Fixing Flash and Sink Marks in Injection Molding

What are flash and sink marks

Flash and sink marks are among the most frequent surface defects that appear in injection molded parts. Flash is an excess of molten polymer that squeezes into gaps or seams and cools into a thin, soft edge. Sink marks are shallow depressions that form on thick sections because the material continues to solidify after the mold has closed or cooled. Both defects affect appearance, fit, and downstream processing such as painting or assembly.

Flash is usually easier to spot. It appears as a thin rib of plastic along a parting line, around a screw boss, or at a gate. Sink marks are harder to catch unless you know where to look. They show up as dimples on the inside of a thick wall, near a rib, or under a boss. In consumer parts, a sink mark can be visible under flat lighting. In automotive or medical parts, the same mark can fail an inspection standard.

The two defects share some causes, but they respond to different fixes. Flash often points to pressure or mold fit problems. Sink marks usually point to cooling or filling issues. Treating them with the same correction can make one problem worse while the other improves. A good troubleshooting approach separates the symptoms, checks the process data, and then applies the smallest change that addresses the root cause.

How to identify the defect from the part

Start by looking at the part under even, angled light. Flash is a ridge. Sink marks are hollows. Touch the area. Flash feels like a thin lip. Sink marks feel like a depression that may have a raised rim around it. If the defect is only visible under magnification, it may still fail an aesthetic requirement. If it is visible to the naked eye, it is almost certainly a customer complaint waiting to happen.

Check the location. Flash along the parting line suggests a mold gap, high pressure, or a worn clamp face. Flash near a gate suggests the gate is too large or the gate pressure is too high. Flash at a rib suggests the rib is too thick or the mold is not holding the rib line tightly. Sink marks on thick sections suggest uneven cooling or too much material in the area. Sink marks near a boss suggest the boss is pulling material while the surrounding wall is already solid.

Record the part number, the mold cavity, and the shot number. Defects are not random. If flash appears only in cavity three, the mold may have a gap in that area. If sink marks appear only on parts made from a new resin lot, the material may have a different melt behavior. Keeping a simple log helps separate process variation from mold or material variation.

Common causes and practical fixes

The table below lists the most common symptoms, the likely causes, and the actions that usually help. Use it as a starting point, not a fixed recipe. Every mold and every process is different.

Symptom	Likely cause	What to do
Thin flash along parting line	Mold gap, high cavity pressure, or worn clamp plates	Check mold alignment, reduce pressure, inspect clamp plates, clean mold
Flash at gate	Gate too large, high gate pressure, or slow cooling	Shrink gate size, lower gate pressure, improve cooling at gate area
Flash at rib or boss	Rib too thick, mold line not sealed, or high fill pressure	Reduce rib thickness, verify mold fit, lower fill pressure
Sink mark on thick wall	Uneven cooling, high injection pressure, or long solidification time	Improve cooling, reduce pressure, add venting, adjust hold time
Sink mark near boss	Boss too thick, poor venting, or high pressure in surrounding wall	Reduce boss thickness, add venting, balance cooling, lower pressure
Flash and sink mark together	High pressure, poor venting, or mold misalignment	Review process settings, check mold condition, verify venting and cooling

The most useful first move is to reduce the pressure. If flash disappears but the part is short, the problem is not pressure. It is mold fit or venting. If the part fills fully and the flash disappears, the pressure was too high. For sink marks, the first move is often to improve cooling. If the thick section cools faster, it shrinks less and the mark shrinks. If cooling is already good, reduce hold pressure or shorten hold time.

Do not chase a single symptom with a single setting. Flash and sink marks can be caused by three or four things at the same time. A common mistake is to lower the pressure, fix the flash, and then increase the hold time to fill the part. The flash comes back, or the sink mark gets worse. Make one change, measure the result, and document it.

Mold design and tooling checks

Mold design determines how much of the defect problem is possible. A well-designed mold with good venting, balanced cooling, and proper rib thickness will tolerate more process variation than a marginal mold. If the same defect appears across multiple machines, the mold is often the limiting factor.

Start with the parting line. A worn or dirty parting line creates flash. Clean the mold, inspect the line for scoring, and verify that the mold closes evenly. If the parting line is worn, the fix is usually a reface or a new mold half. A temporary fix is to reduce pressure, but that is a workaround, not a repair.

Next, check the ribs and bosses. Thick ribs and bosses are the most common sink mark sources. Reduce the rib thickness to a standard ratio of the wall thickness. Keep bosses short and round. A long, square boss traps pressure and pulls material. If the part function requires a thicker boss, consider changing the resin or adding a core post. If the part is for a consumer product, a smaller boss may be acceptable. If it is for an automotive bracket, the design may need a different approach.

Venting is the second factor. A mold with poor venting traps air and raises pressure. The pressure pushes plastic into the parting line and creates flash. It also causes sink marks because the trapped air prevents the material from flowing evenly. Add venting pins, micro-vents, or improve the existing vent lines. Do not rely on a single vent. A good mold has venting at the end of the cavity and along the parting line.

Cooling is the third factor. Sink marks are a cooling problem as much as a pressure problem. The thick section needs to cool faster than the thin wall. If the mold does not have cooling near the thick area, the material stays hot and shrinks more. Add cooling channels, change the coolant flow rate, or use a different mold steel. If the mold is old, the cooling channels may be clogged. A cleanout and flow test can reveal the problem.

Process settings and machine checks

Process settings are the most common cause of flash and sink marks because they are the easiest to change. They are also the easiest to change incorrectly. A small change in pressure, temperature, or hold time can move the defect from one side of the part to the other.

Begin with the injection pressure. High pressure causes flash. Lower the pressure until the flash disappears. If the part is short, do not raise the pressure. Increase the hold pressure or the hold time instead. Hold pressure fills the part after the cavity is full. It is a more targeted way to fill short shots. If the part is still short after increasing hold, the problem is likely mold fit, venting, or material flow.

Next, check the melt temperature. High temperature reduces the viscosity of the melt. The melt flows more easily and creates flash. It also stays molten longer and can cause sink marks. Lower the melt temperature to the recommended range. If the part is short, do not raise the temperature. Increase the hold pressure or fix the mold.

Check the mold temperature. Cold mold temperature solidifies the part faster. This can reduce flash, but it can also cause sink marks if the thick section cools unevenly. A common fix is to raise the mold temperature slightly and reduce the hold pressure. The part fills more evenly and the thick section cools more uniformly.

Check the hold time. Long hold time packs the part and can create sink marks. Short hold time leaves the part short. Find the shortest hold time that fills the part. Measure the part dimension after the hold. If the dimension is stable, the hold time is good. If the dimension changes with hold time, the process is not stable.

Check the clamp force. If the clamp force is too low, the mold opens slightly during injection. Flash appears along the parting line. Raise the clamp force until the flash disappears. If the clamp force is too high, the mold may not close properly, or the machine may be overloaded. A machine that is overloaded will not hold the mold closed. It will creep open under pressure.

Check the machine settings. The screw speed, back pressure, and shear heat affect the melt quality. Too much shear heat raises the melt temperature and creates flash. Too little back pressure causes air entrainment and poor fill. Adjust these settings to the recommended range. If the machine is old, check the screw and barrel for wear. A worn screw leaks pressure and causes flash.

Material selection and lot control

The resin affects flash and sink marks in ways that are easy to ignore. A resin with a lower melt flow rate is thicker and may need higher pressure to fill. That pressure can create flash. A resin with a higher melt flow rate is thinner and may flow into gaps. That flow can create flash too. The trade-off is not a simple better or worse choice. It is a balance.

Sink marks are more sensitive to the resin. A resin with a high shrinkage rate shrinks more as it cools. The thick section shrinks more than the thin wall. The sink mark gets deeper. A resin with a low shrinkage rate shrinks less. The mark is shallower. For thick-walled parts, choose a resin with a lower shrinkage rate. For thin-walled parts, the shrinkage rate matters less.

Check the resin lot. A new lot can have a different moisture content, a different molecular weight, or a different melt flow index. These changes shift the process. If a defect appears after a resin change, run a small test batch. Compare the part dimensions and the surface. If the new lot causes the defect, work with the supplier to find a lot with tighter specifications.

Moisture is a common cause of surface defects. Moisture in the resin creates vapor pockets. The vapor pockets push the melt into gaps and create flash. They also cause uneven cooling and sink marks. Dry the resin to the recommended moisture level. Use a dryer with the correct temperature and time. Verify the moisture with a test or a visual check. If the resin is hygroscopic, store it in a dry cabinet or use a desiccant.

Filler and color also affect the defect. Fillers change the melt viscosity and the shrinkage rate. A high filler content can increase the pressure needed to fill the part. That pressure can create flash. Colorants can change the heat absorption of the melt. A dark color absorbs more heat and can raise the melt temperature. That can create flash. If the part has a dark color, consider a lower melt temperature and a higher hold pressure.

Prevention and quality checks

Prevention is cheaper than repair. A part that fails inspection must be reworked, and rework can damage the surface. A part that is flash-free and sink-free on the first shot saves time and money. The best prevention is to catch the cause before the mold is run.

Start with the design review. Check the wall thickness, rib thickness, boss size, and gate size. A thick wall is a sink mark waiting to happen. A large gate is a flash waiting to happen. A sharp corner traps stress and creates a sink mark. A rounded corner flows more easily. If the design is marginal, fix it before the mold is built. The cost of a design change is low. The cost of a mold change is high.

Second, review the mold design. Check the parting line, venting, cooling, and gate location. A mold with good venting and balanced cooling will produce fewer defects. A mold with poor venting and unbalanced cooling will produce more. If the mold is already built, add venting or cooling where possible. If the mold is new, build it with the defect in mind.

Third, set a baseline process. Run the mold with a known good resin and record the settings. Save the settings as a recipe. When a defect appears, compare the current settings to the baseline. The difference often shows the cause. If the pressure is higher, flash is likely. If the temperature is higher, flash is likely. If the hold time is longer, sink marks are likely.

Fourth, inspect the part regularly. Use a light and a magnifier. Check the parting line, the gate, the ribs, the bosses, and the thick sections. Measure the dimensions. If the dimension changes, the process is not stable. If the surface changes, the mold or the material is changing. Keep a record of the defects and the settings. The record helps identify trends.

Fifth, train the operator. The operator is the first person to see a defect. If the operator knows what to look for, the defect is caught early. If the operator does not know, the defect goes to the next step. Train the operator on the common defects, the common causes, and the first checks. A trained operator can fix a small problem before it becomes a big one.

When to call for help

Some defects are not a process problem. They are a mold problem. Flash along the parting line that does not disappear with lower pressure may be a worn mold. Sink marks that do not disappear with better cooling may be a design problem. If the defect persists after the process changes, stop and review the mold.

A mold repair can fix a worn parting line, a broken vent, or a clogged cooling channel. A mold repair can also fix a misaligned mold. If the mold is misaligned, the parting line is open and flash appears. A mold repair can fix the alignment.

A design change can fix a thick wall, a large boss, or a sharp corner. A design change can also fix a gate that is too large. If the part function allows, reduce the thickness, change the shape, or move the gate. If the part function does not allow, choose a different resin or accept the defect as a cosmetic risk.

A supplier can help with a resin that has a lower shrinkage rate or a better melt flow. A supplier can also help with a resin that has tighter moisture specifications. If the current resin causes the defect, ask the supplier for a different grade. If the supplier cannot provide a different grade, consider a different resin.

Do not ignore a defect because it is small. A small flash can chip during handling. A small sink mark can be visible in use. A small defect can fail an inspection standard. A small defect can become a big complaint. Catch it early, fix the cause, and document the fix.