Compression Molding for Plastic Caps

Finally, compression molding is a mainstream and high-efficiency plastic cap manufacturing process specially used for mass plastic cap manufacturing.

Different from injection plastic cap manufacturing, this technology relies on pressure and temperature to shape plastic materials.

Workers first cut and weigh qualified plastic raw materials into fixed material blocks. Then workers put the plastic blocks into preheated cap molds.

The machine applies continuous and even pressure to squeeze the materials to fill every corner of the mold cavity.

After low-temperature cooling and setting, workers take out complete plastic caps and trim extra edges.

This process brings less internal stress to plastic caps, so finished caps have better toughness and stable sealing performance.

It suits most standard screw plastic caps and daily packaging caps with high production accuracy requirements.

What Plastic Cap Mold Do You Need?

The mold is one of the most important parts of the production system.

A cap mold normally needs to control:

• Cavity number
• Thread geometry
• Gate design
• Cooling channels
• Ejection
• Mold steel
• Parting line
• Automatic demolding

Why Cavity Number Matters

A single-cavity mold produces one cap per cycle.

A 16-cavity mold produces 16 caps.

A 32-cavity mold produces 32 caps.

Therefore:

Output per hour ≈ cavity number × 3600 ÷ cycle time

For example, a 24-cavity mold running at a 10-second cycle has a theoretical output of:

24 × 3600 ÷ 10 = 8,640 caps/hour

Actual output will be lower because of startup, inspection, mold adjustment and production downtime.

This is why mold cavity number and cycle time should be considered together.

How to Choose a Plastic Cap Injection Molding Machine?

Machine selection should start with the mold and production target.

Check the Mold Cavity Number

More cavities mean more parts per cycle.

However, they also increase the required shot size and may increase the required injection speed.

Therefore, a 48-cavity mold should not be matched to a machine simply because the clamping force appears sufficient.

Check the Shot Weight

Calculate:

Shot weight = total part weight + runner weight

Then make sure the injection unit has sufficient capacity.

The machine should also have enough plasticizing capacity to maintain the required cycle time.

Check the Clamping Force

The machine must provide enough clamping force to keep the mold closed during injection.

Insufficient clamping force can cause:

• Flash
• Parting-line defects
• Mold instability

The required force depends on the projected area, cavity pressure and safety margin.

Check Injection Speed

Small caps have relatively thin walls and can have narrow flow paths.

Therefore, the melt may need to fill the cavity quickly.

Stable injection speed can help reduce:

• Short shots
• Weld lines
• Inconsistent filling
• Dimensional variation

Check Cycle Time

In high-volume cap production, even a small reduction in cycle time can have a major impact on annual output.

For example, reducing a cycle from 12 seconds to 10 seconds increases theoretical cycles per hour from:

300 → 360 cycles/hour

That is a 20% increase in theoretical cycle capacity.

Therefore, mold cooling and machine response are both important.

Which Plastic Cap Manufacturing Process Should You Choose?

Use injection molding when you need:

• Complex cap geometry
• Fine threads
• Flip-top designs
• Tamper-evident features
• High dimensional accuracy
• Multi-cavity production

Consider compression molding when you need:

• Very high production volume
• Relatively simple cap designs
• A process suited to the selected material and closure design

In short, the right process depends on the product.

Do not choose the machine first. Choose the cap design and manufacturing process first, then match the mold and machine to the production target.