Mechanical Engineering Perspectives, Part 3: Quantity, Quality, And Time To Market

Here's how to select the most appropriate manufacturing processes.

Development of even a simple product can require hundreds or even thousands of manufacturing decisions. Many options should be selected early in the development cycle, while others can be decided near the end without great consequences. Although we do our best to make manufacturing decisions at the most appropriate time, we should take a moment and verify that our earlier decisions are still valid for the latest manufacturing plan ahead of us.

Forging ahead with a high-volume process like casting or injection molding when a reduced projected annual quantity says we should be machining parts or 3D printing them can minimize profitability to the point of project failure. That said, the opposite is also true.

This article touches on how successful products are manufactured and why they are manufactured the way they are.

MOQ: Why Is This Important?

In the early stages of product development, we may have a general idea of how a component should behave that begins to define the type of material (metal, plastic, ceramic, elastomer), but in order to more precisely define that material, we will likely need to know the manufacturing process. This also drives design constraints important for production; however, the most appropriate manufacturing process may change depending on the minimum order quantity (MOQ). Redefining manufacturing processes midstream is an unfortunate, but sometimes necessary step. Many production manufacturing methods are considered high volume, requiring a high MOQ with substantial upfront costs. Plastic injection-molded parts, for example, require the design and development of an injection mold tool as well as iterative testing and part sampling to prove out the tool and part design. All of this effort has associated costs. The downstream benefit would be lower part costs if you can justify the higher up-front cost by amortizing those costs over a large number of parts. If you are only making a small number of parts, you might be better off machining the parts at higher per-piece costs that are ultimately less expensive than the added upfront tooling costs associated with injection molding.

We don’t always have all of the information required to design a component for manufacturing. In those cases, we have to make an educated guess and move forward at risk. Those risks can sometimes be reduced by selecting a lower-volume process with the understanding that higher-volume processes can be utilized when increased quantities warrant the time and effort required to adjust the component’s design for that process. For instance, machined parts do not normally require a draft, but injection molded parts and cast parts typically do. Changing processes from machined to molded or cast would require adjustment to the design of at least that component, and possibly other parts it mates with. Knowing when to make the decision to design for increased or decreased volumes can be key to maximizing production efficiency.

Beauty Is in the Eye of the Beholder

“How many?” is not always the most pertinent question. When a particular aesthetic or function of a component is paramount, we may need to choose a manufacturing process that best provides that need. We may even choose a more expensive material for a component for no other reason than it looks or feels “better.” Using higher-cost manufacturing methods for key components may be the difference between a “must-have” product and an unknown product. We need to know how best to create what is desired most efficiently without compromising on what is most important, like using liquid metal injection for latch components on a wearable device instead of cast parts to avoid latch failures and streamline the look and feel.

If You Want It Really Bad—You May Get It—Really Bad 

There is a tendency to rush to market. We are all so focused on the project schedule and completing the design for release to manufacturing that we lose sight of the actual goal, which should be a successful product. As soon as there is a minimum viable product, that product gets pushed to the masses, creating a worldwide beta program. Launching too early without a proper design for manufacturing and design for assembly effort can lead to unforeseen issues and hidden costs that outweigh the would-have-been costs of the DFM/DFA efforts.

We may have heard “the last ten percent of product development is ninety percent of the work.” This is true for most highly successful products around the world. Refining the design for manufacturing processes, adjusting those processes to best match the needs and requirements for those components, and refining the product design as a whole for ease of assembly not only makes for a better product, it also increases yield, decreases waste, and increases profit.

Experience Can Pay Dividends

Early in my career, I worked on a product that we designed to be housed in a sheet-metal enclosure. As the project approached completion, we found the tolerance of the sheet metal was just barely adequate to hold the components in their required positions. The sheet-metal enclosure also had sharp edges and gaps that allowed wires to get caught and damaged and allowed liquids to enter the housing. We were ready to build almost 50 units immediately, but we decided to review the enclosure and see whether there were any elegant options we were missing. We found that by simply changing the sheet-metal housing to a housing machined from billet aluminum, all of those issues went away. And, thanks to the mid to low volume of the sheet-metal parts, the costs were actually slightly lower having the parts CNC machined.

By not being afraid to pursue other options on a product that was technically “complete,” we lowered costs, eliminated product failures, and improved performance and aesthetics.

It's not always easy, but taking a moment to catch your breath and grab any low-hanging fruit you find may help produce the product you are hoping for.

This article, written by Product Creation Studio Senior Mechanical Engineer Adam Smith , originally appeared in DesignNews on October 18, 2022.

For other installments in this series, please see Mechanical Engineering Perspectives, Part 1: Material Costs, Function, & Durability, and Mechanical Engineering Perspectives, Part 2: Creative Design Tweaks to Manage Speed, Cost, & Function.