Fixing Sink Marks in Nylon Injection Molds

Why nylon parts develop sink marks

Nylon is a semi-crystalline polymer. It shrinks significantly during cooling as the material moves from a molten state to a solid crystal structure. Sink marks appear when the surface skin freezes before the core of the section is fully solid. The surface contracts while the underlying resin continues to pull inward, leaving a dimple on the finished part.

The defect is most visible on flat surfaces, bosses, ribs, and areas near the gate. In production, even a small dimple can fail a visual acceptance test. Buyers often reject parts that show surface distortion, especially if the part is cosmetic or fits against another component.

Common symptoms of sink mark problems

Inspection teams usually identify sink marks during first article inspection or during routine production checks. The symptoms vary based on part geometry and resin grade.

Symptom	Likely cause	What to do
Dimple near the gate	High injection pressure creates a local shrinkage pocket	Reduce injection speed or adjust gate size to balance fill
Dimple on a thick section	Uneven cooling leaves a hot core that shrinks later	Thin the wall or add cooling channels behind the section
Multiple small dimples across a flat face	Inadequate hold pressure or short hold time	Increase hold pressure and extend hold time to ensure pack
Dimple behind a rib or boss	Resin freezes around the feature before the base cools	Modify rib thickness or add a vent to improve cooling
Uneven sink depth across cavities	Unbalanced cooling or gate flow in a multi-cavity mold	Adjust coolant flow rates and gate design per cavity

When a part shows a single, deep dimple, the issue is usually localized. When dimples appear across multiple areas, the problem is often a process or design mismatch.

Mold design choices that prevent sink marks

Gate placement is the first design decision that affects sink behavior. A gate that is too large allows resin to pack into the part, but it can also freeze late and create a hot spot. A gate that is too small restricts flow and can lead to short shots rather than sink marks.

For nylon, a fan gate or a tapered gate often works well in multi-cavity tools. These gates allow resin to pack into the part without creating a large, visible gate pad. In thin-walled parts, a small pin gate may be easier to remove, but it requires higher pack pressure. The choice depends on the part geometry and the machine capability.

Cavity geometry matters more than most tooling teams expect. A thick boss or a rib that is too deep creates a cooling bottleneck. The resin in the center of the feature stays liquid longer. When the outer skin freezes, the inner resin shrinks and pulls the surface down.

A practical rule is to keep wall thickness as uniform as possible. If a thick section is required for strength, add a relief feature or a taper that allows the resin to cool more evenly. In multi-cavity molds, each cavity must cool at the same rate. If one cavity cools faster than another, the part will shrink differently.

Process settings that fix existing sink marks

If the mold is already in production, process adjustments are the fastest way to reduce sink marks. The goal is to pack the part fully before the surface freezes.

1. Increase hold pressure. This forces more resin into the mold cavity and reduces the void space that causes shrinkage.
2. Extend hold time. A longer hold time allows the resin to pack until the gate freezes, which seals the part and prevents backflow.
3. Raise mold temperature. A warmer mold slows the skin freeze, giving the resin more time to pack. However, this increases cycle time and may affect dimensional stability.
4. Reduce injection speed. A slower fill reduces shear heat and turbulence, which can help the resin pack more evenly.
5. Check material drying. Nylon absorbs moisture from the air. Wet resin flashes and creates voids that look like sink marks but are actually gas pockets.

Do not change one setting at a time without recording the result. Keep a simple log of pressure, time, temperature, and part appearance. A small change in hold pressure can be enough to eliminate a dimple, but a large change can cause a short shot or a flash.

Material and drying considerations

Nylon grades vary in shrinkage behavior. Higher molecular weight grades tend to shrink more. If a part was designed for a lower shrinkage grade and the supplier delivers a higher molecular weight lot, the part may develop sink marks that were not present before.

Drying is a common failure point. Nylon must be dried to a low moisture content before injection. If the resin is wet, the moisture vaporizes during injection and creates voids. These voids can collapse and look like sink marks.

Check the dryer temperature and time. A standard drying cycle for nylon is often in the range of 2 to 4 hours at a specific temperature, but the exact values depend on the grade and the dryer type. Use a moisture meter or a weight loss test to verify that the resin is dry before running production.

Also verify the resin lot. If the sink marks appear after a material change, run a comparison part with the original lot. If the defect disappears, the issue is material-related, not mold-related.

Inspection and acceptance criteria

Define what a sink mark is before production starts. A small dimple may be acceptable on an internal component but unacceptable on a consumer-facing part. Set a clear threshold. For example, a dimple deeper than 0.1 mm may be a reject, while a dimple shallower than 0.05 mm may pass.

Use a light source and a magnifier for inspection. Shine a light at an angle to the surface. Sink marks are easier to see when the light grazes the surface. A flat surface under direct light may hide a shallow dimple.

If the part has a flat face, use a straight edge or a gauge block to check for flatness. Sink marks can cause the surface to curve slightly, which may not be visible but can affect assembly. If the part fits against a mating surface, a slight concavity can cause a gap or a stress point.

Prevention for new mold projects

For new molds, address sink marks during the design phase. Review the CAD model for thick sections, deep bosses, and uneven wall thickness. A mold flow simulation can predict where the resin will cool first and where shrinkage will occur.

The simulation does not replace a trial run, but it helps identify high-risk areas before steel is cut. If the simulation shows a thick section that cools slowly, adjust the design now. It is cheaper to change a CAD file than to re-machine a steel mold.

In multi-cavity molds, balance the cooling channels. Each cavity must have the same coolant flow rate. If one cavity is cooler than another, the part will shrink differently. A mold builder can add adjustable coolant nozzles to fine-tune the flow after the mold is in the machine.

Also consider the gate size. A gate that is too large creates a large gate pad that is difficult to remove and may cause a visible mark. A gate that is too small restricts pack. A balanced gate is the target. For nylon, a gate size that allows full pack without a large pad is usually the best choice.

When to redesign the mold

Some sink marks cannot be fixed with process changes alone. If the part has a thick section that is required for structural strength, the only long-term fix may be to change the geometry. This could mean thinning the wall, adding a rib, or changing the boss shape.

If the mold is a rapid prototype, a small sink mark may be acceptable. If the mold is a production steel tool, the cost of a visible defect is higher. A rework on a production mold can take days and cost money. It is better to fix the design before the tool is finalized.

If the problem persists after process changes, material checks, and inspection adjustments, the mold likely needs a design change. This may include adding cooling channels, changing the gate, or modifying the cavity wall. Work with the mold builder to identify the root cause.

Final checks before production

Before releasing a part to production, run a small batch and inspect it under multiple lighting conditions. Check the part after it has cooled for a few hours. Some parts can develop sink marks after they leave the mold, as they continue to shrink.

Record the process parameters that produce an acceptable part. Save the settings in the machine memory or in a document. If an operator changes the settings, the part quality may change. A consistent process is the best defense against variation.

If the part is used in a safety-critical application, test the sink mark severity under load. A dimple may not be visible, but it may reduce the strength of a rib or a boss. A simple pull test or a visual check under stress can reveal issues that a flat surface inspection misses.

Sink marks in nylon are a common problem, but they are also predictable. The cause is almost always a combination of part geometry, material shrinkage, and cooling behavior. Address each factor, and the defect usually disappears.