Design for Manufacturability: 7 Rules Every Product Designer Should Know

Design for Manufacturability (DFM) is the practice of designing parts with the manufacturing process in mind. In injection molding, applying DFM principles from the start can dramatically reduce tooling costs, shorten cycle times, and prevent defects before a single shot is made.

Here are seven rules every product designer should apply when designing for injection molding.

1. Maintain Uniform Wall Thickness

The single most important DFM rule for injection molding. Inconsistent wall thickness causes:

Sink marks — thick sections cool slower, creating depressions on the surface
Warpage — differential shrinkage causes the part to bow or twist
Long cycle times — the mold must wait for the thickest section to solidify

Recommended wall thickness: 1.5mm – 4mm for most materials. PP and ABS typically run well at 2–3mm.

If you must have varying thickness, transition gradually — aim for a 3:1 maximum ratio and taper the transition over a length equal to the wall thickness difference.

2. Add Draft Angles

Draft is the taper applied to vertical walls so the part can be ejected from the mold without sticking or dragging.

Standard draft: 1–2° per side for most surfaces
Textured surfaces: 3–5° minimum (texture depth increases required draft)
Zero-draft walls possible with side actions, but adds tooling cost

No draft = damaged parts, stuck parts, or excessive ejector pin marks. Always add draft in the direction of pull.

3. Use Ribs and Bosses Correctly

Ribs add stiffness without adding wall thickness. Bosses are the cylindrical features used for screws and fasteners. Both are common — both are commonly done wrong.

Ribs:

Height: max 3× the base wall thickness
Thickness: 50–60% of the adjacent wall
Root radius: minimum 0.25mm (reduces stress concentration)
Use multiple thin ribs rather than one thick rib

Bosses:

Outside diameter: 2× the insert diameter
Fillet the base where it meets the wall
Don't connect bosses directly to walls — use a gusset or rib connection
Boss wall: 60% of main wall thickness

4. Avoid Undercuts Where Possible

An undercut is any feature that prevents the part from being ejected in the direction of mold opening. Examples:

Side holes perpendicular to the pull direction
Internal threads
Snap-fit clips facing inward

Undercuts require side actions (slides or lifters) in the mold, which significantly increase tooling cost and complexity.

Where possible, redesign to eliminate the undercut. Where they're unavoidable, we'll design the mold with appropriate side actions — just factor in the added tooling cost.

5. Specify Generous Radii

Sharp internal corners are stress concentrators — both in the plastic part and in the mold steel. They:

Cause premature part failure under cyclical load
Cause premature mold wear (EDM burn marks at sharp corners)
Are harder to machine precisely

Rule of thumb: Add a minimum radius of 0.5mm on all internal corners. 1.5–3mm on structural features. External corners can remain sharp if needed for assembly.

6. Gate and Parting Line Awareness

The gate is where plastic enters the part. The parting line is where the mold splits. Both leave marks on the finished part.

Gates leave a small witness mark (vestige) — place them in low-visibility locations
Parting lines leave a faint line — consider aesthetics when deciding where the mold splits
Discuss gate and parting line placement with your molder early in the design phase

We review every part's gating strategy as part of our DFM process and flag any issues before cutting the mold.

7. Tolerance Awareness — Plastic is Not Metal

Injection molded plastic parts have typical tolerances of ±0.2mm to ±0.5mm for general features. Tighter tolerances (±0.05–0.1mm) are achievable but require:

Premium steel molds with tighter machining
Strict process control
Post-mold measurement and sorting
Higher cost

Design parts with appropriate assembly tolerance in mind. If two plastic parts must mate, build in generous clearance — especially for parts operating across temperature ranges, as plastics expand and contract more than metals.

Get a Free DFM Review

Before you cut your mold, send us your 3D file. Our team will review it for moldability, flag any potential issues, and suggest modifications — all before you spend money on tooling.

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