Cartridge Filling Defects 101: A Failure-Mode Library (and the SOP Fix for Each)

Why “cartridge filling defects troubleshooting” is really a process-control problem

Most cartridge filling defects aren’t “mystery failures”—they’re repeatable outcomes of uncontrolled variables. When output is low (say 50 units/hour), a skilled operator can compensate with intuition: a slower foot pedal, a quick wipe, a slightly longer dwell, a warmer reservoir.

When you scale to 500+ units/hour, intuition breaks. Small drifts in oil temperature, viscosity, needle gauge, dispense speed, and tare discipline turn into real scrap, rework, and compliance headaches.

This post is a practical failure-mode library you can hand to operators and QA. For each defect, you’ll get:

• What it looks like (symptom)
• Likely causes mapped to controllable variables (viscosity, temperature, needle gauge, dispense speed, technique, reservoir management)
• SOP-level controls you can implement immediately
• Scaling notes—what changes when you go from “small batch” to production

Recommended gear for repeatable results (product plug): Thompson Duke MCF1 semi-automatic filling machine

• Product page: https://www.urthandfyre.com/equipment-listings/thompson-duke-mcf1

The MCF1 is designed specifically for high-viscosity oils with a heated reservoir and heated oil delivery system up to 93°C (200°F) and is commonly deployed in operations that target 3,000–5,000 units/day under stable process conditions (manufacturer literature). It’s a good platform for building “locked” filling recipes because it gives you the core levers: temperature, flow, and repeatable dispense hardware.

External references used in this guide:

• RheoSense/VROC application note showing strong temperature dependence of oil viscosity (viscosity drops substantially from ~40–70°C): https://promo.paralab.pt/wp-content/uploads/2019/05/Nota-Aplica%C3%A7%C3%A3o-VROC-initium-APP27-5-19-Cannabis-Final.pdf
• Needle gauge dimensions and selection references (useful when dialing flow vs control):
• Hamilton needle gauge chart: https://www.hamiltoncompany.com/knowledge-base/article/needle-gauge-chart
• Agilent syringe/needle selection guide (general guidance for viscous samples): https://www.agilent.com/cs/library/selectionguide/public/5990-3563EN.pdf
• Thompson Duke MCF1 spec sheet: https://thompsondukeindustrial.com/wp-content/uploads/2019/12/TDI-MCF1-cspb-winter2020-final.pdf

Scaling rule #1: Define a viscosity window (not just a temperature setpoint)

Temperature is a proxy. Viscosity is the variable that actually controls fill behavior—stringing, bubbles, underfills, overfills, and leaks.

A key takeaway from the RheoSense/VROC temperature sweep data is that oil viscosity can drop multiple-fold across modest temperature changes (e.g., moving from the low-40s °C into the 60–70°C band). That means a “small” 2–5°C drift at the nozzle can produce a meaningful change in dispense dynamics.

SOP control concept:

• Define Product A viscosity window in production terms: a temperature band + a stabilization time + an observation-based acceptance check.
• Treat “reservoir temp” and “nozzle temp” as different things.

Simple measurement habits that scale

Add these to your batch record:

• Nozzle-tip temperature check (IR thermometer can work if used consistently; a probe is better). Record at startup and every set interval.
• Tare verification: confirm scale returns to zero with empty fixture or container.
• First-article inspection: pull the first 5–10 units, verify fill and cosmetics.
• In-process checks every X units: for example every 25–50 units per operator, and tighter when learning a new SKU.

Defect Library #1: Underfill / Overfill

What it looks like

• Underfill: headspace is visibly large or the mass/volume is below target.
• Overfill: oil sits too high, contacts seals, or pushes into the mouthpiece region during assembly.

Likely causes (mapped to controls)

• Viscosity drift (oil too cold → slow flow, incomplete dispense; oil too hot → over-delivery/afterflow)
• Dispense speed / pedal technique inconsistencies
• Needle gauge too small for the viscosity (high backpressure causes incomplete fills)
• Reservoir management issues: low reservoir level, inconsistent priming, or air ingestion
• Scale / verification errors: poor taring, unstable bench, drafts, or sticky fixtures

SOP-level fixes

1. Lock your fill verification method (mass beats “looks full”).

• Create a per-SKU spec: target fill mass and acceptable range.
• Calibrate/verify the scale on a schedule and before runs.

1. Add a startup “prime + purge + first-article” sequence.

• Prime lines until flow is steady.
• Purge the first dispenses into waste until the stream is consistent.
• Run first-article checks and document acceptance.

1. Define an in-process check cadence.

• Example: every 25 units at low throughput; every 50–100 units once stable.
• If a check fails: quarantine last check interval and correct.

1. Control afterflow (the hidden overfill).

• Add a consistent dwell time at the end of dispense before lifting the needle.
• Standardize “needle out” speed and angle.

Scaling note

At 50/hr, an operator can “feather” the pedal. At 500/hr, that becomes your primary source of variance.

Scaling solution: define a narrow process band (temp/viscosity, pressure/flow, needle spec) and train operators to run the band—not improvise.

Defect Library #2: Stringing (tailing, hairlines, oil threads)

What it looks like

• Thin strings of oil form when the needle retracts.
• Oil trails onto threads, outer surfaces, or into the mouthpiece zone.

Likely causes

• Oil too hot / too low viscosity → increased dripping and tailing
• Needle gauge too large for the target control (excess flow at the end of dispense)
• Retract technique: lifting too quickly or without a dwell
• Nozzle contamination: oil buildup changes surface tension behavior

SOP-level fixes

1. Add an end-of-shot dwell and “break” motion.

• Dispense → pause 0.5–1.5 seconds (dial to your oil) → retract slowly.
• Consider a small lateral “wipe-off” motion only if it doesn’t touch cartridge walls.

1. Tune needle gauge to match viscosity and target fill speed.

• Use gauge charts to understand ID/OD constraints and how that impacts backpressure and flow.
• References: Hamilton gauge chart https://www.hamiltoncompany.com/knowledge-base/article/needle-gauge-chart and Agilent guide https://www.agilent.com/cs/library/selectionguide/public/5990-3563EN.pdf

1. Define a nozzle wipe frequency.

• Example: every tray change or every 15–30 minutes.
• Use lint-free wipes and a defined, compatible solvent.

Scaling note

Stringing often “appears” during scaling because higher tempo reduces dwell time and cleaning discipline.

SOP improvement: incorporate dwell and wipe steps into takt time so operators aren’t punished for doing them.

Defect Library #3: Bubbles / Voids

What it looks like

• Visible air bubbles in the oil column.
• Voids that appear after settling, leading to net underfill.

Likely causes

• Air entrainment during stirring, transfer, or reservoir refills
• Cold oil increasing viscosity, trapping air during dispense
• Dispense too fast (turbulence)
• Needle positioning: dispensing above the oil surface instead of down the side/bottom
• Reservoir level too low leading to air ingestion

SOP-level fixes

1. Reservoir management rules.

• Define a minimum reservoir level.
• Refill with a method that minimizes splashing/whipping.

1. Degassing and rest steps (if your formulation allows).

• Add a controlled rest time after mixing or terpene addition.
• Avoid high-shear mixing right before filling.

1. Needle placement standard.

• Insert needle to a consistent depth.
• Dispense along the side to reduce turbulence and bubble capture.

1. Verification: settling check.

• Add a hold step (e.g., 5–15 minutes, SKU-dependent) and re-check a sample for void formation.

Scaling note

At higher throughput, “just top off the reservoir quickly” becomes a common bubble generator.

SOP fix: a refill procedure with acceptance criteria (no visible foam, no air pockets, temperature within window).

Defect Library #4: Leaking at the base (post-fill leak, seepage)

What it looks like

• Oil appears at the base after fill, during capping, or after a short soak.
• Sticky trays or spotting under cartridges.

Likely causes

• Overfill increases internal pressure and forces oil into seals
• Oil too hot at fill → lower viscosity and higher propensity to wick through imperfect seals
• Needle contact damaging internal components or dislodging seals
• Hardware variability: inconsistent cartridge components
• Oil on sealing surfaces from stringing or sloppy handling

SOP-level fixes

1. Set a maximum fill and enforce it with mass checks.

• Overfill is a leading indicator for leak returns.

1. Add a post-fill “soak + inspect” gate.

• Hold upright for a defined time.
• Inspect a sample for base seepage before releasing the lot.

1. Standardize needle depth and “no contact” rule.

• Train operators not to bottom out the needle.

1. Cosmetic controls are leak controls.

• Any oil on threads or outside surfaces is a risk factor for sealing integrity and customer perception.

Scaling note

When you scale, leaks shift from “occasional annoyance” to “mass rework” because leak checks get skipped.

SOP fix: bake leak checks into your batch record and line clearance. No record, no release.

Defect Library #5: Clogging (needle clogs, slow shots, intermittent flow)

What it looks like

• Fill rate slows over time.
• Shots become inconsistent or stop.
• Pressure increases but flow doesn’t.

Likely causes

• Oil too cold (viscosity too high)
• Needle gauge too small for the oil and target rate
• Particulates (crystallization, debris) or residue from prior SKU
• Poor cleaning/changeover leaving incompatible residues
• Temperature gradients between reservoir, line, and nozzle

SOP-level fixes

1. Temperature soak discipline.

• Don’t start the run until reservoir, lines, and nozzle have stabilized.

1. Filter/strain upstream when appropriate.

• If allowed by your product spec, implement a controlled filtration step to remove particulates.

1. Changeover/cleaning validation.

• Define a cleaning method, solvent, contact time, and verification method.
• Make sure cleaning agents are compatible with wetted materials.

1. Spare consumables and rapid swap.

• At production scale, a “clean it in place for 20 minutes” approach destroys OEE.
• Pre-kit needles/tubing assemblies for quick change.

Solvent compatibility note

Compatibility depends on the exact polymers in your wetted path, but as a general rule, PTFE is broadly compatible with many solvents, while other plastics/elastomers can swell or degrade.

For membrane/solvent compatibility references (useful for filtration and some consumables): Restek resource hub charts are a common starting point: https://discover.restek.com/articles/gnss2122/syringe-filters-solvent-compatibility-chart

Always validate your specific materials and your safety program before adopting any solvent.

Defect Library #6: Cosmetic contamination (smears, fingerprints, dust, fibers)

What it looks like

• Oil smears on the outside.
• Fibers trapped in oil.
• Fingerprints, haze, particulate specks.

Likely causes

• Nozzle stringing and careless wipe practices
• Improper PPE/glove discipline
• Dirty trays/fixtures
• Open containers during long runs
• High-traffic environment generating lint/dust

SOP-level fixes

1. Line clearance and 5S before filling.

• Clean bench, trays, tools.
• Define what “clean” means (visual standard).

1. Glove policy and touch points.

• Change gloves on a set cadence and when contaminated.
• Define “no-touch zones” on cartridges.

1. Cover and protect WIP.

• Use covers for trays during holds.

1. Define acceptable vs rejectable cosmetics.

• QA needs a defect standard to avoid subjective decisions.

Scaling note

Cosmetic defects climb when labor is added quickly without standardized handling.

SOP fix: photo-based defect standards + training + periodic audits.

The scaling playbook: what to formalize before you add headcount

If you do only one thing before ramping output, formalize these five controls:

1. Defined viscosity/temperature operating window with stabilization time and nozzle-temp checks.
2. Needle spec per SKU (gauge, length, tip style) and a documented rationale.
3. Verification plan: tare verification + first article + in-process checks every X units.
4. Changeover and cleaning validation SOP (with documented pass/fail).
5. Defect escalation rules: when to stop the line, quarantine, and re-qualify.

Where the Thompson Duke MCF1 fits (and why it’s still a workhorse)

The Thompson Duke MCF1 is built around production realities operators care about:

• Heated reservoir and delivery path up to 93°C (200°F) for viscosity control
• Designed for high-viscosity oils and repeatable volumetric delivery
• Quick changeover claims and replaceable food-safe components (manufacturer literature)

If you’re building a defect-reduction program, the MCF1 is a strong foundation because it supports the two things that reduce defects fastest: stable temperature control and repeatable dispense mechanics.

Explore the listing: https://www.urthandfyre.com/equipment-listings/thompson-duke-mcf1

Urth & Fyre angle: defect reduction is equipment + SOPs + verification

Urth & Fyre helps teams reduce defects by connecting three pieces that are too often handled separately:

• Proven filling equipment (new-to-you and production-ready)
• Operational consulting to build changeover, cleaning validation, and training SOPs
• Verification ecosystem: connecting operators to calibration services and helping implement scale/checkweigher verification habits (especially important where NTEP packaging accuracy and documentation matter)

If your team is scaling and your reject rate is creeping up, the fix is rarely “work faster.” It’s almost always: tighten process windows, standardize technique, and verify more often—using simple, repeatable checks.

To explore equipment listings and consult on your fill/train SOPs, visit https://www.urthandfyre.com.