Design · Sink marks

A sink mark is not a process defect. It is the part telling you where it is too thick.

Every sink mark in injection molding starts the same way. The skin against the cold steel solidifies first. The material behind it is still molten, and as it cools it shrinks — but it is now trapped behind a solid shell, so it pulls that shell inward and leaves a dimple on the outside. Anywhere the section is thicker than its neighbours cools later and shrinks more, which is why sinks appear opposite ribs, bosses and thick junctions rather than at random. Some molders call it a shrink mark, which is the more literal name: it is shrinkage that had nowhere else to go.

The same cause has a second outcome. If the skin is stiff enough to resist being pulled in — a rigid resin, a thick wall, a cold mold — the shrinkage opens a void inside the part instead of a dimple outside it. A part with no visible sink and an internal void has not been fixed; the defect has moved somewhere you cannot see it without sectioning the part.

What the press can do

These are real fixes and they close marginal sinks every day. Each one also has a point where it stops, and knowing that point is the difference between tuning and guessing.

Process

More pack pressure, and why it stops working

Packing forces extra melt into the cavity to make up for the volume shrinkage as it cools. It is the most direct lever there is, and on a marginal sink it often closes it entirely.

Where it stops: It works only while the gate is still open. Once the gate freezes, no pressure at the machine reaches the part — and the gate is usually the thinnest section in the whole system, so it freezes first. A thick section far from the gate can be unreachable before packing has done anything for it.

Process

Longer hold time

Holding pressure until the gate seals makes sure you are using all the packing that is available. A gate-seal study finds that point rather than guessing it.

Where it stops: Past gate seal, extra hold time is pure cycle time. It costs money on every shot and changes nothing about the sink.

Process

Cooler mold, or hotter

A cooler mold builds a stiffer skin sooner, which can resist being pulled in. A hotter mold keeps the path to the thick section open longer so packing can reach it. Which direction helps depends on whether the problem is reach or resistance.

Where it stops: Cooling faster is the move that converts a visible sink into an invisible void, and it will look like a fix on the bench. Colder molds also raise residual stress and can cost you dimensional stability later.

What the geometry can do

Past the limits above, the section is simply too thick and only the part changes that.

Geometry

Thin the rib relative to the wall it springs from

The common rule of thumb is to keep a rib at or below roughly 60% of the thickness of the wall it meets. Above that the junction is a thick spot and telegraphs to the opposite face. Two thin ribs usually carry the same load as one thick one and sink far less.

Where it stops: A rib that is too thin fills poorly and can hesitate, so this trades one problem for another if taken too far.

Geometry

Core out the thick section

A boss or a solid junction does not need to be solid. Coring the middle out leaves a uniform wall wrapped around a void that was designed in rather than formed by accident, and the section cools like the rest of the part.

Where it stops: Coring needs somewhere for the core to come from, which is a tooling decision and sometimes a parting-line one.

Geometry

Gate into the thick section, not away from it

Packing reaches thick sections last and gates freeze first, so a gate placed near the heaviest section keeps the packing path open where it is needed most. This is a decision made once, in CAD, and it constrains everything the press can do afterwards.

Where it stops: The gate leaves a vestige, so this trades against cosmetics and against where a functional surface can be.

Geometry

Move it somewhere nobody looks

Sometimes the honest answer is that the sink stays and the face it lands on is changed — put it on a hidden surface, or texture the face so the eye cannot read the dimple. Texturing genuinely works and is used constantly.

Where it stops: This hides the defect rather than removing it. If the face is a sealing surface or a datum, hiding is not available and the geometry has to change.

If the question is whether the resin leaves you any room at all, the shrinkage calculator shows how much of a tolerance its shrink range takes on its own — and what actually decides a dimension covers the rest of the stack. Otherwise, send the part.

Keep reading

  • Injection molding tolerances

    Why a ±2 µm cavity does not make a ±2 µm part, and how DIN 16901 maps to DIN 16742.

  • Ultem (PEI) injection molding

    Amorphous, stable and low-shrink — and notch-sensitive, which puts the risk in the corners rather than the dimensions.

  • Materials

    PEEK, Ultem, LCP and COC molded; titanium and stainless machined — and what each does at micro scale.

  • Insert molding

    Insert vs overmolding, and the five ways a molded-in insert fails at small scale.

  • The Lab

    Free molding calculators: shrinkage, clamp force, gate seal, cooling time, capability and more.