사출 성형 싱크 마크가 발생하는 이유와 해결 방법

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.

포장 시간 연장

게이트가 동결될 때까지 패킹이 계속되어야 합니다. 게이트 동결 전에 패킹이 멈추면 재료가 캐비티에서 역류하고 두꺼운 단면이 제어되지 않은 채 수축합니다. 압력 센서로 게이트 동결 시간을 모니터링하고 패킹 시간을 최소한 그에 맞게 설정하세요.

용융 온도 낮추기

낮은 용융 온도는 전체 수축이 적음을 의미합니다. 그러나 이는 충전 품질과 균형을 맞춰야 합니다 — 온도가 너무 낮으면 단사, 높은 사출 압력 및 불량한 표면 마무리가 나타납니다. 표준 용융 온도에서 5–10°C 낮추는 것이 일반적으로 싱크 감소를 위한 안전 범위입니다.

냉각 최적화

더 빠르고 균일한 냉각은 싱크가 발생하는 시간 창을 줄입니다. 이는 두꺼운 단면 근처의 냉각 채널 배치 최적화, 국부적 열 추출을 위한 베릴륨 구리 인서트 사용, 적절한 냉각수 유량 보장을 의미합니다. 숙련된 금형사들은 금형 시운전 중 싱크 핫스팟을 해결하기 위해 정기적으로 국부 냉각 솔루션을 추가합니다.

조정	Impact	Risk
패킹 압력 증가	높음	플래시, 잔류 응력
패킹 시간 연장	중간-높음	사이클 타임 증가
용융 온도 낮추기	Medium	단사, 불량 표면
금형 온도 낮추기	Medium	용접선 가시성, 흐름 흔적
사출 속도 감속	Low-Medium	더 긴 주기, 흐름 주저

Which Materials Are Most Susceptible to Sink Marks?

재료 선택은 싱크 마크 가시성에 큰 영향을 미칩니다. 비정질 재료 (ABS, PC, PMMA, PS)는 두꺼운 부분에서 얇은 부분으로 전환되는 구역에서 금형 수축률이 더 높고, 투명하거나 광택 있는 표면 때문에 사소한 함몰도 보이기 때문에 싱크 마크가 더 쉽게 나타납니다.

반결정성 재료(PP, PE, 나일론, POM)는 수축이 더 균일한 경향이 있고 일반적인 표면 마무리가 덜 두드러지기 때문에 다소 관대한 편입니다. 그러나 여전히 연마된 표면에서는 싱크가 나타납니다.

유리 섬유, 광물 또는 활석 필러가 포함된 충전 재료는 싱크가 현저히 적게 나타납니다. 왜냐하면 필러는 체적 수축을 감소시킵니다^[3]. 애플리케이션이 충전 등급을 허용한다면, 부품 설계를 변경하지 않고 싱크를 최소화하는 가장 효과적인 방법 중 하나입니다.

재료 유형	싱크 가시성	설계 권장사항
비정질(ABS, PC, PMMA)	높음	엄격한 50–60% 리브 규칙
반결정성 (PP, 나일론, POM)	Medium	표준 60% 리브 규칙
유리 충전 등급	낮음	최대 70% 허용

싱크 마크가 문제가 될 경우, 무충전 등급에서 10–20% 유리 섬유 충전 등급으로 전환하면 싱크를 완전히 제거할 수 있을 뿐만 아니라 치수 안정성과 강성도 향상됩니다.

Can You Fix Sink Marks After the Mold Is Built?

예, 하지만 옵션이 크게 좁아지고 비용이 증가합니다. 다음은 가장 저렴한 것부터 가장 비싼 것까지 수정 방법의 계층 구조입니다.

1. 공정 최적화: 패킹 압력, 패킹 시간, 용융 온도 및 냉각을 조정합니다. 비용: 시험을 위한 기계 시간. 효과: 경미한 싱크에 대해 중간.
2. 가스 보조 또는 폼 성형: 두꺼운 단면의 경우, 가스 보조 사출 성형은 두꺼운 부위에 질소를 주입하여 내부를 비우면서 표면 품질을 유지합니다. 가스 핀을 위한 금형 수정이 필요합니다. 비용: 보통. 효과: 국부적 두꺼운 단면에 대해 높음.
3. 금형 수정 — 강철 제거: 리브 두께 감소 또는 캐비티에서 강철을 제거하여 두꺼운 단면을 코어 아웃합니다. 이는 가장 효과적인 수정 방법이지만 재가공이 필요합니다. 비용: 캐비티 복잡도에 따라 중간에서 높음.
4. 금형 수정 — 강철 추가: 두께 보상을 위해 특정 부분의 벽 두께를 늘려야 할 경우, 용접 및 재절단이 필요합니다. 비용: 높음. 위험: 생산 금형에서 용접 부위의 완전성.

이 모든 접근법을 통한 핵심 통찰: 싱크 마크를 설계 단계에서 예방하는 것이 금형 제작 후 수정하는 것보다 항상 더 저렴합니다.

How Do You Measure and Evaluate Sink Marks?

싱크 마크를 정량화하는 것은 허용 기준을 설정하고 공정 개선을 추적하는 데 중요합니다. 세 가지 일반적인 방법이 있습니다.

외관 검사 가장 간단한 방법입니다. 부품을 팔 길이로 비스듬한 빛 아래에서 들고 함몰부를 찾습니다. 표준 조명 아래 30cm 거리에서 보인다면 고객에게도 보일 것입니다. 이는 정량적이 아닌 합격/불합격 방식입니다.

표면 프로필 측정 접촉식 또는 광학식 프로파일로미터를 사용하여 함몰부의 정확한 깊이와 너비를 측정합니다. 일반적인 허용 기준은 외관 표면의 경우 최대 0.05mm 깊이, 비외관 표면의 경우 0.10mm입니다.

초음파 두께 측정 싱크 위치에서 벽 두께가 허용 범위를 초과하여 얇아졌는지 확인합니다. 이는 품질 검사^[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	통과 기준
모든 리브 ≤ 기준 벽의 60%	✓
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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