DFM for Injection Molding

Uniform walls.
Proper draft.
Predictable parts.

Injection molded parts fail predictably when DFM rules are violated. Sink marks, warpage, weld lines, short shots — all preventable at design stage. These rules are the difference between a tool that works and one requiring costly rework.

Free DFM Review Injection molding

01 · Wall thickness

Keep walls uniform.

Uneven wall thickness causes differential cooling — thicker sections cool slower, pulling thinner sections inward. Result: sink marks, warpage, internal voids. Target: ±25% variation across the part.

Material	Nominal wall	Max variation	Notes
ABS	2.0-3.5 mm	±25%	Good flow, tolerant of variation
Polycarbonate	2.5-3.5 mm	±20%	Sensitive to thickness — warps easily
Nylon (PA6/66)	1.5-3.0 mm	±20%	Shrinks 1.5%, uniform walls critical
PP	1.0-3.0 mm	±30%	Forgiving, good flow
POM (Delrin)	1.5-3.0 mm	±20%	Very shrinkage-sensitive
PEEK	1.0-5.0 mm	±20%	Expensive material, minimize thickness

02 · Draft angles

Every vertical wall needs draft.

Molded parts shrink onto mold cores as they cool. Without draft, the part grips the core and tears when ejected. Zero-draft walls cause scrapes, ejector pin failures, and surface damage.

0.5°

Minimum smooth

Absolute minimum for polished mold surface. Still may cause ejection issues.

1°

Standard polished

Recommended for SPI A1-A3 polished surfaces. Works for most applications.

2°

Standard textured

For SPI C-D matte/textured surfaces. Larger draft compensates for tool grip.

3°+

Heavy texture

For deep texture (MT-11010, leather grain). More draft prevents drag marks.

5°

Ribs & bosses

Internal features need extra draft — high friction on mating surface.

0°

Only for side-action

Features that release via slides can have zero draft — but adds slide cost to tool.

03 · Ribs & bosses

Structural features done right.

Ribs add stiffness without adding wall thickness — smart DFM. But wrong rib dimensions cause sink marks on the opposite surface.

Rib thickness

60% of wall

Rib thickness at base = 60% of adjacent wall. Thicker causes sink marks on show surface; thinner doesn't add useful stiffness.

Rib height

3× wall thickness max

Height beyond 3× wall is wasted — diminishing returns on stiffness. Add multiple shorter ribs instead of one tall rib.

Rib spacing

2× wall thickness min

Spacing between ribs must be at least 2× wall. Closer spacing causes flow issues and gas traps.

Boss diameter

2× fastener diameter

For M4 screw boss, use 8mm OD. Smaller OD causes weak threads or sink marks on opposite face.

04 · Gate placement

Where plastic enters matters.

Good gate location

• Thickest section (plastic flows to thinner areas)
• Away from cosmetic surfaces (gate leaves mark)
• Enables uniform flow front reaching all features
• Centered for symmetric parts (balanced fill)
• Multiple gates for large parts (reduce flow distance)

Bad gate location

• Thin sections (causes short shot — part doesn't fill)
• On cosmetic A-surface (gate vestige visible)
• At feature creating long flow path
• Creating weld lines across stressed area
• Single gate on very large parts (uneven fill)

FAQ

Why is uniform wall thickness so important?

Thick sections cool slower than thin sections. As the thick section continues cooling, it contracts inward, pulling material from the thinner adjacent area. Result: sink marks on the thick section's opposite surface, warpage as contraction forces distort the part, internal voids if contraction exceeds material supply. All preventable by keeping walls uniform (±20-30% variation).

Can draft angle be less than 1°?

Sometimes. For very polished (SPI-A1) molds with short vertical walls (under 5mm), 0.5° works. Below that, parts drag on ejection — causing surface damage, ejector pin marks, tool wear. Specifying less than 0.5° is high-risk; unless you have specific reason, 1° is safe standard.

How do I avoid weld lines?

Weld lines form where two flow fronts meet. Prevention: (1) Use single gate where possible, (2) Position multiple gates so flow fronts meet at non-critical areas, (3) Avoid features in middle of part that split flow (holes, bosses), (4) Use gas assist for complex flow paths. If weld lines are unavoidable, locate them on hidden surfaces and ensure adequate wall thickness (weld lines are 20-40% weaker than bulk material).

Undercuts — always avoid?

Undercuts require side-action mechanisms (slides, lifters) in the mold. Each slide adds $2,000-8,000 to tool cost and complexity. Simple undercuts (0.5-1mm deep, low volume): consider keeping them, cost manageable. Deep undercuts (>3mm) or complex geometry: redesign to eliminate if possible, as multiple slides compound tool cost quickly. DFM review flags undercut cost implications before tool is built.

Bosses — common mistakes?

Most common: boss OD too small (creates sink mark on opposite face). Rule: boss OD = 2× fastener diameter for heat-set inserts, 2.5× for self-tapping screws. Second mistake: boss too tall (sink marks also increase with height). Third: ribs to boss too thick (causes sink mark where rib meets main wall). Reference: Protolabs and major molders have detailed boss design rules — essentially: keep all features at 60% of nominal wall thickness.

Should I include draft in my CAD?

Yes. Don't assume mold-maker will add draft. Draft affects part functionality — mating features may need different draft angles, datum surfaces may need reference features, visible surfaces have cosmetic requirements. Model the part with draft included so you can verify fit and function. Tool maker will adjust as needed but starts from your draft decisions.