Written by Muqi Wulan
The material provides the further information on design for manufacture (DFM), especially on design for piece-part producibility (DFP) at Level 3.
As known in the DFM material for Level 2, DFP is largely affected by the manufacturing processes and materials for making piece-parts. In order for readers to have a quick and overall understanding on this topic, the content will be divided into several sub-sections. Each sub-section handles a design for one type of manufacturing process, and presents the design guidelines for this particular process. Therefore the following representative topics will be discussed in this material.
• Design for machining
• Design for injection moulding
• Design for die casting
Design for machining
Machining operations are the most common manufacturing processes. Machining processes are classified as shown in Figure 1.
Figure 1 – Classification of metal-cutting processes
Most metals and plastics can be machined, but they vary in the ease with which they can be machined. That is called machinability. Machinability is a property that depends on the workpiece material, the cutting tool material and its geometry, and the type of machining operation (Dieter & Schmidt 2009: 640). Table 1 lists metallic alloys by decreasing order of machinability.
Table 1 – Types of metals and machining processes (Dieter and Schmidt 2009: 641)
The general design guidelines for machining are summarised as follows (Boothroyd, Dewhurst & Knight 2011: 300) (Dieter & Schmidt 2009: 640).
1. Use standard components as much as possible.
2. Choose raw materials that would result in minimum component cost.
3. Try to design a part that can be machined on one machine tool only.
4. Keep in mind the sequence by which the designed part would be machined.
5. Design a part so that the workpiece must have a reference surface suitable for holding it on the machine tool or in a fixture. A surface with 3-point support is better.
6. Specify a machined surface only when it is needed for the functioning of the part.
7. Avoid reclamping the workpiece in the machining process.
8. Whenever possible, the design should be such that existing tools can be used in production.
9. Design a part so that machining is not needed on the unexposed surfaces of the workpiece when the component is gripped in the working-holding device.
10. Make sure that when a part is machined, the tool, tool holder, workpiece, and work-holding device do not interfere with one another.
Design for injection moulding
Injection moulding is a process predominantly used for thermoplastic polymers. It consists of heating thermoplastic material until it melts, then forcing this melted plastic into a steel mould, where it cools and solidifies (Boothroyd, Dewhurst & Knight 2011: 331). Figure 2 illustrates the process of injection moulding with equipment units. Figure 3 is an explored view of mould construction.
Figure 2 – An overview of injection moulding (CustomPartNet 2011)
Figure 3 – Injection mould (CustomPartNet 2011)
Some design consideration and guidelines for injection moulding are listed (El Wakil 1998: 292), and shown in Figure 4.
1. Make the thickness of a part uniform and as thin as possible (Figures 1a, 1b, 1c).
2. Avoid sharp corners for fillers and provide generous radii instead (Figures 2a, 2b).
3. Provide appropriate draft (Figures 3a, 3b).
4. Avoid heavy section when designing bosses (Figures 4a – 4d).
5. Add ribs for structural support, rather than increasing wall thickness (Figures 5a – 5d).
Figure 4 – Examples of poor and good designs for injection moulding (CustomPartNet 2011)
Design for die casting
Die casting, also called pressure die casting, is a moulding process in which molten metal is injected under high pressure into cavities in reusable steel moulds, called dies, and held under pressure during solidification (Boothroyd, Dewhurst & Knight 2011: 423). There are four main types of alloys that are die cast: zinc, aluminium, magnesium, and copper-based alloys. The material selection for die casting depends on the density, melting point, strength, corrosion resistance, and cost of the material. The most commonly used is the aluminium alloys.
Figure 5 – An engine block made of aluminium and magnesium alloys (Wikipedia 2011)
Die casting machines are constructed by several components: die mounting and clamping system, the die, metal-pumping and injection system, metal-melting and storing system, and any auxiliary equipment for mechanisation of the operations such as part extraction and die lubrication. The two basic types of die casting systems are hot-chamber and cold-chamber as shown in Figure 6. Hot-chamber systems are used with alloys having low melting temperatures, such as Zinc. Cold-chamber machines must be used for high melting temperature alloys like aluminium, cooper-based, and ZA Zinc alloys containing a high amount of aluminium (Boothroyd, Dewhurst & Knight 2009: 425).
Figure 6 – Types of die casting machine systems
General design guideline for die castings is listed below. Figure 7 shows the design improvement on a die casting part after these design rules are taken into account.
1. Structure of die castings should be thin walled and uniform. It ensures smooth metal flow and avoid distortion during filling and cooling.
2. The thickness of projections from the main wall of a die casting should not exceed 80% of the main wall thickness.
3. Sufficient draft on side walls and cores should be provided to permit easy removal of the die casting from the die without distortion.
4. Internal wall depressions or internal undercuts should be avoided since these increase die or operational costs.
5. Sharp corners should be rounded.
Figure 7 – Examples of a part (left) and the part redesigned (right) (Madan, Rao & Kundra 2007: 155)
Summary
Design rules or knowledge for manufacture are very diverse on the manufacturing processes and materials, which are chosen to make the parts of a product. In this section for L3, the design for piece-part producibility (DFP) is presented in three sub-sections: design for machining, design for injection moulding, and design for die casting.
Selected References
Boothroyd, G., Dewhurst, P., and Knight, W. A. (2011) Product design for manufacture and assembly. 3rd edn. Boca Raton: CRC Press, Taylor & Francis Group
CustomPartNet (2011) Injection Molding [online] available from [16 August 2011]
Dieter, G.E., Schmidt, L.C. (2009) Engineering design, 4th edn. New York: McGraw-Hill
Madan, J., Rao, P. V. M., and Kundra, T. K. (2007) ‘Computer aided manufacturability analysis of die-cast parts’. Computer-aided design & applications 4(1), 147-158
Pugh, S. (1991) Total design: integrated methods for successful product engineering. Harlow: Pearson Education
Wikipedia (2011) Die casting [online] available from [18 August 2011]
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