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Metal and Plastic Prototype Limitations (and why that’s OK)

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Most parts and products have plastic or metal components, and during new product development, we commonly use rapid prototyping to create development prototypes that we test and validate fit, form, and function. It’s helpful to understand that these prototypes are not the same as production standard products made with injection molding and high-pressure die-casting. So, what limitations can you expect from these prototypes? We’ll show you why they don’t need to matter when you bear in mind that they help you validate your new product.

 

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Why are we producing prototypes to start with?

During the product development process, we produce prototypes to test and validate fit, form, and function. To summarise, Fit is where we investigate if the product is fit for purpose in prototype/s. The form is where we start to get the details of a product into our hands and the idea you have designed and perhaps had down on paper or in CAD drawings is brought to life in a prototype/s. Function is where the user’s interface is developed and investigated in a series of prototypes. (02:06)

 

Plastic prototyping processes.

Production plastic parts are usually plastic injection molded, but the following processes are used for creating plastic prototypes:

  • Conventional 3D printing where layers of resin are laid down to build the product or part is perhaps the first process that comes to mind when we think about prototyping in plastic, however, it is too rough for marketing, sales, showing to customers, etc. Rather, think of it as a way to transfer CAD drawings from the computer to a tangible prototype in your hand that you can examine in the flesh. Just don’t expect the results to be production standard in terms of surface finish, tolerances, and more.
  • For better prototype results in plastic, there are other processes, though. SLA and SLS printing give a higher resolution, tighter tolerances, and a reasonable approximation of the finishing that you want, although still not the production standard that you’d obtain from plastic injection molding. (04:09)

 

3D-printed plastic development prototype limitations.

3D-printed plastic prototypes can suffer from these limitations:

  • Thin sections, such as walls, might bend, warp, or come out of tolerance.
  • Layers produced by 3D printers, even SLS and SLA, are not as tight as plastic injection molding production.
  • They won’t be able to achieve a very glossy finish (although paint, varnish, or lacquer can be added to a prototype to achieve it if the subsequent thickness doesn’t cause a problem with tolerances).
  • Snap fits and hooks or living hinges may not be as good or possible, as the resins used in 3D printing don’t have the same flexible properties as injection-molded PP, for example.

Where mass production is concerned these limitations probably wouldn’t be acceptable for a lot of plastic products, but this shows the difference between using 3D printing to create prototypes that serve the purpose of being used for testing and validation of the product’s Form, Fit, and Finish, and using injection molding to create high-quality production units. (07:02)

 

The trade-off between quality and cost when it comes to prototypes.

If you need a very high-quality prototype that is close to production standards, it is possible to spend time and money tweaking the 3D printing to obtain very tight tolerances and hand-finishing the prototypes to look as close as possible to your final products. These could be used for photos and videos for sales and marketing materials, instead of product renders, for example. However, a regular 3D printed prototype you can hold and use for FFF validation might take days or even hours to make, whereas these could take weeks and require a lot of manual labor, so the cost of doing so is high. You need to decide how much you need prototypes like this during the new product development process to see if the investment of time and money is worthwhile.

Larger products and components being 3D printed will also likely be more challenging to keep within tolerance for FFF testing. They are more prone to warping, for example, which could affect how they fit together or their strength. (12:10)

 

Metal prototyping processes.

A lot of components, such as automobile parts or parts used in machines, gears, etc, are required to be made of metal and will be die-cast under high pressure in mass production, so it follows that their prototypes will often also be made from metal, too. These are the metal prototyping processes to be aware of:

  • CNC machining is the obvious and popular choice, but it may not always have the accuracy to create the undercuts and cavities you would have in a complex die-cast housing, for example.
  • A surprising option is to 3D print the metal parts in plastic first to check that they at least look correct, then you can move on to metal prototypes for checking fit, function, etc.
  • 3D metal sintering is also possible today, that’s similar to plastic printing, and it will produce complex metal parts quite faithfully, even those with complex cavities, embossed logos, etc. (15:34)

 

Metal development prototype limitations.

Prototypes made from metal do have these limitations to be aware of:

  • They will not have the strength or same mechanical properties as finished high-pressure die-cast parts and they will probably break sooner, but they do allow for validating the product before expensive tooling is made.
  • CNC-machining set-up costs for one standalone part to be made are quite high.
  • The metal materials used in machining may not be the same as in high-pressure die-casting, so results from the part may differ.

Although you may be using different materials are you close enough to test and validate FFF satisfactorily? That’s the purpose of these prototypes as opposed to the final die-cast parts. (18:18)

 

Key limitations of development prototypes (summary).

Here’s a rundown of limitations you may have with prototypes in general:

  • Dimensional Accuracy and Surface Finish: 3D printed or CNC machined prototypes usually won’t exactly match the precise dimensions and smooth finish achieved in mass production so don’t necessarily expect them to look, feel, and function the same.
  • Material Properties: Be aware the materials used in prototyping may not be the same as in final production products, so results in functional testing and validation may be misleading.
  • Production Volume and Cost: Prototype methods are typically slower and more expensive for larger quantities, so they should be used appropriately to control costs.
  • Design Complexity: CNC machining in particular might not be able to replicate the intricate geometries achieved by die casting or injection molding.
  • Post-Processing: Prototypes sometimes need to be hand-finished, painted, sprayed, or heat treated to ‘look right’ which is a labor-intensive task adding cost and time compared to minimal post-processing in mass production.
  • Environmental Impact: CNC machining creates metal waste as it removes small pieces of metal from a billet, and this waste has to be handled and may be negative for the environment.
  • Lead Time: CNC machining lead times can be long due to programming and setup requirements, and 3D printing is also slower than injection molding, too. (22:48)

 

Your takeaway: Considerations about prototypes.

  • The ideal choice between prototypes and production methods depends on your specific needs and priorities. If you are never producing in huge quantities and don’t require high accuracy, maybe you don’t need to use mass production processes at all to manufacture your products.
  • Prototypes remain crucial for early design iteration, visual representation, and limited testing. They are ideal for validating that the design looks good and that the product works and functions to specification. It is easier and less costly to iterate the design during the prototyping stages of new product development than after the product is manufactured at scale.
  • When the product has finished prototyping and validation, also test and validate the production-standard products coming off of tooling from a fit, property, and assembly perspective, and make sure that they reach your specifications before launching into mass production, as these may not be the same as your earlier prototypes. (29:35)

 

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