Prototyping techniques practice questions.

A designer shapes a solid block of polyurethane modelling foam into the outline of a handheld torch and paints it. It contains no electrical parts and cannot be switched on. Which type of prototype has been produced?

• An aesthetic model, made to judge form, size and finish
• A functional prototype, made to test the working parts
• A pre-production prototype, made from production tooling
• A proof-of-concept rig, made to test the working principle

Answer: An aesthetic model, made to judge form, size and finish

An aesthetic or form model carries appearance and physical bulk only, so it can answer questions about proportion and grip but nothing about performance. The fidelity of a prototype is judged by the questions it can settle, not by how neatly it is finished.

A casing printed in ABS by fused deposition modelling comes off the machine slightly smaller than the CAD model, and its corners have lifted away from the build plate. Which mechanism explains both effects?

• The nozzle deposits less material than the sliced file specifies
• The build plate expands under the heat of the nozzle and stretches the base
• Support material contracts onto the part as it is peeled away
• Each deposited layer contracts as it cools and pulls on the layers below

Answer: Each deposited layer contracts as it cools and pulls on the layers below

A bead is laid down hot and shrinks as it solidifies, which both reduces the finished size and curls the part off the plate where that pull is greatest. This is why a heated bed and generous corner radii are used, and why printed dimensions are measured rather than assumed.

A test bar printed by fused deposition modelling breaks at a much lower tensile load when it is pulled along the build direction than when it is pulled along the deposited beads within a layer. Which explanation is correct?

• The bond between deposited layers is weaker than the material within a layer
• The polymer filament loses most of its strength while it is melted in the nozzle
• Support material left inside the part carries some of the applied tensile load
• The infill pattern is always aligned with the build direction

Answer: The bond between deposited layers is weaker than the material within a layer

Printed parts are anisotropic: each new bead only partly fuses to the one below, so the interlayer bond governs strength in the build direction. Part orientation on the build platform is therefore a design decision, not just a machine setting.

A team has assessed a CAD model of a kettle handle on screen using virtual prototyping. Which limitation of that assessment does a physical prototype overcome?

• Dimensions of the model cannot be altered once the geometry is complete
• Only one design variant can be examined during a single work session
• Real users cannot judge grip comfort, weight and surface texture on screen
• Material data such as density and colour cannot be assigned to any part of the model

Answer: Real users cannot judge grip comfort, weight and surface texture on screen

Virtual models handle geometry and simulated performance well, but grip comfort, weight and texture can only be judged by a user holding a physical object. The other options are not limitations of CAD at all: geometry can be edited, several variants compared in one session and material data assigned to any part.

What distinguishes rapid prototyping from traditional workshop model making?

• The model is always made in the material specified for the production part
• The model is produced straight from the digital file, with no pattern or jig
• The model is made without any computer-aided design data being prepared first
• The model can be tested by users, which a workshop model cannot be

Answer: The model is produced straight from the digital file, with no pattern or jig

Rapid prototyping means the machine generates the model directly from the digital file, so no pattern, mould or hand shaping is needed and a changed file gives a new model the same day. It says nothing about the material, which is usually not the production one.

Two additive machines are compared on the same test part. Machine P builds in 0.02 mm layers, but its finished parts measure up to 0.40 mm under the stated size. Machine Q builds in 0.10 mm layers and its parts stay within 0.05 mm of the stated size. Which statement uses the terms accuracy and resolution correctly?

• Machine P has the better accuracy and machine Q the finer resolution.
• Machine Q has both the finer resolution and the better accuracy.
• Machine Q has the better accuracy and machine P the finer resolution.
• Both machines have the same accuracy because both work to a stated size.

Answer: Machine Q has the better accuracy and machine P the finer resolution.

Resolution is the smallest increment the machine can form, so the 0.02 mm layer belongs to machine P; accuracy is how close the finished dimension comes to the nominal, and only machine Q holds 0.05 mm. Fine resolution does not guarantee accuracy: a machine can build very small steps and still place them in the wrong position.