Selecting the right material for the right application is critical to the success of any design project, particularly for challenging environments such as those associated with medical equipment and devices. In order to choose the right polymer, you must first understand which plastic material properties are most important for the integrity of your design. 

With so many boxes to check, specifying the right materials can be exceptionally complicated and time-consuming, especially for engineers who lack expertise and experience with plastics . Data sheets allow design engineers to compare key performance properties of different materials, but if you have limited plastic experience and are not familiar with the nuances of polymers, the information stated on material data sheets can be hard to make sense of and may leave you more confused than when you started. 

To help engineers who are new to plastics feel more confident evaluating and selecting materials, we are embarking on a new series aimed at demystifying polymer data sheets. Insight into how polymers are tested will help you properly interpret the data to assess the capabilities and limitations of plastic materials.

First up: melt flow.

 

What is Melt Flow, and Why Does it Matter?

Melt flow index (MFI), also referred to as melt flow rate (MFR) or melt flow number (MFN), is the measure of the ability of a melted polymer to flow under a particular pressure. It is one of the most frequently used tests to provide a quick index of processability and product quality control as it requires little operator training to provide proper results. 

MFI is an indirect measure of relative average molecular weight of a polymer, and it may be the only value on the data sheet that varies significantly from grade to grade within a product offering. A high MFI corresponds to a low molecular weight (MW) and vice a versa. Since MW drives performance in polymeric materials, the higher the MW, the better the performance. Assuming all other factors are kept constant, MFI can be used as a key parameter for lot-to-lot quality certification.

MFI is inversely related to viscosity, and it can provide an indication of the resin’s processability. High-viscosity materials flow with greater resistance and therefore more slowly during melt-processing than low-viscosity materials. Injection molders tend to prefer high-flow (low MW, low viscosity) materials because they make it easier to fill demanding flow paths. Extruders are more likely to prefer low-flow (high MW, high viscosity) materials because they make it easier to control the shape of a complex profile.


Calculating Melt Flow Rate

Melt Flow Index testing may appear to be rudimentary and is often outshined by more complex and modern test methods. Testing is performed using a melt flow extruded plastometer in accordance to either ASTM D1238 (Melt Flow Rates of Thermoplastics by Extrusion Plastometer) or ISO 1133 (Determination of the Melt Mass-Flow Rate and Melt Volume Flow Rate of Thermoplastics). There are small differences between the two methods, but they essentially perform the same function and manufacturers consider them technically equivalent.

The plastometer (figure 1) consists of a temperature-controlled cylindrical annulus through which a polymer melt is forced to flow through a capillary tube of specific length and diameter using a piston actuated by a standard weight. The weight of melt in grams flowing through the capillary in 10 minutes is the MFI of the polymer.

Figure 1. Diagram of plastometer

Plastometer-diagram

 

Interpreting Melt Flow Index

Melt flow data must be interpreted cautiously because the MFI test method has several shortcomings. First, the equipment setup is a pressure-limited configuration. The standardized weights create a constant load that forces the melt through the orifice. It is a constant-shear-stress test that provides a single point on a viscosity curve based on a single shear rate and temperature . Second, and more important, most plastics are shear thinning (meaning their viscosity decreases under shear strain) and they are actually processed at much higher shear rates than those observed during the MFI measurement.

Processing conditions can be accurately optimized by using a capillary rheometer to measure the viscosity of polymer melts as a function of temperature and rate of deformation (figure 2). Considered a controlled-shear-rate test, capillary rheometers ensure that testing conditions are truly representative of processing conditions, especially for high-pressure and high-speed techniques like injection molding.

Figure 2. Capillary rheology of Radel® PPSU with 17 MFI

Radel-ppsu-capillary-rheology-graph

 

 

Limitations of Data Sheets

Data sheets are useful in comparing similar grades of plastics from different suppliers. Unfortunately, test conditions rarely correspond to the product service conditions you have in mind. Data sheets should not be used to compare significantly different materials as they may fail in completely different ways.

Every polymer supplier provides a basic material data sheet that reports properties of a particular grade based on standardized testing. There is no standard for data sheets, however, so the selection of the properties listed and how they’re presented may differ from one to the other. Good plastic material suppliers can help you understand what the numbers mean, and they often have additional data to help you properly evaluate the performance capabilities of the material for your application.

 

Selecting the Right Supplier

The more completely you define the requirements for your medical device application, the better the chance you’ll have of finding the right material for the job. Often, the subtle differences in impact strength, chemical resistance, heat resistance and dimensional stability become the deciding factors in critical applications.

That’s where Solvay can help. With over 30 years of experience as a leading materials supplier to the healthcare industry, we can help you navigate the complexities of the material selection process. As an added resource to your design and engineering teams, we can also provide design guidelines for metal-to-plastic replacement, Moldflow™ process modeling, Finite Element Analysis (FEA), fabrication and assembly techniques, product testing, failure analysis and assistance with global regulatory submissions.