Hustler Series: Design For Manufacturability

10: Design For Manufacturability

Before we get to the manufacturing stage I think it’s important to go over a few techniques of designing for manufacturability. Design for manufacturability, or DFM, is an important part of engineering that ensures your product is manufactured most efficiently. With proper DFM your product will be easier to manufacture, cheaper to manufacture, and quicker to bring to market. So let’s get into the basics of DFM.

Doing a quick Google search for DFM will return more detailed recommendations but I’d like to go over 5 key areas. These are moldability guidelines, part complexity, part tolerances, fasteners, and product assembly. Once you have a basic understanding of these five areas you’ll be on your way to optimizing your product for the first production run. So let’s get started.

If your product has injection-molded parts, moldability is a critical part of DFM that you must carefully consider. The basic premise of optimizing moldability is, as it sounds, to make sure the part can be molded. While this sounds simple, there are a few key things you’ll want to keep in mind. The first principle is trying to maintain uniform wall thicknesses throughout your part. This is necessary for achieving a quality surface finish due to how injection molded parts cool in their mold. Having extreme transitions in wall thickness will cause the thin sections to cool quicker than the thick sections which forms internal stress concentrations and leads to warped parts and sink marks. For non-cosmetic parts this may be acceptable but it won’t fly for any product that a consumer will see or touch. Having uniform wall thickness also has the drawback of making your product less structurally sound. So to compensate for that you’ll want to add ribs to areas that need increased stiffness. Ribs will increase stiffness without increasing wall thickness, which means they won’t create warp or sink marks. In addition to wall thickness you’ll want to minimize undercuts. In order to achieve undercuts you’ll need to add special modifications to the tool that drastically increases startup costs. It may be worth having an undercut if it reduces product complexity but, when in doubt, it’s good practice to minimize undercuts so you can minimize tool complexity and cost. The last moldability principle to follow involves optimizing mold fill and release. For this you’ll want to make sure you avoid sharp corners and perfectly vertical walls on your product. Sharp corners may have trouble filling so it’s recommended that you round them if possible. Additionally, perfectly vertical walls will drag on the mold when ejected from the cavity, which will ruin the surface finish. It’s recommended that you add at least a 1° draft angle to your vertical walls to ensure no mold release issues. I recommend checking out this link if you want more detailed information about moldability.

The next DFM principle is minimizing part complexity. As mentioned with undercuts, complex parts will require complex tooling and have higher associated startup costs. Because of this it’s good practice to minimize part complexity unless you can justify the increased cost. It’s also good practice to minimize the number of parts in your product. Fewer parts mean fewer tools and quicker assembly, both of which reduce cost. However, it’s possible to go too far with minimizing part count to the point that your design becomes weak or flimsy. Everything is a tradeoff. So in most cases you’ll want to minimize part count but there may be instances where an extra part can be justified. Use your judgment, try to optimize for part complexity and count wherever possible, but know when you’re going too far.

The third DFM principle to keep in mind is tolerance optimization. Any plastic or metal product you design will have some sort of tolerance threshold. This is just a part of designing things for the real world. There’s no such thing as perfect precision so it’s impossible to machine parts exactly in line with specified dimensions. Additionally, the hot plastic used in injection molding will shrink slightly as it cools to room temperature. To account for this shrink your manufacturer will design the mold to be slightly larger than your part, but it’s impossible to dial in this compensation perfectly. So every part in your product will have it’s own slight deformities in flatness and length. Optimizing tolerances means designing your product efficiently so you either absorb these deformities or throw out any parts that are too deformed to work. The cheapest option, of course, is allowing a little wiggle room in your product to absorb any deformities. However, if this isn’t acceptable, you’ll have to specify what level of deformation is acceptable and recycle any parts that don’t meet this threshold. Unfortunately this will mean taking time to measure the deformities and also trash parts that fail the test, both of which add considerable cost to your product. Do your best to optimize tolerances and know when a dimension or flatness is absolutely critical to function.

The fourth DFM principle to keep in mind is fastener optimization. If your product has more than one part there’s a good chance it will need screws or some other fastener to hold everything together. Your options are typically screws, snap features, or glue, each with it’s own set of advantages and disadvantages. Screws are the simplest and most robust fastener option. Being made from steel gives them a strong hold but this does come with disadvantages. Mainly screws can add additional weight and assembly time to the product. They also require your product have visible screw heads. If you decide to use screws, it’s good practice to try and use as few screw variations as possible. Ideally you should only use one style and length of screw to ensure fewer headaches during assembly. This may not be possible but it’s something to strive for. Snap features are another option that may be better suited than screws. Molding snap features into your plastic parts will allow you to save on assembly time and weight while also reducing part count. However the snap features typically won’t be as strong as screws and they can make disassembly of your product challenging. Similarly glue makes disassembly nearly impossible. It has the advantage of being low weight and may be necessary if you don’t want visible screw heads in your product, but you will need to be cautious of the curing period and potential mess associated with glue. This adds processing time and labor, which drives up the product cost. Understand the circumstances where each of these fastening options excels and you’ll set your product up for efficient assembly.

The final DFM principle to keep in mind is optimizing the assembly. If you want to mass-produce your product on a large scale, you’ll want to be able to assemble the product as quickly and efficiently as possible. This means optimizing it to have as few parts as possible and as straightforward an assembly as possible. You can achieve this by properly optimizing according to the previous four principles. This is really the overarching goal of design for manufacturing.

Hez Cases required careful consideration of modability. Despite being a simple product there were still many engineering challenges. One challenge was achieving uniform wall thickness. Due to the overmold design, there had to be a thin section in the hard plastic piece where the rubberize material could adhere to. I totally messed this up with my first production run and paid the price for it in the form of noticeable sink marks. But I managed to fix the problem for the second production run by smoothing out the transition in wall thickness. In addition to the wall thickness, the case had quite a few undercuts. The volume and power buttons had minor undercuts; the mute switch, speakers, and charging port had deep undercuts; and the inner walls all had undercuts. Achieving these undercuts meant adding complexity to an otherwise simple mold. Though it was not ideal from a cost perspective, the added complexity was necessary to achieve my vision. It was the only way to get the look and feel I wanted for my product. So for me, the higher cost was not only worth it, but absolutely necessary.

With that you should have the basics of designing for manufacturability. The overarching goal is to optimize every constraint so you can reduce production and assembly costs. There will be tradeoffs with each optimization but use your judgment and you’ll achieve a product that’s cheap and easy to manufacture.