part thermosetting | Metals, ceramics, glass, composites | 5 min to 24 hrs (depending on type) | Very high | Structural bonding, filling gaps, repairing metal |
| Superglue (cyanoacrylate) | Chemical cure | Most materials (not PE or PP) | Seconds to minutes | High (brittle) | Small, precise bonds; model making; quick repairs |
| Contact adhesive (e.g. Evostick) | Solvent-based | Laminates, rubber, leather, wood | Apply to both surfaces, leave to become tacky, then press together | Good | Applying laminate to worktops, shoe repair |
| Hot glue (from a glue gun) | Thermoplastic | Card, fabric, wood, some polymers | Sets in seconds as it cools | Low to moderate | Quick prototyping, craft, temporary bonds |
| Tensol cement | Solvent-based | Acrylic (PMMA) only | Minutes; full cure 24 hrs | Excellent on acrylic | Joining acrylic components (actually dissolves and fuses surfaces together) |
| Fabric adhesive (e.g. Bondaweb) | Heat-activated or solvent | Textiles | Varies | Moderate | Hemming, applique, bonding fabric layers without sewing |
AQA Exam Tip: When selecting an adhesive in the exam, match it to the specific materials being joined. PVA is for timber and paper; Tensol cement is for acrylic; epoxy resin is for metals and mixed materials. If you suggest PVA for joining metal, you will lose marks because PVA does not bond effectively to metal surfaces.
Mechanical fixings use physical interlocking or clamping to hold components together. Most are temporary or semi-permanent, allowing disassembly.
| Fixing | Type | Description | Application |
|---|---|---|---|
| Wood screws | Semi-permanent | Tapered, self-tapping screw driven into timber | Joining timber components, attaching hinges and fittings |
| Machine screws and nuts | Temporary | Screw passes through a clearance hole and is secured with a nut | Joining metal and plastic components; easy to disassemble |
| Bolts and nuts | Temporary | Stronger than machine screws; hexagonal head tightened with a spanner | Structural connections, machinery, furniture |
| Rivets (pop rivets) | Permanent | A mandrel is pulled through a tubular rivet, expanding it to clamp two sheets together | Joining sheet metal; replacing traditional solid rivets; aircraft skins |
| Knock-down (KD) fittings | Temporary | Cam locks, barrel bolts, cross dowels | Flat-pack furniture assembly; designed for repeated assembly and disassembly |
| Nails | Semi-permanent | Driven into timber with a hammer | Simple timber joints, construction framing, attaching trim |
| Clips and press-fits | Temporary | Plastic clips snap together | Electronic enclosures, car interior trim, cable management |
KD fittings are specifically designed for flat-pack furniture and allow the end user to assemble and disassemble the product without specialist tools.
| KD Fitting | How It Works | Example Use |
|---|---|---|
| Cam lock and bolt | A bolt screws into one panel; a cam (rotating disc) in the adjacent panel pulls the bolt tight | IKEA furniture — connects shelf panels to side panels |
| Barrel nut and bolt | A cylindrical nut sits in a cross-drilled hole; a bolt passes through the adjacent panel into the nut | Bed frames, larger furniture |
| Cross dowel | Similar to barrel nut but sits in a wider hole for easier alignment | Cabinet construction |
| Modesty block (corner block) | A plastic block screwed into both panels at a right angle | Simple corner joints in budget furniture |
Welding permanently joins metals (or sometimes thermoplastics) by melting the base materials at the joint, often with the addition of a filler material that also melts.
| Welding Type | Process | Materials | Application |
|---|---|---|---|
| MIG (Metal Inert Gas) | Wire electrode fed through a gun; shielding gas (argon/CO2) protects the weld pool | Mild steel, stainless steel, aluminium | Car body repair, fabrication, structural steel |
| TIG (Tungsten Inert Gas) | Non-consumable tungsten electrode; filler rod added by hand; argon gas shield | Aluminium, stainless steel, titanium | High-quality welds; aerospace, food industry, bicycles |
| Oxy-acetylene welding | Oxygen and acetylene gases burn to produce a hot flame (~3,200 degrees C) | Mild steel, copper, brass | General fabrication, brazing, cutting (with a cutting attachment) |
| Spot welding | Two electrodes clamp sheet metal; high current melts a small spot | Thin sheet steel | Car body production (a car body has ~3,000-5,000 spot welds) |
AQA Exam Tip: MIG welding is the most commonly examined welding process. Know that it uses a consumable wire electrode, an inert shielding gas, and produces strong permanent joints. Also know the safety requirements: welding mask/helmet, leather gloves, fire-resistant clothing, adequate ventilation, and screens to protect others from UV radiation.
Soldering joins metals (usually copper, brass, or electronic components) using a filler metal (solder) that melts at a lower temperature than the base metals. The base metals do NOT melt.
| Feature | Detail |
|---|---|
| Temperature | Soft soldering: 180-230 degrees C (below base metal melting point) |
| Solder | Lead-free solder (tin-copper or tin-silver alloy) — lead solder is now restricted |
| Flux | Chemical cleaning agent applied to surfaces before soldering to remove oxides and help solder flow |
| Tool | Soldering iron (electric, typically 15-60 W) |
| Joint strength | Moderate — not as strong as welding or brazing |
| Applications | Electronic circuit board assembly, plumbing (copper pipe), jewellery |
Brazing joins metals using a filler metal (brazing rod, typically brass or silver alloy) that melts at a higher temperature than solder but below the melting point of the base metals.
| Feature | Detail |
|---|---|
| Temperature | 450-900 degrees C (higher than soldering, lower than welding) |
| Filler | Brass rod (spelter) or silver solder |
| Heat source | Oxy-acetylene torch or brazing hearth |
| Flux | Borax-based flux |
| Joint strength | Strong — stronger than soldering, though not as strong as welding |
| Applications | Joining steel, copper, and brass; bicycle frame brazing; plumbing |
| Feature | Soldering | Brazing | Welding |
|---|---|---|---|
| Temperature | Lowest (~200 degrees C) | Medium (~450-900 degrees C) | Highest (~1,500+ degrees C) |
| Base metal melts? | No | No | Yes |
| Filler metal? | Yes (solder) | Yes (brazing rod) | Optional (filler rod or wire) |
| Joint strength | Moderate | Strong | Strongest |
| Distortion risk | Very low | Low | High (heat affected zone) |
| Typical use | Electronics, light plumbing | Bicycle frames, plumbing, jewellery | Structural steel, car bodies, pressure vessels |
AQA Exam Tip: A classic exam question asks you to compare two joining methods. Use a table format to make the comparison clear, and always relate your answer to the specific materials and product in the question. For example: "Brazing is more suitable than welding for a bicycle frame because it uses a lower temperature, reducing the risk of distortion in thin-walled steel tubes."
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