September 2015

Material selection and design considerations for thin-walled injection molded medical parts

Eric Tan (Engineering Technology Manager), Raymond Wong (Product Technology Manager), Bayer MaterialScience Limited Business Unit Polycarbonates APAC

Material selection is very important for successful plastic applications development. Basic considerations include appearance, colors, and key properties such as mechanical strength and heat resistance to meet the in-use functional requirements. For medical applications, additional considerations in disinfectant compatibility, sterilization requirements, and regulatory fulfillment are also necessary.

Polycarbonate resins and blends have excellent thermal, mechanical and environmental properties, which makes them suitable for a wide variety of medical applications. Some medical parts require glass-like transparency, and polycarbonate resins are a possible choice; while polycarbonate blends (e.g. PC/ABS) are generally opaque. Polycarbonate resins and blends can be pre-colored to achieve different colors and special effects to meet the requirements of various applications.

Bayer offers the following polycarbonate resins and blends products specially developed to meet the special requirements of medical applications – Makrolon® polycarbonate, Apec® high heat polycarbonate, and Bayblend® PC/ABS blends. For typical medical applications of these products, please refer Bayer’s Medical Brochure “Medical Reference Guide – High Performance Materials for the Healthcare Industry” which is available in Bayer MaterialScience Plastics Product Center website: www.plastics.covestro.com/Applications/Medical-Grade-Plastics

Medical devices often come into contact with a variety of substances from medical tubing, drugs, IV (intravenous) fluids and antiseptics. Ensuring the chemical compatibility and sufficient chemical resistance in the expected exposure is essential. Conducting chemical compatibility test on specimen bars, molded parts, or even end-products, is recommended in the case of uncertain.

Medical devices usually require sterilization before use. The most common sterilization methods in the medical industry are:

  1. heat
  2. ethylene oxide gas
  3. high-energy radiation.

Sterilization requirements have strong influence in material selection also. For example, steam sterilization (134°C) is too hot for Markolon® PC and Bayblend® PC/ABS, so high heat PC grade such as Apec® 1745 will be necessary. High-energy radiation sterilization, as another example, will have different yellowing effects on different material grades – those high-energy radiation stable grades (e.g. Makrolon® Rx2430) will have much less yellowing as compared to the general Makrolon® or Apec® grades. Please note that the resistance to different sterilization methods depends not only on the selected material grade, but also on part design, processing and other factors, so the manufacturer must check the suitability in each individual case.

Medical applications, which are intended under normal use to be brought into direct contact with the patient’s body (e.g. skin, body fluids or tissues, including indirect contact to blood), will need to fulfill various regulatory requirements. Bayer offers a range of medical grades which meet certain biocompatibility test requirements of USP Plastics Class VI and ISO 10993-1, and can be considered as potential material candidates for various medical applications. The manufacturer of the final end-use product should do further evaluation, for example testing the final product under actual end-use requirements, to determine the suitability of the selected material grade for a specific medical application. For more details, please again refer Bayer’s Medical Brochure. Moreover, some local regulations should be noticed. For example YY/T 0806-2010 standard in China, it is about “Polycarbonate material for manufacture of infusion, transfusion and injection equipments for medical use and other medical devices”.

Thinner wall thickness is sometimes a high priority requirement for medical parts, for example in high volume production single-use units, in order to reduce material cost and increase production efficiency. Selecting an appropriate material grade is for sure important. At the same time, it is equally important to select a technically competent material supplier to support the development stages for example in part/mold design and CAE analysis.

Table 1 shows the high flow PC grades offered by Bayer for medical devices. For thin-walled design a very good flow grade will be necessary, and low viscosity grade such as Makrolon® 2258 could be considered. This grade has excellent flow, and at the same time still maintains a very good impact performance (with “no break” in Charpy un-notched impact tests).

Thinner wall thickness usually means smaller processing window and reducing mechanical performance, so following the plastic part design rules will be very important. Figure 1 shows some examples of plastic injection molding part design guidelines.

  • Proper ribs and bosses designs are important to prevent filling and ejecting problems, which will be usually more critical for thin-walled parts. Please note that sometimes it is necessary to adapt the design rules to thin-walled scenario, for example, ribs can be thicker without causing sink mark problem in thinner parts.
  • Keeping uniform part thickness is another important rule for plastic part design, especially for thin-walled parts because of less stiffness and easier for warpage. One common way to keep uniform thickness is by coring out thick sections. These thick sections should be removed because they will not only cause warpage, but also cause sink mark and long cooling time.
  • Corner radii are necessary to prevent weak point due to stress concentration. Radii at both inner and outer corners should originate from the same point; otherwise a uniform thickness cannot be maintained at the corner.

Proper gating location is another important consideration for thin-walled design – the target is to have acceptable cavity filling pressure and balanced-flow to prevent over-packing, otherwise there will be a risk of high internal stress which can significantly reduce the part’s performance and quality. Figure 2 shows the theoretical flow-length of Makrolon® 2258 at different thickness. The flow-length (distance from gate to end-of-filled area) is considered to be acceptable if the corresponding cavity filling pressure is within the recommended limit, which is normally taken as 65 MPa for general molding, and can vary depending on the specific requirements of each application. Such flow-length diagram is a simple but effective tool for preliminary gating feasibility checking.

For more complicated part design, the flow-length is less well defined, and CAE mold-filling analysis software will be necessary for a more accurate estimate of filling pressure; such analysis can also predict the filling pattern and determine whether the flow is balanced or not. Figure 3 shows some examples of cavity filling pressure results, in which the one with unbalanced-flow will have a large area of over-packing (the red area). Such over-packing will cause high internal stress and shrinkage variation resulting in higher tendency of warpage.

Summary

Selecting an appropriate material grade and having proper design are essential for successful development of thin-walled medical parts. To select the appropriate material grade, one should consider basic properties such as mechanical strength and chemical resistance, and make a balance with the flow behavior. Additional considerations are also necessary in order to meet the sterilization and regulatory requirements for the specific medical application. Regarding to design, following the basic plastic design rules and selecting a proper gating location are some of the most important considerations, and use of CAE is recommendable especially for relatively complicated parts.

For more information, visit plastics.covestro.com or email plastics@covestro.com.

« Back to Overview