Fig 7 shows a rapid prototype model of a new mobile phone case. The model was produced using 3D printing (additive manufacturing). (a)(i) Describe how a 3D ...

(a)(i) How a 3D printer produces a product from a digital design file:

1. A 3D digital model (CAD file) of the phone case is created on computer, then slicing software divides the model into many thin horizontal layers (typically 0.1-0.3 mm thick). [1]
2. The printer builds the product layer by layer: for FDM (fused deposition modelling), a thermoplastic filament is melted and extruded through a nozzle, depositing a thin bead of material that traces the cross-section of each slice. [1]
3. Each new layer bonds to the one below as the build platform lowers (or the print head rises) by one layer height. The process repeats, building up the three-dimensional shape progressively. [1]
4. When all layers are complete, the finished object is removed from the build platform and any support material is broken away or dissolved. [1]

(a)(ii) The digital file format commonly used:

STL (stereolithography) file. [1] This format represents the surface geometry of the 3D model as a mesh of small triangles, which the slicing software then processes into layer-by-layer instructions for the printer.

(b) Two advantages of 3D printing a prototype compared to making it by hand:

1. A prototype can be produced directly from the CAD file in a matter of hours, much faster than hand-shaping from raw material using workshop tools. [1]
2. Complex shapes with internal features, thin walls or undercuts can be produced that would be very difficult or impossible to achieve by hand. [1]

(c) Why 3D printing is not suitable for mass production of the phone case:

1. 3D printing is slow, producing one item at a time over several hours per part, making it far too slow for mass production volumes. [1]
2. The unit cost remains high regardless of quantity (no economies of scale), and the surface finish and mechanical properties of 3D-printed parts are generally inferior to injection-moulded parts. [1]

(d)(i) The missing stage (dashed box) in the production flow chart:

Quality control inspection / check finished product [1] - visual and dimensional inspection of the moulded cases to verify they meet specification before packaging.

(d)(ii) Why this stage is essential in mass production:

1. It catches defective products (dimensional errors, surface defects, flash, warping) before they are packaged and dispatched to customers. [1]
2. In mass production, a single undetected fault in the mould or process settings could produce thousands of faulty products. Without this quality gate, defective stock would reach the market, causing customer returns, reputational damage and financial loss. [1]

(e) Lean manufacturing explained:

1. Lean manufacturing is a systematic approach that aims to minimise all forms of waste (materials, time, movement, overproduction, inventory, defects) while maximising productivity and value. [1]
2. It focuses on producing only what is needed, when it is needed, to the quality required, eliminating activities that do not add value to the final product. [1]
3. A specific lean technique: Just-In-Time (JIT) delivery, where raw materials arrive only when needed for immediate production, reducing inventory holding costs and warehouse space requirements. Other valid techniques include Kaizen (continuous improvement), 5S workplace organisation, and Kanban scheduling. [1]

(f) The role of CNC in producing the injection mould:

1. A detailed CAD model of the mould cavity (the negative of the phone case shape) is created on computer, specifying all dimensions, draft angles, gate locations and cooling channels. [1]
2. CAM software converts this 3D model into a precise program of tool paths and cutting instructions (G-code). [1]
3. The CNC milling machine follows these instructions automatically to cut the complex mould cavity from a solid block of tool steel, achieving the high precision (typically ±0.01 mm) needed to produce accurate, repeatable mouldings. [1]

(g)(i) Comparing quality control and quality assurance:

• Quality control (QC) inspectors check samples of finished products against the specification to identify defects after production has occurred. QC is reactive, catching problems after they happen. [1]
• Quality assurance (QA) is a proactive, preventive system embedded throughout the entire production process. It establishes standards, procedures, training and documentation at every stage to prevent defects from occurring in the first place. [1]

(g)(ii) A statistical quality control method:

1. Random sampling: At regular intervals (e.g. every 100 units, or every 30 minutes), a set number of products are randomly selected from the production line. [1]
2. These samples are measured and tested against the specification. If the defect rate in the sample exceeds a pre-defined acceptable quality level (AQL), the entire batch is flagged and the production line may be stopped for investigation and correction. [1]

(h) Organising production to switch between three colours efficiently:

1. The factory uses batch production for the different colours, manufacturing all cases of one colour before switching to the next. [1]
2. Between colour changes, the injection moulding machine barrel is purged of the previous colour granules by running the screw mechanism until only the new colour material is emerging. [1]
3. Production is planned to minimise changeover waste: lighter colours are typically produced first (e.g. white, then yellow, then black), because a small amount of residual dark pigment in a light batch is more visible and wasteful than light residue in a dark batch. [1]