Injection Molding Defects and Troubleshooting
Identify short shots, flash, sinks, burns, splay, warp, and other defects, and trace each one to a likely cause and fix.
Table of contents
Almost every molding defect comes down to temperature, pressure, speed, time, material or the mold itself. The hard part is not knowing the list of causes. It is staying disciplined when parts are going bad and people want a quick fix. A molder who changes three things at once may fix the part but never learn why, and the problem returns next shift. Learning to read a defect, find what changed and change one thing is the skill that makes a process tech.
What You Will Be Able to Do
- Name the common defects by sight and say what causes each one.
- Use a step-by-step method instead of guessing.
- Read process numbers like cushion and fill time as clues.
- Tell a process problem from a material, machine or mold problem.
- Make a correction, test it and record it.
A Method That Works
Only change settings you are authorized to change. If you are not, your job is to spot the defect, hold the parts and report what you found.
- Hold suspect parts. Put them aside and label them so they do not ship.
- Describe the defect. Where on the part is it? Does it happen on every shot or only some? In every cavity or just one?
- Compare the process to the setup sheet. Check real numbers: cushion, fill time, peak pressure, melt and mold temperature, dew point, water flow.
- Ask what changed. New resin lot, more regrind, a new shift, a dryer alarm, a cooling water problem, a mold repair.
- Pick the most likely cause, and make one change. Keep the step small. For melt temperature, 5 to 10 degrees F is plenty.
- Let it settle. Speed or pressure changes show up in about 10 shots. Temperature changes take longer, often 20 to 30 minutes.
- Check the result. If it did not help, put the setting back before you try the next idea.
- Record it. Write down what you saw, what you changed and what happened.
Change one thing at a time. If you change the speed and the temperature together and the part improves, you do not know which did it, and you may have added a new problem that will show up later. If the sheet numbers are all still right and the part is bad, resist the urge to adjust around it. Look for what is different, such as the material, water or the mold.
Patterns That Point to a Cause
| What you see | Where to look |
|---|---|
| Same spot, every shot | Mold, gate, vent or a steady setting |
| Random, comes and goes | Material, dryer, check valve, heaters |
| One cavity only | That cavity's gate, vent or water |
| All cavities | Machine, material or overall settings |
| Started after a change | Whatever changed |
Reading the Process Numbers
| Number | If it changes | Possible meaning |
|---|---|---|
| Cushion | Falling to zero | Leaking check valve, shot too small |
| Cushion | Growing | Less plastic going in, blocked gate or nozzle |
| Fill time | Getting longer | Colder or thicker melt, hitting the pressure limit |
| Peak pressure | Climbing | Colder melt, blocked flow path |
| Recovery time | Getting longer | Low hopper, pellets bridging at the throat |
| Part weight | Dropping | Low pack, leaking check valve, short hold |
| Dew point | Rising | Dryer bed used up, filter or hose problem |
Quick Reference
| Defect | What you see | Most common cause | First things to try |
|---|---|---|---|
| Short shot | Part not fully filled | Not enough plastic or flow | More shot, speed or heat, check vents |
| Flash | Thin extra plastic at edges | Mold pushed open | Less pack, check clamp and mold |
| Sink mark | Dimple over a thick area | Not enough packing | More pack or hold, check cushion |
| Void | Bubble inside thick section | Not enough packing | More pack, slower fill |
| Burn mark | Black or brown at end of fill | Trapped air | Slow end of fill, clean vents |
| Splay | Silver streaks from the gate | Wet resin | Check dryer and dew point |
| Warp | Bowed or twisted part | Uneven cooling or shrinkage | Balance mold temps, cool longer |
| Weld line | Line where flows meet | Fronts too cold to bond | More melt heat and speed |
| Jetting | Snake pattern near the gate | Melt squirting through gate | Slow first stage of fill |
Fill Problems
Short Shot
A short shot is a part with missing plastic, because the cavity did not fill. It usually shows in the last areas to fill, such as thin sections or spots far from the gate.
| Likely cause | Fix |
|---|---|
| Shot size too small | Increase shot size |
| Melt too cold | Raise melt temperature in small steps |
| Speed too low, plastic freezes early | Raise injection speed |
| Pressure limit too low | Raise the limit |
| Blocked gate or runner | Check and clear them |
| Poor venting | Clean vents |
| Worn check valve | Check cushion, report it |
Flash
Flash is thin extra plastic at the parting line, around ejector pins or along slide shut-offs. It means plastic got out of the cavity, because the mold halves were pushed apart or do not seal where they should.
| Likely cause | Fix |
|---|---|
| Too much injection or pack pressure | Lower pressure, check transfer point |
| Not enough clamp tonnage | Raise tonnage to the sheet value |
| Melt too hot and thin | Lower melt temperature |
| Worn or damaged parting surfaces | Send the mold for repair |
| Debris or degraded resin on the mold face | Clean the mold face |
Do not keep adding clamp to stop flash on a worn mold. It hides the problem and damages the mold further.
Weld Lines
A weld line, also called a knit line, forms where two flow fronts meet. This happens with several gates and around holes or cores. The fronts have cooled, so they may not bond fully. The line can be visible and it is usually weaker than the rest of the part.
Fixes: raise melt temperature, injection speed or mold temperature so the fronts are hotter when they meet. Add or clean a vent at the line. If it still falls in a stressed spot, talk to the mold designer about moving a gate.
Jetting
Jetting is a wiggly, snake-like line near the gate. The melt squirts through the gate like water from a hose, instead of spreading along the wall. The cause is a gate that is too small, injection that starts too fast or a gate that aims into open space.
Fixes: slow the first stage of injection. The lasting cures are a larger gate or a gate that aims the flow at a wall.
Flow Lines
Flow lines are wavy bands on the surface, often near the gate or at thin to thick changes. The melt is cooling too quickly and rippling along the wall. Raise the melt or mold temperature, or increase speed.
Shrinkage Problems
Sink Marks
Sink marks are dimples on the surface, usually across from thick spots such as ribs, bosses and wall junctions. The outer skin freezes first. As the thick core keeps shrinking, it pulls the skin in.
| Likely cause | Fix |
|---|---|
| Pack pressure too low | Raise pack |
| Pack time too short, gate seals early | Raise hold, check gate seal study |
| Zero or unstable cushion | Fix shot size or check valve |
| Melt too hot | Lower melt temperature |
| Cooling too short | Lengthen cooling |
| Thick design | Core out or thin the ribs, an engineering change |
Inside a thick wall, the same shrinkage can pull a void, or bubble, instead of a sink. Treat it the same way.
Warpage
Warpage means the part bows or twists after ejection. All plastic shrinks. Warp happens when different areas shrink by different amounts.
| Likely cause | Fix |
|---|---|
| A and B halves running at different temperatures | Check water flow and temperatures, balance the halves |
| Ejected too early, still soft | Lengthen cooling time |
| Uneven wall thickness | Design change, thick areas shrink more |
| Gate location gives lopsided fill | Work with the mold designer |
If the walls are uniform and the part still warps, look first at the temperature difference between mold halves and at whether it is coming out too hot. The side that stays hotter shrinks more and pulls the part toward it. Measure both faces with a surface pyrometer. Some plants hold warm parts on a cooling fixture, but that treats the symptom.
Gas and Moisture Problems
Burn Marks
Burn marks are dark brown or black marks, usually at the end of fill, in thin sections or at spots where flows trap air. When the plastic pushes air into a corner faster than it can escape, the air is squeezed so hard that it becomes extremely hot, and it scorches the plastic. This is sometimes called dieseling.
| Likely cause | Fix |
|---|---|
| Vents clogged or too shallow | Clean or service vents |
| Fill speed too high at end of fill | Slow the last stage of fill |
| Too much clamp crushing vents | Lower tonnage to the sheet value |
| Degraded material | Purge the barrel |
The usual fix for a burn at end of fill is to lower speed there and check vents, then clean them. If the burn is spread across the part rather than at the end of fill, suspect degraded resin instead of trapped air.
Splay
Splay, or silver streaks, are silvery lines fanning out from the gate. Gas in the melt is being smeared along the surface as the plastic fills.
| Likely cause | Fix |
|---|---|
| Wet resin, the most common cause | Check dew point, temperature and time |
| Overheated resin giving off gas | Lower melt temperature |
| Long time in the barrel | Check that shot size fits the barrel |
| Screw speed too high, air pulled in | Lower RPM, raise back pressure a little |
With hygroscopic resins such as nylon, polycarbonate and PET, check the dryer first, on every streak. Wet resin also loses strength, so splay can be a warning that parts will be brittle, not just ugly.
Material and Contamination Problems
| Defect | What you see | Likely cause | Fix |
|---|---|---|---|
| Black specks | Dark spots in the part | Degraded resin in the barrel | Purge, check heat, clean screw |
| Color streaks | Uneven color | Poor mixing | More back pressure, check blender |
| Delamination | Skin peels off in layers | Wrong resin mixed in | Purge, check the resin |
| Brittle parts | Part snaps easily | Wet or degraded resin, too much regrind | Check dryer, heat, regrind limit |
| Stringing or drool | Plastic leaks from the nozzle | Melt or nozzle too hot | Lower nozzle heat, add decompression |
Contamination takes patience. If specks keep coming after a purge, the source may be burnt plastic stuck on the screw or the check valve. That needs a screw pull, which is maintenance work.
Ejection Problems
| Defect | What you see | Likely cause | Fix |
|---|---|---|---|
| Sticking | Part stays in the mold | Overpacked, hot, scratched mold | Reduce pack, cool longer, polish |
| Pin marks | Pushed-in or white spots | Part is too soft | Cool longer, slow the ejectors |
| Drag marks | Scratches along the part | Mold damage, draft problem | Send the mold for repair |
| Stress whitening | Pale marks at corners | Forced ejection, sharp corner | Lower pack, slow the ejectors |
Warning: A part stuck in the mold is a common reason people reach in. Follow your plant's procedure. Use the gate, stop the machine, and lock out if the procedure calls for it. Use brass or soft tools so you do not scratch the steel, and never use a tool that can damage the mold.
When the Process Is Not the Problem
Settings cannot fix everything. If you keep pushing settings against a hardware problem, you just move the defect around. Suspect hardware if:
- The defect appeared suddenly and the numbers match the sheet.
- Cushion keeps wandering. The check valve may be worn.
- One zone will not hold temperature. A heater band or thermocouple may be bad.
- Flash shows at one spot. The parting line or a shut-off may be damaged.
- Vents keep filling with residue. The mold needs cleaning or service.
- Water flow is low in one circuit. A line may be blocked or kinked.
Call maintenance or the toolroom, and tag the mold with what you saw.
Common Mistakes
| Mistake | What it costs | How to avoid it |
|---|---|---|
| Changing several settings at once | Nobody knows what fixed it | One change, then check |
| Not waiting for it to settle | Wrong conclusions | Wait 10 shots or more |
| Adding clamp to stop flash | Crushed vents, mold damage | Find the real cause |
| Raising heat to fix wet resin | Worse splay, degraded resin | Check the dryer first |
| Leaving a change that did not help | Drifted process | Put it back |
| Not writing it down | The next shift repeats it | Log each change |
| Letting bad parts mix with good | Customer rejects | Segregate first |
Check Your Understanding
1. PA66 parts have silver streaks fanning out from the gate on every shot. What is your first check?
The dryer. Nylon absorbs moisture, so check dew point, drying temperature and how long the resin has been in the hopper. Wet resin is the most common cause of splay.
2. Thin-walled parts have black marks at the far end, the last place to fill. Everything else looks good. What do you try?
That is trapped air being compressed and scorching the plastic. Slow the injection speed toward the end of fill, and check and clean the vents.
3. Parts are coming out with sinks and weighing light, and the cushion reads zero. What is going on?
The screw has bottomed out, so it cannot deliver pack pressure. Find out why. The shot size may be too small, or the check valve may be leaking. Adding pack pressure alone will not help.
4. A flat part with even walls bows the same way after every ejection. The A side reads 110 F and the B side reads 140 F. What do you do?
The two halves are too far apart in temperature. Check water flow and temperature on both sides, and bring them together. Also make sure the part is not ejecting too early. Make one change at a time.
5. You raised melt temperature to stop a short shot. The short shot is no better, and there is now a little flash. What next?
Put the temperature back. It did not fix the problem, and leaving it changes the process. Move to the next likely cause, such as vents, shot size or a worn check valve.
Key Takeaways
- Hold suspect parts first, then compare the process to the setup sheet.
- Ask what changed. Then change one thing and let it settle.
- If a change does not help, put it back and record it.
- Short shots lack plastic, flash means the mold was pushed open or is damaged, and sinks mean too little packing.
- Burns at end of fill are trapped air. Slow the end of fill and check vents.
- Splay means check the dryer first, and uniform-wall warp points to uneven mold cooling or early ejection.