Por Que Surgem Marcas de Afundamento na Moldagem por Injeção — e Como Corrigir?

Sink marks — those annoying depressions on the surface of an otherwise perfect molded part — are one of the most common and visible defects in injection molding. They appear directly opposite thick sections, ribs, and bosses, and they’re the first thing any customer notices on a cosmetic surface.

In this guide, I’ll explain what causes injection molding sink marks, how to prevent them through design and processing, and what to do when they’ve already appeared on your parts. This guide draws on decades of production experience across thousands of mold sets.

What Are Sink Marks in Injection Molding?

Sink marks are localized depressions or dimples on the surface of a molded part, typically appearing opposite features like ribs, bosses, thick wall sections, or changes in geometry. They occur when the inner material shrinks more than the outer skin during cooling, pulling the surface inward.

The mechanism is straightforward: when a thick section of plastic cools, the outer skin solidifies first while the interior remains molten. As the interior material cools and contracts, it pulls the already-solidified skin inward, creating a visible injection molding defect^[1]. The deeper the thickness differential between the feature and the surrounding wall, the more pronounced the sink mark.

What Causes Sink Marks During Molding?

Sink marks have both design and processing causes. Understanding which is responsible in your specific case is the first step toward eliminating them.

Design Causes

The most common design cause is excessive material thickness at ribs, bosses, or wall transitions. When a rib is too thick relative to the nominal wall, it creates a thermal mass that cools much slower than the surrounding area. The rib material shrinks as it cools, pulling the opposite surface inward.

Other design causes include abrupt wall thickness transitions (without gradual tapers), oversized bosses that aren’t cored, and thick gusset plates at mounting points. Any feature that adds significant local thickness relative to the nominal wall is a sink mark candidate.

Processing Causes

Insufficient packing pressure is the primary processing cause of sink marks. During the packing phase, additional material is forced into the cavity to compensate for shrinkage. If the packing pressure^[2] is too low or the packing time is too short, the material in thick sections continues to shrink without compensation, producing sink marks.

Other processing factors include excessive melt temperature (which increases total shrinkage), insufficient cooling time (ejecting before thick sections have fully solidified), and slow injection speed (which can cause premature freeze-off at gates, reducing the effective packing distance).

How Do You Prevent Sink Marks Through Design?

Design prevention is always more effective than processing fixes because it addresses the root cause rather than the symptom. Here are the key design strategies, ordered by impact.

Control Rib Thickness

This is the single most effective design measure. Ribs should be 50–60% of the nominal wall thickness at their base. At 50%, sink marks are barely visible on most surfaces. Above 70%, they become noticeable. At 100% (where the rib is as thick as the wall), deep sink marks are virtually guaranteed on the opposite surface.

Designers often make ribs too thick because they’re thinking about structural stiffness, not manufacturability. The solution: use multiple thin ribs instead of one thick rib. You get equivalent or better stiffness with a fraction of the sink risk.

Core Out Thick Sections

Wherever possible, hollow out thick sections to maintain uniform wall thickness. This applies especially to bosses (core the center with a pin), thick mounting pads (add a recess on the non-cosmetic side), and structural gussets (use a triangular cross-section with a cored center).

Use Gradual Transitions

When wall thickness must change, use a 3:1 taper ratio. For every 1 mm of thickness change, provide at least 3 mm of gradual transition. This distributes the shrinkage differential over a larger area, making any surface depression shallower and less visible.

Add Surface Texture Strategically

Texture on the cosmetic surface can effectively hide minor sink marks. A medium-to-deep texture (VDI 21–27) breaks up the visual pattern enough that a 0.05–0.10 mm depression becomes imperceptible. This isn’t a fix — it’s a camouflage technique that works when design constraints prevent complete elimination.

How Do Processing Adjustments Reduce Sink Marks?

When design changes aren’t possible — perhaps the mold is already built or the feature thickness can’t be reduced without compromising function — processing adjustments become the primary tool for sink mark reduction.

Increase Packing Pressure

This is the most direct processing fix. Increasing packing (holding) pressure forces more material into the thick sections during the compensation phase, reducing the volumetric shrinkage that causes sink. The limitation: excessive packing pressure can cause flash, increased residual stress, and dimensional growth beyond tolerance.

Extend Packing Time

Packing must continue until the gate has frozen. If packing stops before gate freeze, material flows backward out of the cavity, and the thick section shrinks unchecked. Monitor gate freeze time with pressure sensors, and set packing time to at least match it.

Reduce Melt Temperature

Lower melt temperatures mean less total shrinkage. However, this must be balanced against fill quality — if the temperature is too low, you’ll see short shots, high injection pressure, and poor surface finish. A reduction of 5–10°C from the standard melt temperature is usually the safe range for sink reduction.

Optimize Cooling

Faster, more uniform cooling reduces the time window during which sink develops. This means optimizing cooling channel placement near thick sections, using beryllium copper inserts for localized heat extraction, and ensuring adequate coolant flow rates. Experienced molders regularly add localized cooling solutions during mold commissioning to address sink hotspots.

Which Materials Are Most Susceptible to Sink Marks?

Material choice significantly affects sink mark visibility. Amorphous materials (ABS, PC, PMMA, PS) show sink marks more readily because they have higher mold shrinkage in the thick-to-thin transition zones and because their transparent or glossy surfaces make even minor depressions visible.

Semi-crystalline materials (PP, PE, Nylon, POM) are somewhat more forgiving because they tend to shrink more uniformly and their typical surface finishes are less revealing. However, they still show sink on polished surfaces.

Filled materials — those with glass fiber, mineral, or talc fillers — exhibit significantly less sink because the fillers reduce volumetric shrinkage^[3]. If your application allows a filled grade, it’s one of the most effective ways to minimize sink without changing the part design.

When sink marks are a concern, switching from an unfilled to a 10–20% glass-filled grade often eliminates sink completely while also improving dimensional stability and stiffness.

Can You Fix Sink Marks After the Mold Is Built?

Yes, but the options narrow significantly and costs increase. Here’s the hierarchy of fixes, from cheapest to most expensive.

1. Process optimization: Adjust packing pressure, packing time, melt temperature, and cooling. Cost: machine time for trials. Effectiveness: moderate for minor sink.
2. Gas assist or foam molding: For thick sections, gas-assist injection molding introduces nitrogen into the thick feature, maintaining surface quality while hollowing the interior. Requires mold modification for gas pins. Cost: moderate. Effectiveness: high for localized thick sections.
3. Mold modification — steel removal: Reduce rib thickness or core out thick sections by removing steel from the cavity. This is the most effective fix but requires re-machining. Cost: moderate to high depending on cavity complexity.
4. Mold modification — steel addition: If you need to increase wall thickness somewhere to compensate, this requires welding and re-cutting. Cost: high. Risk: weld integrity in production molds.

The key insight across all these approaches: it’s always cheaper to prevent sink marks in the design phase than to fix them after the mold is built.

How Do You Measure and Evaluate Sink Marks?

Quantifying sink marks is important for establishing acceptance criteria and tracking process improvements. There are three common methods.

Visual inspection is the simplest method. Hold the part at arm’s length under angled light and look for depressions. If you can see them at 30 cm under standard lighting, they’ll be visible to your customers. This is a pass/fail method, not quantitative.

Surface profilometry uses a contact or optical profilometer to measure the exact depth and width of the depression. Typical acceptance criteria are 0.05 mm maximum depth for cosmetic surfaces and 0.10 mm for non-cosmetic surfaces.

Ultrasonic wall thickness measurement at the sink location reveals whether the wall has thinned beyond tolerance. This is useful for inspeção da qualidade^[4] between sink (surface depression with maintained wall thickness) and void (actual material absence inside the wall).

What Should You Check to Prevent Sink Marks?

Use this checklist before and during mold commissioning to keep sink marks under control.

A thorough DFM review that checks every item above is the most cost-effective way to eliminate sink marks before they appear — because the cheapest sink mark fix is the one you never need.

Frequently Asked Questions About Sink Marks?

Are Sink Marks Always Visible?

Not always. Sink marks under 0.02 mm are generally imperceptible, even on glossy surfaces. On textured surfaces (VDI 21+), sink marks up to 0.05 mm can be hidden. The key factor is the combination of depth and surface finish — polished, glossy surfaces show even 0.02 mm depressions clearly.

Can Sink Marks Be Removed After Molding?

Not economically. Unlike flash or burrs, sink marks are volumetric — the material has shrunk below the intended surface level. Painting can partially camouflage minor sink but won’t eliminate it. The only reliable fix is addressing the root cause through design or processing changes.

What Is the Difference Between Sink Marks and Voids?

Sink marks are surface depressions; voids are internal cavities. They share the same root cause (differential cooling shrinkage in thick sections) but manifest differently. In thin-wall parts, you tend to see sink marks because the thin outer skin gets pulled inward. In very thick parts, the shrinkage creates internal voids instead because the outer skin is rigid enough to resist the pull.

Does Gate Location Affect Sink Marks?

Yes. Gate location determines the flow path and packing efficiency. A gate far from a thick section means packing pressure drops before reaching it, increasing sink risk. Ideally, gates should be positioned near the thickest cross-section to maximize packing compensation exactly where it’s needed most.

How Does Wall Thickness Affect Sink Mark Depth?

Thicker nominal walls produce shallower sink marks for the same rib thickness ratio because there’s more material to absorb the shrinkage differential. Conversely, thin-wall parts (under 1.5 mm) are extremely sensitive to even small rib thickness increases — a 1.0 mm rib on a 1.5 mm wall will sink noticeably.

Is Gas-Assist Injection Molding Effective for Sink Prevention?

Very effective, particularly for thick sections that can’t be redesigned. Gas assist hollows out the interior of thick features while maintaining surface quality. It adds cost and complexity to both the mold and the process, but it’s the most reliable way to eliminate sink in structural ribs and large bosses without reducing their dimensions.

Bottom line: Keep ribs at 50–60% of nominal wall, maintain uniform thickness, and address sink in the design phase — not after the mold is built. Design prevention is always cheaper than processing fixes or mold modifications.

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