How to Prevent Low Foaming Ratio in Foamed Cable Extrusion

2025-12-30

Low foaming ratio is one of the most misunderstood problems in foamed cable production. On paper, everything looks correct: the gas is injected, the extruder is running, temperatures are “within range”, and yet the insulation comes out heavier than expected, with poor dielectric performance and unstable density.

Many factories react by increasing gas input or blaming the foaming agent. In reality, low foaming ratio is almost never caused by gas shortage alone. It is usually the result of pressure, temperature, melt condition, and timing not working together.

This article explains why foaming ratio drops, how to identify the real limiting factor, and how manufacturers stabilize foaming without chasing parameters blindly.

1. What “Low Foaming Ratio” Really Means in Practice

A low foaming ratio means the polymer does not expand as designed after exiting the die. Typical symptoms include:

Insulation density higher than target
Reduced dielectric constant improvement
Stiffer insulation than expected
Inconsistent foam structure across the cross-section
Higher material consumption per meter

Importantly, low foaming ratio does not always mean “no foam”. Many lines still show microcells, but not enough expansion to reach design targets.

2. Why Increasing Gas Injection Usually Doesn’t Fix It

This is the most common mistake.

If foaming ratio is low, operators often:

Increase N₂ or CO₂ flow
Increase chemical foaming agent dosage

What happens next:

Pressure becomes unstable
Cells collapse
Surface roughness increases
Foaming becomes inconsistent

Key reality:

Foaming is limited by melt condition and pressure release, not by how much gas you force in.

If the melt cannot release gas at the right moment, extra gas simply dissolves or escapes unevenly.

3. Core Causes of Low Foaming Ratio (What Actually Blocks Expansion)

3.1 Excessive Melt Pressure at the Die (Primary Cause)

Foaming depends on a sudden pressure drop at the die exit.

If die pressure is too high:

Gas stays dissolved
Cell nucleation is suppressed
Expansion happens too late or not at all

Common reasons for high pressure:

Die gap too small
Excessive output rate
Melt viscosity too high
Over-packed screw compression zone

Rule of thumb:

Stable foaming prefers moderate pressure + sharp pressure release, not maximum pressure.

3.2 Melt Temperature Profile Is “Correct” but Wrong

This one confuses a lot of engineers.

Yes, temperatures may be within supplier recommendations — but foaming needs a specific thermal window:

Too cold → melt too stiff, gas cannot expand
Too hot → gas escapes early, cells collapse

Common hidden issues:

Overheated metering zone
Underheated adapter or die
Large temperature gradient between zones

Foaming does not like thermal inconsistency.
It likes controlled softness, not extreme heat.

3.3 Screw Design Not Supporting Foaming

Not every extrusion screw is foaming-friendly.

Problems include:

Compression ratio too high
Insufficient mixing for gas dispersion
Excessive shear heating
No dedicated gas-holding section

Result:

Gas dissolves unevenly
Cell nucleation becomes unstable
Expansion efficiency drops

This is why the same material foams well on one line and poorly on another.

3.4 Incorrect Gas Injection Timing or Location

Gas injection must happen when:

Melt is fully plasticized
Pressure is stable
Mixing is sufficient but not violent

If gas is injected:

Too early → gas escapes or degrades
Too late → gas cannot distribute uniformly

Small timing shifts can reduce foaming ratio dramatically, even if gas flow stays constant.

3.5 Excessive Cooling Immediately After Die Exit

This is often overlooked.

If cooling:

Starts too close to the die
Is too aggressive

The foam structure “freezes” before full expansion.

This leads to:

Dense outer layer
Under-expanded core
Lower overall foaming ratio

Foam needs a short free expansion window before solidification.

4. How to Diagnose the Real Limiting Factor

4.1 Measure Density First, Not Just Appearance

Do not rely on surface look.

Measure:

Actual insulation density
Weight per meter
Cross-section foam uniformity

Low foaming ratio can hide behind a smooth surface.

4.2 Reduce Output by 10–15% (Test Only)

This is a powerful diagnostic step.

If foaming ratio improves immediately:

Pressure or residence time is the issue

If nothing changes:

Look at temperature or gas dispersion

4.3 Slightly Increase Die Temperature Only

Increase die temperature by:

+5 to +8°C

If foaming improves:

Expansion was thermally restricted at exit

If surface degrades:

Gas release timing is wrong

5. Proven Process Fixes That Actually Work

5.1 Optimize Die Pressure, Not Maximize It

Actions:

Slightly enlarge die gap
Reduce unnecessary output load
Smooth melt flow path

Target:

Stable pressure with sharp release, not extreme pressure buildup

5.2 Fine-Tune Temperature Profile (Zone by Zone)

General direction:

Slightly lower compression zone temperature
Stabilize metering zone
Increase adapter/die temperature moderately

This helps gas stay dissolved until the correct moment.

5.3 Improve Gas Dispersion, Not Gas Quantity

Better results come from:

Improved mixing
Stable injection pressure
Consistent melt flow

More gas without control = unstable foam.

5.4 Adjust Cooling Position and Intensity

Move first cooling point slightly downstream
Reduce initial water or air intensity
Allow controlled free expansion zone

Even 20–30 mm of extra free space can change foaming ratio significantly.

5.5 Accept That Speed Has a Foaming Limit

Foaming is sensitive to:

Residence time
Pressure relaxation
Expansion window

At some point:

Higher speed simply kills foaming efficiency.

Many stable lines run slower but lighter, saving material overall.

6. Why Low Foaming Ratio Often Appears Gradually

Factories often say:

“Foaming used to be fine.”

Because:

Screws wear
Pressure slowly increases
Cooling systems get upgraded
Output creeps up over time

Foaming fails slowly, then suddenly becomes obvious.

7. Real Production Insight

A data cable factory struggled to reach target foaming ratio despite increasing nitrogen input.

Actual fixes:

Reduced output by 12%
Increased die temperature by 6°C
Delayed cooling start by 25 mm