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

FAQ

What Conformal Cooling Actually Is

Most buyers picture cooling as drilled holes. That picture is twenty years old.

A conventional circuit is a set of straight drilled lines. The drill only goes straight, so each channel sits wherever the drill can reach, not where the heat sits.

A conformal channel bends with the cavity and core walls. It holds a steady distance from the plastic along the whole part, including ribs, bosses, and deep cores that drills cannot reach.

You cannot drill a curve. Shops build these channels with metal 3D printing (DMLS), vacuum brazing, or diffusion bonding instead of a drill press.

The payoff is even heat removal. Hot spots shrink, warpage drops, and the cycle shortens. A conformal channel goes where the heat is; a drilled channel goes where the drill can reach.

Filling gets the attention, but cooling decides the economics of the injection molding process.

Why Cooling Owns Your Cycle Time

Cooling takes 50–80% of a typical molding cycle. Fill, pack, and eject take seconds. Cooling takes the rest.

Cooling time grows with the square of wall thickness. A 4 mm wall needs roughly four times the cooling of a 2 mm wall.

That square law is why thin-wall parts fly and thick bosses drag. It is also why no press upgrade beats a better cooling layout. A faster machine cannot cool plastic sooner than physics allows.

Cooling time grows with the square of wall thickness, so the wall decides the cycle before the press is even chosen. This is where part design rules pay off: uniform walls cool evenly and fast.

Try the interactive cooling simulator below. Change the wall thickness and watch the cooling time move.

Interactive Cooling Layout & Heat Transfer Simulation by Topworks Plastic Mold

The Design Rules That Hold Up

Three dimensions control a conformal layout: diameter, pitch, and wall distance. The table below gives the working rules.

A published design-of-experiments study found the best cooling at 6 mm diameter, 8 mm pitch, and 4 mm wall distance. When mold strength counted too, the optimum shifted to 6 mm, 12 mm, and 6 mm.

Start the circuit at the gate area. That zone runs hottest, so it deserves the coldest water.

Prefer series layouts over parallel splits. Parallel branches invite uneven flow, and one starved branch ruins the whole circuit.

Pitch at three to four diameters and wall distance at two to three diameters is the layout every simulation then refines.

How the Channels Get Built

A beautiful channel design means nothing if the shop cannot build it. Three methods dominate: DMLS metal printing, vacuum brazing, and diffusion bonding.

DMLS prints the insert layer by layer in maraging steel, often grade 1.2709. The shop then machines and polishes the working surfaces to standard mold finish.

Vacuum brazing joins machined plates with the channels milled into the joint faces. Diffusion bonding fuses the plates without filler metal. Both suit larger inserts where printing gets expensive.

Most shops build conformal cooling into the insert only, then fit it into a conventional mold base. Ask your supplier which method they run in-house. A subcontracted print adds weeks and a second margin.

Most suppliers print or braze only the core or cavity insert. The rest of the mold stays conventional, which bounds the cost premium.

Cycle-Time Gains With Names Attached

NyproMold cut a high-cavitation tool from 23 seconds to 16. That is over 40%, confirmed by flow simulation before any steel was cut (MoldMaking Technology).

Cimatron’s car reflector case went from 40.3 to 33.4 seconds, down 17%. Warpage fell 62% and sink-mark area fell 50% (MoldMaking Technology).

Star Rapid’s transparent parts saw cooling time drop 60%, about 38 seconds. Manual air cooling was no longer needed (engineering.com).

Consultant Bob Beard puts the typical range at 20–40% cycle reduction across programs. Treat vendor claims above 50% with care. They usually describe cooling time, not total cycle time.

A 40% cut on a 20-second cycle saves about 7 seconds a shot, and that compounds across millions of shots. Use these numbers when comparing mold quotes, not the brochure headline.

Doing the ROI Math

Conformal cooling pays back in press hours, not in tool price. The tool costs more. Beard estimates roughly 20% extra on cavity and core work.

Take a worked example with round numbers. A 4-cavity cap mold runs 2 million shots a year. The cycle drops from 30 to 22.5 seconds, a 25% cut. Press time costs $35 an hour. The conformal work adds $18,000 to the tool.

That program runs 500,000 cycles a year. Each cycle saves 7.5 seconds, which totals about 1,040 press hours, worth roughly $36,000. Payback lands near six months.

Payback (months) = tool premium ÷ monthly press-hour savings.

Below about 500,000 shots a year, the same $18,000 premium takes over two years to repay. Volume decides.

Conformal cooling is a volume game: high shots per year repay the premium in months, while low volumes may never repay it. Pair it with the hot runner vs cold runner decision, since both target cycle economics.

When Conformal Cooling Does Not Pay

The most expensive cooling channel is the one the part never needed.

Flat parts with uniform walls cool fine on straight drilled lines. Curves earn the premium, not flat plates.

Low-volume programs cannot amortize the extra tool cost. The math in the last section shows why.

Conformal channels are harder to descale and flush than drilled lines. Poor water treatment will choke them first, so ask about your plant’s water quality before approving.

I’ve seen buyers spec conformal cooling on a flat cover where four drilled lines would have matched it. If straight drilled channels already cover the part evenly, conformal cooling buys cost without benefit.

Before you approve the premium, ask your supplier to show the simulated cycle with and without it.

Frequently Asked Questions