In the previous post I outlined some of the current developments in rapid manufacturing, and what lessons could be learned from the consumer adoption of technologies in the past. Access to a technology is only part of the picture though; if these technologies are to be used by consumers it requires that non-experts are able to design products and supply data to rapid manufacturing machines in a form that the machines understand. So in this part I will look at why the design rules which apply to rapid manufacturing makes it easier to design products.

One of the common features of mass manufacturing processes is that the means of production require substantial initial investment, however once in place the cost of manufacturing a single part or product (relative to the initial investment) is negligible. It is therefore a basic principle of mass manufacturing that as the number of parts produced increases, the cost of production of each individual part decreases. This inevitably leads to uniformity, since even small design changes require significant reinvestment in tooling. To get a return on the investment in the tooling, the number of parts produced must typically be in the tens or even hundreds of thousands. This makes manufacturing one-off or batch volume products virtually impossible without reverting to craft-like technologies.

Because of the absence of tooling, rapid manufacturing dramatically reduces the costs involved in producing a part or product. A rapid manufacturing machine doesn’t care whether it’s making 100 identical parts or 100 unique parts; with rapid manufacturing variety comes “for free.” But there is still the issue of the skills required to design the part in the first place.

To understand the complexities involved in designing a part which is to be mass manufactured, it’s useful to look at some of the design requirements of a typical mass manufacturing technology such as injection moulding. (Much of the following is taken from two papers: ‘Implications on design of rapid manufacturing’¹ and ‘Impact of Rapid Manufacturing on Design for Manufacture for Injection Moulding’²). Some of the rules an industrial designer and mechanical design engineer will need to incorporate in a part’s design include:

- Minimisation of features in non line-of-draw faces to reduce complexity and cost
- Minimisation of re-entrant features (ie undercuts) to reduce complexity and cost
- Uniform wall thicknesses to avoid stresses and weaknesses
- Avoidance of sharp corners to prevent stresses and weaknesses
- Avoidance of weld lines to prevent stresses, weaknesses and visual defects
- Avoidance of visible witness lines to prevent visual defects
- Avoidance of sink marks to prevent visual defects
- Avoidance of ejector pin marks to prevent visual defects
- Avoidance of flashing at the tool’s parting line to prevent visual defects
- Siting of gating / sprue points on non-visual surfaces to prevent visual defects
- Design of wall thicknesses, ribs, bosses etc to provide a structurally sound part

Although the designer will be considering issues such as yield, cycle time, cost etc, essentially these design rules are aimed at achieving three things: that the part can be removed from the tool once it’s been moulded, that the part doesn’t break in use, and that the part looks acceptable to the consumer. Now look at which of the rules apply to a tool-less rapid manufacturing technology:

- Minimisation of features in non line-of-draw faces to reduce complexity and cost
- Minimisation of re-entrant features (ie undercuts) to reduce complexity and cost
- Uniform wall thicknesses to avoid stresses and weaknesses
- Avoidance of sharp corners to prevent stresses and weaknesses
- Avoidance of weld lines to prevent stresses, weaknesses and visual defects
- Avoidance of visible witness lines to prevent visual defects
- Avoidance of sink marks to prevent visual defects
- Avoidance of ejector pin marks to prevent visual defects
- Avoidance of flashing at the tool’s parting line to prevent visual defects
- Siting of gating / sprue points on non-visual surfaces to prevent visual defects
- Design of wall thicknesses, ribs, bosses etc to provide a structurally sound part

Obviously the part or product still needs to work and not break under normal conditions of use. But none of the rules which govern how a plastic part is moulded from molten material, and which ensure the part has visual and structural integrity, apply any longer. This will make it much easier for designers and consumers alike to design products: most industrial designers will have experienced how just three degrees of draft can have a profound influence on the fit of internal components or the way surfaces transition from one plane to another. And this new manufacturing mindset will open up huge possibilities in terms of aesthetic design: not having to consider a part’s removal from a tool allows the kinds of complexities being investigated by the likes of Freedom of Creation.

© Freedom of Creation

This is not to say there aren’t new rules. For example 3D printers often manufacture parts at different resolutions in each of their three axes - this requires skill in understanding which surfaces are visually most important. Those technologies which utilise a support structure during a part’s construction will in most circumstances require the structure to be removed, meaning enclosed (ie hollow) volumes must be avoided. But as the performance of RM technologies improves, and as software is developed which accounts for these requirements, they are likely to become less of an issue. Rapid manufacturing thus removes two of the biggest hurdles to any person or company which wants to manufacture a product: cost and complexity.

1. Hague, R; Campbell, R.I. and Dickens, P. (2003) ‘Implications on design of rapid manufacturing’ in Proceedings of the Institute of Mechanical Engineers, Part C, Journal of Mechanical Engineering Science, 217(1), pages 25-302.

2. Mansour, S. and Hague, R. (2003) ‘Impact of Rapid Manufacturing on Design for Manufacture for Injection Moulding’ in Proceedings of the Institute of Mechanical Engineers, Part B, Journal of Mechanical Engineering Science 217(4), pages 453-461