Acrylic Polymer Injection Molding Primer
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.
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.
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 firstname.lastname@example.org, Rissel at 610-878-6216 or email@example.com, and Aubart at 610-878-6691 or firstname.lastname@example.org.