How a Plastic Cap Is Injection Molded, Step by Step

What is plastic cap injection molding? Plastic cap injection molding is the process that makes bottle caps: plastic granules (usually PP) are melted, injected into a multi-cavity steel mould, cooled until solid, and the finished caps are released and taken out. The cycle repeats automatically, around the clock.

Watch: the process running

What you will see: the mould opening, a robot taking out a row of flip top caps and setting them on a conveyor, and the lids being closed in-line as the caps travel along it.

This page explains how plastic caps are injection molded and what decides whether a cap is good. If you are choosing a machine and mould for caps, the last section shows how the two are matched. AMAN cuts cap moulds in its own workshop and supplies the injection molding machines they run on. We have done this since 2007.

The seven steps of the cap molding cycle

Figure 1. The cap molding cycle.

1. Melt the granules. The screw turns granules into one uniform melt. Plastic granules (PP is the usual material for caps) fall from the hopper into a heated barrel. A rotating screw carries them forward, and the heat of the barrel plus the shear (the rubbing and squeezing) of the screw melt them into a uniform melt in front of the screw tip.

Figure 2. Step 1: hopper, heated barrel and rotating screw.

2. Close and clamp the mould. The clamp has to hold harder than the melt pushes outward. The two mould halves close and the clamping unit holds them shut. If it does not, melt leaks out between the mould halves (this leak is called flash), so the clamping force (in tons) is what decides the machine size.

Figure 3. Steps 2 to 5: the mould closes, fills, cools and opens; the ejector pushes the caps off the core.

3. Inject into every cavity. The melt fills all the cavities at once. The screw pushes forward and drives the melt through the runner system (the channels that feed each cavity) into all the cavities at once. A cap mould has several: for example, our drawings show an 8-cavity tool for a 200 ml flip cap (PP, 7.5 ± 1 g per cap) and a 6-cavity tool on a 450 × 450 mm plate for a 400 ml flip cap (PP, 12 ± 1 g). A hot runner keeps the melt heated all the way to the cavities, so it reaches every cavity evenly and there is no cold-runner scrap.

Figure 4. Step 3: cold runner and hot runner (schematic; not a real mould layout).

4. Hold and cool. Cooling is normally the longest part of the cycle. The screw keeps pressure on the melt while the cap solidifies, to make up for shrinkage, and water channels in the mould carry the heat away. On a flip top cap the hinge sets the cooling: how the tool is cooled decides whether the hinge sets the same way in every cavity.

5. Open and release the cap. A screw cap is usually unscrewed from its core. The mould opens and the caps are pushed off the cores. A screw cap is usually different: its thread would lock it on the core, so the tool unscrews the cap as it releases it — in the footage below, an unscrewing mould turns the cores, with a hydraulic motor on the tool, to free eight caps at a time.

Watch the footage

Footage: the unscrewing mould. The cores turn, driven by a hydraulic motor on the tool, and free eight caps at a time.

Figure 5. Step 5: the core turns and the cap comes off.

6. Take out and, for a flip top, close the lid. A robot lifts the caps out and sets them on a conveyor. A flip top cap is molded with its lid open; on our footage the lids are closed in-line on the conveyor, so the cap leaves the machine ready to pack.

Watch the footage

Footage: a robot lifts the caps out and sets them on a conveyor.

7. Collect and pack. The caps drop into a bin or carton and the cycle repeats. The cycle runs automatically, around the clock.

What decides whether a cap is good

A cap mould is judged on three things: whether the closure seals on the neck it has to seal on, whether every cavity gives the same cap, and whether it keeps doing both once the machine runs fast. All three are decided when the cavity, the thread shape and the shut-off faces (the faces where the two mould halves press together) are cut to the drawing. That is why AMAN cuts the cavities in its own workshop in Jiangsu, runs the tool on a machine there, and ships the trial caps with it.

Figure 6. 'Seals on the neck' means the thread and the sealing surface fit the bottle neck.

Different caps, different tools

Machine and mould as one set

AMAN supplies plastic cap injection molding machines with the matching cap mould cut in-house: our page lists 16 machines, including a 16-cavity PCO 1880-1881 cap injection molding machine. The mould takes 40–60 days from approved drawings; machine and mould ship as one set, tested and tuned together, with the trial caps. Send your cap drawing and we quote the machine together with the mould.

A real example

A closures maker in Pakistan needed two-colour threaded caps matched to its bottles. The moulds were multi-cavity with hot runners, so the melt stays balanced across the cavities and the cavities stay identical. Read the case: multi-cavity two-colour and threaded cap moulds, Pakistan.

FAQ

Related: cap moulds · flip top cap moulds · two colour cap moulds · how an injection molding machine works