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 catch and classify a defect is first article sampling (T1/T2 mold trials), while corrective actions are still manageable before full production launch. Whether the cavity steel is already through-hardened at T1 depends on the mold steel grade and shop practice: pre-hardened steels (such as P20 or NAK80) and aluminum tools are machined in their final hardness state, whereas through-hardened tool steels (such as H13 or S136) are typically rough-machined, heat-treated, and finish-ground or EDM’d before T1, though some complex tools undergo a preliminary soft tryout (T0) prior to final hardening or texturing. Confirm your toolmaker’s exact heat-treatment and texturing sequence before authorizing steel revisions at T1.

A defect that appears intermittently in full production (present on some shots, absent on others) is a serious process-control warning. It usually points to unstable machine parameters, inconsistent material preparation (such as moisture or regrind variation), or a worn tool operating without a locked process window. Use the Defect Evidence Generator and Supplier Defect Review Sheet to require root-cause data rather than accepting temporary machine tweaks.

Buyer FAQs

How do I tell whether a molding defect requires a tool change or a process adjustment?

Ask the supplier to demonstrate how the defect responds to a controlled process window study (varying pack pressure, melt temperature, or injection speed) and compare the defect location against the part geometry. Defects tied directly to fixed geometry—such as sink opposite a rib that exceeds 60% of nominal wall, a weld line formed behind a through-hole, or end-of-fill burns where no vent channel exists—rarely disappear permanently through machine settings alone and typically require steel or CAD modification.

Who pays for tooling changes to fix defects at T1?

Responsibility follows root cause and DFM history. If the defect stems from mold construction errors (such as undersized vents, poor gate land geometry, or cooling imbalance) or a geometry issue the supplier failed to flag during a contracted DFM review, the supplier typically covers the rework. If the buyer insisted on un-cored thick sections, zero draft, or restricted gate placement against the molder’s written DFM warnings, the engineering change order (ECO) is normally billed to the buyer.

When is a visual molding defect acceptable to ship?

Only when it falls within a written cosmetic standard agreed upon by both parties. Industry practice separates parts into Class A (primary visible show faces), Class B (secondary faces seen occasionally), and Class C (hidden internal surfaces); see cosmetic surface classes and defect acceptance criteria. Signed boundary samples (limit samples) at T1/T2 prevent subjective disputes during production inspection.

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.