May 2009 Design Tip

First Impressions

The word “prototype” comes from the Greek, protos meaning “first” and typos meaning “impression.” Like many words, its meaning has changed over time, so that a single finished product can be preceded by a number of “first impressions.” And while we're often told the importance of making a great first impression, not all prototypes need to be great; some need only be adequate for a specific task. (Of course before committing to production, you might want at least one great first impression.)

Prototype plastic parts serve a variety of purposes. They can be used to test:

Form: Appearance, including overall shape, surface texture, and color.

Fit: The ability to interconnect with other parts of an assembly.

Function: The ability to withstand various kinds of stress under varying conditions, such as mechanical fatigue, heat, radiation or chemicals.

Manufacturability: The ability to be made using standard high volume production methods such as machining or injection moulding.

Viability: The ability to appeal to the market. This means getting a production equivalent part into the hands of the consumer for testing.

Because product development is an iterative process, it can include multiple prototyping steps, each serving a different function.

1. Virtual Prototyping

Virtual prototyping is supported by advanced 3D CAD software and actually produces a simulation of the part being designed. It is ideal for early conceptualization.

Pros: It allows parts to be designed, revised, virtually fitted together, and tested under simulated stress using finite element analysis. It lets a designer create and revise a design in real time at no cost except that of the software.

Cons: It is entirely digital, so going directly to high volume production from this point is very risky.

2. Stereolithography Apparatus (SLA)

SLA is an additive process that uses a computer controlled laser to cure layers of photopolymer resin. The process is suitable for making concept models or prototypes to support presentations or trade shows.

Pros: The process is a relatively inexpensive and fast way to make a single part, produces a good surface finish, and can reproduce very complex (even unmanufacturable) geometries. It is a good choice for testing the form and fit of a part.

Cons: It only works with a very limited range of proprietary resins and produces a fragile end product whose dimensional stability suffers over time.

3. Selective laser sintering (SLS)

SLS uses a computer-controlled laser to fuse powdered material. As is the case with SLA, SLS is suitable for making initial prototypes for demonstration purposes.

Pros: The process is relatively quick and inexpensive, produces more durable parts than SLA, and can also reproduce very complex geometries. It is a good choice to test form, fit and, to some extent, function.

Cons: It works with a very a limited range of materials, and the resulting parts have a rough finish. Although the parts tend to be more durable than those made using SLA, they are weaker than injection moulded or machined parts. For these reasons it is not a good choice to test manufacturability or viability.

4. Fused deposition modeling (FDM)

FDM uses a thermal print head to deposit and fuse layers of resin. The process can produce both demonstration parts as well as low volume production parts for some applications.

Pros: It is a relatively inexpensive way to make prototype parts that are stronger than either SLA or SLS, and can also produce very complex geometries. It can be a good choice for testing form, fit and sometimes function. The material properties are better than SLA or SLS.

Cons: The process is much slower than the other additive processes and suffers from the same stair-stepped surface finishes (although this can be addressed to some extent with post-processing). It also only works with a small number of proprietary materials and cannot produce parts with the standard mechanical properties of CNC machined or injection moulded parts. It is usually not a good choice for testing, manufacturability or viability.

5. Three dimensional printing (3DP)

3DP uses a print head to lay down a plaster-like material. As is the case with SLA, it is well suited for producing conceptual models during the early stages of design.

Pros: This is the fastest and least expensive of the additive processes. It allows the production of colored models and is ideal for testing form.

Cons: Parts have a rough surface finish and are very fragile. The material choices are even more limited than with other additive processes, and for these reasons it is not a good choice for testing fit, function, manufacturability or viability.

6. Polyjet (PJET)

PJET is similar to SLA, using computer controlled UV light to cure layers of photopolymer. As is the case with SLA, PJET is used primarily as a concept modeling process.

Pros: The process offers the same advantages of SLA, but the process is less expensive to operate and is more office-friendly.

Cons: It has the same disadvantages as SLA, and is much more limited in the size of parts that can be made.

7. CNC machining

CNC machining uses standard computer controlled equipment to cut parts from a solid block of material. With the advent of First Cut’s automated toolpath generation technology, the process is useful for demonstration parts through low volume production.

Pros: It is as fast as the additive processes, and because it uses standard materials as feedstock it produces parts comparable to injection moulding. For these reasons the process is a good choice for testing form, fit, function and viability.

Cons: The process is generally not well suited for production quantities in excess of hundreds of parts (see rapid injection moulding).

8. Rapid injection moulding

Rapid injection moulding involves the use of proprietary software to automate the process of quoting, designing and manufacturing injection moulds. It is useful in the production of small to medium quantities of parts for testing or bridge tooling prior to production.

Pros: Rapid injection moulding produces real injection moulded parts in as little as one business day at a fraction of the cost of conventional injection moulding. The parts are ideal for testing function, manufacturability and viability.

Cons: The non-recurring cost to manufacture the mould can make this process more expensive than additive prototyping processes for low volumes.

For a free, detailed report on prototyping processes, including detailed information on material strengths, surface finishes and process selection, visit our web site.