Fig 10 shows a cordless electric screwdriver intended for domestic use. (a) (i) Describe the injection moulding process used to produce the ABS casing halve...

(a)(i) The injection moulding process for producing ABS casing halves:

1. ABS granules are loaded into a hopper and gravity-fed into a heated barrel. [1]
2. A rotating screw (Archimedean screw) pushes the granules along the barrel, where heater bands progressively melt the polymer into a viscous liquid. [1]
3. The screw then acts as a ram, injecting the molten ABS under high pressure into a cooled steel mould cavity shaped to form one casing half. [1]
4. The mould is water-cooled so the polymer solidifies rapidly. The mould opens and ejector pins push out the finished casing half. [1]

(a)(ii) Two reasons why ABS is suitable for the outer casing:

1. Good impact resistance / toughness - ABS can withstand being dropped onto hard surfaces without cracking, protecting the internal components from damage during normal use. [1]
2. Good surface finish straight from the mould - ABS produces an attractive, smooth finish without requiring additional surface treatment, giving the product a professional appearance. [1]

(a)(iii) Why injection moulding is appropriate for 50,000 units per year:

1. Although injection moulding has a high initial tooling cost (the precision steel mould), this cost is spread across 50,000 units, resulting in a very low unit cost. [1]
2. The process is fast (cycle times of seconds) and highly repeatable, producing consistent, dimensionally accurate parts with minimal waste, making it economically viable at this volume. [1]

(b)(i) One alternative thermoplastic for the casing:

Polycarbonate (PC) [1] - polycarbonate has excellent impact resistance (even greater than ABS) and can also be injection moulded into complex shapes with fine detail. [1] High-impact polystyrene (HIPS) or polypropylene (PP) with valid reasoning are also accepted.

(b)(ii) Difference between thermoplastic and thermosetting polymers:

• A thermoplastic polymer softens when heated because its polymer chains are held together by weak intermolecular forces. It can be reshaped repeatedly and recycled by remelting. [1]
• A thermosetting polymer undergoes an irreversible chemical change (cross-linking) when first heated and cured. The strong covalent cross-links mean it cannot be reshaped or recycled by remelting; it will char and decompose if overheated. [1]

(c)(i) Why self-tapping screws are used rather than adhesive:

1. Self-tapping screws allow the casing to be opened for repair, battery replacement, or recycling at end of life without damaging the halves. [1]
2. Adhesive would create a permanent bond, making disassembly difficult or impossible and potentially destroying the casing if forced apart. [1]

(c)(ii) A design feature for alignment of the two halves:

Locating pins (or pegs) moulded into one casing half that fit precisely into corresponding holes moulded into the other half. [1] These registration features ensure the two halves are aligned accurately in all three axes before the screws are tightened, preventing misalignment of internal components and external seam lines. [1]

(d)(i) Why the casing must provide electrical insulation:

1. The casing must prevent the user from coming into contact with live electrical components (the circuit board, motor connections, and battery terminals) inside the screwdriver, eliminating the risk of electric shock during use. [1]
2. ABS is an excellent electrical insulator - it does not conduct electric current, so no voltage can reach the outer surface that the user grips. [1]

(d)(ii) Two other functions the casing performs:

1. Ergonomic grip - the casing is shaped to provide a comfortable, controllable hand grip, reducing user fatigue and improving accuracy during use. [1]
2. Aesthetic identity - the casing gives the product its visual identity, branding and colour scheme, influencing the consumer's purchase decision and brand recognition. [1]

(e)(i) A suitable polymer for the sliding switch component:

Nylon (polyamide) [1] - nylon has a naturally low coefficient of friction (it is self-lubricating) and excellent wear resistance, allowing the switch to slide smoothly and reliably over many thousands of operating cycles without degrading or generating excessive friction. [1]

(e)(ii) A durability test for the switch mechanism:

1. Set up a fatigue/cycle test where an automated machine repeatedly operates the switch through its full travel (pressed and released) for a specified number of cycles, for example 10,000 or 50,000 cycles. [1]
2. Throughout the test, monitor the force required to actuate the switch and the accuracy of the switch position (fully engaged and fully released) to check whether the mechanism still functions correctly. [1]
3. At the end of the test, inspect the switch for signs of wear, cracking, deformation, or failure to engage. Any degradation in function or physical damage is recorded as a defect indicating the design does not meet the required durability specification. [1]