Pourquoi les marques d'affaissement en moulage par injection apparaissent-elles — et comment les corriger ?

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.

Prolonger le temps de maintien

Le maintien de pression doit continuer jusqu'à la congélation de la vanne. Si le maintien de pression s'arrête avant la congélation de la vanne, le matériau s'écoule en sens inverse hors de la cavité, et la section épaisse se contracte sans contrôle. Surveillez le temps de congélation de la vanne avec des capteurs de pression, et fixez le temps de maintien de pression pour au moins correspondre.

Réduire la température de fusion

Des températures de fusion plus basses signifient un retrait total moindre. Cependant, cela doit être équilibré avec la qualité de remplissage — si la température est trop basse, vous verrez des incomplétions de moulage, une pression d'injection élevée et un fini de surface médiocre. Une réduction de 5 à 10°C par rapport à la température de fusion standard est généralement la plage sûre pour réduire les enfoncements.

Optimiser le refroidissement

Un refroidissement plus rapide et plus uniforme réduit la fenêtre de temps pendant laquelle l'enfoncement se développe. Cela signifie optimiser le placement des canaux de refroidissement près des sections épaisses, utiliser des inserts en cuivre au béryllium pour une extraction de chaleur localisée, et garantir des débits de liquide de refroidissement adéquats. Les moulistes expérimentés ajoutent régulièrement des solutions de refroidissement localisées lors de la mise en service du moule pour traiter les points chauds d'enfoncement.

Ajustement	Impact	Risk
Augmenter la pression de serrage	Haut	Barbes, contraintes résiduelles
Prolonger le temps de maintien	Moyenne-élevée	Temps de cycle plus long
Réduire la température de fusion	Moyen	Injections incomplètes, finition médiocre
Réduire la température du moule	Moyen	Visibilité des lignes de soudure, marques de flux
Vitesse d'injection plus faible	Low-Medium	Cycle plus long, hésitation d'écoulement

Which Materials Are Most Susceptible to Sink Marks?

Le choix du matériau influence significativement la visibilité des marques d'affaissement. Les matériaux amorphes (ABS, PC, PMMA, PS) montrent les marques d'affaissement plus facilement car ils ont un retrait au moule plus élevé dans les zones de transition épaisse-fine et parce que leurs surfaces transparentes ou brillantes rendent même les dépressions mineures visibles.

Les matériaux semi-cristallins (PP, PE, Nylon, POM) sont un peu plus tolérants car ils tendent à se contracter plus uniformément et leurs finitions de surface typiques sont moins révélatrices. Cependant, ils montrent encore des affaissements sur les surfaces polies.

Les matériaux chargés — ceux avec des fibres de verre, minéraux ou charges de talc — présentent significativement moins d'affaissement parce que le les charges réduisent le retrait volumique^[3]. Si votre application le permet, l'utilisation d'une matière chargée est l'un des moyens les plus efficaces de minimiser les enfoncements sans modifier la conception de la pièce.

Type de matériau	Visibilité du retrait	Recommandation de conception
Amorphe (ABS, PC, PMMA)	Haut	Règle stricte des nervures 50–60 %
Semi-cristallin (PP, Nylon, POM)	Moyen	Règle standard des nervures 60%
Qualités chargées de verre	Faible	Acceptable jusqu'à 70%

Lorsque les marques de retrait sont préoccupantes, le passage d'une qualité non chargée à une qualité chargée de 10–20 % de verre élimine souvent complètement le retrait tout en améliorant la stabilité dimensionnelle et la rigidité.

Can You Fix Sink Marks After the Mold Is Built?

Oui, mais les options se réduisent considérablement et les coûts augmentent. Voici la hiérarchie des correctifs, du moins cher au plus cher.

1. Optimisation du procédé: Ajuster la pression de maintien, le temps de maintien, la température de fusion et le refroidissement. Coût : temps machine pour les essais. Efficacité : modérée pour les affaissements mineurs.
2. Assistance gazeuse ou moulage avec agent d'expansion: Pour les sections épaisses, le moulage par injection avec assistance gazeuse introduit de l'azote dans la zone épaisse, préservant la qualité de surface tout en évident l'intérieur. Nécessite une modification du moule pour les cannes à gaz. Coût : modéré. Efficacité : élevée pour les sections épaisses localisées.
3. Modification du moule — enlèvement d'acier: Réduire l'épaisseur des nervures ou évider les sections épaissees en retirant du métal de la cavité. C'est la correction la plus efficace mais nécessite une remachination. Coût : modéré à élevé selon la complexité de la cavité.
4. Modification du moule — ajout de métal: Si vous devez augmenter l'épaisseur de paroi à un endroit pour compenser, cela nécessite une soudure et une rectification. Coût : élevé. Risque : intégrité des soudures dans les moules de production.

L'idée clé dans toutes ces approches : il est toujours moins cher de prévenir les marques d'affaissement lors de la phase de conception que de les corriger après la construction du moule.

How Do You Measure and Evaluate Sink Marks?

Quantifier les marques d'affaissement est important pour établir des critères d'acceptation et suivre les améliorations du processus. Il existe trois méthodes courantes.

Inspection visuelle est la méthode la plus simple. Tenir la pièce à bout de bras sous une lumière inclinée et rechercher les dépressions. Si vous pouvez les voir à 30 cm sous un éclairage standard, elles seront visibles pour vos clients. C'est une méthode de type accepté/rejeté, non quantitative.

Profilométrie de surface utilise un profilomètre à contact ou optique pour mesurer la profondeur et la largeur exactes de la dépression. Les critères d'acceptation typiques sont une profondeur maximale de 0,05 mm pour les surfaces esthétiques et 0,10 mm pour les surfaces non esthétiques.

Mesure d'épaisseur de paroi par ultrasons à l'emplacement de l'affaissement révèle si la paroi s'est amincie au-delà de la tolérance. Ceci est utile pour l'inspection de la qualité^[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.

Check Item	Critères de validation
Toutes nervures ≤60% de l’épaisseur nominale du mur	✓
Bosses cored to uniform wall	✓
Wall transitions use 3:1 taper	✓
Packing pressure optimized	✓
Packing time ≥ gate freeze time	✓
Cosmetic surface texture specified	✓ or N/A
Material selection reviewed for shrink	✓

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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