What Are LDR and MFI? A Complete Technical Guide for Developing PE and PP Formulations
What Are LDR and MFI? A Complete Technical Guide for Developing PE and PP Formulations
For process engineers and production managers in the polymer industry, two parameters are absolutely fundamental — LDR and MFI. Choosing them incorrectly results in the most expensive types of failures: color mismatch, surface defects, equipment damage and, worst of all, loss of customer trust. Miscalculating LDR or ignoring the MFI of the masterbatch is the biggest mistake companies make when trying to "save" on additives.
In this guide, as a technological partner of Caucasus Polymer Group, we explain in detail what these values mean and how proper selection for polyethylene (PE) and polypropylene (PP) directly affects your product quality and total production cost.
What Is LDR — the Let Down Ratio?
LDR (Let Down Ratio) represents the percentage dosage of a masterbatch or additive in the base resin. It is calculated by weight.
For example, an LDR of 2% means that to produce 100 kg of final compound, you need 98 kg of your natural polymer (PE or PP) and 2 kg of masterbatch.
Why LDR Is Not Simply “More” or “Less”
Choosing the right LDR is always a balance between economics and quality:
- Impact on cost: If one supplier recommends an LDR of 4% for a color masterbatch and another supplier offers 2%, the second option — even if more expensive per kg — becomes **twice as cost-efficient** overall. We always calculate the real coloring cost per 1 kg of finished product.
- Impact on functional properties: If you dose only 0.5% of a UV stabilizer or HALS instead of the recommended 1.5%, your agricultural film or outdoor furniture will fail before the warranty period ends. Too much Slip additive causes printing and adhesion issues.
- Impact on color: Too low LDR of white (TiO₂) or black (Carbon Black) masterbatch results in poor opacity, low coverage and unwanted grey tones.
What Is MFI — the Melt Flow Index?
MFI (Melt Flow Index), also known as MFR (Melt Flow Rate), is a key parameter that defines the viscosity of a polymer in the molten state. It shows how many grams of polymer pass through a standard die in 10 minutes under defined temperature and pressure.
A simple rule:
- High MFI = low viscosity → polymer is very **fluid**. Suitable for thin-wall injection.
- Low MFI = high viscosity → polymer is **thick**. Suitable for pipe extrusion or blow molding jerrycans.
The Golden Rule of Polymer Processing: MFI Compatibility
This is the most crucial and most underestimated part of any formulation.
The MFI of the masterbatch must match the MFI of your base polymer (PE or PP).
If you mix materials with incompatible MFI values, they cannot homogenize properly in the screw. You will get defects even if the LDR was calculated perfectly.
Practical Guide: How LDR and MFI Work Together
Case 1: HDPE or LLDPE Film Extrusion
Task: Produce a thin HDPE “T-shirt” bag with 20% CaCO₃ mineral filler.
Problem: The film tears, shows surface defects, or develops “shark skin” — technically melt fracture.
Analysis: Most likely, the MFI of the CaCO₃ masterbatch and the MFI of the HDPE base resin differ significantly. Additionally, in LLDPE/HDPE film extrusion, solving “shark skin” requires not only the right MFI but also a Process Aid (PPA) additive.
Our solution: We select HDPE and CaCO₃ grades with closely matched MFI and introduce 1–2% Process Aid. Result — stable bubble formation and a smooth, defect-free film.
Case 2: Injection Molding with PP Homo or Block
Task: Mold a thin-wall IML container.
Problem: Long cycle time and warpage of the molded part.
Analysis: The base PP resin has too low an MFI — the material is too viscous and crystallizes slowly.
Our solution: We recommend high-MFI PP Homo (e.g., 25–40 g/10 min) and add a nucleating masterbatch (Nucleator). The nucleator accelerates crystallization.
Result: Cycle time reduced by 15–20% and stable part geometry.
Why Formulation Audit Matters More Than Price per Kilogram
Never select a masterbatch or raw material solely by its price per kg. A “cheap” masterbatch with incorrect MFI or a high LDR will cost your factory thousands of lari due to defects and machine downtime.
At Caucasus Polymer Group, we follow an engineering-first approach. Before recommending any product, we perform a full audit of your formulation. We analyze your equipment, your base PE/PP resin, and your end application. Only after that do we recommend a masterbatch with the correct LDR and compatible MFI, provide samples, and support your pilot production.
Facing a difficult technical challenge? Contact us — we will prepare a precise formulation calculation for your production.
