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 primary question is not about naming the defect. The critical question is determining ownership: Is this a failure of the part design, the tooling, the process, or the material?

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

This guide outlines the common injection molding defects, categorizes their root causes, and details the specific levers a buyer controls. Understanding these mechanics elevates your supplier discussions from “fix this part” to technical inquiries about wall thickness, gate location, or processing parameters. For the design-side preventive measures, consult the plastic part design for manufacturing guide.

Identifying the Common Defects

Most defects buyers encounter fall into a recognizable set. Each is covered in depth in its 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 systematically through five distinct domains. This model is highly useful for buyers: when a supplier claims they “cannot process out” a defect, you must determine which of these five domains is acting as the hard limit.

1. Machine: Is the press correctly sized and maintained? Utilizing an undersized machine, exceeding barrel capacity, or relying on inconsistent hydraulic responses manifests directly as part defects. 2. Process: Injection velocity, hold pressure, cooling timers, and back pressure. A highly capable molder can resolve a significant percentage of defects solely through disciplined process optimization. 3. Temperature: This includes both the melt temperature (barrel and nozzle) and the mold temperature. Running too hot, too cold, or with extreme gradients generates characteristic failures like degradation or warpage. 4. Mold (Tooling): Gate sizing and location, venting layout, cooling channel design, and steel fit. Many defects are literally 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 masquerades as a process failure.

The Buyer’s Three Levers

As a buyer, you operate entirely upstream of the molding press. You control three specific levers to eliminate defects:

1. Optimize the design before tooling. A massive share of sink marks, warpage, weld lines, and short shots trace directly to geometry—specifically uneven walls, heavy sections, and awkward flow paths. Conducting a rigorous design review before cutting steel resolves these issues when they are merely CAD revisions. See wall thickness and ribs, gate design, and parting line layouts.

2. Select process-disciplined suppliers. A molder employing documented, data-driven methodologies—often referred to as scientific or decoupled molding—will rarely ship inconsistent parts. Use the supplier capability checklist during sourcing. Review the tooling and production guides to understand the qualification stages where defects must be surfaced and closed.

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

Timing Determines the Severity of the Signal

The ideal time to discover a defect is during the first article inspection (T1), before the tool is hardened for production. Finding and resolving defects at T1 means the qualification process is functioning correctly.

Conversely, a defect that appears intermittently during full production—present on some shots, absent on others—is a severe warning sign. It almost always indicates a process operating out of control or a tool functioning at the very edge of its capability. Address intermittent production defects immediately before they escalate into 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.