Injection Molding Defect Troubleshooting: Short Shots, Flash, Warpage & Sink Marks — Root Causes & Process Fixes
The Four Defects That Cost You Money
Every injection molding technician knows these four defects by sight. Short shots that leave parts incomplete. Flash that requires manual trimming or scraps the part. Warpage that makes assembled parts refuse to fit. Sink marks that look like manufacturing defects even when the part is structurally sound. This guide covers root cause diagnostics for each — but more importantly, it tells you when the problem is your process parameters vs. when it is your mold design. You can spend weeks adjusting injection speed, holding pressure, and mold temperature on a mold that was designed with a gate too small or cooling channels too shallow — and you will never fully fix the defect because you are treating the symptom, not the cause.
Short Shots: Incomplete Filling
What it looks like: The part is incomplete — material did not reach the end of the cavity before solidifying. The edges of the unfilled area are typically smooth and rounded (the melt front froze, it did not fracture).
| Root Cause | Process Fix | Mold Design Fix (if process cannot resolve) |
|---|---|---|
| Insufficient injection pressure or speed | Increase injection pressure and/or injection speed. The melt must reach the end of the cavity before it solidifies. | — (this is a process parameter issue) |
| Material too viscous (too cold) | Increase melt temperature (barrel temperature) by 5-10°C increments. Verify actual melt temperature with a needle probe — controller setpoint may not match actual. | — |
| Gate too small or restricted | Increase injection speed to shear-thin the material through the gate. This is a band-aid — the gate needs to be enlarged. | Enlarge the gate. Gate size should be 50-80% of the wall thickness for most materials. PEEK and other high-viscosity materials need gates at the upper end. |
| Mold too cold | Increase mold temperature. For semi-crystalline materials (PEEK, PPS, nylon), the mold temperature is critical — too cold and the material freezes before filling. | Add or improve mold heating. For high-temp materials (PEEK, Torlon), mold temperatures of 150-200°C are required — standard mold heaters may be insufficient. |
| Inadequate venting | Reduce injection speed slightly — this gives trapped air more time to escape through existing vents. | Add vents at the end of fill (the last area to fill is where air is trapped). Vent depth: 0.02-0.05mm for most materials. Vent width: 3-6mm. |
Flash: Material Escaping the Cavity
What it looks like: A thin film or fin of material extending from the parting line or around ejector pins. Flash is material that escaped the cavity — it indicates that the mold was not fully sealed during injection.
| Root Cause | Process Fix | Mold Design Fix |
|---|---|---|
| Clamp force insufficient for injection pressure | Reduce injection/holding pressure. If flash disappears, the injection pressure was exceeding the clamp force capacity. Consider a larger machine or reducing the cavity pressure. | — (machine sizing issue) |
| Mold parting line damage or wear | — (no process fix for mechanical damage) | Inspect the parting line under magnification. Re-grind or replace damaged mold plates. Worn parting lines allow material to escape at pressures far below the machine's clamp rating. |
| Overpacking during holding phase | Reduce holding pressure and/or holding time. Flash that appears only during the holding phase indicates overpacking — the cavity is full, and additional material is being forced in, opening the mold slightly. | Reduce the gate size — a smaller gate freezes sooner, preventing overpacking during the holding phase. |
| Material viscosity too low (too hot or wrong grade) | Reduce melt temperature. If the material is too fluid, it will find escape paths that a properly viscous material would not. Check that the correct material grade is being used — a high-flow grade may be inappropriate for this mold. | If the material must be low-viscosity (e.g., LCP for thin-wall connectors), the mold must be built to tighter tolerances at the parting line. |
Warpage: Parts That Will Not Stay Straight
What it looks like: The part is dimensionally distorted — bent, twisted, or bowed — after ejection and cooling. Warpage is not a cosmetic defect; it is a dimensional defect that prevents assembly and indicates residual stress in the part.
| Root Cause | Process Fix | Mold Design Fix |
|---|---|---|
| Differential shrinkage (thick and thin sections cooling at different rates) | Reduce holding pressure and increase cooling time. This gives the part more time to cool uniformly. However, differential shrinkage from geometry is primarily a design problem. | Redesign the part with uniform wall thickness. If thick sections are unavoidable, core them out or add ribs. Differential cooling between thick and thin sections is the #1 cause of warpage. |
| Asymmetric cooling (one side of mold colder than the other) | Balance mold temperatures between the fixed and moving halves. A temperature difference of 10°C between mold halves can cause significant warpage in semi-crystalline materials. | Add or balance cooling channels. Ensure both mold halves have equivalent cooling. For complex parts, conformal cooling channels (3D-printed) can follow the cavity contour. |
| High mold temperature for semi-crystalline materials | For PEEK, PPS, and other semi-crystalline materials: INCREASE mold temperature. Paradoxically, a hotter mold reduces warpage because it allows the material to crystallize more completely and uniformly before ejection. | Mold temperature control must be capable of reaching and maintaining 150-200°C for PEEK, 130-150°C for PPS. Standard mold temperature controllers often max out at 120°C — this is insufficient for high-temperature semi-crystalline materials. |
| Molecular orientation (flow-induced anisotropy) | Reduce injection speed — high-speed filling orients polymer chains in the flow direction, causing anisotropic shrinkage (more shrinkage perpendicular to flow than parallel). | Reposition the gate so that flow is more uniform and the primary flow direction aligns with the part's long axis — where shrinkage has the least dimensional impact. |
Sink Marks: Surface Depressions
What it looks like: A shallow depression on the part surface, typically above a thick section, rib, or boss. Sink marks are caused by the surface layer of the part solidifying while the interior is still cooling and shrinking — the surface collapses inward as the interior contracts.
| Root Cause | Process Fix | Mold Design Fix |
|---|---|---|
| Insufficient holding pressure or time | Increase holding pressure and/or extend holding time. The holding phase must continue until the gate freezes — otherwise, material flows backward out of the cavity as the part shrinks, leaving no material to fill the shrinkage void. | Increase the gate size — a larger gate stays molten longer, allowing more holding pressure to be transmitted to the cavity before gate freeze. |
| Holding pressure not reaching the thick section | Increase holding pressure significantly — but if the thick section is far from the gate, the holding pressure may drop off along the flow path because the material near the gate freezes first. | Move the gate closer to the thick section. For parts with unavoidable thick sections remote from the gate, consider multiple gates or a hot runner system to deliver holding pressure directly to the thick area. |
| Melt or mold temperature too high | Reduce melt temperature and/or increase cooling time. A hotter melt shrinks more as it cools — the shrinkage is proportional to the temperature change from melt to ejection. | — (this is a process issue, not mold design) |
| Part geometry — thick section without adequate cooling | Extend cooling time. However, if the thick section (e.g., a boss or rib root) is significantly thicker than the nominal wall, process fixes will be limited. | Redesign: core out thick sections, reduce rib thickness to 50-60% of wall thickness, add cooling channels near thick areas. For bosses, consider gas-assist molding to hollow out the thick interior. |
When to Stop Tweaking the Process and Fix the Mold
Process technicians often spend hours adjusting parameters to fix a defect that originates in mold design. Here is the rule of thumb: if you can eliminate the defect by adjusting a process parameter — injection speed, holding pressure, mold temperature — and the new parameter values are within the material supplier's recommended range, the process adjustment is valid. If you must adjust a parameter outside the recommended range to fix the defect — for example, running PEEK at 420°C when the recommended maximum is 400°C, or running the mold at 80°C when the material needs 160°C — the problem is in the mold or part design, not the process. Continuing to run outside recommended parameters will degrade the material, produce inconsistent parts, and eventually cause a field failure that costs far more than fixing the mold would have.
Disclaimer: This guide provides general injection molding troubleshooting guidance. Specific applications require consultation with material suppliers, mold designers, and process engineers. Propprose does not provide engineering certification or process validation services.
References & Industry Standards
- ASTM International. Standard Specifications for Engineering Plastics & Thermoplastics. astm.org
- ISO. ISO 1043 — Plastics — Symbols and Abbreviated Terms. iso.org
- National Institute of Standards and Technology (NIST). Polymer Properties Database. nist.gov
- UL Prospector. Plastics & Elastomers Material Database. ulprospector.com
- MatWeb — Material Property Data. matweb.com