Injection Molding Defects & Solutions: Sink, Weld, Flash

Every molder eventually sees the same five defects walk through the door: sink marks, weld lines, flash, splay, and flow marks. They are not random. Each one is a readable signal from somewhere in the system — mold design, process window, or material condition — and each one is fixable when you diagnose the source instead of chasing the symptom with a temperature knob.

This guide is written from the shop floor up. It covers what causes the common injection molding defects, how to correct them, and how to make sure a corrected defect stays corrected across thousands of parts and dozens of batches. Maidder Technology Co., Ltd. runs custom plastic injection molding from Hengli Town, Dongguan City, Guangdong, China, with an engineering team that has spent years on exactly this problem set.

Why Defects Appear: Three Root Causes, Not One

Before touching any machine setting, separate the cause into three buckets.

Mold design: wall thickness transitions, gate location and size, cooling layout, venting, ejection balance, steel strength in high-pressure areas.
Process: fill speed, packing pressure and time, melt and mold temperature, screw recovery, clamp tonnage, cooling time.
Material: moisture content, melt flow rate, regrind ratio, colorant and additive loading, contamination, lot-to-lot variation.

A defect that responds to a process change is not necessarily a process defect. Sink marks that disappear when you increase hold pressure but reappear whenever the machine drifts are a design symptom: an overly thick section that no amount of packing will fully compensate. Chasing that with pressure alone produces a process window so narrow it will not survive a normal production week. The correct answer is to fix the root cause and then lock in a window wide enough to run 24-hour shifts without babysitting.

Maidder's design department runs free Moldflow analysis for complex, high-precision products and issues a DFM report before steel is cut, which is where the majority of these defects are cheapest to eliminate.

Sink Marks

What causes sink marks

A sink mark is a localized depression on a cosmetic surface, caused by volumetric shrinkage in a region that is still cooling while the rest of the part has already frozen.

Non-uniform wall thickness. Thick ribs, bosses, and deep sections cool last and pull the adjacent surface down.
Insufficient packing. Hold pressure or hold time too low to push material into the shrinking core.
Gate seal-off too early. A small gate freezes before the thick section finishes packing, so pressure never reaches it.
High melt temperature. More thermal shrinkage to compensate for.
Low mold temperature. Slow skin formation on the cosmetic side, so the surface yields to the shrinking core.
Semi-crystalline materials. PP, PE, and PA shrink more than amorphous resins and show sink marks sooner.
Poor cooling on the thick side. Uneven cooling creates a differential shrink gradient.

How to solve sink marks

Design side:

Core out thick sections and keep nominal wall as uniform as possible. Rib thickness is commonly held to roughly 50–60% of the adjacent wall.
Reduce boss wall thickness relative to the mating wall and add radii at the base to spread the shrink.
Gate into or near the thickest section so packing pressure reaches the last region to freeze.
Enlarge the gate or move to a hot runner with valve-gate sequencing so seal-off happens after packing is complete.
Run Moldflow to predict sink locations before committing to steel.

Process side:

Increase hold pressure and extend hold time until the gate seals.
Lower melt temperature and raise mold temperature on the cosmetic side to even out the cooling gradient.
Increase cooling time on the thick section specifically, not universally.
Increase injection speed to fill before the flow front freezes.

Material side:

Switch to a lower-shrinkage grade or a filled compound if the geometry cannot be changed.
Control regrind percentage; high regrind content widens shrinkage variation.

Weld Lines and Knit Lines

What causes weld lines

A weld line forms wherever two flow fronts meet and fuse. It is a cosmetic issue on visible surfaces and a structural issue when it lands in a load path.

Flow splitting around cores, holes, and inserts.
Multiple gates causing fronts to meet in the middle of a part.
Low melt or mold temperature, so the fronts cool before they merge.
Low injection speed, increasing the time the front spends cooling.
Contamination — mold release, moisture, or incompatible colorant — weakening the bond.
Poor venting at the meeting point, trapping gas between the fronts.

How to solve weld lines

Relocate the weld. Move gates so the meeting point falls in a non-cosmetic, low-stress area. This is a mold design fix, and it is the most reliable one.
Add an overflow well at the weld location so the cold front is pushed out of the part and trimmed afterward.
Increase injection speed and melt temperature to keep both fronts hot when they meet.
Raise mold temperature locally with a dedicated cooling or heating circuit.
Vent at the weld line. Trapped gas is often what turns a cosmetic weld into a visible streak.
Use sequential valve gating so fronts are controlled rather than random.
For structural parts, review material choice. Some blends and filled grades produce inherently weak knit lines; a compatibilizer or a different grade can recover a significant percentage of base strength.

Flash

What causes flash

Flash is excess material escaping the mold cavity, usually along the parting line, at vent landings, or around ejector pins.

Insufficient clamp tonnage relative to projected area and cavity pressure.
Worn or damaged parting lines on a mold that has run for years.
Mold deflection in weak plates or unsupported cores under high pressure.
Excessive injection pressure or a velocity profile that spikes at the end of fill.
Melt temperature too high, lowering viscosity so material flows into gaps it should not enter.
Vent depth too deep for the material's viscosity.
Unbalanced multi-cavity molds, where one cavity overpacks while another starves.
Tie bar or platen alignment issues that let the mold breathe unevenly.

How to solve flash

Calculate required clamp tonnage from projected area and cavity pressure rather than defaulting to the machine maximum.
Reduce peak injection pressure and tune the velocity-to-pressure transfer point.
Inspect and refurbish parting line surfaces; flash that appears after a long production run almost always points to wear.
Match vent depth to the material. Low-viscosity grades need shallower vents than high-viscosity ones.
Balance the runner system so all cavities fill at the same time.
Verify platen parallelism and tie bar condition as part of scheduled maintenance.

Flash is also a safety and cost issue: it consumes material, adds a trimming operation, and hides more serious tooling problems. Maidder treats parting line condition as a scheduled maintenance item, and molds belong to the customer with free maintenance and one year of free storage.

Splay (Silver Streaks) and Gas Marks

What causes splay

Splay appears as silvery streaks radiating from the gate, often with a burn or discoloration near the end of fill.

Moisture in the resin. The single most common cause, especially with PA, PC, ABS, and PET.
Thermal degradation from excessive melt temperature or long residence time.
Excessive shear at a small gate or a restricted flow path.
Trapped gas from poor venting or an improperly placed gate.
Contamination — purging compound residue, mixed resins, or degraded regrind.
Excessive back pressure or screw speed generating frictional heat.

How to solve splay

Dry the material to the resin supplier's specification using a desiccant dryer, and verify with a moisture analyzer rather than trusting the timer.
Lower melt temperature and reduce screw speed and back pressure.
Enlarge the gate or reduce the flow restriction at the gate.
Improve venting at the last point to fill.
Purge thoroughly between material changes and segregate regrind by resin type.
Check the barrel for degraded material if splay persists across multiple lots.

Because material condition is central here, Maidder maintains a melt flow indexer and a colorimeter in-house, so resin lots and color consistency can be verified before a run starts rather than diagnosed afterward.

Flow Marks, Jetting, and Record Grooves

Flow marks are visible lines or waves on the surface that follow the direction of material flow.

Causes:

Low injection speed causing the flow front to hesitate and cool.
Cold mold walls, especially on long, thin sections.
Gate too small, producing jetting — a rope of material that snakes into the cavity instead of filling it smoothly.
Abrupt wall thickness transitions that stall the front.
Material with a viscosity that does not suit the wall thickness.

Solutions:

Increase injection speed and adjust the velocity profile so the front moves at a consistent rate.
Raise mold temperature.
Enlarge the gate or switch to a fan gate, tab gate, or valve gate, depending on part geometry.
Add gradual transitions and radii between thick and thin sections.
For multi-gate parts, use sequential valve gating to control where fronts meet.
Confirm the fill pattern with Moldflow before cutting steel.

Dimensional and Performance Defects

Not every defect is visible. The following show up in inspection or in the customer's assembly line.

Warpage. Caused by differential cooling, uneven shrinkage, poor packing, or fiber orientation in filled materials. Fixed with balanced cooling design, symmetric gate layout, adjusted packing, and fixture cooling where appropriate.
Short shots. Insufficient fill from low pressure, low temperature, poor venting, or inadequate gate size. Diagnose by checking whether the short location is at the end of fill or in a thin section.
Burn marks. Trapped gas compressed at the end of fill. Fixed with venting, reduced injection speed at the end of fill, and gate repositioning.
Dimensional drift. Often a packing or cooling time problem rather than a mold problem; verify with a documented process window before touching the tool.
Reduced impact strength at weld lines. A performance defect that no cosmetic inspection will catch. This is where design review and material selection matter most.
Compliance failures. For food-contact and children's products, tell us your application and target market; we will confirm suitable materials and documentation for your program. Certificates available upon request.

Preventing Recurrence: How QC Stops a Defect From Coming Back

Fixing a defect once is process adjustment. Keeping it fixed is a system.

First-article inspection. Full dimensional and cosmetic approval before production release, with results compared against the drawing and the PPAP approval process.
In-process patrol inspection with traceability. Parts checked at defined intervals during the run, not only at the start and end.
Process testing. Machine parameters recorded and monitored against the validated window, so drift is caught before it becomes scrap.
Per-product batch trace codes. Every batch can be traced back to machine, mold, material lot, and operator shift.
Measurement capability. CMM (coordinate measuring), vision measuring instruments, calipers, a colorimeter, and a melt flow indexer, with Minitab statistical software used to analyze process capability rather than eyeball trends.
Batch quality issues answered within one day. When a customer reports a problem, the response is fast and it comes with traceability data attached.

Quality management runs under ISO 9001 and ISO 14001:2015, with injection machines from Yizumi and Tairui in the 130–500T range operating on 24-hour shifts.

Production Stability: Beating Batch-to-Batch Variation

Most recurring defects are actually lot-to-lot variation.

Resin lot variation. Melt flow rate differs between lots. Verify with an MFI check on incoming material and adjust the process window if needed.
Regrind ratio inconsistency. Set a maximum regrind percentage and document it; uncontrolled regrind is a leading cause of shrinkage drift.
Drying consistency. Verify moisture, not drying time.
Mold temperature control. Check controller performance and cooling circuit flow; a partially blocked circuit will change dimensions across a shift.
Mold wear. Weld lines, flash, and dimensional drift often trace back to gate wear or parting line wear. Scheduled maintenance catches this before it reaches the customer.
Documented setup sheets. A locked process window keeps output consistent across operators and shifts.

Mold and Process Engineering at Maidder

Maidder Technology Co., Ltd. was founded in 2016 and is based in Hengli Town, Dongguan City, Guangdong, China. The company provides custom plastic injection molding ODM/OEM as a one-stop service: product development, 3D printing, mold and fixture manufacturing, injection molding, CNC machining, EDM, wire cutting, and assembly.

Mold capability includes multi-cavity molds, auto-unscrewing molds, 2K molds, overmolds, collapsible-core molds, and gas-assist molds, with molds up to 15 tons.

Company project experience includes an HP printer base ECN project in 2020 covering part F0V63-90005 — an engineering change involving add/remove steel, welding and remaking CAV/COR inserts, moving from Rev 4.9 to Rev 12.1 at a modification cost of USD 25,000 with a documented three-week lead time and customer approval recorded as "HP - Proceed this modf." Related capability areas include hard cooler ODM with secondary foam injection, large-format containers, medical device components, and automotive overmolding and 2K parts.

That experience translates directly into defect control on visible, structural, and regulated parts. The current hard cooler line — a 10QT hand-carry at 380x255x270 mm and a 16QT shoulder-strap at 380x356x270 mm, both company-reported: ice remains unmelted for 60 hours at 25C without frequent opening — depends on sink control across large flat walls, weld line placement away from visible surfaces, and consistent dimensional control at the lid interface. Both are custom molds, with capacity, color, and logo configurable.

Maidder's founder worked as a mold design engineer from 2006 to 2016 at Junyi, Foxconn-group Fugang, and Fuyu — pre-2016 career experience, before the company was founded. That background shapes the current approach: read the mold design first, then the process, then the material.

According to company information, a Cambodia production base provides OEM Cambodia service as an alternative manufacturing origin for buyers concerned about tariff exposure. Primary markets are North America, Europe, and Japan.

Commercial terms: no MOQ (negotiable), with a machine setup fee on orders under 3000 pcs; molds belong to the customer with free maintenance and one-year free storage; batch quality issues answered within one day.

Get a Defect Assessment Before You Cut Steel

If you are seeing sink marks, weld lines, flash, splay, or flow marks in a current project, or if you are about to commit to a mold and want the fill pattern verified first, send the part drawing or 3D file to the Engineering Team. Maidder provides DFM reports and free Moldflow analysis for complex, high-precision products, and quotes include the process window plan rather than just a piece price.

Contact the Sales Team Email: [email protected] Phone: +86 137 13040698 Location: Hengli Town, Dongguan City, Guangdong, China

Tell us the defect, the material, and the annual volume. You will get an engineering answer, not a sales pitch.

Injection Molding Defects & Solutions: Sink, Weld Lines, Flash, Splay