What is Quality in AM design?

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This text is an opinion rather than a statement of fact.

One aspect of Quality in DfAM is its value. Just because AM allows ‘design freedom’ doesn’t mean any design is ok! And in fact, with AM, not designing things right can get expensive very quickly. In addition, MEX has some real weaknesses when challenging conventional polymer manufacturing, e.g. Injection Moulding (IM). The texture, the sheen, the colour, the materials, the perfection of conventionally manufactured objects, the consistency of form and of material structure across all feature sizes. These are things that MEX can’t beat. A design for MEX needs to add value to make sense. Then it can compete with IM or other conventional manufacturing processes and even be better.

So by a design that incorporates ‘value’ we mean one that makes things better than they were before that design existed. In the majority of cases that means something that is designed to the strengths of AM and away from its weaknesses, that encompasses a benefit and that is designed to be manufactured with the lowest ‘cost’. ‘Cost’ includes manufacturing risk and manufacturing time as well as overall piece price.

By ‘manufacture’ we mean everything that needs to be done to that part to give it to a user, and that is not just ‘printing’ it. A quality part design should leave your workstation and go to the shop floor with all the accompanying information for people who will receive and manufacture your design, from build preparation, support placement, post-processing, inspection.

The data for the downstream process steps will comprise:

  • A model of the net shape part (usually a CAD model).
    • Depending on the company processes this could be the native CAD model but is usually a file format that the slicer can process. Some slicers process STEP files but most need a tesselated file. STL is a legacy file format that is prone to errors and doesn’t carry metadata. So we would recommend (provided downstream process steps accept it) that you use 3MF since it can carry additional information about supports, build file orientation , lattices etc. For more information on this topic, see BP026.
  • Accompanying information that build preparation and manufacturing need. This will include:
    • Build setup variables
    • Post-processing and inspection requirements as per conventional manufacturing Product Modelling Information (PMI)

So if you put those things together, a Quality design is the right application, with the right choice of process and the right material, the right geometry and the right instructions to people downstream in the process chain, such that the experience for everyone involved is seamless. That seamlessness is dependant on the choices embedded in the design by the designer. Such a design will be adding value, and minimizing the downside / risks of MEX.

In the case of prototyping, where the aim is just get fast iterations in your hand, to see what the design concepts look like in reality, or when you a further along the design process, to check form, fit and function, then the requirements for a quality design don’t really apply. It’s all about playing in a sandbox, using a printer to help evaluate your ideas and then discarding them and moving on.
It’s worth saying that good AM design isn’t defined by complexity. A simple bracket that replaces a conventionally manufactured one can be a good quality design. It adds value if its function or aesthetic is improved, or it’s opened new markets for the seller of the bracket, or it’s made manufacturing faster or cheaper, less dependant on the supply chain, or has a lower environmental impact, or any combination of the above. If that is coupled with an absence of design-induced manufacturing risk, or difficulty in post-processing & inspection, then we would call that a quality AM design.

So in summary, a quality AM design:

  •  Adds value through use of AM instead of conventional manufacturing
  • Minimises manufacturing cost and risk along the whole AM process chain i.e. print file preparation, printing, post-processing and inspection
  • Is a file of the part geometry accompanied by complete and unambiguous information that the downstream process steps will need.

Sending your design to a bureau 

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If you’re planning to use a bureau to print your design, you could just send over your CAD model in a neutral file format like STEP, 3MF or STL. However, when translating your file to the actual gcode that will drive the 3d printer, the bureau will choose the values of different variables that will affect the final part properties, so we recommend you try and specify as many of those as you can. There are also post-processing steps to consider that will affect your final part quality. Specifying what you require at each step will help you get as close as possible to your expected result.

Stage

Variable

What it affects in the final part

How to define it

Build layout

Orientation

Amount of support material, surface texturemechanical properties (see BP033).

Note that this also affects build time and therefore, cost.

Send instructions to use the part’s orientation as supplied

Supports

Where to permit supports, what type (tree etc).

Send a 3MF file that includes your part and the supports as separate items

Slicing

Nozzle diameter

Resolution of sharp corners, wall thickness (number of passes needed), minimum/maximum layer height. A bureau may favour a large nozzle for speed, but you may a small nozzle for definition.

Indicate the minimum feature resolution you need and the required tolerance, to ensure the correct nozzle size is used.. Bear in mind that loaded filaments such as wood, CF or GF filament may require a large nozzle, e.g. 0.8mm

Layer thickness

Vertical resolution (stepping), surface texture.

Note that this also affects build time and therefore, cost.

Request a specific layer thickness

Wall thickness

Part strength

Note that this also affects build time and therefore, cost.

Specify a minimum number of inner shells

Infill

Part strength

Note that this also affects build time and therefore, cost.

Specify the infill percent (and pattern if you know the slicer that will be used) or indicate that the part needs to be mechanically strong.

If the bureau agrees, you could even use your own slicer to create a gcode file and send that, for the bureau to use as a reference.

Post-processing

Support removal

You may want the part shipped with supports attached.

Indicate if you want supports removed or you prefer to do it yourself

Heat treatment

Thermally treating some materials (eg PLA) can improve their mechanical properties.

If the bureau does provide this service, there should be a number of standard treatments to choose from. Otherwise it may need to be done inhouse.

Vapour smoothing

Vapour smoothing improves texture and even water tightness of the part, but can adversely affect the mechanical properties if not done correctly. More information here

If the bureau offers this process, then you should first discuss the expected loss of detail/ dimensional changes to the part.

Plating/coating

Plating or coating can change the appearance and function of the part, for example electroplating would make the part electrically conductive

These are normally specialist services not offered directly by a print bureau. You should discuss your requirements directly with the service provider.

For a list of what else to check before sending your file, such as making sure the scale of your model is correct, Shapeways have a good guide here.