Rib and Boss Design for Injection Molded Parts: Avoiding Sink Marks

Good rib design injection molding practice eliminates sink marks and warpage before the first steel is cut. The single most violated rule we see on incoming part prints is rib thickness: ribs cut at 75% or 80% of nominal wall almost guarantee a visible sink on the show surface. Hold rib thickness to 60% of nominal wall and you eliminate that defect on the first tool trial roughly 80% of the time.

Why Rib Thickness Rules Exist and What Happens When You Break Them

A rib adds stiffness by increasing the section modulus without adding bulk to the nominal wall. The problem is that every rib root creates a local mass concentration. Plastic cools from the outside in, and a heavy rib root shrinks inward after the skin freezes, pulling the surface down into what we call a sink mark rib defect.

The rib-to-wall ratio quantifies this risk directly. SPI guidelines and most resin supplier design guides set the maximum rib thickness at 60% of the adjacent nominal wall. For a 3.0 mm nominal wall, that means a rib no thicker than 1.8 mm at its base. Push that to 2.4 mm (80%) and you are doubling the local volume at the root. Sink depth climbs fast from there.

Shrinkage rate compounds the problem. Semi-crystalline materials like 30% glass-filled nylon 66 shrink at 0.5% to 1.0% in the flow direction and 1.5% to 2.0% transverse. A thick rib root in a transverse orientation on that material will sink visibly even at modest pack pressures. Amorphous materials like ABS shrink at a more uniform 0.4% to 0.7%, so they tolerate slightly heavier ribs, but the 60% rule still applies as a starting point.

Rib Height, Draft, and Spacing: The Numbers That Govern Moldability

Rib height should not exceed three times the nominal wall thickness. On a 3.0 mm wall that caps ribs at 9.0 mm tall. Taller ribs increase ejection force, raise the risk of drag marks in the tool, and flex during packing in ways that cause warpage after ejection.

Draft angle on rib faces is non-negotiable for reliable ejection. We run a minimum of 0.5 degrees per side on polished tool steel and 1.0 degree per side as the standard on untextured ribs. Textured surfaces need an additional 1.0 degree per 0.001 inch of texture depth, per standard SPI/SPE texture guidelines. A rib that is 9.0 mm tall with 0.5 degrees of draft loses only 0.08 mm of width from root to tip, which is negligible structurally.

Rib spacing matters as much as individual rib geometry. Ribs placed closer than two times the nominal wall thickness create a pocket that traps heat, raises local cycle time, and causes differential shrinkage between rib fields and open wall areas. Space ribs at least 2x nominal wall apart, center to center, measured from rib face to rib face.

Parameter	Recommended Value	Maximum Limit
Rib thickness (% of nominal wall)	50% to 60%	60%
Rib height (x nominal wall)	2x to 3x	3x
Draft angle per side (untextured)	1.0 degree	0.5 degree minimum
Root radius	0.25x nominal wall	0.5x nominal wall
Rib-to-rib spacing (face to face)	2x nominal wall minimum	No upper limit

Root radius is frequently omitted from part prints. A sharp rib root concentrates stress and creates a notch that acts as a crack initiation site under cyclic loading. Radius the root at 0.25x to 0.5x the nominal wall. At 0.25x on a 3.0 mm wall, that is a 0.75 mm radius. The toolmaker cuts it with a ball end mill during the CNC roughing pass at no added cost if it is on the print from the start.

Boss Design Injection Molding: OD, ID, and the Connection Problem

Boss design injection molding follows the same mass-concentration logic as ribs. A boss is a hollow cylinder that accepts a self-tapping screw, heat-set insert, or press-fit pin. The wall of the boss is the critical dimension.

The standard boss wall calculation: boss outer diameter equals the screw or insert major diameter multiplied by 2.0 to 2.5. For a No. 6 screw (major diameter 0.138 inch), that gives a boss OD of 0.276 inch to 0.345 inch. Boss wall thickness should follow the same 60% rule applied to ribs: keep the boss wall at 60% or less of the nominal part wall. A 3.0 mm nominal wall calls for a boss wall no thicker than 1.8 mm.

Boss height should not exceed 2.5x the boss OD without additional support. A freestanding boss 12 mm tall with a 5 mm OD will deflect during screw-drive assembly and can crack at the base if the boss wall is undersized. When height exceeds the 2.5x guideline, connect the boss to the nearest wall or rib with a gusset.

Gussets on bosses should be thinner than the boss wall, not equal to it. We target 75% of the boss wall on gussets. A boss wall of 1.8 mm gets gussets of 1.35 mm. Gusset height matches boss height, and gusset draft mirrors the surrounding rib draft at 1.0 degree per side minimum.

Connecting Ribs to Bosses: The Geometry That Kills Tooling Budgets

Connecting a rib directly to a boss at full rib thickness creates a T-intersection with a fat root. That intersection sinks, and it also creates a complex parting or shutoff geometry in the tool that adds $800 to $2,500 per boss location in hand-finishing time at a typical Chinese mold shop. We have seen 24-boss parts where this detail alone added $28,000 to the tool cost because the buyer’s design team ignored it until after tool design was locked.

The fix is straightforward. Run the connecting rib into the boss at the same rib thickness (60% of nominal wall) and terminate it with a radius equal to the rib half-thickness. Do not widen the rib as it approaches the boss. The tool insert for the pocket between rib and boss is then a simple shape that a CNC machine handles in one pass.

For structural bosses that carry high pull-out loads, use three or four gussets at 90 or 120 degree spacing rather than a single connecting rib. This distributes the load symmetrically and avoids the differential shrinkage that causes a single-rib boss to pull slightly off-axis after ejection. That off-axis shift is typically 0.05 mm to 0.15 mm, small enough to pass dimensional inspection but large enough to cause cross-threading at assembly.

Material-Specific Adjustments to Standard Rib and Boss Rules

The 60% rib-to-wall ratio is a starting point, not a universal constant. Material selection shifts the number up or down by a meaningful amount.

Unfilled polypropylene (PP) shrinks at 1.5% to 2.5% and tolerates a rib thickness of only 50% to 55% of nominal wall before sinking becomes visible at standard cosmetic requirements. Filled materials behave better. A 20% glass-filled PP can hold the full 60% limit with good pack pressure because the fiber network resists shrinkage. Acetal (POM) runs at 2.0% to 3.5% shrink and needs ribs at 50% or less; we treat POM boss walls the same way and reduce them to 55% of nominal wall as a default in our shops.

PC/ABS blends are the forgiving end of the spectrum. Low shrink (0.4% to 0.6%), good flow, and wide process windows mean ribs at 60% to 65% of nominal wall rarely sink under normal pack conditions. Still, never go above 65%. The cosmetic risk is not worth the marginal stiffness gain.

Material	Typical Shrinkage (%)	Max Rib-to-Wall Ratio	Notes
ABS	0.4% to 0.7%	60%	Standard rule applies
PC/ABS	0.4% to 0.6%	60% to 65%	Low shrink; cosmetic surfaces only to 60%
Unfilled PP	1.5% to 2.5%	50% to 55%	Reduce further on show surfaces
20% GF PP	0.6% to 1.2%	60%	Fiber network reduces sink risk
Nylon 66 (unfilled)	1.5% to 2.0%	50%	Hygroscopic; dry parts before measuring
Acetal (POM)	2.0% to 3.5%	50%	Boss walls to 55% max

Running a DFM Review Before Tool Kick-Off

Every rib and boss defect listed above is detectable in a design for manufacturability review before any steel is touched. In our experience managing Chinese tool programs, rib and boss geometry changes after T1 steel cut cost an average of $1,200 to $3,500 per modification, depending on insert complexity and whether the change requires new EDM electrodes. Catching ten violations in DFM costs nothing. Fixing them in steel costs $12,000 to $35,000.

A solid DFM review checks every rib for the rib-to-wall ratio, every boss for OD/ID and wall compliance, all root radii, draft angles on both rib faces and boss ODs, and every rib-to-boss intersection geometry. It also flags any rib taller than 3x nominal wall that lacks a taper compensation in the ejection layout.

Our project managers run this review as a red-line markup against the native CAD file, not a PDF. PDF markups miss wall thickness measurements that only show up in a section cut. If your supplier is reviewing DFM on a PDF print, that is a process gap worth correcting now.

Frequently Asked Questions

What is the rib-to-wall ratio and why does 60% matter?

The rib-to-wall ratio is the rib base thickness divided by the adjacent nominal wall thickness. At 60%, the rib root volume is low enough that pack pressure and gate placement can compensate for the local shrinkage differential in most materials. Above 60%, the mass concentration outpaces what packing can fix, and a sink mark rib defect appears on the opposite surface.

Can I reduce a sink mark rib defect by increasing pack pressure instead of redesigning the rib?

Sometimes, but not reliably. Higher pack pressure can reduce sink depth on amorphous materials with low shrink rates like ABS or PC/ABS. It rarely eliminates sink on semi-crystalline materials like nylon or acetal where volumetric shrinkage is high. Increasing pack pressure also raises residual stress and can cause warpage, which trades one defect for another.

What draft angle should I specify on boss outer diameters?

Use 0.5 degrees per side as a minimum on boss ODs for untextured surfaces. Specify 1.0 degree per side as the standard if the part is not dimensionally constrained at the boss OD. The taller the boss, the more important draft becomes; a 12 mm tall boss with zero draft will require polished tool steel and high ejection force, both of which raise tool cost and cycle time.

When should I add gussets to a boss instead of connecting ribs?

Add gussets when boss height exceeds 2.5x the boss OD, when the boss carries axial pull-out loads from self-tapping screws or heat-set inserts, or when a single connecting rib would create a non-symmetric shrinkage pattern. Three or four gussets at equal angular spacing hold the boss concentric after ejection far better than a single rib, which improves screw-drive assembly yield at the line.

Does boss design injection molding change for heat-set inserts versus self-tapping screws?

Yes. Heat-set inserts require a tighter boss ID tolerance, typically the insert supplier’s recommended hole diameter plus 0.0 to 0.05 mm. The boss OD must be large enough to keep the boss wall at 60% of nominal after the insert bore is cut. Self-tapping screws allow a slightly larger pilot hole, which means a smaller boss ID and more boss wall material, making the self-tapping boss geometry easier to hold to the rib thickness rules.