Rapid prototyping process selection

The success or failure of a prototype depends on your selection of rapid prototyping processes for the new product development. There are various ways in which engineering product design prototypes can be made varying from simple cardboard mock-ups to fully machined metal sub-assemblies.

3d printing materials
3d printing materials (source:https://jayconsystems.com)

Prototyping is crucial to any engineering product design, especially new product development. It is the process of making rough models of the product, for instance, to test its functionality, shape, size etc. Read more about the various types of prototypes and their importance here.

This article is about part-based prototypes i.e. how individual parts can be made to create system-level product prototypes.


5 key factors to consider when selecting a Rapid Prototyping Process

Prototypes vary in so many ways as each project, product and product design element are different. As the success of any prototype will depend on the selection criteria of prototyping processes, these 5 key factors should be considered at the start of any rapid prototyping process selection.

5 key factors of rapid prototype process selection
5 key factors of rapid prototype process selection
  1. Purpose
  2. Quality
  3. Quantity
  4. Complexity
  5. Budget/cost

Let’s delve into these key factors to understand why they are essential in choosing your next prototyping process.

1.     Purpose

Within an engineering product design and development project, prototypes are created for four purposes, according to Ulrich and Eppinger (2008). They are Learning, Communication, Integration and Milestones. These purposes of the prototyping will vary depending on which of the 4 new product development (NPD) stages you are in. Each stage of the NPD will have its feature and functionality requirement to eliminate risk. This will then define the fidelity type of the prototype required, which is the quality of the prototype discussed later.

Rapid prototyping stages
Rapid prototyping stages (source:dyson.com)

First, it would depend on the planned tests or risk mitigation exercises such as test types, customer interaction feedback etc. If the product would go through rigorous testing, outside deployment and product verification then the material selection would play a key part in the selection of prototyping techniques.

Secondly, any functional aspect you would like on your prototype needs to be considered. Are you planning on functional tests or do you have any moving parts? This will dictate the selection and assembly.

Third, changes and modifications. It’s highly unlikely that your prototype is going to be a success without a few tweaks. So, consider how easy or difficult it would be to modify to get the prototypes working.

NPD stage

  • Product planning and clarifying task
    • This stage generally requires very early proof of concept mock-ups, demonstration units and industrial design prototypes
    • Prototyping techniques
      • CNC machined foam models
      • Cardboard mock-ups
      • 3D printed parts and assemblies (FDM, SLA, SLS etc)
Paper prototype
Paper prototype (Credit: www.interaction-design.org)
  • Conceptual design
    • At this stage, you probably need scaled parts or assemblies of the design along with some user interface and limited functionally
    • Again, FDM parts are great at getting a feel for the shape and size. If you need more accuracy then you could move into the next level of 3D printing i.e. SLA, SLS and poly jet parts. If the parts are metal, then CNC at this stage is your best bet. In some cases, sheet metal fabrication is also worth considering
  • Embodiment design
    • This is the development phase where you need to explore fully functional (form, fit and function) prototypes, hence details are important. At this stage, it is more than likely that the prototypes are going to be working assemblies containing a lot of parts
    • At this stage, you would also need more than one unit for testing purposes, and it is also worth considering the final manufacturing techniques so they could be simulated
    • Consider vacuum casting and high-resolution 3D printing such as SLS and SLA for plastic parts
    • SLM/DMLS parts are ideal for simulating casting parts (Sand, investment and die casting)
  • Detailed design


2.     Quality

As discussed previously the fidelity or the accuracy of the product required will dictate what type of process and post-processing you would need. The quality of the prototype as compared to your final product or subcomponent also needs to be considered. As high-fidelity prototypes cost more, they should be considered in terms of return on investment.

Prototype qualities FDM VS SLA
Prototype qualities ( credit: google images)

For example, if you have a thread feature on a part then SLA is better than FDM but would cost more.

The life of the prototype is also crucial when deciding the technology. For example, if the parts have fasteners that will be used frequently, then machined or metal inserts are a better option than 3D printed threaded or self-tapping holes.

Material selection also plays a vital role in terms of the quality of the prototype. If the functional elements are linked to special material properties, such as surface finish and durability, then choosing additive manufactured parts might not be the best choice. The general material choices for the different manufacturing methods are as follows:

3d printingCNCVacuum casting
Nylon, PLA, ABS, ULTEM, ASA, TPUABS, Nylon, Polycarbonate, PEEKABS, Nylon Nylon HT
Aluminium, Stainless Steel, Titanium, InconelAluminium, Stainless Steel, Titanium, BrassN/A

If the prototype is made of more than one part, then the tolerance of the prototyped parts will have to be considered for ease of integration.

3.     Quantity

The number of required prototype parts are essential in deciding the process as some prototyping technologies are only cost-effective for smaller quantities. For additive manufacturing parts volume also plays a crucial part in costing as bigger parts will require more time to print compared to smaller parts. As a rule of thumb, the following rules apply.

Plastic parts

Prototype processes
Plastic partsQuantity
Low (1's)Medium (10's)
SizeSmall3D printingCNC machining (simple)
3D printing (complicated)
Large3D printingVacuum casting
CNC machining

Metallic parts

Prototype processes
Metallic PartsQuantity
Low (1's)Medium (10's)
SizeSmallCNC machining
3D printing
CNC machining
Investment casting
LargeCNC machiningCNC machining

4.     Complexity

The complexity of the part and intricacy of the features will also dictate the rapid prototype process selection. Additive manufacturing is good for producing very complicated small parts, but one should be cautious about the final design because of complicated means very expensive mass production.

ProcessTolerance (mm)Minimum wall thickness (mm)
FDM±0.20 – ±0.500.8 -1.0
SLS/SLA±0.20 – ±0.300.7 – 1.0
SLM/DMLS±0.100.4 – 0.5
Binder jetting±0.201.5 mm – 2.0 mm
CNC *±0.0120.5
Vacuum casting±0.10.9 – 1.0

Please note that these tolerances and minimum wall thickness are typical values and hugely vary depending on the material choice and feature design.

AM complex parts
AM complex parts (source:https://fts.gr)

Parts that are eventually produced by injection moulding, various forms of castings can be prototyped using 3D printing while machined parts can be 3d printed or manufactured using conventional forming or subtractive manufacturing processes.

5.     Cost

Finally, the resources available; the objective of the prototype would go hand in hand with the resources available. Time, money and man-hours need to get the prototypes manufactured and working, needs to be considered while choosing the rapid prototyping process or technology.

Things to ponder:

  • Most of the time, time consumed by post-processing or to get the part working from low-quality prototypes will be more than that of high-quality prototypes
  • Some processes such as 3D printing might need some post-processing time but it’s relatively quicker and cheaper while a vacuum casting would give parts almost identical to that of injection moulding and can be used without post-processing. However, this will be more expensive with the tooling cost
  • Cost of CNC is proportional to the complexity of the part while the cost of AM is directly proportional to the volume and size
  • The overall cost would also be tightly linked to the quantities more for a process like CNC because of the higher set-up cost


New engineering product development almost always involves making prototypes to test ideas, functionalities etc. But the quality of your testing and subsequent decision making will highly depend on how well your prototype simulates the final product. So, selecting the right process to make the prototype is crucial to the success of any engineering product.

Once you have a clear understanding of the above 5 key factors, you can decide on the type of process to explore. There are so many ways prototypes can be made, and every prototyping process will have its advantages and limitations. Thus, choosing the correct one is vital to your prototyping success.

Rapid prototyping selection process steps

  • Define the purpose of the prototype
  • Establish the level of approximation (quality and complexity)
  • Outline the evaluation method and plan to identify quantities
  • Ensure your cost is kept within your prototype budget




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