The pipette is a glass tube in every case, cut to the bottle height with the tip 5 mm clear of the base; the decision is the bulb. Use a silicone bulb for facial oils, essential oils, high-ethanol serums and most actives; use a TPE bulb only for plain water-based serums (hyaluronic acid, niacinamide), because TPE and NBR swell in oils. Drop size is set by tip bore and viscosity: 15–30 µl precision, 30–50 µl serum, 50–100 µl facial oil, verified per bottle-pipette pairing over 50 sequential drops. The test that decides: the assembled dropper conditioned in your actual bulk at 48 °C for 30 days with no swelling, hardening, odour or drop-volume drift.
What a dropper is made of
A dropper assembly has four parts. The cap is PP or ABS, sometimes with an aluminium-over-plastic collar that can be anodised or sprayed. The collar carries the thread that engages the bottle neck, 18/400 or 20/400 on glass, 18/410 or 20/410 on PET, and holds the gasket that actually seals. The pipette is a glass tube, straight or bent-tip, cut to the bottle height so the tip sits 5 mm clear of the base, with a length tolerance of +2/−1 mm. The bulb is the elastomer on top that the consumer squeezes, and it is the only part where you have a material decision to make. Everything else is fixed by the bottle drawing; the pipette diameter is set by the bottle's inner bore, the I dimension, which is why the closure is bought to the bottle and never the reverse (see dropper bottle neck finishes explained).
Silicone vs TPE: what each survives
Two bulb materials are common on cosmetic droppers. TPE (thermoplastic elastomer, and the older NBR nitrile rubber) is cheaper, moulds fast and is fine in water. In oils, essential oils and silicone-heavy serums it swells: the bulb softens, its return stroke weakens, the seal at the collar loosens, and low-surface-tension oils then creep past the thread. In ethanol above about 30% it softens and can take on odour. Silicone costs more, but it is inert to oils, essential oils and alcohol at the concentrations skincare uses, holds its return force, and keeps the drop volume stable through the pack life.
| Formula | Bulb | Gasket / seal | Bottle | Why |
|---|---|---|---|---|
| Facial oil, 100% oil phase, essential oils | Silicone | Silicone or fluoroelastomer | Amber glass 15–30 ml, 18/400 or 20/400 | TPE and NBR swell in oils; low surface tension creeps past loose threads |
| Vitamin C, retinol (oxygen- and light-sensitive) | Silicone | Silicone | Amber or opaque glass; opaque carton | Retinol is oil-phase; if oxidation is the priority consider airless |
| Serum above ~30% ethanol | Silicone | PTFE-coated or solvent-grade; ethanol rating (e.g. ≥50%) in the spec | Glass or alcohol-rated PETG; never standard PET | TPE softens; PET swells and deforms above ~30% ethanol |
| AHA/BHA, pH below 4 | Silicone or TPE | Standard | Glass or PP | Nothing attacks the bulb; the dropper avoids the metal spring an acid corrodes |
| Water-based serum: hyaluronic acid, niacinamide, peptides in water | TPE acceptable; silicone if in doubt | Standard | High-flint or frosted glass; PET/PETG for e-commerce | Nothing attacks the bulb; the question is drop size and seal |
| Acetone or fragrance concentrate | Purpose-made plug and liner, not a bulb dropper | Solvent-grade | Glass only | Dissolves PET, PS, AS, PMMA and most standard gaskets |
The rule reduces to one line: silicone for anything with oil or alcohol; TPE only for plain water-based serums. If a supplier quotes a "rubber bulb" without naming the material, ask. The nine material rules behind this table are on the formula × packaging compatibility matrix.
Drop size and who sets it
The consumer's dosing instruction ("3 drops, morning and night") only works if the drop is repeatable. Drop size is set by the tip bore of the glass pipette and the viscosity of the bulk, so the same pipette gives a different drop in a thin essence and a thick oil. Working bands: 15–30 µl for precision actives, 30–50 µl for a standard serum, 50–100 µl for a facial oil. The number is verified per bottle-and-pipette pairing on 50 sequential drops from a fresh bottle, to a ±20% acceptance, and measured again after conditioning, because a bulb that has softened in the formula delivers a different drop than a fresh one. A calibrated pipette with printed volume marks (0.5 ml, 1 ml) adds cost and reinforces clinical positioning; a child-resistant press-and-turn collar is available where the destination market requires it for essential-oil products.
The tests that decide before the run
| Test | Method | Pass threshold |
|---|---|---|
| Bulb compatibility | Assembled dropper conditioned in the actual bulk at 48 °C for 30 days; 25 °C control; ≥3 units per condition | No swelling, hardening, discolouration or odour; drop volume within the agreed tolerance after conditioning |
| Drop volume consistency | 50 sequential drops from a fresh bottle, weighed | ±20% across the 50; within the 15–30 / 30–50 / 50–100 µl band specified |
| Seal integrity | Filled, capped, held at 45 °C for 24 h inverted and on its side | No weep at neck or dropper; weight loss ≤0.5% |
| Vacuum leak | Vacuum chamber, lying and inverted | −0.06 MPa for standard bulks; −0.08 MPa for 15 min, loss ≤0.5% for alcohol or volatile bulks |
| Dropper seal, squeeze test | 20 samples per batch: squeeze, release, invert | No air entry, no backflow; nothing that fails at 40 °C leaves the factory |
| Opening torque | Torque meter after application | 0.2–0.7 N·m; reverse torque ≥2.5 N·m; application 0.6–1.2 N·m |
| Pipette length | Measured to the bottle drawing | Tip 5 mm clear of the base; +2/−1 mm |
Water passes all of these on a bulb that will fail in your oil in week three. The only test that counts is the one run with the actual formula, on the assembled dropper, for 30 days. That test sits inside sampling, before the golden sample is signed, and its result belongs in the specification alongside the glass grade and neck finish. The full battery and the four inspection gates are on our QC standards page.
Writing the dropper into the spec
- Bulb material by name: "silicone" or "TPE", not "rubber"; gasket material alongside it.
- Neck finish and I dimension from the bottle drawing, so the pipette diameter and the collar thread are cut to the glass, not guessed. Glass neck tolerances are wider than injection-moulded ones, so the closure factory needs the glass drawing; see neck finish and closure standards.
- Drop-size band and the 50-drop acceptance, with the viscosity of the bulk it was measured on.
- Conditioning result: 48 °C / 30 days in the named formula, pass, dated.
- Collar finish and its minimum: stock colours from about 500 pieces; sprayed or anodised 2,000–5,000; a second anodised batch rarely matches without a control sample, so buy the range's collars in one lot. Minimums by route are in 30 ml dropper bottle MOQ from China.
Glass grades, capacities and dropper options are on the dropper bottles page; the serum, oil and cream range planned as a set is on skincare packaging solutions; and what to check about a supplier before paying a deposit is in how to evaluate a China packaging supplier.
Vella is a Guangzhou-based beauty packaging solutions provider, a sourcing partner working with vetted partner factories operating to ISO 22716 / GMP. For dropper programmes we specify the bulb and gasket to the formula, cross-check the bottle and dropper drawings, and run the 30-day soak and 50-drop test on samples filled with the client's bulk before production.
Common questions
Launching a facial oil or active serum?
Send the formula, we'll spec the dropper.
Tell us the oil phase, ethanol percentage, pH and target drop size. We reply within 24 hours with the bulb, gasket and glass recommendation and a 30-day compatibility test plan on samples filled with your bulk.
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