Sink marks are one of the most common cosmetic defects in plastic injection molding. They often appear as shallow depressions on an otherwise smooth surface, particularly opposite ribs, bosses, thick walls or other areas where the plastic mass is greater than the surrounding section.
For OEM buyers and product engineers, sink marks can become a recurring problem because the visible defect may originate from geometry hidden on the opposite side of the part. Changing machine settings can sometimes improve the surface, but a design-related sink mark often returns unless the underlying cause is addressed.
This guide explains how sink marks form, which product features create the highest risk, how material and molding parameters influence the result, and what can be done before tooling to reduce the problem.
What Are Sink Marks in Injection Molded Plastic Parts?
Sink marks are shallow depressions or dimples that appear on the surface of an injection molded plastic part. They are most commonly found in areas opposite ribs, bosses, thick walls, corners or intersections where a larger mass of plastic cools more slowly than the surrounding material.
A sink mark is primarily a shrinkage-related surface defect. During molding, the outer skin of the plastic part contacts the mold surface and solidifies first. The material inside the thicker section continues to cool and contract. If enough molten material cannot be packed into that area while it is shrinking, the internal contraction pulls the already-solidified surface inward. The result is a visible depression.
On a matte industrial component, a small sink mark may be acceptable if it does not affect function. On a glossy housing, automotive trim, appliance cover or customer-facing enclosure, the same defect may be unacceptable because reflected light makes the depression very visible. For this reason, sink-mark requirements should be discussed together with both dimensional and cosmetic requirements.
Why Do Sink Marks Form During Injection Molding?
The basic cause of a sink mark is local volumetric shrinkage. However, several design and processing conditions determine whether that shrinkage becomes visible on the part surface.
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Alt: Thick plastic sections cool and shrink more slowly, increasing sink mark risk
Thicker sections contain more molten plastic and therefore take longer to cool. As the core of the thick area contracts, the surface needs additional material from the gate to compensate for that shrinkage. If the gate freezes too early, packing pressure is too low, the hold time is insufficient, or the flow path makes the area difficult to pack, the surface can be pulled inward.
This is why sink marks often appear in predictable locations. A buyer may see a small depression on the outside of a housing and assume the mold surface is damaged. In reality, the root cause may be a thick boss or rib located on the inside of the component directly behind the visible mark.
The most reliable way to solve sink marks is therefore to identify the plastic mass that is creating the local shrinkage, not simply to polish the visible surface.
How Does Wall Thickness Affect Sink Marks?
Wall thickness is one of the most important design factors in injection molding. Uniform walls cool more consistently and generally produce more predictable shrinkage. Sudden transitions from thin sections to thick sections create different cooling rates inside the same part.
A common example is a housing with a generally thin wall and one locally reinforced block that is several times thicker than the surrounding area. The thick block remains hot longer, shrinks more as it cools and can pull the opposite cosmetic surface inward.
Increasing wall thickness is not always the best way to improve strength. In many components, stiffness can be improved more efficiently by using ribs, geometry or material selection while keeping the main wall reasonably uniform.
When a thick section is functionally necessary, it can sometimes be cored out to remove unnecessary plastic mass. This reduces cooling time, material consumption and sink-mark risk at the same time.
Why Do Ribs and Bosses Often Cause Sink Marks?
Ribs and bosses are extremely useful injection molding features. Ribs increase stiffness without requiring a completely thick wall, while bosses provide locations for screws, threaded inserts, alignment pins or assembly features.
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Alt: Rib and boss design comparison for reducing sink marks in injection molded plastic parts
The problem occurs when these features create too much local material at the point where they connect to the main wall. A thick rib can behave like a thick wall section. A solid boss can create an especially large plastic mass where the boss meets the base surface.
When a visible sink mark appears directly opposite a rib or boss, the correct response is usually to review the feature geometry. A thinner rib, cored boss, smoother transition or revised support structure may reduce the material concentration while maintaining the required mechanical function.
The exact rib and boss dimensions should be evaluated according to the base wall, material, load and molding conditions rather than applying one fixed ratio to every product. This is where DFM review is valuable: the molding supplier can identify high-risk areas before mold steel is cut.
Do Plastic Materials Affect Sink-Mark Risk?
Yes. Different plastics have different shrinkage behavior, crystallinity, flow characteristics and sensitivity to processing conditions.
Semi-crystalline materials can show greater molding shrinkage than many amorphous plastics. Materials such as PP, PE, POM and some nylon grades therefore require careful consideration of local thickness and packing. ABS and PC can also develop sink marks when the geometry contains excessive material mass, even though their shrinkage behavior is different.
Reinforcement and fillers can change shrinkage as well. Glass-filled materials often behave differently from unfilled grades, and fiber orientation may introduce directional shrinkage or warpage. A resin change after tooling can therefore affect not only strength and appearance but also dimensions and surface quality.
For a critical cosmetic component, the exact production resin grade should be confirmed before final mold tuning whenever possible.
How Do Packing Pressure and Hold Time Affect Sink Marks?
After the mold cavity fills, the molding machine normally maintains pressure for a period of time. This packing or holding stage supplies additional material to compensate for shrinkage while the plastic is cooling.
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Alt: Packing pressure, gate location, cooling and mold temperature factors affecting sink marks
If packing pressure is insufficient, or if hold time ends before the gate freezes, a thick section may not receive enough compensation material. Increasing packing pressure or hold time can sometimes reduce a mild sink mark.
However, more pressure is not automatically better. Excessive packing may create flash, internal stress, difficult ejection or dimensional problems. The correct process window depends on the part geometry, resin, gate design and mold.
This is why processing should be optimized systematically rather than by simply increasing one parameter until the surface looks better.
Why Does Gate Location Matter?
The gate is the path through which molten plastic enters the mold cavity. Its size and location affect filling, pressure transmission and how long material can continue to pack different regions of the part.
If a thick area is located far from the gate, pressure loss along the flow path may make it harder to pack that area effectively. If the gate freezes before the thick section has finished shrinking, additional material can no longer enter the cavity.
For components with important cosmetic surfaces or large differences in section thickness, gate location should therefore be considered during DFM and mold design. The objective is not merely to hide the gate mark. The gate also needs to support stable filling and packing.
A supplier may use flow analysis, previous tooling experience and mold trials to determine whether the proposed gate location is appropriate.
Can Cooling Reduce Sink Marks?
Cooling affects both cycle time and dimensional stability. Thick areas require more time to cool than thin walls, so removing a part too early can make surface depressions more noticeable after ejection.
Increasing cooling time may improve some parts, but it does not eliminate the underlying geometry problem when a section contains too much plastic. A very thick boss will still cool more slowly than the surrounding wall even if the entire cycle is made longer.
The mold cooling system also matters. Well-designed cooling channels help keep mold temperature more uniform. Poor cooling around a local feature can exaggerate differences in shrinkage and may contribute to sink marks or warpage.
For repeat production, the goal should be a stable cooling strategy rather than an unnecessarily long cycle. Long cycles increase unit cost, so design improvements that allow the part to cool more uniformly can improve both quality and manufacturing efficiency.
Can Sink Marks Be Solved by Processing Alone?
Sometimes, but not always.
If the sink mark is mild and the part design is generally suitable, adjustments to packing pressure, hold time, melt temperature, mold temperature or cooling time may improve the surface.
When the root cause is an excessively thick wall, rib intersection or solid boss, process adjustments usually have limited ability to eliminate the defect completely. The molding machine cannot fully compensate for poor geometry after the gate has frozen.
This distinction is important for buyers. If a supplier repeatedly changes process settings but the defect returns during long production runs, the design or mold may need to be modified. A robust solution should remain stable across normal production conditions rather than depending on a very narrow process window.
How Can You Prevent Sink Marks Before Tooling?
The most cost-effective time to prevent sink marks is before mold manufacturing begins.
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Alt: How to prevent sink marks with uniform walls, cored thick areas, optimized ribs and T1 validation
Start by reviewing wall thickness. Remove unnecessary thick sections and avoid abrupt transitions where possible. Core out heavy areas that do not need to be solid.
Next, review ribs and bosses. These features should provide the required strength or assembly function without creating excessive material concentration on the opposite side of a visible wall.
Identify cosmetic surfaces clearly. If a housing has one customer-facing side and one hidden internal side, tell the mold manufacturer. This information can influence gate location, rib placement and acceptable appearance standards.
Material should also be confirmed early. Different resin grades have different shrinkage behavior, and changing resin after the mold is tuned can change the result.
Finally, request a DFM review before tooling. A good DFM review should identify thick areas, high-risk ribs and bosses, gate locations, parting lines, ejector positions and other factors that may affect the final surface.
How Should Sink Marks Be Checked During T1 and Sample Approval?
The first mold trial provides the first opportunity to evaluate the real interaction between the part design, resin, mold and process.
Samples should be inspected under consistent lighting, especially when the surface is glossy or textured. Reflections can make shallow sink marks easier to detect than ordinary overhead factory lighting.
The buyer and supplier should agree which surfaces are cosmetic and which are non-cosmetic. A small depression on a hidden internal surface may have no practical effect, while the same mark on a visible cover may be unacceptable.
For subjective appearance requirements, an approved physical sample is often useful. Once the customer approves the acceptable surface quality, that sample can become part of the production reference for later orders.
If a sink mark is unacceptable, the engineering team should determine whether the corrective action is process tuning, mold modification or product-design modification before mass production begins.
Do Sink Marks Affect Part Strength?
A visible sink mark is primarily a surface and dimensional symptom, and a small cosmetic sink does not automatically mean that a component will fail mechanically.
However, the feature causing the sink may still matter structurally. For example, an excessively thick boss may contain residual stress, cool slowly or contribute to other dimensional problems. Conversely, reducing a rib or boss too aggressively simply to improve appearance could weaken the component.
The solution should therefore balance appearance, strength, manufacturability and cost. Engineering plastic parts should be evaluated as complete functional components rather than optimized for appearance alone.
What Should Buyers Include in an RFQ to Control Cosmetic Defects?
For a custom injection molding project, the drawing and RFQ should communicate more than material and quantity.
Provide the 3D model and 2D drawing. Identify visible cosmetic surfaces. State the required material grade, color and surface finish. If certain areas must be free of sink marks, flow marks or gate marks, identify those zones clearly rather than relying on a general statement such as "good appearance."
If you have an existing physical sample that represents the desired appearance, tell the supplier. A reference sample can help establish a realistic cosmetic standard.
Also provide the initial order quantity and expected annual demand. This helps the manufacturer design appropriate tooling and cooling for repeat production rather than optimizing only for a few samples.
How Should You Work With an Injection Molding Supplier on Sink Marks?
A capable injection molding supplier should be able to discuss the relationship between the visible defect and the underlying geometry.
If a supplier sees a sink mark opposite a large rib, thick wall or boss, the discussion should include the product design rather than only machine settings. The supplier should also be able to explain whether the proposed correction affects tooling, dimensions, strength or appearance.
For new projects, early communication is the easiest way to reduce risk. Sharing the application, material, cosmetic requirements and critical dimensions during quotation allows the supplier to evaluate the project before mold manufacturing.
For existing molds, clear photos, marked defect locations and sample measurements help the supplier identify whether the issue is localized or process-related.
How Can SWKS Help With Custom Injection Molded Parts?
Sink marks are easier to prevent than to correct after tooling is complete. Uniform wall thickness, efficient rib and boss design, suitable material selection, appropriate gate design and stable molding conditions all contribute to better surface quality.
SWKS supports custom plastic injection molding projects from drawing review and DFM through mold development, T1 sample evaluation and mass production.
If you are developing a plastic housing, bracket, gear, guide, equipment component or other custom molded part, send us your 3D drawing, 2D drawing, material requirement, quantity, surface requirements and application information.
The more complete the project information, the easier it is to identify high-risk features before tooling and prepare a manufacturing solution based on the actual requirements of your part.
Quick FAQ for Website Publishing
Are sink marks only a cosmetic problem?
Not always. Small sink marks are often mainly cosmetic, but the thick feature causing the sink can also influence cooling, stress and dimensional stability.
Can polishing remove sink marks?
Polishing the mold surface does not remove a sink mark caused by internal shrinkage. The geometry, packing or cooling condition must be addressed.
Why do sink marks appear opposite ribs?
The rib creates additional plastic mass at the wall intersection. As that thicker area cools and shrinks, it can pull the opposite surface inward.
Can longer cooling time eliminate sink marks?
It may improve some parts, but cooling alone usually cannot fully solve a severe sink mark caused by excessive local thickness.
Should cosmetic requirements be shown on the drawing?
Yes. Identifying cosmetic zones helps the mold manufacturer evaluate ribs, bosses, gate location and acceptable surface standards before tooling.
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