Keep walls uniform.
| 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 |
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.
Every vertical wall needs draft.
Minimum smooth
Absolute minimum for polished mold surface. Still may cause ejection issues.
Standard polished
Recommended for SPI A1-A3 polished surfaces. Works for most applications.
Standard textured
For SPI C-D matte/textured surfaces. Larger draft compensates for tool grip.
Heavy texture
For deep texture (MT-11010, leather grain). More draft prevents drag marks.
Ribs & bosses
Internal features need extra draft — high friction on mating surface.
Only for side-action
Features that release via slides can have zero draft — but adds slide cost to tool.
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.
Structural features done right.
60% of wall
Rib thickness at base = 60% of adjacent wall. Thicker causes sink marks on show surface; thinner doesn't add useful stiffness.
3× wall thickness max
Height beyond 3× wall is wasted — diminishing returns on stiffness. Add multiple shorter ribs instead of one tall rib.
2× wall thickness min
Spacing between ribs must be at least 2× wall. Closer spacing causes flow issues and gas traps.
2× fastener diameter
For M4 screw boss, use 8mm OD. Smaller OD causes weak threads or sink marks on opposite face.
Ribs add stiffness without adding wall thickness — smart DFM. But wrong rib dimensions cause sink marks on the opposite surface.
Where plastic enters matters.
Good gate location
Bad gate location
FAQ
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).
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.
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 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.
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.
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.
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