Engineering Insights: Understanding Melt Flow
Melt flow is an essential measurement in plastics manufacturing that identifies the rate at which a material flows when melted. But what does it mean for your injection molding operations? Nick Rose, Technical Development Engineer at M. Holland, explains what MFR and other related measurements can reveal about a polymer. Read on to learn how injection molders can use information about melt flow to enhance the material selection process.
What is Melt Flow Rate?
Melt flow rate (MFR) is a measure of a polymer’s viscosity and is regularly included in technical data sheets and certificates of analysis. Expressed in grams per 10 minutes (g/10min), MFR has an inverse relationship to viscosity — as the MFR increases, the viscosity decreases. With injection molding, a material with low viscosity and a high MFR will flow more freely when melted, while a material with high viscosity and a low MFR will be harder to work with.
“Although MFR was originally used to assess a material’s average molecular weight, today it is primarily used to gauge a plastic’s processability,” Nick explained. “A polymer with a high average molecular weight and a low MFR will often showcase improved material properties, such as strength and toughness, dimensional stability, and chemical and heat resistance.”
The use of fillers, glass and additives can influence the density of a polymer and dramatically affect MFR without changing the molecular weight. For this reason, some suppliers refrain from publishing MFR for filled materials.
If MFR is not published, it can be found by conducting one of two standard tests: ASTM D1238 and ISO 1133.
Melt Flow and Melt Volume
Over the years, melt volume rate (MVR) has slowly made its way onto data sheets, either alongside or in place of MFR. MVR is a measure of volumetric flow rate expressed in cubic centimeters per 10 minutes (cm3/10min). MVR is also determined through the ASTM D1238 and ISO 1133 tests and has the same inverse viscosity relationship as MFR. A polymer with a higher MVR will have lower viscosity, meaning it will flow better in injection molding applications according to Nick.
“Most data sheets will include a polymer’s solid-state density but not necessarily its melt density,” Nick advised. “The melt density measurement helps injection molders determine the barrel capacity for a given material — an important step to avoid under-sizing or over-sizing barrel capacities, which can lead to quality issues and reduce production efficiency. However, you can calculate the melt density of a polymer by dividing the material’s MFR by its MVR.”
Alternatives to MFR and MVR
For some materials, suppliers will use intrinsic viscosity or viscosity number instead of MFR and MVR. These alternate measurements focus on molecular weight, with higher numbers indicating higher viscosity. Intrinsic viscosity is found using ISO 1628, while viscosity number is determined by ISO 307. Both tests indicate the average molecular weight of the material.
“Intrinsic viscosity and viscosity number are more commonly used for hygroscopic materials like PET and nylon,” Nick said. “Hygroscopic polymers absorb moisture, so traditional MFR testing, which requires materials be very dry, can produce widely varying results depending on the level of moisture in the material. Instead, hygroscopic materials are tested by dissolving a small amount of the polymer in an appropriate solution. This way, moisture content will not affect the results.”
Using Melt Flow in Material Selection
For injection molders, a material with high viscosity can be more difficult to process because it flows like cement. For extruders, however, high viscosity can be advantageous. That’s why it is incredibly important for molders to understand the definitions of MFR, MVR, intrinsic viscosity and viscosity number.
“If the melt flow of a material is not readily available, I strongly recommend molders run the appropriate tests on their own. Even if you have access to the measurements, testing the melt flow on your own is a proactive step that can be beneficial for incoming quality inspection. It’s not an expensive test and can help verify a material’s melt flow before further investments are made.”
Careful consideration of melt flow is important when selecting a material for production. The manufacturing process, part/tool design and intended part performance can be impacted by any material’s given melt flow. Understanding this early in the material selection process can help save time, money and potential headaches.
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Disclaimer: The information provided in this post is intended solely for general guidance. Outcomes vary based on individual circumstances, and M. Holland Company, LLC does not ensure a specific result. Clients shall use their own independent skills and expertise when testing any application of technical support. M. Holland is not responsible and will not be liable for any discrepancies between expected and actual outcomes. M. HOLLAND DISCLAIMS ALL WARRANTIES, WHETHER EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION ANY WARRANTIES OF MERCHANTABILITY OR FITNESS OF A PRODUCT FOR A PARTICULAR PURPOSE.
Nick delivers technical plastics expertise in material selection, engineering, process improvements, part/mold design and more. His 17-year career spans the automotive, packaging, injection molding and extrusion markets, covering process engineering, quality and continuous improvement. Nick earned a B.S. degree in Plastics Engineering Technology from Ferris State University, along with Master Molder I and Master Molder II certifications.