When a capsule serum batch is rejected, the reason is usually visible at the seam. Oil on the outside, a thickened or ragged edge, a white haze on the shell, capsules that have hardened since approval - almost all of it originates where the two ribbons were pressed together.
This article covers what each defect looks like, what causes it in the process, and the checks that catch it before a batch leaves the factory. It is written for buyers who have received a sample that looked wrong and want to know what actually happened.
Most capsule defects show up at the seam, not in the fill
The seam is where two ribbons of gel mass are pressed, heated and cut in a single motion. It is the weakest part of the capsule by construction, and it is where any mismatch in timing, temperature, fill volume or ribbon condition becomes visible.
That is useful diagnostically. If capsules are leaking, the first question is not whether the formula is wrong - it is whether the sealing conditions were right for that formula.
What a good seam looks like
A well-formed seam is a narrow, even band with a consistent width around the whole capsule. There should be no product visible on the outside, no ragged or feathered edge, and no step where one ribbon overlapped the other unevenly. Held up to light, the seam should not show a channel.
In practice, buyers rarely see a defined seam standard. Ask for one. A written description plus a retained reference sample is the difference between "this looks wrong" and "this is outside specification".
Leakage: product on the surface
Surface oil is the most common and most damaging defect. It appears as shiny patches on the capsule, as capsules sticking together in the pack, or as oil transferred to the inside of a bottle or blister.
The usual causes, in order of frequency:
- Product trapped in the seal. Fill was injected slightly early or late, so a small amount sat in the sealing band and prevented a clean weld.
- Over-filling. More fill than the pocket can hold comfortably leaves material at the seal.
- Insufficient sealing temperature or pressure. The weld is partial and opens later, often during transit when the pack is warm.
- Fill incompatibility with the shell. Some fills interfere with the shell material at the interface and weaken the seal.
Note the third cause: capsules can pass inspection and still leak in transit. A partial weld holds at room temperature and fails in a hot container. This is why transit simulation belongs in packaging testing.
Blooming: the haze that is not mould
Blooming shows as a dull, whitish or cloudy appearance on the shell surface, sometimes mistaken for microbial growth. It is not. It is usually migration - components of the fill moving into the shell, or moisture movement changing the shell's surface structure.
Blooming is a compatibility signal. It often appears when a fill contains components that are partly soluble in the shell material, or when the moisture balance between fill and shell is unstable. It can also appear weeks after packing, which means a sample that looked clean at approval can bloom before it reaches a customer.
The control is compatibility testing during development, with the actual fill and the actual shell, observed over time rather than on the day of production.
Hardening and brittleness over time
Capsules firm up as they age. Some of that is expected: moisture equilibrates and the shell settles. The problem is when hardening goes far enough to change the user experience - a capsule that is difficult to twist open, or that cracks rather than breaks cleanly.
Excess hardening usually traces to one of three things: the shell was over-dried during production, the plasticiser level is too low for the formulation, or the product lost moisture to the packaging and the environment after packing. In the last case the packaging is the variable, not the capsule.
This is a stability question. Only real-time and accelerated data on your specific combination will tell you how far hardening goes and whether it stays acceptable.
Seam thickening and ragged edges
A seam that is wider than expected, or that has a feathered edge, points to die condition and ribbon alignment rather than to the formula. Causes include worn die pockets, ribbons that are not aligned as they enter the dies, and gel mass temperature outside the working window.
Ragged edges also come from cutting conditions. If the capsule is cut rather than cleanly separated, the edge tears. This is a maintenance and setup issue, and it is usually visible early in a run - which is why start-up inspection matters more than end-of-run inspection.
The process variables that control seam quality
| Variable | Symptom when wrong | Control |
|---|---|---|
| Injection timing | Product in seam, leakage | Timing check at setup and after any stop |
| Die temperature | Weak seal or scorched shell | Set per gel mass, monitored during run |
| Fill volume | Bulging capsules, trapped product | Fill weight specification with in-process checks |
| Ribbon thickness | Uneven seam, thin spots | Ribbon gauge checked at setup |
| Gel mass temperature | Poor weld, variable seam width | Holding temperature controlled and logged |
| Ribbon alignment | Off-centre or stepped seam | Alignment check at start-up |
How seam quality is checked
Visual inspection under good lighting catches most of it. Better operations add methods that make leaks visible: holding capsules on absorbent material and looking for transferred oil, or a vacuum or pressure test on a sample.
The important variable is when. Inspection at start-up catches setup errors before most of the batch is made. Inspection only at the end tells you what you have already paid for. Ask how often sampling happens and what triggers a line stop.
Transit behaviour deserves separate testing. A leak test that only runs at room temperature will miss partial welds that fail at elevated temperature. Our packaging compatibility guide covers what to commission.
What leakage does to your brand in the market
A leaking capsule is not a minor cosmetic defect. It damages the packaging, it makes the product unpleasant to handle, and it is visible in reviews with photographs. On a single-use premium product, where the format is the reason for the price, leakage undermines the reason to buy.
It is also expensive to resolve after launch. The cost of preventing leaks in development - compatibility testing, transit simulation, a defined seam standard - is small compared with a recall or a reformulation after the product is in market. Product recall handling is covered in our complaints and adverse events guide.
What to specify and what to ask
- A written seam description: width, appearance, and what counts as a defect.
- An allowable defect rate per batch, expressed per thousand units.
- Sampling frequency during the run, and the action that triggers a stop.
- A leak test method, including whether it is run at elevated temperature.
- Compatibility testing between your fill and the shell, observed over time.
- A retained approval sample from the first commercial batch.
Where our capsule capability fits
We run capsule serums with defined seam and appearance criteria, across formats including softgel face serum capsules, AM/PM retinol and vitamin C softgels and repair serum oil capsules. If you have received samples elsewhere that leaked or bloomed, send us the details - the symptom usually identifies the cause.
We work with brands on private label (your brand on a proven base), OEM/ODM contract manufacturing, and full custom formula development - whichever matches where your brand is today.
Describe what you saw, tell us the fill and the market, and we will tell you whether the format can be made to hold. Start at our inquiry page, email capsulebiotech@163.com, or message the right team on WhatsApp: Capsule Series (capsules only) +86 158-7655-5319, Cosmetics Series (serum, hair care, body care, fragrance) +86 158-7655-5319.
This article is general manufacturing information, not regulatory or legal advice. Defect limits and acceptance criteria should be agreed in writing with your manufacturer.