Cap Liners Explained: F217, PE Foam, Induction, and More
The liner inside your cap is easy to overlook until it causes a product failure. Wrong liner choice leads to leaks, contamination, or failed tamper-evidence — all of which are expensive to discover after you’ve placed a large order. Here’s what you need to know.
What Is a Cap Liner?
A cap liner (also called an insert or wad) is a disc of material inside the cap that creates a seal when the cap is tightened. The liner contacts the top of the bottle or jar’s opening (the finish) and prevents leakage, contamination, and evaporation. Different liner materials have very different chemical compatibility, moisture resistance, and functional properties.
Liners are typically specified by a short code (F217, TE, PS, etc.) that tells a supplier exactly which material to use. When sourcing caps from China, you must specify the liner explicitly — “standard liner” is not a meaningful specification.
F217 / PE Foam (Most Common)
F217 is a foam liner consisting of a layer of low-density polyethylene (LDPE) foam laminated to a layer of polyethylene film. You’ll also see this called “PE foam,” “foam liner,” or “F217 foam” — they all refer to the same thing.
F217 is the default for most liquid and dry applications. It provides a good seal, resists moisture, and is compatible with a wide range of products including personal care, food, pharmaceutical, and industrial liquids.
Best for:
- Shampoos, conditioners, lotions
- Water-based liquids
- Pharmaceutical and supplement bottles
- Most general-purpose applications
Avoid for:
- Essential oils and solvents
- Hot-fill applications (>160°F)
- Products requiring tamper evidence
- High-concentration acids or bases
Induction Seals (TE Liners)
Induction seals use an aluminum foil layer that is heat-bonded to the bottle opening by an induction sealing machine after filling. The cap is applied loosely at first; the induction sealer passes the filled and capped bottle through an electromagnetic field that heats the foil and fuses it to the bottle rim.
The result is a hermetic, tamper-evident seal. When the consumer opens the bottle, the foil must be punctured or peeled away — any tampering is immediately obvious.
Common for: dietary supplements, pharmaceutical products, salsa and condiments, any product where tamper evidence or extended shelf life is important.
Pressure Sensitive (PS) Liners
Pressure sensitive liners have a thin adhesive layer that bonds to the bottle’s opening when the cap is tightened — no heat or equipment required. The liner stays with the bottle when the cap is removed, providing a tamper-evident, resealable seal.
PS liners are a good no-equipment alternative to induction seals for products that need tamper evidence but don’t require a hermetic seal. Less common for liquids because they’re not as leak-proof as F217 or induction seals.
Plastisol Liners
Plastisol is a PVC-based compound used in metal caps, particularly continuous thread (CT) and lug caps for glass jars and bottles. It flows to conform to the jar opening during capping and creates a vacuum seal — the distinctive “pop” when you open a jar of jam is the vacuum releasing. Plastisol is the standard liner for home canning lids and commercial food packaging in metal caps.
Pulp and Foil Liners
A cardboard pulp backing laminated with aluminum foil. Common in caps for dry products like spices, coffee, powdered supplements, and vitamins. Good oxygen and moisture barrier. Not appropriate for liquids. Often seen in pharmaceutical caps as the standard liner before induction sealing became widespread.
Ester / Polyester Liners (Chemical Resistance)
For products that would dissolve or degrade a foam liner — essential oils, certain solvents, concentrated cleaners, and some agricultural chemicals — you need an ester or polyester liner. These liners resist the penetration and swelling that hydrocarbon-based products cause in PE foam.
If you’re packaging essential oils or solvents and you specify F217, expect the liner to swell, leak, and potentially contaminate your product within weeks. Always specify a solvent-resistant liner (polyester, PTFE, or Buna-N depending on the specific chemical) for these applications.
Unlined Caps
Some caps are sold without any liner — typically used where the bottle itself provides the seal (like a flip-top dispensing cap that clicks shut) or where the product doesn’t require a seal. Unlined caps are also used as overcaps over pumps and sprayers. Make sure to specify “unlined” explicitly if that’s what you need, or you may receive caps with a foam insert.
Quick Choosing Guide
| Your product | Recommended liner |
|---|---|
| Shampoo, lotion, liquid soap | F217 / PE foam |
| Supplements, pills, powder | F217 or pulp/foil |
| Salsa, condiments, food jars (metal cap) | Plastisol |
| Essential oils, solvents | Polyester / ester |
| Products requiring tamper evidence (no equipment) | Pressure sensitive |
| Products requiring tamper evidence (with sealer) | Induction seal |
| Pump or sprayer overcap | Unlined |
| Dry spices, coffee | Pulp/foil |
China Sourcing Note
When sourcing caps from China, liner type must be specified explicitly in your purchase order. Don’t assume — confirm the liner spec with your supplier in writing before production. Request a physical sample with the liner installed and test it with your actual product before approving the run. A mislabeled liner won’t show up until your product is already packaged and on a shelf.
