Introduction to Polymer Stock Forms

Imagine you wanted to bake a cake, but you had to grow the wheat and milk the cow yourself first. It would take forever! In Design and Technology, stock forms are like buying flour and eggs from the shop. They are the standard shapes and sizes that polymers are sold in by suppliers.

Using stock forms makes life easier because they are cheaper to buy, easy to transport, and ready to be used in machines like laser cutters or vacuum formers. In this chapter, we will look at the different forms polymers come in and how to calculate exactly how much you need for a project.

Common Stock Forms of Polymers

Polymers (plastics) don't just come in flat sheets. Depending on what you are making, you might buy them in several different forms:

  • Sheet: This is the most common form for school workshops. It is a flat, thin piece of plastic. Example: Acrylic sheets used for laser cutting or HIPS (High Impact Polystyrene) for vacuum forming.
  • Bar and Rod: These are solid shapes. Rods are usually circular, while bars can be square or rectangular. They are great for making spacers or axles.
  • Pipe and Tube: These look like rods, but they are hollow. We use these for things like plumbing pipes or lightweight frames.
  • Granules and Powder: Small plastic "beads" or dust. These are used in industrial machines. They are melted down to be injection moulded into shapes like LEGO bricks.
  • Resin: This is a thick liquid. Polyester resin is often used with glass fibre to make strong shapes like boat hulls.
  • Film: Extremely thin sheets of plastic. Think of the thin plastic used for food packaging or PET water bottle labels.
  • Mouldings: These are pre-shaped sections, like skirting boards or plastic trim, that are ready to be cut to length and fitted.

Key Takeaway: Choosing the right stock form saves time and reduces waste. You wouldn't buy a solid rod if you really needed a hollow tube!

Understanding Sizes and Measurements

When ordering polymers, you need to speak the "language" of sizes. In the UK, we almost always use millimetres (mm) for D&T calculations.

Important Dimensions to Know:

  • Length and Width: Used for sheets (e.g., \(1220 \text{ mm} \times 610 \text{ mm}\)).
  • Thickness: How "deep" a sheet is (e.g., \(3 \text{ mm}\) acrylic is very common).
  • Diameter: The distance across a round rod or pipe.
  • Wall Thickness: For tubes and pipes, this is the thickness of the "shell" of the pipe.

Working within Tolerance: In manufacturing, nothing is ever 100% perfect. Tolerance is the small amount of "wiggle room" allowed. For example, if a plastic rod is supposed to be \(10 \text{ mm}\) thick with a tolerance of \(+/- 0.1 \text{ mm}\), it is acceptable as long as it is between \(9.9 \text{ mm}\) and \(10.1 \text{ mm}\).

Quantity Calculations

One of the most important skills in the exam is being able to calculate how much material you need. This helps you work out the cost and ensures you don't run out halfway through making your prototype!

1. Calculating Area

To find out how much flat sheet you need, use this formula:
\( \text{Area} = \text{Length} \times \text{Width} \)

Example: If you need a piece of acrylic that is \(200 \text{ mm}\) by \(150 \text{ mm}\):
\( 200 \times 150 = 30,000 \text{ mm}^2 \)

2. Calculating Volume

If you are using a 3D form (like a block or a resin mould), you calculate volume:
\( \text{Volume} = \text{Length} \times \text{Width} \times \text{Depth} \)

3. Calculating Percentage Waste

When you cut shapes out of a sheet, there is always some plastic left over (the "offcuts"). This is called wastage. To calculate the percentage of material wasted:
\( \text{Percentage Waste} = \frac{\text{Wasted Area}}{\text{Total Area}} \times 100 \)

Pro Tip: To minimize waste, designers use tessellation (or "nesting"). This means fitting the shapes as closely together as possible on the sheet before cutting. It saves money and is better for the environment!

Quick Math Review for the Exam

Don't worry if the math seems tricky. Just remember these three steps:

  1. Check your units: Make sure everything is in mm before you start. If the question gives you cm, multiply by 10!
  2. Show your workings: Even if you get the final answer wrong, you can get marks for the right formula.
  3. Ratios: Sometimes you might be asked for a ratio of materials (e.g., \(2:1\) ratio of resin to hardener). This means for every \(2 \text{ parts}\) of resin, you add \(1 \text{ part}\) of hardener.

Common Mistakes to Avoid

  • Mixing units: Adding \(10 \text{ cm}\) to \(10 \text{ mm}\) and getting \(20\) (The answer should be \(110 \text{ mm}\)).
  • Forgetting the "kerf": When a laser cutter or saw cuts through plastic, the blade or beam actually removes a tiny bit of material (the width of the cut). If you don't account for this, your pieces might be slightly too small!
  • Ordering exactly the right amount: Always allow for about \(10\%\) extra material for mistakes or "test pieces."

Summary Checklist

- Can you name three stock forms? (e.g., Sheet, Rod, Powder)
- Do you know the standard units? (Millimetres)
- Can you calculate the area of a rectangle? (\(\text{L} \times \text{W}\))
- Do you understand nesting? (Arranging parts to reduce waste)

Note: For more information on how these polymers are turned into products, check out the "Polymers: Processes and Scales of Production" chapter.