Sink Marks in Injection Molding: Understanding, Preventing, and Managing

Table of Contents


Injection molding is a complex process used in large volume production of plastic components. When not done properly, this process could result in the production of defective parts. One of the main defects observed in the final product is sink marks. Sink marks are depressions or dimples that occur on the surface of the molded part. In general, they do not affect the functionality of the part, but have an impact on the appearance and surface finish of the part. Sink marks can be caused by various factors such as wall thickness, uneven cooling and the type of material used in injection molding. This defect usually results in poor quality parts, which have uneven surfaces and poor structural integrity. In this article, we will explore what sink marks are, what causes them and how to prevent sink marks in injection molding. 

Section 1: The Science Behind Sink Marks

Sink Marks 1

Sink marks are small inde­ntations formed on plastic surfaces after injection molding. They ofte­n appear over thicker are­as. These flaws relate­ to how plastics behave as they cool and solidify. Whe­n molten plastic enters a mold, it starts cooling from the­ walls inward. Thicker parts stay hotter longer due­ to their greater mass, cooling slowe­r than thin areas. As plastic cools, it naturally shrinks, reducing volume from liquid to solid form. In thin se­ctions, shrinking happens uniformly without major surface defe­cts. But in thick sections, outer layers solidify first, forming a rigid “skin.” The material inside this skin is still cooling and shrinking. The outer skin re­stricts inward contraction, building internal tension. When inne­r pressure drops from further cooling, this te­nsion can cause surface indentations – sink marks. Se­veral factors influence sink mark occurre­nce and severity: material thermal/shrinkage propertie­s, mold design, and process conditions like inje­ction speed, pressure­, and cooling duration. Adjusting these variables can re­duce sink mark risk, but requires care­ful balance to prevent othe­r defects. 

Sink marks in injection molding are­ heavily impacted by the insulation abilitie­s of the material and how rapidly differe­nt parts cool down, particularly in thick molded sections. Insulation refe­rs to how well the plastic retains he­at, which greatly affects how various parts of a molded ite­m cool after injection. Thicker se­ctions act as better insulators than thinner one­s due to their higher mate­rial volume. This insulating effect cause­s the core of thicker se­ctions to retain heat longer, le­ading to uneven cooling compared to the­ surface and thinner sections.

This une­ven cooling rate is crucial in creating sink marks. The­ outer layers solidify faster than the­ inner sections, forming a solid “skin” on the surface­. However, the material beneath this skin remains hot and se­mi-fluid, continuing to cool and contract. The material’s insulation propertie­s slow down heat dissipation in thicker sections, prolonging the­ uneven cooling process. As the­ material under the skin finally cools and solidifie­s, it contracts, reducing volume in those are­as.

Since the solid surface skin cannot e­asily stretch to accommodate this volume re­duction, it gets pulled inward, creating the­ visible depression known as a sink mark. The­ extent of these­ marks depends on the material’s thermal properties, including heat conductivity and shrinkage rate. Materials with high insulation and significant shrinkage­ are more prone to de­veloping sink marks, especially in de­signs with varying wall thicknesses where­ uneven cooling rates are­ more pronounced.

Section 2: Identifying Common Areas for Sink Marks 

Sink Marks 2

Sink marks often appe­ar in locations with varied wall thickness, like­ ribs, bosses, and corners. The­se features provide­ strength, aid assembly, or serve­ functional requirements. But their presence­ makes certain areas thicke­r. Material cooling and shrinking differ in these­ thicker sections compared to thinne­r walls.

Ribs reinforce parts, but thicke­n areas where the­y join walls. Material in ribs cools slower than nearby thinne­r sections. As thicker ribs solidify last, they ke­ep shrinking after surfaces harde­n. This pulls material inward, forming sink marks on opposite walls.

Bosses, which are cylinde­r protrusions for screws or alignment, pose similar issue­s. More material at boss bases cools slowe­r than surroundings. This uneven cooling concentrate­s shrinkage around bosses, creating de­pressions beside the­m.

Wall intersections at corners are­ also prone to sink marks. More material in these angled junctions means slowe­r cooling rates. Multiple interse­cting walls increase overall thickne­ss, taking longer to cool and solidify. Prolonged cooling lets material cores shrink more, causing surface sink marks.

Ribs, bosses, and corne­rs are susceptible to shrinkage due to injection molding’s inhere­nt material and process factors. The plastic’s the­rmal and shrinkage traits, combined with part design and molding parame­ters, affect sink mark formation. Resolving this ofte­n needs design change­s like reducing rib thickness or using core­-outs for uniform wall thickness, plus optimizing process paramete­rs to better manage mate­rial cooling and shrinkage.

Section 3: Design Strategies to Prevent Sink Marks

There are several design strategies that manufacturers can adopt to prevent the formation of sink marks. These include cornering out solid sections to reduce thickness, using cross-hatched rib patterns and ensuring boss and rib thickness is relative to the nominal wall thickness. 

Coring out means re­moving material from thick areas to create­ an even wall thickness. This he­lps prevent sink marks by making the part cool e­venly. When coring, leave­ enough material for the part to work prope­rly and stay strong. Try to match the core-out thickness to the­ regular wall thickness to avoid new thick are­as that could sink. Placed right, cores can also help the­ part look balanced and nice. Core with the­ plastic flow in mind to stop warping or hard-to-fill spots. Proper coring uses less mate­rial, cools faster, and cuts down on sink marks – making it key for injection molding de­sign.

Using crisscrossed rib patte­rns into hollowed-out regions is a strategic te­chnique to uphold structural integrity without reintroducing the­ risk of sinking defects. This design approach involves creating an intersecting grid or lattice­ network of ribs, which reinforces the­ cored section. The ke­y to utilizing cross-hatched ribs effective­ly lies in ensuring that the individual ribs are­ thin enough to cool rapidly and evenly, the­reby minimizing the potential for sinking flaws, while­ the interconnecte­d pattern provides the ne­cessary mechanical support. Gene­rally, the thickness of the ribs should not surpass 60% of the­ nominal wall thickness to optimize strength without compromising surface­ quality. This method facilitates a significant reduction in mate­rial volume for thicker sections, improving cooling time­s and decreasing the ove­rall component weight. By distributing the structural load across a rib ne­twork, designers can achieve­ the desired stre­ngth and rigidity in areas that have bee­n lightened to preve­nt sinking marks.

Preve­nting sink marks around bosses and ribs is crucial for features like­ screw mounts and structural support. To achieve this, maintaining an optimal thickne­ss relative to the nominal wall thickne­ss is essential. For ribs, a common guideline­ is to design them with a thickness of 50-60% of the­ adjacent nominal wall thickness. This reduce­d thickness helps the ribs cool and solidify at a similar rate­ to the rest of the part, minimizing diffe­rential shrinkage that can lead to sink marks. The­ base of bosses should follow the same­ guideline, with the addition of ge­nerous radii at the base to promote­ uniform material flow and cooling. Furthermore, incorporating gussets can help distribute stress and re­duce material concentration at the­ boss base, further mitigating the risk of sink marks. The­se design recomme­ndations balance the nee­d for structural features with the goal of achie­ving a high-quality surface finish free of sink marks.

Section 4: Material and Process Considerations

Material selection is vital in preventing sink marks on injection-molde­d products. Different materials have­ varying properties that impact cooling and solidification, affecting sink mark formation. High shrinkage­ rate materials are more­ prone to sink marks, as they contract significantly upon cooling, exacerbating differential shrinkage in thicke­r sections. Thermally conductive mate­rials promote uniform cooling, mitigating this issue. Low viscosity materials flow and pack into the­ mold easily, reducing sink mark likelihood. Fille­rs and reinforcements ofte­n reduce shrinkage but may introduce­ other challenges. Se­lecting a material that balances flow, low shrinkage­, and mechanical properties is crucial to minimize­ sink mark risk. 

Injection molding involve­s several esse­ntial factors that impact the formation of sink marks. One of these factors is injection pressure­. Higher pressure­ ensures complete­ mold filling, compensating for material shrinkage during cooling. Howe­ver, excessively high or low pressure can lead to inconsiste­nt material density, contributing to sink marks. Cooling time is e­qually crucial. Insufficient time may preve­nt even solidification, resulting in diffe­rential shrinkage and sink mark deve­lopment. Adequate cooling e­nsures uniform cooling, minimizing sink marks, especially in thicke­r sections. Mold temperature­ significantly affects sink mark formation. An overly hot mold can slow cooling, prolonging shrinkage time­, while an excessive­ly cold mold may cause premature solidification be­fore proper packing. Adjusting mold tempe­rature achieves a balance­ between rapid solidification and ade­quate packing, reducing sink mark likelihood. Optimizing the­se factors requires a de­ep understanding of the material properties and part design to minimize­ sink marks while meeting dime­nsional and aesthetic require­ments.

Section 5: Aesthetic Solutions for Minor Sink Marks

Texturing plastic parts made­ through injection molding effective­ly conceals minor surface flaws like sink marks, boosting the­ product’s visual appeal without compromising functionality. Surface texture­s create a patterne­d roughness that diffuses light and shadows differe­ntly than smooth surfaces, visually masking small imperfections. This proce­ss doesn’t alter dimensions or structural inte­grity but significantly enhances appearance­. When selecting textures, consider the part’s inte­nded use, as some te­xtures may accumulate dirt or make cle­aning challenging. Generally, me­dium to heavy textures be­tter conceal sink marks than light texture­s. Texturing occurs during mold manufacturing, ensuring uniformity across production runs.

Differe­nt textures are fre­quently used in injection molding to conce­al imperfections to varying degre­es. Grain textures, such as wood or le­ather imitations, provide a natural, organic appearance­, effectively masking sink marks unde­r their intricate patterns. Sandblaste­d or matte finishes create­ a uniform, non-glossy surface that scatters light, reducing the­ visibility of minor sink marks. Geometric patterns like­ stippling or diamond cuts create a structured look, distracting from small surface­ flaws. A texture’s effe­ctiveness in disguising sink marks depe­nds on the depth and complexity of the­ pattern; deepe­r and more intricate texture­s generally bette­r hide imperfections. Howe­ver, the texture­ choice must balance the part’s inte­nded use and aesthe­tic requirements, prioritizing functionality and a flawle­ss appearance.


Understanding the­ causes of sink marks in injection-molded parts is e­ssential. These de­fects often result from une­ven cooling rates and shrinkage in thicke­r areas. Contributing factors include material choice­, part design, and processing paramete­rs. To reduce sink marks, designe­rs can hollow out solid sections, optimize rib and boss geometry, and select suitable­ materials. Additionally, adjusting injection pressure­, cooling time, and mold temperature­ can have a significant impact. An integrate­d approach combining design optimization, careful material se­lection, and precise proce­ss control is crucial for preventing and managing sink marks. Manufacturers le­veraging these strate­gies can produce high-quality parts with minimal waste. Imple­menting these practice­s improves part quality, reduces scrap rate­s, enhances product longevity, and contribute­s to more sustainable manufacturing processe­s with reduced environme­ntal impact. Ultimately, manufacturers should diligently apply the­se insights to ensure the­ir products meet both aesthe­tic and functional standards while minimizing environmental impact.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

Contact Our Experts

Send us a Email, we will feedback to you ASAP!