Addition and deforming (AQA GCSE Design and Technology): Revision Notes
Addition and deforming
Understanding how to work with polymers involves learning various manufacturing and shaping techniques. These processes allow us to transform raw polymer materials into useful products by adding material, reshaping existing forms, or joining pieces together.
Traditional forming methods
Line bending
Line bending is a straightforward technique used to create precise bends in thermoplastic sheets. The process relies on applying controlled heat along a specific line where you want the bend to occur.
The method uses strip heaters to warm the thermoplastic material until it becomes flexible. These heaters provide consistent, focused heat that softens only the area you want to bend, leaving the rest of the sheet rigid. Once the material reaches the right temperature, you can use a jig or guide to bend it to the desired angle. The plastic holds this new shape once it cools down.
Worked Example: Line Bending Process
Step 1: Set up the strip heater along the desired bend line Step 2: Heat the thermoplastic sheet until it becomes pliable Step 3: Use a jig to bend the material to the required angle Step 4: Hold in position while the plastic cools and sets
This technique works particularly well for creating clean, sharp folds in materials like acrylic sheets.
This technique is popular for fabricating display cases, signs, and architectural models due to its precision and clean results.
Press forming (plug and yoke)
Press forming creates three-dimensional shapes by pressing heated thermoplastic between two complementary moulds. This process involves three main steps that work together to achieve consistent results.
First, the thermoplastic sheet gets heated until it becomes pliable and ready for shaping. Next, the warm material is positioned between a male plug (the positive form) and a female yoke (the negative form). Finally, these two parts get clamped together, forcing the plastic to take on the desired shape. After cooling, the former gets removed, leaving behind a precisely shaped component.
The male plug and female yoke work as a matched pair, with the plug fitting exactly into the yoke. This arrangement ensures uniform wall thickness and sharp detail reproduction in the finished product.
Vacuum forming
Vacuum forming transforms flat plastic sheets into complex three-dimensional shapes using air pressure differences. The process begins by softening a plastic sheet with heat, making it flexible enough to conform to a mould shape.
The heated sheet gets clamped over a former (the mould), and then air gets removed from beneath the plastic. This creates a vacuum that draws the softened material tightly around the former's contours. High Impact Polystyrene (HIPS) works particularly well for this process because it responds predictably to heat and maintains good detail reproduction.
When designing formers for vacuum forming, several important factors need consideration. Draught angles help the finished part release easily from the mould, while radiused corners prevent stress concentrations that could cause cracking. Vent holes allow air to escape completely, ensuring the plastic contacts all surfaces of the former. A smooth finish on the former transfers to the final product, creating professional-looking results.
Critical Design Considerations:
- Avoid undercuts that would trap the formed plastic
- Keep height proportions reasonable to prevent the material from stretching too thin
- Include proper draught angles for easy part removal
- Ensure adequate vent holes for complete air evacuation
Composite materials
Laminating polymers
Laminating combines thin layers of different materials to create composites that are stronger than any individual component. This technique produces materials with enhanced properties by taking advantage of each layer's unique characteristics.
A common example involves laminating polyvinyl butyral (PVB) or ethylene-vinyl acetate (EVA) between two layers of glass. The vinyl interlayer sits between the glass sheets, with all three layers bonded together through heat and pressure. This creates safety glass that holds together even when broken, as the polymer layer catches glass fragments.
Another application uses melamine formaldehyde as a laminated polymer for worktops. The process bonds decorative surfaces to strong core materials, creating durable surfaces that resist scratches, heat, and moisture.
Worked Example: Safety Glass Lamination
Materials needed:
- Two sheets of glass
- PVB or EVA interlayer
- Heat and pressure equipment
Process:
- Place PVB/EVA layer between glass sheets
- Apply controlled heat and pressure
- Bond all layers together permanently
- Result: Shatterproof safety glass
The advantages of laminated polymer-glass combinations include impressive safety features. The composite can block up to 99% of harmful ultraviolet light while the polymer layer creates shatterproof glass. The material also provides excellent insulation from heat, cold, and sound. Additionally, it can incorporate heating elements for applications like heated car windscreens.
Modern manufacturing
3D printing
3D printing revolutionises manufacturing by converting digital CAD models into physical objects through layer-by-layer construction. Several different 3D printing methods exist, each with specific advantages for different applications.
Fused Deposit Modelling (FDM) represents the most common 3D printing method found in schools and workshops. This technique uses thermoplastics such as ABS and polystyrene, which get heated and extruded through a moving nozzle. The CAD programme controls a liquefier head that moves in horizontal planes (X and Y axes), while the build platform can move vertically (Z axis). This allows the system to print complex geometries layer by layer.
Worked Example: FDM 3D Printing Process
Step 1: Create or import a digital CAD model Step 2: Generate support material where needed for overhangs Step 3: Heat thermoplastic filament and extrude through nozzle Step 4: Build object layer by layer following programmed path Step 5: Remove support material after printing completes
The process often requires support material to hold up overhanging sections during printing. This support dissolves away after printing completes, leaving only the desired object. While printing large objects takes considerable time, the process generally works faster than traditional hand-manufacturing methods.
Stereolithography offers another approach where liquid resin polymer gets selectively hardened by laser light to build up three-dimensional shapes layer by layer. This method can achieve very fine detail and smooth surface finishes.
Laser sintering uses powdered polymer that gets selectively melted by heat from a focused laser beam, then solidifies to build up the final shape. This technique works well for creating strong, functional parts.
Support Materials in 3D Printing
Support materials play a crucial role in 3D printing success. When printing objects with overhangs, bridges, or complex internal structures, temporary supports prevent sagging or collapse during the printing process. These supports get designed to remove easily once printing finishes.
Joining methods
Welding
Welding creates permanent joints between thermoplastic components using two main approaches: chemical welding and heat welding.
Chemical welding uses specially selected solvents to dissolve and blend polymer surfaces together. The process involves applying the appropriate solvent along the joint line, where it melts the polymer surfaces. As the solvent evaporates, the dissolved polymers fuse together, creating a strong permanent bond. Different polymers require specific solvents for effective welding.
Dichloromethane methyl methacrylate serves as a liquid solvent that gets drawn along joints, fusing polymers where they contact but leaving gaps unfilled. This creates quick curing joints but works only where parts fit tightly together.
Tensol 12 cement provides a thicker solvent that can fill small gaps between components, though it requires longer curing time compared to liquid solvents.
Heat welding applies to polymers like HDPE, LDPE, ABS, PP, or rigid PVC. The process uses a heat gun to soften the parts that need joining, along with a filler rod made from similar polymer material. As both the components and filler rod soften, they fuse together to create a strong joint.
Safety Considerations for Welding
- Ensure proper ventilation to remove potentially harmful fumes
- Use protective equipment to prevent skin contact with solvents
- Avoid contact with hot materials during heat welding
- Work in well-ventilated areas when using chemical solvents
Resin casting
Resin casting creates solid objects by pouring liquid thermosetting polymer into moulds where it hardens into the mould's shape. The process begins with mixing liquid resin with a catalyst (hardener), which triggers the curing reaction.
Once mixed, the liquid gets poured into prepared moulds and left to cure completely. The chemical reaction transforms the liquid into a rigid solid that retains the mould's exact shape and surface texture. After curing completes, the finished part gets removed from the mould.
This technique works particularly well for creating multiple identical parts, decorative objects, or components with complex internal details that would be difficult to machine or form through other methods.
Key Points to Remember:
-
Heat makes thermoplastics flexible - Most traditional forming methods rely on controlled heating to soften materials before shaping them
-
Vacuum forming uses air pressure differences - The process pulls heated plastic around moulds by removing air from one side
-
3D printing builds objects layer by layer - This allows creation of complex internal geometries impossible with traditional manufacturing
-
Laminating combines materials for enhanced properties - Multiple thin layers create composites stronger than individual components
-
Welding creates permanent joints - Both chemical and heat welding methods fuse polymer pieces together permanently, unlike mechanical fasteners