A manufacturing choice is a match between what the product needs and what the material offers. Name the need, name the property, then state the match.
This lesson belongs to industrial chemistry. It builds on comparing polymers, glass and ceramics, and leads to interpreting industrial-process diagrams.
What is the criteria-first method?
List what the product must do before you look at the materials. For example, a bicycle frame must be strong, light and affordable.
Then read the data for each material against each need. The choice is whichever material meets the most important need, and your answer says which need you ranked first.
Worked example 1: mass of a frame
Original data are supplied for two materials. Steel has a density of 7.8 g/cm³ and aluminium alloy has a density of 2.7 g/cm³. A frame needs 500 cm³ of material.
Step 1: write the formula. Mass = density × volume.
Step 2: calculate for steel. 7.8 g/cm³ × 500 cm³ = 3 900 g = 3.9 kg.
Step 3: calculate for the aluminium alloy. 2.7 g/cm³ × 500 cm³ = 1 350 g = 1.35 kg.
Step 4: compare. The steel frame is 3.9 − 1.35 = 2.55 kg heavier. If lightness is the main need, the aluminium alloy is the better match.
Step 5: state the assumption. The comparison assumes both frames use the same volume of material and have similar strength for that volume.
Worked example 2: cost matters too
A pipe of 30 m is needed. Polymer pipe costs RM2 per metre and does not rust. Iron pipe costs RM6 per metre and rusts when wet.
Polymer: 30 × RM2 = RM60. Iron: 30 × RM6 = RM180. The polymer pipe is RM120 cheaper, and it avoids rust.
The conclusion depends on the criteria. If the pipe carries hot water that would soften the polymer, the question would supply a temperature limit and the decision could change. Read what is supplied.
How does shaping affect the choice?
A property can make a method possible. A polymer softens when heated, so it can be melted and moulded into a bottle. Hot glass can be blown or pressed into a jar. A ceramic is shaped before firing, because it cannot be bent afterwards.
If a question asks why a polymer is used for a toy with a complicated shape, the answer links the moulding method to the softening property, then adds low cost if the question mentions it.
The mistake that costs marks
The common slip is to say a material is best without a criterion. “Aluminium is better than steel” earns nothing on its own.
| Wrong | Right |
|---|---|
| “Aluminium is the better material” | “For a light frame, aluminium alloy suits better because its density is lower” |
| “Plastic is cheap, so it is used” | “The polymer is used because it is light, rust-free and moulds into the shape” |
| “Glass is good” | “Glass is used because the contents must be visible and the glass does not react” |
The repair is a sentence that starts with the need, “For a …”, and follows with the property.
Check yourself
A school buys water bottles. Bottle P weighs 40 g and costs RM3. Bottle Q weighs 95 g and costs RM8.
The school needs 50 bottles for students to carry. Find the total cost of each and recommend one, stating the criterion.
Answer
Total cost of P: 50 × RM3 = RM150. Total cost of Q: 50 × RM8 = RM400.
Bottle P is RM250 cheaper, and each one is 55 g lighter (95 − 40), which matters for bottles students carry. Recommend P, with the criterion “low cost and low mass for carrying”. If the question adds a need such as survives a fall onto concrete, the recommendation could change.
What to study next
Move on to interpreting industrial-process diagrams, where the same ideas appear as inputs and outputs of a factory. The graph evidence and fair-comparison lab offers data to practise on.
To rehearse criteria-first answers with a teacher, see online one-to-one Science tuition.