DFM Design Guidelines by Process

Rules make DFM concrete

Design for manufacturing can sound abstract until you reach the rules, which are specific and physical. Each exists because a real machine has real limits, and following them is what keeps a part cheap. Start with the universal rules, then apply the ones for your chosen process.

Universal rules (any process)

• Keep it simple. Every added part, feature, and finish adds cost. Aim for the fewest parts and steps that still deliver the product.
• Reduce part count. Combine parts that don't need to be separate (see the minimum-part test in DFMA).
• Use standard components. Off-the-shelf fasteners and parts are cheaper, available, and proven.
• Specify only what you need. Tight tolerances and fine finishes cost real money, so call them out only where the part truly requires them.
• Design around the process from the start. Pick your process early and shape the part to it, rather than designing freely and forcing it later.

Injection molding rules

Molded plastic parts have to fill, cool, and release cleanly from a two-part mold:

• Uniform wall thickness, about 1.2 to 3 mm. Thick sections cool slowly and cause sink and warping, so keep walls even and transition with chamfers or fillets.
• Draft angle, 1 to 2 degrees on vertical walls (more for texture), so the part releases instead of dragging.
• Rounded corners, since sharp internal corners concentrate stress and disrupt flow.
• Avoid undercuts, which block the part from ejecting in one pull; if you can't design them out, the mold needs costly side-actions or lifters.
• Ribs for strength, around 60 percent of the wall thickness, instead of thick solid sections.

CNC machining rules

Machined parts are cut from solid stock by a rotating or fixed tool:

• Radius inside corners. The cutting tool is round, so it can't make a sharp internal corner; design in a radius.
• Avoid deep, narrow pockets and very thin walls, which need special tooling and slow, careful cutting.
• Keep features reachable. The tool has to physically get to every surface, so avoid geometry that forces extra setups or odd angles.
• Loosen tolerances where you can. Machine time rises sharply with precision, so reserve tight tolerances for the fits that matter.

Why the rules are really about cost

Every one of these rules maps to money. A missing draft angle means a part that won't release, so the mold needs rework. A sharp internal corner a mill can't cut means a slower process or a redesign. An unnecessary tight tolerance means more machine time and inspection on every unit. Because most of a product's cost is decided in design, following the guidelines is one of the highest-leverage things you can do. The mindset behind them is in design for manufacturing.

The short version

DFM guidelines are specific and physical: keep it simple and standard, then follow your process's rules, uniform walls and draft for molding, radii and reach for machining. Pick your process early and design to it. For the reasoning see design for manufacturing, and for cutting part count, DFMA.