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March 11, 2018

Design for Manufacturing

Anyone can design a part (well, almost anyone). But, is it manufacturable?  Which manufacturing method should be used?  Better yet, why manufacture the part ourselves when we can buy it off-the-shelf? These are all matters of DFMA – design for manufacturing and assembly.

Production Volume

There are many ways to make a part: injection molding, stamping, machining, sheet metal forming, cutting… the list goes on. Production quantities (AKA volume) is one of several major factors influencing which manufacturing method is chosen.

In terms of cost, different manufacturing methods make sense at different production volumes. Manufacturing methods range from labor-centered (high labor per part, low tooling cost, typically used in low-volume production) to capital-centered (low labor per part, high tooling cost, typically used in high-volume production).  Part cost is a function of tooling cost and labor cost.

High volume production: Customized tooling is often used to produce parts in high volume. These tools are costly. However, part piece-prices are low because the labor to produce “one more part” from said tooling is very small. Many units must be sold before ROI for custom tooling is attained. This approach makes more sense when producing high part volumes. Example: Factory-made cookies

Low volume production: Alternatively, more manual processes (such as hand-building or machining individual parts) cost little or nothing to begin production. But, part piece-prices are high due to high labor per part. This approach makes more sense for producing low volumes. Example: Homemade cookies

To create a simple example: Homemade cookies have a high labor cost and a low start-up cost compared to factory-made cookies, which have a low labor cost and a high start-up cost. It would not make sense to buy a cookie making machine if you’re only producing 10 cookies. But, if you’re producing 10,000,000 cookies, you should buy a machine instead of making them by hand.

Example: At some point, injection molding becomes more cost-effective than vacuum forming

(Source: www.rayplastics.com)

There are pros and cons to each production approach. Oftentimes, those with a tight budget and/or limited market research will initially opt for low-volume production in order to minimize financial risk while testing market reception. If the product fails or if parts need to be modified, the financial impact is minimal.

In order for us to make the best recommendations possible, we ask that you inform us of your production strategy and anticipated volumes.

Manufacturability

Each manufacturing process has its own limitations. Injection molding, for example, is fraught with design restrictions (draft, undercut, and mold flow challenges, to name a few). Naturally, some suppliers are more capable and innovative than others, offering convenient workarounds to traditional manufacturing challenges. Experienced product engineers should have their own internal rankings of suppliers based on cost, capability, and quality.

Make vs. Buy

Why concern yourself with the myriad challenges of manufacturing when you can simply buy a part off-the-shelf? Indeed, purchasing pre-made components is oftentimes far more economical than making them on your own. Take a bolt, for example: It’s more or less common sense that buying an existing bolt is preferable to designing one custom (and designing a custom bolt for a bolt supplier to produce is still infinitely better than trying to start your own bolt factory, if that even needs to be said). Only extremely high production volumes or an extremely demanding design would justify creating a customized bolt, as there are already thousands upon thousands of bolt varieties already stocked and ready to ship. For truly custom parts, like a molded housing, the choice is equally clear in the opposite direction: This part must be custom-made. “Make vs. buy” is almost a misnomer in the sense that for most companies launching a product, even custom parts (like a molded housing) still technically fall under the “buy” category because they are produced by  and purchased from specialized suppliers.

Common manufacturing methods in product design

Injection molding

Typical usage: Plastic parts with complex geometry

Tooling cost: High

Piece cost: Low

Primary design restrictions: Draft angle, undercuts, part thickness

Vacuum/thermoforming

Typical usage: Thin plastic housings/shells with approximately uniform wall thickness

Tooling cost: Moderate

Piece cost: Moderate

Primary design restrictions: Draft angle, undercuts

(CNC) machining

Typical usage: Thick, 3-dimensional metal parts

Tooling cost: Low

Piece cost: High

Primary design restrictions: Undercuts, fixturing

Sheet metal bending

Typical usage: Simple housings, brackets

Tooling cost: Low

Piece cost: Low

Primary design restrictions: Order of bend operations, very limited geometry, uniform wall thickness

Sheet metal stamping

Typical usage: Complex thin housings/shells

Tooling cost: High

Piece cost: Low

Primary design restrictions: Uniform wall thickness

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