The Importance of Gravimetric Blending in Injection Molding

Learn about the benefits of consistent material dosing in injection molding

For years, blending discussions in plastics processing focused heavily on one thing: dosing accuracy.

Can the blender hit the target ratio?

Today, especially in injection molding, processors are asking a more important question:

How consistent is the material from one shot to the next?

That distinction matters because even small variations in additive concentration, colour dispersion, or material composition can directly affect finished part quality, process stability, and material usage.

In extrusion processes such as blown film or sheet extrusion, the screw provides additional mixing time as material moves through the barrel. Minor inconsistencies upstream can sometimes be partially absorbed during processing.

Injection molding offers far less margin for variation.

Every shot is a separate event with limited time between them. The material entering the machine throat at that moment has an immediate influence on the finished part. If the blend changes from shot to shot, the finished parts can change too.

When blending consistency drifts, processors often see the effects quickly:

Colour streaking or shade variation
Inconsistent part weights
Weak points in molded parts
Failed drop or impact tests
Higher additive and masterbatch consumption
Increased scrap and rework

In many cases, operators compensate by increasing additive percentages to mask inconsistency. While this may temporarily improve appearance or performance, it also increases material costs without solving the root problem.

As material costs rise and sustainability targets increase, maintaining stable material composition becomes increasingly important.

Why Consistency Matters More Than Average Accuracy

Many blending systems promote accuracy figures based on averaged batch results.

The problem is that averages can hide variation between individual samples.

In injection molding, this matters because processors do not ship averages. They ship individual parts.

What determines part quality is not simply whether the overall batch average is accurate, but whether each individual shot receives a consistent material composition.

This becomes even more important when processing:

Recycled materials
Regrind
Multiple additives
Tight colour tolerances
Technical or high-performance parts
As recycled content targets increase and tolerances become tighter, sample-to-sample consistency becomes critical for maintaining stable production.

The Role of Gravimetric Blending
Gravimetric blending helps address this challenge by measuring and controlling materials by actual weight rather than volume.

TSM’s gravimetric approach combines precise dosing with a patented two-stage mixing process designed to homogenise material before it enters the machine throat.

The goal is not simply accurate ingredient delivery, but stable material composition from shot to shot.

This supports:

More uniform masterbatch dispersion
Improved shot consistency
Stable screw feed conditions
Reduced material overconsumption
Greater overall process repeatability
Improved process confidence when using recycled content
For injection molding environments specifically, the system also incorporates vibration compensation technology designed to maintain weighing stability despite clamp movement and machine vibration during operation.

This is particularly important because even small weighing disturbances can influence blend consistency over time.

TSM Blender and Injection Molding machine

What About Volumetric Blending?
Volumetric blending remains a practical and cost-effective solution for many applications running stable virgin materials with less demanding tolerances.

For straightforward applications, it can still provide acceptable performance.

However, as processors introduce:

Regrind
Low bulk density materials
Multiple additives
Higher recycled content
Tight colour tolerances
…the limitations of volume-based dosing become more visible.

Because volumetric systems dose by volume rather than actual weight, changes in material density can alter the true additive ratio entering the process.

In injection molding, even relatively small fluctuations can affect part consistency, appearance, and material efficiency over time.

TSM Mixing Consistency
To showcase the superior performance of TSM’s patented mixing technology, TSM conducted a 100-sample consistency evaluation in which individual samples were collected and measured separately rather than averaged together.

The objective was to identify sample-to-sample stability under REAL injection molding conditions, not simply overall batch accuracy.

The results showed TSM’s mixing consistency as having highly stable additive control around the 5% target with minimal variation between individual samples.

This distinction matters because consistent blending performance directly influences:

Part quality
Material efficiency
Colour consistency
Process stability
Scrap reduction
Additive consumption
For processors running high-volume production, the tiniest percentage point improvement in consistency can have a real, significant impact on cost savings.

Consistency as a Competitive Advantage
As processors continue balancing quality requirements, sustainability targets, and material costs, consistency is becoming increasingly important across injection molding operations.

The challenge is no longer simply achieving accurate dosing on paper. It is maintaining stable, material conditions throughout production.

To learn more about how TSM’s injection molding solutions help processors improve blending consistency, reduce material waste, and optimise production performance, book a discovery call with our team!