Top Design Tips for DFM

By Michael Gist

When you design a part, there are many factors to consider. Beyond the usual fit, form and function, and material selection; an engineer needs to be thinking about Design for Manufacturing (DFM). Including DFM up front will lead to a better solution for your final product. In this paper you will learn more about DFM, why it’s important, and discover tips on how to optimize your design for manufacturing and assembly processes. Experience and practice have helped SIGMADESIGN develop robust practices for DFM that will help you reduce the cost and difficulty of producing your product while maintaining its quality.

WHAT IS DFM?

Design for Manufacturing is a key activity in taking a component or assembly from design concept to a physical product that can be produced and assembled in an efficient and cost effective manner. DFM is critical in the product development cycle since it involves optimizing the design of the product for its fabrication and assembly processes. DFM unifies the design requirements of the product with its production method. Your concept may start as a an idea sketched on paper, or as a CAD design, but the art of design for manufacturing is in making sure the product can in fact be easily and efficiently fabricated and/or assembled. Moreover, good DFM should reduce the cost of producing a product while maintaining its critical dimensions and functional outputs, i.e. Quality.

WHY IS DFM IMPORTANT?

In a nutshell, cost reduction. Most product manufacturing costs stem from design decisions regarding material selection and manufacturing method. The remaining costs usually flow from production decisions such as process planning and tool selection. At SIGMADESIGN we focus on design optimization in order to reduce the cost of manufacturing for our clients.‍

WHERE DOES DFM FIT IN THE DESIGN PROCESS?

There are two answers to this question. First, DFM applies to the design process starting from the very beginning; the engineer should know how they plan on producing and assembling the part as they start the design. The part production process selection will drive the tolerance capabilities (think machining vs. molding) and part cost. Second, portions of DFM can be left to the fabricators when they start quoting the design. For example, the engineer may not care where all the ejector pins in a molded part are located. You can let the tool designer figure that out, as it affects other aspects of the tool design, and therefore the cost. A good fabricator will know how to adjust as needed to further reduce costs.

DFM TIPS FOR DIFFERENT PART TYPES

As mechanical engineers, we have expertise in a wide range of part types and production processes. We evaluate and consider the manufacturing processes involved and how various factors will affect the fabrication and quality. Following are some tips to keep in mind when designing for manufacturability on a few of the more common processes:

Injection Molded Plastic Part Design and Die Cast Part Design:

• Know how the tool is going to work. Where are the parting lines, where will the gate or gates be, where will ejector pins go?
  • Don’t forget draft. Surfaces perfectly parallel to the tool opening direction will almost never come out cleanly. 3° is a great general purpose starting point for most plastics. 0.5° is good if your molder is friendly and the surface is critical. As much as 10° is necessary on textured or appearance critical surfaces.

• Undercuts are impossible to mold without tool action (slides, lifters, etc.) or bypass conditions. If you have to have them, the tooling becomes much more expensive, and the part design is more complicated to compensate for the action.
  • Fiber reinforced plastics are often chosen as a cost effective way to get a stronger part. However, they typically have non-isotropic flow and mechanical properties. When it comes to DFM, this translates to your tolerances being different in the flow direction of the plastic vs. the perpendicular directions. Good tool designers can mostly compensate for this, but it is never perfect.
  • Keep your wall thicknesses constant or decreasing, and avoid thick sections. These will sink when the plastic cools.

Sheet Metal Part Design:

• Is the tool going to be a progressive die, or press-brake? They have different implications.
  • Press-brake tools typically have shorter lead times, but a higher piece price. The tooling itself is comparatively inexpensive.
  • Progressive die tooling usually has a long lead time, but low piece prices and better repeatability.  The tooling itself is typically comparatively expensive.
  • Sheet metal is like origami, if you can fold it into the shape you want, then you can probably make it. However, the caveat to this is that you have to be able to get the tooling into the spots you want to fold the sheet metal. Steel is much harder to bend than paper after all.
  • Know that features need to be separated from each other, otherwise the flow characteristics will cause them to warp and deform. Your round holes become ovals, or worse, close up completely.

Machined/Fabricated Part Design:

• Machined part costs are typically dictated by the number of setups required to make the part and the accuracy requirement
  • Try to keep as many cuts coming from the same direction as possible. If you have to machine all six faces of a box for example – consider instead making six separate plates and attaching them together.
  • Material selection can greatly impact machining costs, as harder materials take longer to cut than soft.  It is typically easier to achieve a higher accuracy on harder parts though, so it is a trade-off.

3D Printed Part Design:

• Some people like to think that 3D printing can be used to make anything, and DFM isn’t relevant any more. This isn’t the case.
  • Each 3D printing process has different characteristics to keep in mind when designing. For example, a tall cylinder behaves differently when printed with an HP MJF if it is printed vertically (circular build layers) vs. horizontally (rectangular build layers).
  • FDM parts, if they have any overhangs, need to have support structures integrated into the designed.
  • Most 3D print processes do not like overly thick sections – they can sink, warp, or deform severely.

DFM SUCCESS

When designing for DFM, experience and practice will help you succeed. Engineering schools don’t typically teach DFM, so many engineers learn it on the job when they start designing components. Reviewing designs with peers and other experts and having fabricators review the design will help in creating the best parts. At SIGMADESIGN we effectively make an incredible array of different parts required for many diverse industries. Our design abilities are enhanced by the range of experts we have in-house including mechanical engineers, fabricators, and assembly experts, with wide ranging backgrounds. The engineers, fabricators, and assemblers work collaboratively to continually improve the DFM process. We enjoy applying our experience and expertise to assist clients with product design ensuring all of the details that inform making a successful part are part of the process, including DFM.