How Sink Marks Form After the Skin Freezes

The skin does not fail. The core does.

Plastic shrinks as it cools. A rib, boss, or heavy wall holds a pocket of molten material long after the surrounding skin has frozen solid. As that pocket cools, it contracts. Nothing replaces the lost volume, so the skin above it dips inward. That dip is the sink mark.

This is why sinks almost always sit opposite ribs, bosses, internal fillets, or other heavy sections. The defect confuses people because the surface can look fine at ejection and the dip shows up later. It looks like a molding problem. Often it is a geometry problem wearing a process disguise. Diagnose in the right order and you stop chasing the wrong cause.

Step 1: Map the Wall-Thickness Ratio

Start with geometry, not the press.

Find the exact feature under each sink. Measure the nominal wall, then the thick section behind the mark, and compare the two. The standard diagnostic sequence runs wall-thickness ratio first, then gate freeze, packing, cooling, and material. A rib nearly as thick as the wall it stands on will sink on any press at any setting.

Record the ratio before touching any machine setting. These limits are part of basic injection molding part design. Check them before the mold is cut. If the sink sits over a 1:1 rib-to-wall section, you already know the ending. The steps below still matter: they tell you whether the process is masking the geometry or magnifying it.

Step 2: Check Whether the Gate Froze Too Early

Packing cannot cross solid plastic.

This is the most skipped step in sink diagnosis. The gate must stay open long enough for packing to feed the thick zone. Gate type and runner system decide how long that is. A small gate freezes much faster than an edge gate. Once the gate seals, the cavity is on its own. Raising holding pressure after that point changes nothing in the part.

If the gate froze early, more holding pressure only loads the machine harder.

Run a gate seal study. Raise holding time in steps and weigh the parts. Part weight climbs while the gate is open, then flattens. The flat point is gate freeze. Any packing beyond it is wasted cycle time, not extra quality.

Step 3: Read the Packing Profile

Packing feeds shrinkage. Holding stops backflow.

These are two different jobs, and mixing them up means tuning the wrong one. Packing pressure compensates for shrinkage by pushing extra material into the cavity as the plastic cools. Holding pressure, the lower of the two, keeps the melt from flowing back out before the gate freezes.

A short packing phase leaves more shrinkage in the part, which means more or larger sink marks.

Watch the pattern. If the sink fades when you raise packing pressure but returns when the cycle speeds up, your profile is marginal. Fix the window, not the setpoint. And know the limit: packing alone may never pull a bad geometry into spec.

Step 4: Check Cooling Balance on Both Sides

Cooling rarely causes sink. It decides where it shows.

The dip appears on the side whose skin stayed soft longest. A hot mold surface opposite a rib lets that skin yield while the core contracts. Check whether the cooling circuit behind the cosmetic face is weaker than the circuit behind the rib side. A few degrees of mold temperature difference across the wall can move or erase a marginal sink.

Lowering mold temperature on the sink side is the cheapest experiment in this whole flowchart.

If one side of the tool runs hot, find out why before blaming the process. The cause is often a blocked, scaled, or missing cooling channel. Balanced cooling gives the biggest single improvement for surface defects on parts with thick features.

Step 5: Rule Out Material and Regrind

Some resins shrink harder than others.

Semi-crystalline resins like PP, nylon, and PE shrink more than amorphous resins like ABS or PC. The same mold can run sink-free on ABS and sink badly on PP. If the defect appeared after a resin lot change or a higher regrind ratio, suspect the material before the tool. Coarse or excessive regrind changes flow and shrinkage behavior.

When a sink appears mid-production on a mold that ran clean for months, ask what changed first. Check the resin lot, regrind ratio, and mold temperatures before suspecting the steel.

Barrel temperature set too high adds volumetric shrinkage. Dropping melt temperature slightly can flatten a marginal sink. It also raises viscosity, so watch fill at the same time.

When the Process Cannot Fix It

Some sinks are born in CAD, not on the press.

If all five steps check out and the sink stays, stop tuning. The geometry itself defeats packing. The durable fixes are steel fixes, not process tweaks. Core out solid bosses and replace thick sections with ribbed structures. Move the gate closer to the heavy zone so packing reaches it before freeze, or enlarge an undersized gate. Keep wall transitions gradual instead of abrupt.

When packing alone cannot pull a bad geometry into spec, the fix is steel, not settings.

Walk through the same six steps on your own part with the interactive tool below. Then decide whether your answer is a process window or a design change.

Interactive Injection Molding Defect Troubleshooter by

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