Masterbatch Dosing – LDR & Mixing Guide 2026

Introduction to Masterbatch Dosing

Masterbatch dosing calculation is the critical bridge between color formulation and consistent molded parts. Whether you're producing automotive trim, consumer electronics, or medical components, precise color control can make or break your product's market success. In this comprehensive guide, we'll break down the exact formulas for let-down ratio calculation, dosing system calibration, and screw design optimization for maximum color homogeneity.

Masterbatch technology has evolved from simple volumetric dosing to sophisticated gravimetric systems with real-time adjustment capabilities. Understanding the LDR formula and mixing principles will help you achieve the color consistency required for premium applications while optimizing material costs.

Let-Down Ratio (LDR) Fundamentals

The Let-Down Ratio (LDR) is the percentage of color concentrate in your total shot weight. This seemingly simple metric controls everything from color strength to production costs and part consistency. LDR is expressed as a percentage and typically ranges from 0.5% to 6%, depending on the application requirements.

LDR = (Weight of Masterbatch ÷ Total Shot Weight) × 100%

The LDR directly impacts your color cost per part. A 2% LDR means you're paying for 2% masterbatch and 98% base resin, but achieving the target color strength. Understanding this relationship is crucial for cost optimization and quality control.

Why LDR Matters for Quality Control

LDR affects more than just color intensity:

• Color Consistency: Precise LDR control ensures uniform color across all parts
• Mechanical Properties: Excess masterbatch can affect impact strength and surface finish
• Production Costs: Optimal LDR minimizes expensive masterbatch usage
• Regulatory Compliance: Critical for food contact and medical applications

Basic LDR Calculation Formula

The fundamental masterbatch dosing formula is straightforward but requires precise measurements:

W_masterbatch = W_shot × (LDR ÷ 100)

Where:

• W_masterbatch = Weight of masterbatch to add (grams)
• W_shot = Total shot weight including base resin and masterbatch (grams)
• LDR = Let-Down Ratio (percentage)

This formula assumes you're adding masterbatch to pure base resin. When working with regrind or pre-colored materials, you must account for the existing color concentration.

Advanced LDR Formula with Regrind

When incorporating regrind, the calculation becomes more complex:

W_masterbatch = W_total × (Target LDR - Existing LDR_regrind) ÷ (100 - Target LDR)

This formula ensures you achieve the target color strength while maximizing regrind usage for cost savings.

Step-by-Step Dosing Calculation

Let's work through a comprehensive example for a polypropylene automotive component.

Example: Dashboard Trim Panel

Part Specifications:

• Part weight: 250 grams
• Runner weight: 45 grams
• Target LDR: 2.5%
• Masterbatch concentration: 50% pigment
• Base resin: PP homopolymer

Step 1: Calculate Total Shot Weight

W_shot = Part weight + Runner weight = 250g + 45g = 295g

Step 2: Calculate Masterbatch Weight

W_masterbatch = 295g × (2.5 ÷ 100) = 7.375g

Step 3: Account for Masterbatch Concentration

If your masterbatch contains 50% pigment, you need 7.375g of masterbatch to deliver 3.6875g of pure pigment. This affects your total shot weight calculation for precision molding.

Step 4: Calculate Dosing Rate

For a 30-second cycle time, the dosing unit must deliver 7.375g every 30 seconds, or 0.2458 g/second. This rate determines your equipment selection and calibration requirements.

Complex Example: Multi-Component Color Matching

For a custom color requiring three masterbatches:

• Black masterbatch: LDR = 1.2%
• White masterbatch: LDR = 0.8%
• Special effect: LDR = 0.3%
• Total shot weight: 180g

Individual weights:

• W_black = 180 × 0.012 = 2.16g
• W_white = 180 × 0.008 = 1.44g
• W_effect = 180 × 0.003 = 0.54g

Total masterbatch weight = 4.14g (2.3% combined LDR)

Dosing Unit Calibration & Accuracy

Dosing accuracy is critical for color consistency. Even small variations can cause visible color differences. Calibration involves measuring actual dispensed weight against target weight over multiple cycles.

Calibration Procedure

1. Zero the Scale: Ensure dosing unit and collection container are at zero
2. Set Target Weight: Program the dosing unit for your calculated masterbatch weight
3. Run Test Cycles: Dispense 10-20 shots and weigh the collected material
4. Calculate Accuracy: Compare actual vs. target weight
5. Adjust Parameters: Fine-tune screw speed, metering time, or calibration factor

Accuracy Standards by Application

Application	Required Accuracy	LDR Range	Typical Dosing Method
Automotive Exterior	±0.5%	1.0-3.0%	Gravimetric
Consumer Electronics	±0.3%	0.5-2.0%	Gravimetric
Medical Devices	±0.1%	0.2-1.0%	High-precision gravimetric
Packaging	±1.0%	2.0-6.0%	Volumetric

Calibration Frequency Guidelines

• Initial Setup: Daily for first week
• Production: Weekly or after material changes
• Critical Colors: Before each production run
• After Maintenance: Immediately after dosing unit service

Gravimetric vs. Volumetric Dosing Systems

The choice between gravimetric and volumetric dosing significantly impacts color consistency and operational costs.

Gravimetric Dosing Advantages

• High Accuracy: Weighs actual material dispensed (±0.1%)
• Density Compensation: Accounts for material variations
• Real-time Adjustment: Continuous weight feedback
• Suitable for Low LDR: Essential for <1% formulations

Volumetric Dosing Characteristics

• Cost Effective: Lower initial investment
• Simple Operation: No load cells required
• Higher LDR Applications: Better for 2%+ formulations
• Density Sensitive: Requires consistent bulk density

Hybrid Systems

Modern dosing units combine both technologies: volumetric feeding with gravimetric verification. These systems offer the best of both worlds with automatic density compensation and high accuracy.

Mixing Screw Design for Color Homogeneity

Screw design is the final frontier for achieving color consistency. The right screw ensures uniform masterbatch distribution throughout the melt.

Screw Geometry Fundamentals

Mixing Efficiency = Function of:

• Channel depth variations
• Flight clearance
• Compression ratio
• Mixing section design

Standard Mixing Screw Types

Screw Type	Mixing Mechanism	Best for LDR	Pressure Drop
Dalton Mixing	Secondary channels	1-4%	Low
Maddock Mixing	Chaotic mixing	0.5-3%	Medium
Barrier Mixing	Melt filtration	All ranges	High
Distributive Mixing	Laminar flow	>2%	Low

Melt Residence Time Considerations

Color homogeneity requires sufficient melt residence time for diffusion:

Residence Time = (Shot Volume × Number of Flights) ÷ (Screw Speed × Channel Volume)

Typical residence time for good color mixing: 2-4 minutes at processing temperatures.

Static Mixers vs. Dynamic Mixing Screws

The choice between static mixing nozzles and dynamic mixing screws depends on your production requirements and budget constraints.

Static Mixing Nozzles

Advantages:

• Lower Cost: Retrofit to existing screws
• No Pressure Loss in Screw: Maintains plasticizing capacity
• Easy Cleaning: No screw disassembly required
• Suitable for: Simple color changes, standard LDR ranges

Disadvantages:

• Pressure Drop: 10-30% increase in injection pressure
• Limited Mixing: Less effective than screw mixing
• Not for Low LDR: Poor performance below 1%

Dynamic Mixing Screws

Advantages:

• Superior Mixing: Better homogeneity than static mixers
• No Pressure Drop: Mixing integrated into screw design
• Better for Low LDR: Effective down to 0.1%
• Process Flexibility: Handles various materials and colors

Disadvantages:

• Higher Cost: Specialized screw design required
• Complex Maintenance: Screw changes more involved
• Material Restrictions: May not suit all resins

Selection Criteria

Factor	Choose Static Mixer	Choose Dynamic Screw
LDR Range	>1%	<1%
Color Changes	Frequent	Rare
Budget	Limited	Premium
Part Tolerance	Standard	Critical

Impact of Regrind on Color Matching

Regrind incorporation is economically essential but complicates color control. The existing color in regrind affects your LDR calculations and mixing requirements.

Regrind Color Analysis

Before using regrind, measure its color concentration:

Regrind LDR = (Pigment in Regrind ÷ Total Regrind Weight) × 100%

This value is typically 80-95% of the original masterbatch LDR due to pigment loss during processing.

Color Correction Formula

W_{masterbatch correction} = W_total × (Target LDR - Regrind LDR × Regrind %) ÷ (100 - Target LDR)

Regrind Guidelines

Regrind Percentage	Color Adjustment Required	Mixing Considerations
0-20%	Minimal	Standard mixing sufficient
20-40%	Moderate	Enhanced mixing recommended
40-60%	Significant	Special mixing screws required
>60%	Major	Color matching may be impossible

Tederic Control Integration

Tederic injection molding machines offer advanced dosing integration for precise color control.

Smart Dosing Features

• Real-time LDR Monitoring: Continuous weight verification
• Automatic Calibration: Self-adjusting dosing parameters
• Recipe Management: Stored color formulations with one-click recall
• Quality Integration: Color measurement feedback loops

Tederic Dosing Synchronization

The Tederic control system synchronizes dosing with screw recovery, ensuring consistent masterbatch addition regardless of cycle time variations. This eliminates color variations caused by timing differences.

Troubleshooting Color Inconsistencies

Systematic troubleshooting is essential for maintaining color quality.

Common Color Problems

Problem	Symptoms	Root Cause	Solution
Color Streaks	Visible color bands	Inadequate mixing	Increase back pressure, check screw design
Color Variation	Inconsistent shade	Dosing inaccuracy	Recalibrate dosing unit, check masterbatch quality
Poor Coverage	Spotted appearance	Low LDR or poor dispersion	Increase LDR, improve mixing
Color Shift	Gradual color change	Material degradation	Check processing temperatures, reduce residence time

Color Measurement Standards

• Spectrophotometer: Lab-grade color measurement (CIELAB system)
• Colorimeter: Production floor measurement
• Visual Standards: Approved color chips for comparison
• Statistical Control: Cpk > 1.33 for critical colors

Summary & Best Practices

Masterbatch dosing is both a science and an art. The LDR calculation provides the foundation, but successful implementation requires attention to equipment calibration, screw design, and process control.

Key Takeaways

• Calculate Precisely: Use the formula W_masterbatch = W_shot × (LDR ÷ 100)
• Calibrate Regularly: Verify dosing accuracy before each production run
• Choose Right Equipment: Gravimetric for low LDR, volumetric for high LDR
• Design for Mixing: Select appropriate screw geometry for your application
• Account for Regrind: Adjust formulations for existing color concentration
• Monitor Continuously: Use statistical process control for color consistency

By following these principles and leveraging Tederic's integrated control systems, you can achieve the color consistency required for premium molded products while optimizing your production costs.

This comprehensive guide covers the essential calculations and best practices for masterbatch dosing. For specific application assistance, consult with Tederic technical specialists or masterbatch suppliers.