Rapid Oil Filling Without Rework: Build a Viscosity Window That Operators Can Actually Run

“Feel” doesn’t scale—windows do

When cartridge filling is low-volume, a skilled operator can compensate for a lot. They’ll intuit when oil is “running slow,” tweak heat, swap a needle, or change the way they pull away from the fill port. That works—until you scale.

At higher throughput, fill quality controlled by feel turns into:

• Underfills and weight drift as viscosity changes during the shift
• Bubbles / air entrainment from poor reservoir handling or pull-away technique
• Stringing and mess that slows output and increases rejects
• Inconsistent dwell times leading to top-off behavior (rework)
• SKU changeover chaos (cross-contamination risk + lost time)

The fix isn’t “better operators.” It’s giving operators an operator-friendly viscosity window that tells them exactly what “good” looks like: an acceptable temperature band, dwell time, needle gauge, and reservoir management rules that produce consistent fills without rework.

Recommended gear (Product Plug): Thompson Duke MCF1 semi-automatic filler — https://www.urthandfyre.com/equipment-listings/thompson-duke-mcf1

This article will show you how to build a cartridge filling viscosity window that operators can run on day shift, night shift, and with mixed-SKU schedules.

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Why viscosity is the hidden “master variable” in cartridge filling

Viscosity is what your process is fighting—every other control is basically a way to manage it.

• Temperature changes viscosity dramatically in viscous oils.
• Needle gauge / inner diameter determines restriction and shear.
• Dwell time (how long you keep the needle seated after dispense) controls backflow, drip, and air pull.
• Reservoir head pressure (how full the reservoir is, and how the oil is handled) changes flow stability.

If viscosity isn’t bounded, your operators will continuously “trim” the process. Trimming creates variability, and variability creates rework.

Your goal: define a repeatable range where your oil flows reliably and your equipment is stable.

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The workcell mindset: define a viscosity window operators can execute

A practical viscosity window is not a lab report. It’s a short set of rules that answer:

1. What temperature range is acceptable at the reservoir?
2. What needle gauge is required for that SKU?
3. What dispense speed + dwell time prevents bubbles and drips?
4. How do we manage the reservoir during long runs?
5. How do we measure drift quickly without lab gear?

The Thompson Duke MCF1 is built for repeatable semi-automatic filling: it’s foot-pedal operated, uses a luer-lock style syringe/needle connection, and per the manufacturer cut sheet, it includes a 140 mL glass reservoir and digital temperature control up to 200°F (93°C), supporting high-viscosity oils with adjustable flow rate and fast product changeover (see PDF: https://thompsondukeindustrial.com/wp-content/uploads/2019/12/TDI-MCF1-cspb-winter2020-final.pdf).

The machine is only half the story. The other half is your “window.”

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Step 1: Pick 2–3 operator-setpoints, not 12 knobs

If you give operators too many degrees of freedom, they’ll create their own process.

For most semi-auto cartridge filling workcells, the controllable setpoints that matter are:

• Reservoir temperature setpoint (with a defined acceptable band)
• Needle gauge (and tip style/length if applicable)
• Dispense setting (volume + flow rate)
• Dwell time (time the needle stays seated after dispense)

Everything else becomes a rule or checklist item.

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Step 2: Build the “temperature band” (not a single temperature)

Operators shouldn’t chase an exact number; they should stay in a band.

How to set the band

Start by choosing:

• A target reservoir temperature that gives smooth, non-bubbly flow
• A lower limit where fill time starts to increase or underfills show up
• An upper limit where dripping/stringing increases, or oil becomes too “thin” and messy

Rule of thumb for window design: Your band should be wide enough that operators can stay inside it during normal shift variation, but tight enough that the failure modes (bubbles, drips, stringing) sit clearly outside the band.

Practical operator language

Instead of: “Run at 68°C.”

Use: “Run at 65–72°C at the reservoir. If you hit 64°C, pause and reheat. If you hit 73°C, reduce heat and verify first-off cartridges.”

Don’t ignore warm-up and soak time

A common failure mode is measuring temperature at the controller but filling before the entire fluid path is at equilibrium.

Add a warm-up rule:

• “After reaching setpoint, wait X minutes soak before first production fill.”

And a restart rule:

• “If idle > Y minutes, purge 1–2 test shots before resuming.”

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Step 3: Needle gauge selection—make it a controlled variable

Needle choice is one of the fastest ways to “change viscosity” without touching temperature.

From the Urth & Fyre listing for the MCF1, included consumables often include 14 ga, 16 ga, and 18 ga needles. That’s a good practical range.

What operators need to know

• Larger bore (smaller gauge number) reduces restriction, lowers fill pressure, and can reduce bubble formation in very viscous oils.
• Smaller bore (larger gauge number) can improve control for lower-viscosity oils but is more prone to slow fills, pressure spikes, and stringing if the oil is borderline thick.

Turn needle choice into a SKU rule

For multi-SKU operations, write it as:

• “SKU A: 14 ga only”
• “SKU B: 16 ga”
• “SKU C (low viscosity): 18 ga + shorter dwell”

If you allow operators to freestyle needle selection mid-run, your window collapses.

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Step 4: Define dwell time like a process engineer (but write it like a trainer)

Dwell time is the pause after dispensing while the needle remains seated in the cartridge.

This small pause can:

• Reduce stringing by allowing filament to retract
• Reduce drip from residual pressure in the needle
• Improve fill level consistency by letting the meniscus settle

Simple dwell rules

• High viscosity oil: dwell 0.5–1.5 seconds
• Lower viscosity oil: dwell 0.2–0.8 seconds

Then validate with first-article inspection: clarity (no bubbles), consistent fill level, no oil on threads, and no tails.

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Step 5: Reservoir management rules that prevent bubbles and drift

A heated reservoir is not just a tank—it’s a control surface.

Common ways reservoirs create defects

• Introducing air during refill (pouring too fast, splashing, or “stirring”)
• Reheating repeatedly (thermal cycling changes flow behavior and increases odor/volatiles loss for some formulations)
• Running too low (head pressure changes, flow becomes less stable)

Operator-friendly reservoir SOP rules

1. Refill threshold: “Refill when reservoir reaches 25–30%—do not run to empty.”
2. Refill method: “Pour along the wall; do not splash; avoid introducing foam.”
3. Hold-time discipline: “Do not keep oil at temp longer than X hours without QA approval; document start/stop times.”
4. De-bubble step: “After refill, wait 2–5 minutes for bubbles to rise; purge 1–2 shots to waste.”

If you’re seeing persistent microbubbles, the fix is often upstream: pre-warm and degas the oil before it ever reaches the filler.

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Step 6: Simple viscosity measurement methods operators can run

You don’t need a rheometer to control a workcell. You need a repeatable proxy.

Two practical methods:

Method A: Cup viscometer (fast, low-cost)

Flow cups (e.g., Zahn-type) measure efflux time—how long it takes fluid to drain through an orifice. They’re widely used in coatings because they’re fast and repeatable.

Key practice notes:

• Select a cup where your efflux time lands in a stable range (commonly 20–200 seconds for many cups; follow the cup’s guidance).
• Run the test at a controlled temperature.

Reference for general flow-cup use and timing guidance: https://labomat.eu/gb/faq/594-how-to-use-a-viscosity-cup.html

Reference for Zahn cup ranges and operating instructions (example document): https://d163axztg8am2h.cloudfront.net/static/doc/62/c6/e09d9db34bf07ef4f44d610fbf2f.pdf

How to operationalize it:

• Define a “GO” time range at your target temperature (e.g., “At 70°C, efflux time must be 35–50 s using Cup #X”).
• If outside range, adjust temperature or quarantine the batch for formulation review.

Method B: Timed dispense test (best for semi-auto fillers)

This is often the most intuitive proxy because it measures the whole system (oil + heat + needle + pneumatic/flow settings).

Procedure:

1. Set reservoir to target temp and allow soak.
2. Using the production needle, dispense a fixed volume (or a fixed pedal actuation if volumetric is set) into a tared container.
3. Record time-to-deliver and observe tail/string.
4. Repeat 3 times; average.

Define acceptance:

• “At setpoint temp and Needle X, delivery time must be Y ± Z seconds for the standard test shot.”

This turns “feel” into a number operators can defend.

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Troubleshooting: stringing, dripping, air entrainment (operator chart)

Use this as a simple line-side guide. Train operators to change only one variable at a time.

Stringing (tails) at pull-away

What you see: a filament from needle to cartridge; mess on threads.

Likely causes:

• Oil too hot / too low viscosity
• Dwell time too short
• Needle too small (higher restriction can increase elastic tailing)

Fix steps (in order):

1. Increase dwell time by 0.3–0.5 s
2. Reduce reservoir temp by 2–5°C (stay inside band)
3. Move to a larger bore needle (e.g., 16 → 14 ga) if allowed for that SKU
4. Check for residue on needle tip; clean/replace

Dripping between fills

What you see: droplets forming at needle tip; inconsistent start volume.

Likely causes:

• Oil too hot
• Residual pressure in the system
• Worn seals or contamination at luer connection

Fix steps:

1. Reduce temp by 2–5°C
2. Add/extend dwell time; pull away slower and straighter
3. Inspect/retighten luer connections; replace consumables if needed
4. Purge a small shot to re-stabilize

Air entrainment / bubbles in cartridge

What you see: visible bubbles, foam, collapsing fill level after cap.

Likely causes:

• Air introduced during reservoir refill
• Filling too fast for the geometry (turbulence)
• Needle tip not staying below the liquid level during dispense
• Oil too cool (pulsing/poor flow leading to voids)

Fix steps:

1. Slow dispense rate (if adjustable) and keep needle seated deeper
2. Increase temperature within window to stabilize flow
3. Implement reservoir refill rules (pour method + settling time)
4. If persistent: pre-warm and degas batch upstream; verify no leaks at fittings

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Multi-SKU operations: cleaning and changeover discipline (GMP-adjacent)

As soon as you run multiple SKUs—different formulations, flavors, or additives—your risk becomes:

• Cross-contamination
• Label/lot mix-ups
• Residue-driven defects (stringing, clogging, drifting fill volumes)

Even for small teams, borrow proven GMP concepts: line clearance, documented changeovers, and cleaning verification.

A useful general reference on line clearance / changeover SOP structure (pharma example, adaptable to regulated production): https://www.pharmaguideline.com/2011/05/sop-for-procedure-for-changeovers.html

A practical changeover checklist (adapt to your facility)

• Line clearance: remove all previous SKU components, packaging, labels, WIP.
• Parts segregation: dedicate or color-code needles, reservoirs, and tubing assemblies where feasible.
• Disassemble contact parts: syringe, needle, reservoir interfaces.
• Clean: use approved solvents and tools; avoid lint; follow a defined contact time.
• Rinse/inspect: visual inspection under adequate lighting.
• Dry: ensure no solvent remains trapped.
• Reassemble + torque/fit check: luer connections seated.
• First-off verification: 5–10 cartridges checked for bubbles, fill level, and weight.
• Documentation: record lot, operator, time, temp setpoint, needle gauge, and first-off results.

This discipline is what turns a semi-auto filler from “a tool” into a reliable production asset.

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Workcell SOP framework (what to standardize around the MCF1)

If you want consistent output, write SOPs the way operators work—by station.

1) Start-up SOP (10 minutes)

• Verify air supply quality and pressure (instrument-air quality where required)
• Preheat reservoir to setpoint; start soak timer
• Confirm needle gauge per SKU traveler
• Run 2–3 purge shots to waste
• First-article inspection + record results

2) In-process control SOP (every 30–60 minutes)

• Check reservoir temperature is within band
• Run timed-dispense test (or quick cup test) and record
• Check cartridge appearance: bubbles, stringing, drip
• Confirm reservoir level above refill threshold

3) Shut-down SOP

• Controlled cool-down (don’t “bake” residual oil overnight)
• Remove and clean product-contact parts
• Stage spare consumables for next shift

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Spare parts planning: the simplest way to prevent downtime

Semi-automatic filling lines often lose the most time to small things:

• Needles clogging or bending
• Luer fittings loosening
• Syringe seals wearing
• Heat lamp bulbs (if used) burning out

Build a small spares kit per workcell:

• Needles (14/16/18 ga) + backups
• Extra tubing assemblies
• Spare seals/gaskets for product-contact points
• Cleaning tools (brushes, caps)
• A labeled “changeover tote” per SKU family

The Urth & Fyre listing for the MCF1 notes a set of consumables and spare parts often included; whether yours comes with them or not, the point is the same: stock the wear items so operators don’t improvise.

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Where Urth & Fyre fits: turning semi-auto into scalable production

Equipment is necessary—but workcell engineering is what makes it profitable.

Urth & Fyre can help you:

• Build workcell-style SOPs (start-up, in-process, changeover, shutdown)
• Define a viscosity window per SKU: temperature band, needle gauge, dwell time, reservoir rules
• Implement spare parts planning and shift-handoff routines
• Optimize line layout (ergonomics, WIP control, reject handling) so 3,000–5,000 units/day is sustainable
• Reduce rework with first-off/interval checks that operators can actually run

If you’re evaluating hardware, start with the listing here: https://www.urthandfyre.com/equipment-listings/thompson-duke-mcf1

And if you want help making it run like a system—not a bench tool—explore equipment listings and consulting at https://www.urthandfyre.com.