Design for manufacture strategies practice questions.

A torch housing is redesigned so that the two halves clip together with moulded snap features instead of four self-tapping screws, and the line time per unit falls. Which design for assembly principle has been applied?

• Standardising components across the product range
• Eliminating separate fasteners from the assembly
• Adding chamfers to the spigots that locate the printed circuit board
• Modularising the product so that sub-assemblies can be tested separately

Answer: Eliminating separate fasteners from the assembly

Each separate fastener adds a part to buy, a handling operation and an insertion operation, so integrating the joint into an existing moulding removes all three at once. The trade-off is repairability, because a snap fit is harder to open without damage.

A small moulded spacer is redrawn to be symmetrical about both of its axes before it is fed into an automated assembly cell. Which manufacturing problem does this change remove?

• Incorrect orientation of the part during feeding and insertion
• Warping of the moulding as the thicker section cools and shrinks
• Excessive cost of cutting the mould cavity
• Wear of the mould surface over a long production run

Answer: Incorrect orientation of the part during feeding and insertion

Symmetry removes the decision a feeder or an operator has to make, so a wrong orientation becomes impossible rather than merely unlikely. Its purpose here is assembly, not moulding: warping is controlled through uniform wall thickness, and tooling cost and tool life are unchanged.

An injection moulded lid is redrawn with a wall of constant 2.5 mm instead of a wall that varies between 2 mm and 6 mm. Which defect is this change intended to avoid?

• Flash escaping along the parting line of the mould
• Short shots caused by insufficient injection pressure
• Scratching of the surface as the part is ejected from the cavity
• Sink marks and warping caused by uneven cooling

Answer: Sink marks and warping caused by uneven cooling

Thick sections cool and shrink after thin ones, pulling the surface inwards as sink marks and bending the part as it sets. Uniform wall thickness also shortens the cycle because the whole moulding solidifies at the same time.

A shaft is specified to plus or minus 0.02 mm although plus or minus 0.2 mm would still let the bearing run correctly. What is the most likely consequence for manufacture?

• Unit cost rises: a slower process and added inspection are needed
• Unit cost falls because fewer parts are rejected at final inspection
• Cycle time falls because less material has to be removed per part
• Tooling life increases because each cut removes a finer chip

Answer: Unit cost rises: a slower process and added inspection are needed

Tolerance is a cost decision: tighter limits push the part onto a more capable, slower process and add gauging, so a designer specifies the widest tolerance the function allows. Unnecessary precision is one of the most common design for manufacture faults.

A start-up needs 300 identical polymer enclosures with a flat back and an open front. An injection mould would cost 18 000 USD, while a vacuum forming tool costs 400 USD but leaves an edge that must be trimmed. Which process should be chosen and why?

• Injection moulding, because the surface finish is better straight from the tool
• Vacuum forming, because tooling cost dominates at this volume
• Injection moulding, because the trimming operation is then avoided
• Rotational moulding, because a hollow shell is produced in one piece

Answer: Vacuum forming, because tooling cost dominates at this volume

At 300 units the injection mould alone adds 60 USD to every enclosure, far more than the cost of trimming a vacuum formed shell; tooling cost per unit only becomes small when it is spread over thousands of parts. A shallow open shell is also exactly what vacuum forming produces.

A manufacturer fits the same 12 V motor and the same rocker switch to six different models of hand tool. Which design for manufacture strategy is being used?

• Modular design, because each tool is built from interchangeable blocks
• Design for disassembly, because the motor can be removed for service
• Standardisation, because one bought-in part fits the whole range
• Concurrent engineering, because tooling is developed alongside the design

Answer: Standardisation, because one bought-in part fits the whole range

Standardisation reuses one part across many products, which raises order quantities and cuts the number of tools, drawings and stock lines to manage. Modular design is about exchanging whole functional blocks between products, which is a separate decision.