Advertisement
Articles
Advertisement

Acrylic Polymer Injection Molding Primer

Thu, 10/20/2011 - 12:45pm
Joseph L. Mitchell, Charles Rissel, and Mark Aubart

Medical devices are molded from acrylic polymers to meet the requirements of a broad range of applications. Many of these devices are complex and challenge the skills of the injection molder with complicated mold designs that are difficult to fill. These challenges can be overcome with the selection of the proper grade of acrylic polymer and appropriate injection molding parameters.

Acrylics (polymethyl methacrylate or PMMA) are engineering polymers that exhibit excellent optical, thermal, mechanical, electrical, and chemical resistance properties. They are biocompatible and can be formulated to retain their water-white clarity when gamma irradiated. In addition, they can be bonded to many substrates by common chemical and thermal processes. The combination of excellent performance properties and low cost make these materials an easy choice for transparent, injection molded, disposable medical devices.

 

chart1.jpg
Table 1
These premium properties are only useful, however, because acrylics are also easy to injection mold. After drying at 180°F for four hours, they can be molded into a variety of simple or complex, thick- or thin-walled shapes. To illustrate the processability of representative grades, consider the melt flow and typical injection molding parameters for the Plexiglas Medical Resins shown in Table 1. These resins fall into two distinct categories: chemical resistant (Plexiglas CR Grades) and high flow (Plexiglas SG Grades).

Note that there is a significant difference in melt flow rate (MFR) between the CR and SG grades. Based solely on this data, one might assume that the CR grades are difficult to process; however, this is not the case. MFR data alone can often be misleading because it only provides melt flow characteristics at one shear rate. To complicate matters further, the MFR shear rate is extremely low and not representative of high shear rate, injection molding processes.

To better assess how the polymer will process under real world injection molding conditions, it is more informative to consider the spiral flow behavior of the polymer. During this test, the distance the polymer melt travels is measured when the material is injection molded into a long spiral mold cavity under standardized injection pressure, melt temperature, and mold temperature. This test better replicates the shear rates developed in the injection molding process and provides a more accurate indication of how well the polymer can fill a mold cavity. Spiral flow measurements for the CR Grades (shown in Table 1) are lower than, but similar to, the SG grades. These measurements are more in line with the observed injection molding performance of the CR grades.

 

chart.jpg
Table 2
To fully understand polymer melt flow, it is best to examine its viscosity across a broad range of shear rates. Consider two polymers, Plexiglas CR30 with an MFR of 0.8 g/10 min and a medical grade copolyester with an MFR of 5.0 g/10 min. Comparing MFR data alone, one might expect the copolyester to process more easily than Plexiglas CR30. However, just the opposite is true. The process is illustrated in the rotational rheometry plot of viscosity vs. shear rate (Table 2). At very low shear rates, typical for an MFR test, Plexiglas CR30 is more viscous. However, at the more applicable, higher shear rates developed during the injection molding process, Plexiglas CR30 is significantly less viscous than the copolyester, allowing for easier filling of mold cavities. Such variation is due to the exceptional shear thinning properties of Plexiglas Medical Polymers.

Acrylics have been used for decades in the medical market and are known for easy processing compared to other polymers. They can be processed in molds built for polycarbonate or copolyester and they can easily fill complex and thin walled molds because of their exceptional shear thinning properties. The combination of easy processing, excellent properties, and low cost has allowed acrylic polymers to occupy a prominent place in the transparent and disposable medical device market.

At Altuglas International, Joseph L. Mitchell and Charles Rissel are senior technical service engineers and Mark Aubart is research manager. Mitchell can be reached at 610-878-6979 or joseph.mitchell@arkema.com, Rissel at 610-878-6216 or charles.rissel@arkema.com, and Aubart at 610-878-6691 or mark.aubart@arkema.com.

Advertisement

Share this Story

X
You may login with either your assigned username or your e-mail address.
The password field is case sensitive.
Loading