The surface looks fine until it’s painted. Then the sink marks appear as visible shadows, rejecting a batch of cosmetic parts that passed dimensional inspection. Paint acts like a detective, revealing imperfections that bare plastic hides. This is why sink mark prevention matters long before parts reach finishing operations, and why what looks acceptable at the molding machine may not survive downstream processing.
Sink marks form through physics that can’t be negotiated away: plastic shrinks as it cools, thick sections shrink more than thin sections, and if the surface follows that shrinkage, a depression appears. The question isn’t whether thick sections will want to sink but whether design and process can prevent that tendency from becoming a visible defect.
Why Sink Marks Form
The mechanism is straightforward. Molten plastic occupies more volume than solid plastic; the material shrinks as it transitions from melt to solid. In uniform thin walls, this shrinkage is accommodated by pack pressure forcing additional material into the cavity as the material contracts. The surface remains flat because shrinkage occurs uniformly through the wall thickness.
Thick sections disrupt this balance. The outer skin solidifies first, forming a rigid shell while the interior remains molten. As the interior finally solidifies and shrinks, it pulls the outer surface inward. If the skin is thin enough relative to the shrinkage forces, the surface deflects, creating a depression.
Pack pressure can partially compensate by continuing to push material into the cavity during cooling. But pack pressure only reaches areas connected to liquid material; once the gate freezes, no additional packing is possible. Thick sections far from the gate are particularly vulnerable because pack pressure may never reach them effectively.
The critical window is the time between skin formation and gate freeze. During this period, pack pressure can push additional material into the shrinking interior. After gate freeze, the cavity is isolated from the injection unit, and any remaining shrinkage creates either sink marks (surface depressions) or voids (internal bubbles, if the skin is too rigid to deflect).
Design Factors
Part geometry determines sink mark risk. Some designs avoid thick sections entirely; others make thick sections inevitable. Understanding where risk comes from enables mitigation.
Ribs are the classic sink mark source. A rib intersecting a wall creates a thick section at the junction. If the rib is too thick relative to the wall, visible sink marks appear on the opposite surface.
The guideline: rib thickness should be 40 to 60 percent of nominal wall thickness. For high-gloss surfaces where sink marks are most visible, 40 percent is safer. For textured surfaces that hide minor imperfections, 60 percent may be acceptable.
| Surface Finish | Rib Thickness Rule | Notes |
|---|---|---|
| High gloss (SPI A-1, A-2) | Maximum 40% of wall | Sink marks highly visible |
| Semi-gloss (SPI B-1, B-2) | Maximum 50% of wall | Moderate visibility |
| Textured (SPI C, D) | Maximum 60% of wall | Texture hides minor sinks |
Bosses create similar thick sections. A solid boss for a screw or insert creates a cylinder of thick material that shrinks more than surrounding walls. Coring bosses to reduce wall thickness at the base addresses this, but core pin placement and structural requirements constrain options.
Gussets and support features intended to strengthen ribs often create their own thick sections at intersections. Multiple features converging at a single point can create massive thick sections that no amount of process optimization can handle.
Wall thickness transitions from thick to thin areas concentrate shrinkage. Abrupt transitions are worse than gradual ones. The recommendation to limit thickness transitions to a 3:1 ratio with gradual change applies here.
Process Factors
Process parameters provide the tools to compensate for design-driven sink mark risk, but process capability has limits.
Pack pressure and time are the primary controls. Higher pack pressure forces more material into shrinking areas, partially compensating for volume loss. Longer pack time extends the window for compensation. But excessive pack pressure creates its own problems: part sticking, warpage from stress, and gate blush.
Gate location affects which areas receive effective packing. Gates should be positioned near thick sections that need compensation, not far from them. If thick sections are remote from gates, packing pressure may never reach them before gate freeze.
Gate freeze-off timing determines the end of the packing window. Larger gates freeze later, extending pack time. Hot runner valve gates can control freeze timing precisely. Cold runner gates freeze based on size and cooling conditions.
Mold temperature affects cooling rate and, consequently, the time available for packing. Higher mold temperatures slow skin formation, extending the window for pack pressure to compensate. The tradeoff is longer cycle time.
Melt temperature affects material viscosity and shrinkage behavior. Higher melt temperatures may improve packing but can also increase total shrinkage. The net effect depends on the specific material and geometry.
Prevention Through Design
Preventing sink marks through design is more reliable than compensating through process.
Core out thick sections to eliminate the root cause. If a boss doesn’t need to be solid, core it. If a junction between walls creates a thick section, remove material from the interior. Design for uniform wall thickness eliminates differential shrinkage.
Use proper rib proportions from the start. Designing ribs at 50 percent of wall thickness prevents sink marks rather than requiring process compensation for oversized ribs.
Position features strategically. Locate ribs and bosses where sink marks are least visible or where texture can hide them. The same rib that creates visible sink marks on an exterior surface may be acceptable on a hidden interior surface.
Consider gas-assist molding for thick parts that can’t be redesigned. Gas-assist hollows out thick sections by injecting nitrogen into the molten core, eliminating the thick section that would cause sink marks. This adds process complexity but enables geometries that conventional molding can’t achieve without defects.
Use texture strategically. A textured surface hides minor sink marks that would be obvious on a glossy surface. If the design requires thick sections and the application allows texture, specify texture on the surface opposite the thick sections.
Prevention Through Process
When design creates sink mark risk, process optimization minimizes visible defects.
Optimize pack pressure profile. Start with a gate seal study to determine when gate freeze occurs. Then optimize pack pressure for the window before freeze. Profile the pressure to provide maximum compensation early, then taper to minimize stress as the part solidifies.
Extend hold time to the point of diminishing returns. Hold time beyond gate freeze has no effect, so extending hold time only helps if the gate hasn’t frozen. For hot runner systems with controlled gate freeze, hold time extension can be effective.
Increase mold temperature to slow skin formation and extend the packing window. Monitor actual surface temperatures, not just controller settings. Higher mold temperature increases cycle time but may be justified for cosmetic parts.
Optimize gate size for the application. Larger gates extend the packing window by freezing later. The tradeoff is larger gate vestige and potentially more gate blush. For cosmetic parts, valve gates with controlled timing offer the best combination of packing capability and vestige quality.
Quantifying Acceptable Limits
Not all applications require zero sink marks. Defining acceptable limits before production enables realistic quality criteria.
Measurement methods range from visual inspection to profilometry. Visual inspection under specified lighting conditions provides quick assessment but is subjective. Profilometry quantifies depression depth in microns, providing objective data.
Functional acceptance may tolerate sink marks that cosmetic acceptance rejects. A structural bracket with minor sink marks may function perfectly. A cosmetic housing with the same sink marks may be unacceptable. Define acceptance criteria based on actual application requirements.
Paint and finish interaction matters for parts receiving secondary operations. Some paint systems fill minor sink marks; others highlight them. Testing actual parts through actual finishing processes determines real-world acceptance better than assumptions.
Sample approval at project start establishes the baseline. Approve samples representing actual production capability, not hand-picked best shots from extended process development. This sets realistic expectations for ongoing production quality.
Sink marks are predictable consequences of geometry. Design decisions that create thick sections are design decisions to accept sink marks unless compensating measures are included. Understanding this relationship early in product development prevents surprises during production startup.
Sources
- Rosato, Donald V. “Injection Molding Handbook.” Springer.
- Beaumont, John P. “Runner and Gating Design Handbook.” Hanser, 2004.
- Protolabs. “Design Guidelines for Injection Molding.” https://www.protolabs.com/
- RJG Inc. “Controlling Sink Marks.” https://rjginc.com/
- Plastics Technology. “Sink Mark Prevention.” https://www.ptonline.com/