Introduction to Stock Forms and Calculations

In Design and Technology, we don't just pick up any random piece of fabric and hope for the best! Designers need to know exactly how materials are sold and how much they need before they start a project. This helps to save money, reduce waste, and make sure the final product is fit for purpose.

In this chapter, we will look at the standard ways textiles are sold (stock forms), how we measure them, and the math you'll need to pass your exam and plan your projects.

What are Stock Forms?

A stock form is a standard shape or size that a material is produced in by a factory. Think of it like buying milk: you usually buy it in \(1\) litre or \(2\) litre bottles rather than just asking for "some milk."

Using stock forms makes life easier because:

  • It is cheaper for factories to produce things in standard sizes.
  • Designers can calculate exactly how much they need easily.
  • It ensures consistency (the fabric you buy today will be the same width as the fabric you buy next week).

Common Stock Forms in Textiles

Textiles aren't just big pieces of cloth; they come in several different forms depending on what you are making:

  • Rolls: Most woven and knitted fabrics (like calico or denim) are sold by the "linear metre" on large rolls.
  • Yarns and Threads: These are sold on cones, reels, or skeins.
  • Fibre/Batts: Non-woven materials like felt or stuffing for cushions can be sold in sheets or loose bags.

Standard Sizes in Textiles

When you buy fabric from a roll, the length can be whatever you want, but the width is fixed. This is a crucial "stock size" to remember.

1. Fabric Widths

Standard fabric widths usually include:

  • Narrow: \(90\text{ cm}\)
  • Standard: \(115\text{ cm}\)
  • Wide: \(150\text{ cm}\)

Top Tip: Always check the width of your fabric before calculating how much length you need to buy!

2. Denier

Denier is a specific unit of measurement used for the thickness of yarns or threads (especially in synthetic materials like nylon). It tells us the weight in grams of \(9,000\) metres of the yarn.

  • Low Denier (e.g., \(10\) to \(20\)): Very fine, thin, and often "sheer" (see-through), like thin tights.
  • High Denier (e.g., \(80\) to \(100+\)): Thicker, stronger, and more durable.

Quantity Calculations: The Math of Design

To pass Section B of your exam, you need to be able to calculate how much material you need. Don't worry if you find math tricky—just follow it step-by-step!

Step 1: Calculating Area

If you have a rectangular piece of fabric, you find the area using this formula:
\(Area = Length \times Width\)

Example: A student needs a piece of fabric that is \(2\text{ m}\) long and \(1.5\text{ m}\) wide.
\(Area = 2 \times 1.5 = 3\text{ m}^2\)

Step 2: Adding a Waste Allowance

In the real world, you always buy a little bit more than you need. This is for seam allowances (the bit of fabric inside the stitch line) or to cover mistakes. This is often calculated as a percentage.

Example Question: You need \(5\text{ metres}\) of fabric, but you want to add \(10\%\) for waste. How much should you buy?
1. Find \(10\%\) of \(5\text{ m}\): \(\frac{10}{100} \times 5 = 0.5\text{ m}\)
2. Add it to the total: \(5 + 0.5 = 5.5\text{ metres}\)

Step 3: Calculating Costs

Fabric is usually priced "per metre."
Example: If denim costs \(\$12\) per metre and you need \(3.5\text{ metres}\):
\(Cost = 3.5 \times 12 = \$42\)

Efficient Layouts (Minimising Waste)

When cutting out pattern pieces (like the sleeve or front of a t-shirt), designers use a lay plan. This is the "Tetris" of textiles!

Key Rules for Layouts:

  • Tessellation: Fit the shapes close together to leave as few gaps as possible.
  • Grain Line: Most pattern pieces must face a certain way so the fabric stretches correctly.
  • Width Constraints: You cannot make a pattern piece wider than the stock width of the roll (e.g., \(115\text{ cm}\)).

Did you know? Using Computer-Aided Design (CAD), factories can calculate the most efficient layout in seconds, saving thousands of dollars in wasted fabric!

Common Mistakes to Avoid

  • Mixing Units: Always make sure you are working in the same units. Don't multiply centimetres by metres! (e.g., convert \(50\text{ cm}\) to \(0.5\text{ m}\) first).
  • Forgetting Seam Allowances: If you cut a piece exactly the size of a cushion, it will be too small once you've sewn the edges together!
  • Ignoring the "Nap": Some fabrics (like velvet) look different from different angles. You must lay all pieces in the same direction, even if it wastes more fabric.

Quick Review: Key Takeaways

1. Stock Forms: Textiles are usually sold in rolls (\(90\text{ cm}\), \(115\text{ cm}\), or \(150\text{ cm}\) wide) or as yarn.

2. Denier: Measures the thickness of yarn. Higher number = thicker yarn.

3. Calculations: Always calculate the area (\(L \times W\)) and add a percentage for waste.

4. Efficiency: Good "lay plans" save money and help the environment by reducing waste.

If you want to learn more about the fabrics themselves (like the difference between Woven and Knitted), check out the "Textiles: Materials, sources and properties" chapter!