What Causes Injection Molding Defects?
Every injection molding project runs into defects at some point. The key is knowing what causes them and how to fix them fast.
Most defects fall into one of four categories: flow issues, cooling issues, material issues, or tooling issues. This guide covers the most common ones and gives you practical fixes for each.
Sink Marks: The Wall Thickness Problem
Sink marks are depressions on thick part surfaces. They happen when the outer skin cools and hardens while the inner core is still shrinking.
The root cause is almost always uneven wall thickness. When a thick section connects to a thin section, the thick part shrinks more and pulls the surface inward.
| Fix | Effort | Effectiveness |
|---|---|---|
| Re-evaluate wall thickness distribution | High | Best |
| Increase hold pressure and time | Low | Good for minor sinks |
| Move gate closer to thick section | Medium | Good |
| Use gas-assist or foam molding | High | Very Good |
If you are designing a new part, stick to uniform wall thickness. This single rule prevents more defects than any other design guideline. Precision injection molding teams always review wall thickness during the DFM stage before cutting steel.
Flash: The Clamp Force Problem
Flash is thin excess plastic that escapes from the mold cavity along the parting line. It happens when the clamp force is too low or the mold faces do not meet evenly.
Possible causes: clamp force too low for the projected area, worn or damaged parting line surfaces, injection pressure exceeding the clamp force, or mold flex under high pressure.
Start by checking the clamp force. You need roughly 4 to 6 tons per square inch of projected area for most commodity plastics. If the parting line is clean and the force is correct, check for mold wear.
Gas Traps: The Venting Problem
Gas traps, also called burn marks or void gas, happen when air inside the cavity has no way to escape. As the melt front pushes forward, the trapped air compresses and can cause burning or incomplete fill.
Vent depth depends on the material. For most engineering plastics, 0.02 to 0.05 mm is the range. POM and nylon need shallower vents. PVC and flexible materials need deeper ones. Vents should be placed at the last point to fill, around ejector pins, and along the parting line.
If you see burn marks at the same spot on every shot, check that location for venting. Adding 0.02 mm deep vents in that area usually solves the problem.
Weld Lines: The Flow Front Problem
Weld lines form when two melt fronts meet inside the cavity. They are structurally weaker than the surrounding material and can be visible on the part surface. The key to managing weld lines is understanding where they form and how to improve the bond.
| Strategy | Approach | Best For |
|---|---|---|
| Relocate gate | Move gate to shift weld line location | Non-functional areas |
| Increase mold temperature | Higher temp = better polymer bonding | Strength critical parts |
| Overflow wells | Add small cavities past the weld line | Cosmetic surfaces |
| Increase injection speed | Faster fill = hotter melt fronts | Simple geometries |
In multi shot injection molding, weld lines between layers require special attention. The temperature of the first shot surface directly affects the second shot bond quality.
Warpage: The Cooling Problem
Warpage is distortion in the final part. It happens when different areas of the part shrink at different rates. The most common causes are uneven cooling, fiber orientation, and residual stress from the filling phase.
Check your cooling channel layout first. The temperature difference across the cavity should be no more than 5 degrees Celsius. If one side is hotter than the other, that side shrinks less and the part bends toward the hot side.
For glass-filled materials, the flow pattern determines fiber orientation. Mold flow analysis is the best way to predict and adjust fiber alignment before building the mold.
Short Shot: The Flow Problem
A short shot happens when the melt does not completely fill the cavity. The result is an incomplete part with missing features. Causes include insufficient injection pressure, low melt temperature, blocked gate, or inadequate material volume.
The fix depends on the pattern of the short shot. If it always appears at the same location, the flow path at that point is probably too restrictive. Increase the gate size or adjust the wall thickness at that location. If it happens randomly, check for material consistency, temperature stability, or a partially blocked nozzle.
Jetting: The Gate Problem
Jetting creates snake-like lines on the part surface. It happens when the melt entering through the gate does not immediately contact the cavity wall. Instead, it shoots straight into open space before folding over.
The fix is to redirect the incoming flow so it hits the cavity wall right after entering. Use a fan gate, a tab gate, or an oversized gate to reduce the jetting effect. Slowing the initial injection speed also helps.
A Systematic Approach to Defect Troubleshooting
The best approach to injection molding defects is a structured one. Do not change parameters randomly. Collect data, identify the pattern, and fix the root cause.
Start with the part geometry. Check wall thickness, draft angles, and ribs. Then check the mold design. Gate location, cooling channels, and venting. Then check the process. Temperature, pressure, speed, and cooling time. Fix the root cause, not the symptom.
Die casting projects follow the same systematic approach. The same principle applies: vacuum assist and proper gate design prevent gas porosity. Controlled cooling prevents distortion. SHINY Mold applies consistent quality systems across both technologies.
SHINY Mold has been solving injection molding problems since 2003. With 120 plus engineers and 100 plus molding presses from 80 to 1800 tons, the team has the experience and equipment to handle complex defect challenges. ISO 9001/14001/13485/IATF 16949 certified processes keep every project on track.
Whether you are troubleshooting an existing mold or designing a new one, the SHINY engineering team can help with DFM analysis, mold flow simulation, and practical production solutions.
