Injection Molding Defects: A Buyer's Guide to Causes and Fixes

When the first molded samples arrive and something looks wrong (a dimple on a smooth face, a faint line across a panel, or a short edge), the main question is not the defect name. It is ownership: Is this a failure of the part design, the tooling, the process, or the material?

Without a framework, buyers either accept compromised parts or blame the supplier for a defect the CAD model made inevitable. If you are looking at an unidentified mark, the Defect Diagnostic Wizard narrows the likely mechanisms.

Name the defect, assign it to a domain (machine, process, temperature, mold, or material), then decide who pays. For design-side prevention, see the plastic part design for manufacturing guide.

Identifying the Common Defects

Most defects buyers see fall into a recognizable set. Each has a dedicated guide.

Defect	Visual Characteristics	Primary Technical Drivers
Sink marks	Depressions over thick areas, ribs, or bosses	Wall thickness, inadequate packing pressure, poor cooling
Warpage & shrinkage	The part bows, twists, or misses dimensional tolerances	Uneven cooling, asymmetric geometry, material shrinkage rates
Flash	Thin excess plastic escaping along the parting line	Poor tool fit, insufficient clamp tonnage, excessive injection pressure
Weld (knit) lines	A visible line where two flow fronts converge	Gate location, flow distance, low melt temperature
Short shot	Incomplete cavity fill; missing geometry	Insufficient fill pressure, inadequate venting, restricted flow paths
Splay / silver streaks	Silvery streaks radiating outward from the gate	Moisture in the resin, material degradation, excessive shear
Burn marks	Scorched discoloration, typically at the end of fill	Trapped air (dieseling), poor venting, excessive injection speed

The Five Domains of Root Causes

Experienced molders troubleshoot through five domains. When a supplier says they “cannot process out” a defect, ask which domain is the hard limit.

1. Machine: Is the press correctly sized and maintained? An undersized machine, exceeding barrel capacity, or inconsistent hydraulic response shows up as part defects. 2. Process: Injection velocity, hold pressure, cooling timers, and back pressure. A capable molder can resolve a large share of defects through disciplined process work alone. 3. Temperature: Melt temperature (barrel and nozzle) and mold temperature. Too hot, too cold, or steep gradients produce characteristic failures such as degradation or warpage. 4. Mold (Tooling): Gate sizing and location, venting layout, cooling channel design, and steel fit. Many defects are cut into the steel and cannot be processed away without a tooling revision. 5. Material: Resin that is inadequately dried, contaminated, highly degraded, or overloaded with regrind often looks like a process failure.

The Buyer’s Three Levers

You work upstream of the press. You control three levers:

1. Optimize the design before tooling. A large share of sink marks, warpage, weld lines, and short shots trace to geometry: uneven walls, heavy sections, and awkward flow paths. A design review before cutting steel fixes these as CAD revisions. See wall thickness and ribs, gate design, and parting line layouts.

2. Select process-disciplined suppliers. A molder with documented, data-driven methods (often called scientific or decoupled molding) will rarely ship inconsistent parts. Use the supplier capability checklist during sourcing. Review the tooling and production guides for the qualification stages where defects must be surfaced and closed.

3. Define explicit cosmetic and dimensional requirements. Specify cosmetic surfaces (Class A vs. non-appearance) and critical tolerances in the RFQ. A sink mark is a critical defect on an A-surface and irrelevant on an internal boss. Set the standard early so both sides inspect to the same criteria.

Timing Determines the Severity of the Signal

The right time to find a defect is first article inspection (T1), while changes are still cheaper than full production scrap and launch delays. On hardened production steel (for example H13), T1 usually happens after the tool is already heat-treated, not “before hardening.” Soft aluminum or unhardened bridge tools are the exception. Finding and fixing defects at T1 means qualification is working. [need primary source: shop T1 procedure / tool steel heat-treat sequence]

A defect that appears intermittently in full production (present on some shots, absent on others) is a serious warning. It usually means a process out of control or a tool at the edge of capability. Address intermittent production defects immediately before they become high-volume scrap.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, diagnose production problems remotely, or certify suppliers. Confirm defect causes and corrective actions with your supplier against your specific part, tool, and process.