Technical Foundation
What Is a Screw Cap?
Screw cap fitment is defined by a two-number thread finish designation (such as 24-400), where the first number is the nominal neck finish diameter in millimeters and the second identifies the specific thread series (400 series being the common continuous-thread standard for many bottle and vial applications) — both numbers must match the container's neck finish exactly, since caps are not universally interchangeable across different thread series even at the same diameter. Cap material (polypropylene, phenolic, metal) is chosen based on chemical compatibility with the container's contents and the sealing method needed.
Liner material inside the cap does much of the actual sealing work: a PTFE-faced liner resists a broad range of solvents and chemicals for lab and analytical use, foam liners provide a general-purpose seal for less aggressive contents, and induction-seal liners bond to the container mouth under heat to create a tamper-evident, leak-proof factory seal that's broken on first opening. Child-resistant and tamper-evident features add mechanical or visual indicators required by certain regulatory categories, such as pharmaceutical and household chemical packaging.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Thread Finish | Two-number designation, e.g. 24-400, 38-430 | Both diameter and thread series number must match the container neck exactly |
| Cap Material | Polypropylene, phenolic, metal | Must be chemically compatible with the container's contents to avoid degradation or contamination |
| Liner Type | PTFE-faced, foam, induction seal, none | Determines chemical resistance, seal integrity and whether the cap provides a tamper-evident factory seal |
| Closure Feature | Standard, tamper-evident, child-resistant | Certain regulated products (pharma, household chemicals) require specific closure features by law |
| Torque Application | Manual vs. machine-applied cap torque rating | Under- or over-torquing compromises the liner seal even with correct cap fitment |
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Applications
Where Screw Caps Get Used
Screw caps are specified by matching thread finish exactly to the container neck, then selecting liner and closure features for the contents and regulatory category.
Laboratory & Analytical Chemistry
PTFE-lined screw caps sealing sample vials and reagent bottles against solvent attack and contamination.
Pharmaceutical & Nutraceutical Packaging
Child-resistant screw caps meeting regulatory closure requirements for medication and supplement containers.
Industrial Chemical Packaging
Chemically compatible caps sealing bulk and sample containers of solvents, acids and process chemicals.
Food & Beverage Bottling
Tamper-evident induction-sealed caps providing a factory seal on consumer packaged goods.
Cosmetics & Personal Care Packaging
Lined dispensing caps sealing liquid and cream product containers against leakage.
Household Chemical Packaging
Child-resistant caps meeting Poison Prevention Packaging Act requirements for consumer chemical products.
FAQ
Screw Caps Questions, Answered
What does a thread finish designation like 24-400 mean?
The first number (24) is the nominal outer diameter of the container's neck finish in millimeters, and the second number (400) identifies the specific thread series — different series can share the same diameter but have different thread pitch and profile, so a cap won't seal properly, or may not thread on at all, unless both numbers match the container exactly.
What's the difference between a foam liner and a PTFE-faced liner?
A foam liner provides a general-purpose compressible seal suited to less chemically aggressive contents like water-based solutions. A PTFE-faced liner has a chemically inert film facing the container's contents, resisting attack from a much broader range of solvents and reactive chemicals — the standard choice for laboratory and analytical chemistry applications where content compatibility with the seal itself matters.
Do I need a tamper-evident cap or a child-resistant cap for my product?
Tamper-evident features (like an induction seal that must be broken to access contents, or a perforated band that separates on first opening) provide visual proof a container hasn't been previously opened. Child-resistant caps use a mechanical mechanism (like a push-and-turn action) specifically designed to be difficult for young children to open. Certain product categories — pharmaceuticals, household chemicals — have regulatory requirements for one or both; confirm your product's regulatory category before selecting.
Can I use a plastic cap on a container holding a strong solvent?
It depends entirely on the specific polymer and the specific solvent — some plastics degrade, swell or become brittle when exposed to certain solvents, which can compromise the seal or contaminate the contents. Always verify chemical compatibility between the cap material (and liner) and your specific contents using a chemical compatibility chart before packaging, rather than assuming general 'chemical resistant' labeling covers your specific solvent.
Why does over-torquing a cap sometimes cause a leak instead of preventing one?
Over-torquing can crush or distort the liner beyond its effective sealing range, or crack a plastic cap's threads, both of which compromise the seal rather than improving it. Caps are generally rated for a specific torque range depending on whether they're applied by hand or machine — staying within that range gives the most reliable seal.
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