Cracked shells, sticky capsules, caps that pop off, weights that drift: most capsule filling problems come down to a handful of causes, and most of them are easy to fix once you know what to look for. Whether you fill by hand or on an automatic machine, use this guide to diagnose the problem and get your line running smoothly again.
The short answer
Most problems with empty hard gelatin capsules trace back to humidity (too dry makes shells brittle, too damp makes them soft and sticky), fill level (overfilling stops capsules closing), powder on the rims, or machine set-up (wrong size parts or too much closing force). Control the room at 35 to 65 percent relative humidity and most issues disappear.
Quick troubleshooting table
| Problem | Likely cause | How to fix it |
|---|---|---|
| Cracked or split shells | Shells too dry (stored below about 30% RH or near heat) | Keep capsules sealed until use; condition at 35 to 65% RH; keep away from heaters and air-conditioning vents |
| Soft, sticky or dented shells | Too much humidity, or a fill that draws in moisture | Lower room humidity; use a dehumidifier; keep opened packs sealed between runs |
| Capsules will not separate | Wrong size parts, over-locked capsules, worn tooling | Check that tooling matches the capsule size; inspect and clean the parts |
| Caps will not close | Overfilling, or powder on the body rims | Reduce the fill or tamp less; scrape the rims clean before closing |
| Caps open after filling | Not fully locked, overfilled, or shells too dry | Apply even closing pressure; reduce fill; check humidity |
| Telescoping (body pushes through cap) | Too much closing force, misaligned plates | Reduce closing force; re-align cap and body plates |
| Uneven fill weights | Poor powder flow, uneven spreading or tamping | Improve blend flow; spread in several passes; tamp evenly |
| Dusty finished capsules | Loose powder after filling | Dedust or gently polish filled capsules before packing |
Cracked and brittle capsules
A healthy hard gelatin capsule holds about 13 to 16 percent moisture, which keeps the shell strong but flexible. If capsules are left open in a very dry room, near a heater, or in direct sun, they lose moisture and crack when they are separated or closed. Keep capsules in their sealed packaging until you need them, reseal opened bags immediately, and let cartons that have been in a cold or hot store reach room temperature before opening them. Our guide on how to store empty capsules explains the ideal conditions.
Soft, sticky or misshapen capsules
The opposite problem happens in damp conditions. Above about 65 percent relative humidity, gelatin absorbs moisture, softens and can stick together or to the machine. Some fills, especially hygroscopic powders and extracts, also pull moisture out of the shell or into it. Control the filling room with a dehumidifier or air-conditioning, and fill hygroscopic products as quickly as possible after opening the capsule packs.
Capsules that will not close, or open later
If the caps will not lock, the bodies are usually overfilled or have powder on the rims. Scrape the plate clean before closing, and reduce the fill or tamping pressure. If capsules close but pop open later, check that they are being pressed fully to the locked position, and that the shells have not dried out. Choosing the next size up can also help if your dose only just fits; see our size 0 vs size 00 comparison.
Separation and machine problems
Capsule sizes are an international standard, so the most common machine problem is simply using change parts for the wrong size, or parts that are worn or dirty. Check that bushes and plates match the capsule size, clean them regularly, and adjust closing pressure gently. If problems continue with good settings and a controlled room, run a sample of capsules from another batch to see whether the issue follows the capsules or the machine.
Uneven fill weights
Weight variation usually comes from the powder, not the capsule. A blend that flows poorly or contains clumps fills unevenly. Mix thoroughly, sieve out lumps, spread the powder in several passes rather than one, and tamp with the same light pressure each time. Weigh a sample of 10 to 20 capsules regularly and adjust as you go.
Frequently asked questions
Why are my empty capsules cracking?
Almost always because they have dried out. Store them sealed at 35 to 65 percent relative humidity and away from heat.
Why are my capsules sticking together?
The room is too humid or the capsules have been left open. Lower the humidity and keep packs sealed between uses.
What humidity is best for filling gelatin capsules?
Between about 35 and 65 percent relative humidity, at 15 to 25 °C.
Can damaged capsules be reused?
No. Cracked, dented or deformed capsules should be discarded, as they may not protect the fill or close properly.
Still having trouble on your line?
Tell us your capsule size, machine and filling conditions. Our team can help you find the cause and send samples to test.
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