Introduction to Joining Techniques
In Product Design, deciding how to put parts together is just as important as choosing the material itself. Whether you are building a skyscraper, a piece of flat-pack furniture, or a plastic model, the joining method determines how strong, durable, and repairable the product will be. We usually group joining into two categories: permanent (you can't take it apart without breaking it) and semi-permanent (you can take it apart for repair or transport).
In this chapter, we will explore four main ways to join materials: adhesives (glues), mechanical fixings (screws and bolts), heat (welding and soldering), and wood joints.
1. Adhesives (Glues)
Adhesives are used to bond materials together. The secret to a good bond is selecting the right glue for the right material. Here are the specific adhesives you need to know for your Edexcel exam:
Common Adhesives and Their Uses
Polyvinyl Acetate (PVA): This is the standard "wood glue." It is water-based, soaks into the wood fibres, and creates a bond stronger than the wood itself once dry. It is not usually waterproof unless specified.
Key use: Joining timber and wood-based boards.
Contact Adhesive: This is used for large areas like sticking a plastic laminate to a kitchen worktop. You apply it to both surfaces, let them become "tacky," and then press them together. The bond is instant.
Key use: Bonding large sheets of different materials.
Epoxy Resin (ER): A "two-part" adhesive consisting of a resin and a hardener. You mix them in equal parts to start a chemical reaction. It is incredibly strong and can join almost any material (wood, metal, plastic).
Key use: Joining dissimilar materials or where high strength is needed.
Acrylic Cement (Tensol): This is a solvent. It actually melts the surface of the acrylic, "welding" the two pieces into one solid part. It is very thin and watery.
Key use: Permanent bonding of acrylic (PMMA).
Cyanoacrylate (Superglue): A very fast-acting adhesive that cures instantly when it reacts with moisture in the air.
Key use: Small repairs or joining non-porous materials like plastics and rubber.
Hot Melt Glue: Used in glue guns. The glue is a plastic stick that melts with heat and hardens as it cools. It is great for modelling and prototyping because it is fast, but it isn't very strong.
Key use: Rapid prototyping and temporary fixes.
Polystyrene Cement: Similar to acrylic cement, this is a solvent that melts the surface of high-impact polystyrene (HIPS).
Key use: Plastic model kits.
Quick Review: Adhesive Tips
Don't forget: Most adhesives require the surfaces to be clean and keyed (slightly roughened with sandpaper) to create a better mechanical grip!
2. Mechanical Joining
Mechanical joints use physical parts to hold things together. Many of these are semi-permanent, meaning you can take the product apart to fix it or recycle it later.
Mechanical Fixings to Know:
Screws: Used for wood and metal. Wood screws have a coarse thread to grip the fibres. Self-tapping screws for metal cut their own thread as they are driven in.
Nuts, Bolts, and Washers: Used when you need a very strong, removable joint. The bolt goes through a hole, the nut tightens onto it, and the washer helps spread the pressure so the material doesn't get crushed.
Rivets: These are permanent. A metal pin is placed through a hole and the end is "mushroomed" or flattened to lock it in place. "Pop rivets" are commonly used for thin sheet metal where you can only reach one side.
Analogy: Think of a rivet like a permanent staple for metal.
Press (Interference) Fits: This is joining by friction. One part is slightly larger than the hole it’s being pushed into. If the size difference is \(0.01mm\), the parts will stay together purely through pressure.
Key Takeaway
Mechanical fixings are essential for maintenance. If a product is glued shut, it's often destined for the bin if it breaks. If it's bolted, it can be repaired.
3. Joining with Heat
When you need a permanent bond between metals, heat is usually the answer. This is divided into welding (melting the actual metal) and brazing/soldering (using a "glue" made of melted metal).
Welding Techniques
In welding, the base metals are melted together, usually with a filler rod to bridge the gap.
Oxy-Acetylene Welding: Uses a mixture of Oxygen and Acetylene gas to create a flame hot enough to melt steel. It’s versatile but slower than modern methods.
MIG (Metal Inert Gas) Welding: This is the most common industrial method. An electric arc creates the heat, and a thin wire "electrode" is fed automatically into the joint to act as the filler. A gas (like Argon) shields the weld from oxygen to prevent it from bubbling.
Why use it? It's fast, easy to learn, and produces very strong joints in steel and aluminium.
Brazing and Soldering
In these processes, the base metal does not melt. Instead, a filler metal with a lower melting point is melted into the joint.
Brazing: Uses a brass filler rod. It requires higher temperatures than soldering but creates a much stronger joint. It is often used for steel tube frames (like bicycle frames).
Hard Soldering: Commonly used in jewellery (silver soldering). It uses a silver-based alloy filler.
Soft Soldering: Uses a tin-based alloy. It has a very low melting point and is used primarily for electronic circuits or joining copper pipes in plumbing.
4. Wood Joints
Wood joining is a classic part of DT. You need to distinguish between traditional joints (which rely on surface area and glue) and knock-down (KD) fittings (modern furniture fixings).
Traditional Wood Joints
These are chosen for their strength and aesthetics (how they look).
Examples:
• Dovetail Joint: Very strong and looks beautiful; used for high-quality drawers.
• Mortise and Tenon: A "peg in a hole" style joint; used for heavy-duty frames like tables and chairs.
• Lap Joint: A simple joint where the two pieces overlap; used for simple frames.
Knock-Down (KD) Fittings
If you have ever built IKEA furniture, you have used KD fittings! These allow products to be sold in flat-pack form and assembled by the customer using simple tools (like an Allen key).
Advantages of KD Fittings:
1. Distribution: Flat-pack boxes take up less space in shipping containers, reducing transport costs and carbon footprint.
2. Cost: The manufacturer doesn't have to pay workers to assemble the product.
3. Replacement: If one part of the wardrobe breaks, it’s easier to replace that single part.
Quick Review: The "Why" of Joining
Exam Tip: If a question asks you to "Justify" a joining method, think about:
• Strength: Does it need to hold a lot of weight? (Welding/Epoxy)
• Appearance: Does the joint need to be hidden? (Dovetail/Acrylic cement)
• Maintenance: Does it need to be taken apart? (Screws/Bolts/KD fittings)
• Speed: Is it for a mass-produced product? (MIG welding/Pop rivets)
Chapter Summary
Adhesives: Chemical bonding. Choose based on material (e.g., PVA for wood, Tensol for acrylic).
Mechanical: Physical fixings. Use for strength and maintenance/disassembly.
Heat: Metal joining. Welding melts the metal; soldering/brazing melts a filler alloy.
Wood Joints: Traditional for quality/permanence; KD fittings for flat-pack/modern convenience.