Stock caps from a catalog
- Available for common finishes at a low unit price
- Shape, color, and branding are limited to what is offered
- Seal and torque behavior are set by someone else's design
Molded part guide: bottle caps
Injection Molded Bottle Caps
A screw cap is defined by the bottle before anyone draws it. The neck finish sets the thread, the sealing lip, and the ring the tamper band locks under. The cap then has to seal on the filling line, still seal after weeks in a warm warehouse, and open with one twist of the hand.
Start at the neck
The neck finish comes first
Water, juice, personal care, household chemical, and pharmacy bottles all start from a neck finish.
Bottle necks follow published finishes. Common screw finishes are named by diameter and style, such as 28-400 or 38-400, where the second number describes the thread style and how far it wraps. Carbonated drink bottles use their own finishes, such as PCO 1881. Designing a cap to a named finish means it fits bottles from any supplier who blows that finish.
Sealing happens at the top of the neck, not in the threads. Threads only pull the cap down. A liner, a disc of foam or film in the cap top, seals by compressing on the neck lip. A linerless cap seals with molded features instead: a plug that enters the bore, a flexible fin that wipes the lip, or both. Linerless designs save a component and an insertion step, but the seal shape has to be right in the steel.
The seal is loaded by torque. Capping torque turns into clamp force through the threads, and the plastic then relaxes under that clamp. That is why a bottle that was tight at the filler can open easily, or weep, a month later, and why removal torque should be measured on aged bottles. Polypropylene and HDPE, the usual cap resins, both relax, and heat speeds it up.
Gas is the other load. Carbonated and gassing products build pressure under the cap. Vent slots cut across the threads let that pressure escape while the threads are still engaged, so the cap does not fly off at the last turn.
Cap features
What each feature on a screw cap is for
Every feature below is molded in, so each one is decided before the tool is cut.
| Feature | Purpose | Design note |
|---|---|---|
| Liner or linerless seal | Seal the neck lip | Linerless seal geometry is fixed in the steel |
| Tamper-evident band | Show the bottle has been opened | Bridges must break on opening, not on capping |
| Vent slots across the threads | Release gas before the cap disengages | Needed on carbonated and gassing products |
| Outer knurl | Grip for the hand and the capping chuck | Also keeps the cap from turning in some tool designs |
| Child-resistant features | Make the cap hard for children to open | Performance is proven by protocol testing of the filled package |
Tamper evidence
How a tamper band breaks at the right moment
The band is molded with the cap and joined to it by thin bridges.
- Flaps or a bead inside the band ride over the neck ring when capped and catch under it when opened
- Opening lifts the cap while the band is held, so the bridges break and the band stays on the bottle
- Bridges that are too strong make the cap hard to open; too weak and they break on the capping line
- Some designs mold a continuous band and cut the bridges after molding in a separate slitting step
Sourcing
Stock caps from a catalog or caps from your own tool
Stock caps are fine until the brand, the seal, or the supply needs to be yours.
Custom caps on your own tool
- Shape, grip, color, and top marking are designed for the brand
- You own the tool, so supply is not tied to one catalog
- Multi-cavity steel tools suit volume, with no MOQ on repeat orders
Drawing
What to send for a bottle cap
Most of these come from the bottle and the filling line, not the cap.
The neck finish designation, or the bottle neck drawing
The product, and whether it is carbonated, gassing, hot filled, or oily
Liner type if you use one, or a request for a linerless seal
Capping method, application torque, and target removal torque
Tamper-evident or child-resistant requirements
Colors, knurl style, and any printing or marking on the top
Keep reading
Related guides
Other caps, the containers they close, and cap resins.
Closures
Flip-tops and dispensing tops.
ClosuresThreaded caps
How internal threads leave the core.
Threaded capsCaps
Push-on caps held by friction.
CapsContainers
Molded jars and tubs the caps close.
ContainersMolded-in threads
Thread form rules for molded parts.
Molded-in threadsPolypropylene
The most common cap resin.
PolypropyleneHDPE
Softer caps and linerless seals.
HDPEInjection vs blow molding
Why bottles are blown and caps are molded.
Injection vs blow moldingMolded parts hub
Every part-type guide, grouped by family.
Molded parts hubFAQ
Questions about molded bottle caps
Can you mold the bottle as well?
Bottles are blow molded, which NetProto does not offer. Caps, closures, and wide-mouth jars are injection molded and can be quoted together.
Why is removal torque lower than capping torque?
The plastic relaxes under the clamp load after capping, and a liner takes a compression set. Removal torque falls over the first days and weeks, so specify and test it on aged bottles.
Should the cap be lined or linerless?
Linerless saves a component and an insertion step, and suits many drinks and household products. Liners, including induction seal liners, suit products that need a sealed inner membrane or extra barrier.
The cap touches food or drink. What should we specify?
Cap resins are available in grades with food contact documentation from the resin supplier. Name the grade, or the documentation you need, in the upload so the quote is made on that resin.
Next step
Quote caps to your neck finish
Upload the cap and the neck finish. DFM feedback covers the seal design, thread fit, tamper band bridges, and how the cap releases from the core.