Troubleshooting

Something feels wrong. Start here.

Almost everything that goes wrong with one of these pens has a short list of causes, and most of them are the tip or the temperature rather than the pen. Find your symptom in the table, then read the section underneath it if you want the reasoning.

1. Quick fixes

What you're seeing Usually because What to do
Very stiff to press Fine or clogged tip, or cold solution Fresh tip, let it warm up. See section 2.
Stiff only on the first push of a new cartridge The stopper is breaking loose from the glass Normal. It eases immediately after.
Drips from the tip after dosing Tip withdrawn before pressure equalised Keep it in place a few seconds. See section 3.
You press and nothing happens No cartridge seated, or never primed Seat a cartridge, then prime. See section 4.
Air still coming out at the tip Not enough priming cycles Keep priming needle-up until solution appears.
Plunger won't go back into the body You're pushing it straight in Let it rotate — it's a corkscrew thread.
Cartridge won't thread on Overfilled past 3 mL Push the stopper back level with the glass.
Cartridge holder has vanished You unscrewed before pulling apart Reassemble, then pull straight apart first.

2. Stiff to press

This is the most common complaint and it is almost never the pen. Resistance is the sum of several things, and they stack — a fine tip on a cold, slightly viscous solution feels far worse than either problem alone.

The seven factors

Factor Effect What to do about it
1. Tip gauge Biggest single factor. Flow drops off sharply as the bore narrows. Step down a gauge and compare.
2. Wall thickness Two tips of the same gauge can flow very differently. Try a thin-wall or extra-thin-wall tip at the same gauge.
3. Temperature Cold solution is thicker. Straight from the fridge is the worst case. Let the pen sit out before use.
4. Solution viscosity Whatever you loaded has its own thickness, and that sets the floor. Match the tip to the solution, not the other way round.
5. A partially blocked tip A reused or previously dried tip restricts or blocks flow entirely. Fresh tip every session. Never reuse.
6. Stopper break-loose First push on a new cartridge needs extra force. Expect it once. It should not persist.
7. Assembly A cross-threaded or overtightened holder binds the mechanism. Unthread, restart it straight, tighten gently.

Work through it in order

  1. Swap the tip first. It's the cheapest test and it resolves most cases outright.
  2. Let the pen reach room temperature and try again before changing anything else.
  3. Check the assembly. Unscrew the cartridge holder and rethread it slowly and straight.
  4. Try a wider bore — thin-wall at the same gauge, or one gauge down.
  5. Still stiff on every push with a fresh tip and a warm pen? That's when to email us.

How gauge and wall thickness affect flow

3. Dripping after the dose

Why it happens

When the dial returns to zero the mechanism has finished travelling, but the fluid inside hasn't finished moving. There's still pressure in the cartridge and the tip, and it takes a moment to equalise. Withdraw the tip at the instant the dial hits zero and the remainder comes out afterwards — as a drip.

Two other causes worth ruling out:

  • A loosely threaded tip. If it isn't seated squarely, solution escapes around the join rather than through the needle.
  • Air still in the cartridge. Trapped air compresses during the push and then expands afterwards, pushing solution out behind you. That's a priming problem.

The fix

Wait a few seconds after the dial reaches zero before withdrawing the tip. Counting to ten covers it comfortably. This one habit eliminates the large majority of drip complaints.

Then take the tip off and dispose of it. Don't store the pen with a tip attached.

4. Nothing comes out

The pen won't actuate

If you dial, press, and the pen simply doesn't move, the most likely explanation is that there's no cartridge seated. The mechanism needs resistance to actuate — with an empty cartridge holder it will feel dead, and a brand-new pen tested this way looks broken when it isn't.

Seat a cartridge, then test again.

Air at the tip

Air coming out instead of solution means the pen hasn't been fully primed. Hold it needle up, tap the glass gently so bubbles rise, dial a few clicks and press. Repeat until solution appears at the tip and no more air comes out.

If you're priming repeatedly and still getting air, the cartridge probably wasn't vented properly during the fill.

Priming and venting walkthrough

5. Assembly problems

Plunger won't go back into the body

The plunger rides a corkscrew thread and has to rotate as it travels. Guide it in and let it turn under your finger. Forcing it straight in will bend or snap it, and that's avoidable damage rather than a warranty claim.

Cartridge won't seat or won't thread on

Two usual causes:

  • Overfilled. If the stopper was pushed past the bottom edge of the glass during the fill, the cartridge won't sit correctly. Reinsert a syringe and gently push the stopper back level with the bottom edge.
  • Wrong cartridge. These pens take 3 mL cartridges with an 11mm long stopper. A short-stopper cartridge from another supplier will not seat, no matter how similar it looks.

A new pen's threads can also be stiff enough to feel like a mis-fit. Start the thread straight, turn slowly, and don't force it — cross-threading an aluminum body is permanent.

“Where did my cartridge holder go?”

It's inside the cap. This happens when the pen gets unscrewed before it gets pulled apart. Screw the whole thing back together, then pull it straight apart first, and unscrew the holder second. No harm done.

6. When it's actually a defect

Work through the relevant section above first — it resolves the overwhelming majority of cases. But stop and contact us straight away if:

  • The dial slips, skips clicks, or won't hold a position.
  • The pen leaks from anywhere other than the needle.
  • The plunger is visibly bent, or the threads are damaged.
  • The cartridge holder is cracked.
  • It's stiff on every push with a fresh tip and a room-temperature pen.

Don't try to repair or modify it. No printed parts, no improvised spacers, no substitute holders — see the safety and warnings guide for why that matters.

Email tfortech@modtechsupply.net and we'll sort it out. Everything is covered by the 30-day money-back guarantee.

7. Where to go next

Still stuck?

Email tfortech@modtechsupply.net. A real person answers within one business day, and that person is the owner.

MODTECH products are precision hardware sold for laboratory and research use only. They are not medical devices. No substances are included with or sold alongside any product, and nothing on this page constitutes medical advice or instructions for administering any substance to humans or animals.