January 2018

Emerging technology improves surface quality of medical housings

Mark Matsco, director of application development, Polycarbonates – North America, Covestro LLC Jessica Boyer, application development engineer, Polycarbonates – North America, Covestro LLC

What do skylights, automotive headlights, electronics and medical devices have in common? All utilize polycarbonates and polycarbonate acrylonitrile butadiene styrene (PC+ABS) blends – versatile materials that feature remarkable thermal and mechanical properties. For the medical market, polycarbonate resins offer optical clarity for components that require glass-like transparency. Polycarbonate resins and PC+ABS blends can also be pre-colored to achieve different colors and special effects required for a wide range of medical applications

Medical device original equipment manufacturers (OEMs) and their customers demand high-quality surfaces. A plastic component’s surface quality plays a vital role in the functionality and aesthetics of medical devices and equipment. The surface quality depends on three variables: the properties of the thermoplastic, the mold surface finish and the molding process.

Injection molding is often used for mass production of polycarbonate parts. This technology offers several benefits to medical device OEMs, including fast cycle time, excellent part-to-part repeatability and the ability to produce large quantities from a single injection mold. Injection molding also allows engineers and designers to design polycarbonate components of nearly any shape with tight tolerances and varying geometric complexity.

There are different methods to enhance the plastic part’s surface quality in the tool, including an emerging technology known as rapid heating and cooling molding (RHCM). This advancement in injection molding helps medical device OEMs and their customers improve surface appearance for medical housings while reducing manufacturing costs.

Rapid Heating and Cooling Molding
During the typical injection molding process, a thin, “frozen” skin layer is formed due to the temperature difference between the mold tool surface and the plastic melt when they come into contact during melt injection. A large temperature difference does not allow the molten plastic to fully replicate the mold surface before it solidifies, which can cause poor surface quality and highly visible weld lines in some instances. Weld lines on a medical device can be considered unacceptable in some applications from both a visual and structural integrity perspective. To improve part surface quality, a mold temperature closer to the glass-transition point of the polymer is needed. However, this can lead to longer cycle times and poor part quality using conventional molding technology.

FIGURE 1: Mold temperature profile using conventional injection molding (CIM) vs. RHCM.

In the RHCM process, the mold surface is superheated before the plastic is injected into the mold. Once the cavity is filled, the mold is cooled rapidly during the packing and cooling phases before the part is ejected. Figure 1 compares the RHCM process with conventional injection molding (CIM).

RHCM technology together with PC and PC+ABS blends provides an opportunity to achieve high-quality surfaces featuring high- and low-gloss finishes on the same part.
FIGURE 2: RHCM technology together with PC and PC+ABS blends provides an opportunity to achieve high-quality surfaces featuring high- and low-gloss finishes on the same part.

The benefits of RHCM for medical devices include the ability to create a high-quality surface by eliminating defects such as fillers at the surface, gate blush and weld lines while also being able to generate high- and low-gloss parts – all within a single cavity as shown in Figure 2. Surface imperfections can give a “low-quality” impression to an assembled device, which is not the perception medical device manufacturers want to give to patients and health care professionals.

Thin-walled parts typically have a higher reject rate with conventional molding due to poor surface quality and other factors. Many medical devices feature thin-wall design and/or varying degrees of wall thickness within the same part. Wall thickness changes can cause difficulty in surface replication and cooling. RHCM can significantly improve the surface quality of thin-wall parts, helping to achieve defect-free surfaces, even when using glass-filled polycarbonate blends.

Part surface quality using CIM (left) and RHCM (right).
FIGURE 3: Part surface quality using CIM (left) and RHCM (right).

Covestro has developed a multi-texture demonstration tool to highlight the benefits of RHCM as shown in Figure 3. This illustrates how the RHCM technology enhances surface replication to eliminate cosmetic defects such as weld lines, gate blush and rib read-through, as well as achieve high and low gloss on the same part.

Close-up of RHCM part.
FIGURE 4: Close-up of RHCM part.

Figure 4 shows a part molded with a high-gloss PC+ABS blend. The RHCM process allows for designers and molders to control the gloss level through different textures and polishes by adjusting the mold temperature used during the injection molding process. Despite the material naturally exhibiting high gloss, a matte finish was achievable through the use of laser texturing and micro-etching on a portion of the cavity while maintaining the mirror-like, high-gloss finish on the polished portion of the cavity.

Special equipment is required to control the rapid heating and cooling of the mold. The most commonly used approaches include fluid-, electrical- and induction-based temperature control systems. We recently installed a pressurized water-based dynamic mold temperature controller at our Pittsburgh-based North American headquarters to demonstrate improved surface aesthetics and quality of polycarbonates and PC+ABS blends. We also added an induction-based system to further expand the potential of RHCM for polycarbonate-based materials.

The pressurized water-based system’s dynamic mold temperature controller has a hot and cold water circuit. This allows the user to adjust each temperature to optimize the heating and cooling of the mold during all phases of the injection molding process. At the start of the cycle, the tool surface temperature can reach up to 180 degrees Celsius during injection. Typically, once the filling phase is complete, the controller is signaled to switch over from the hot to cold circuit to rapidly decrease the temperature prior to mold open. When this switchover occurs, there is a time delay until the mold temperature drops. This happens because the cold water must replace the hot water present in the lines between the valve station to the manifold and the manifold to the cooling channels within the mold. To optimize cycle time and reduce the delay, the manifold should be placed as close to the mold as possible with short hose connections to improve efficiency.

An induction-based temperature control system.
FIGURE 5: An induction-based temperature control system.

As shown in Figure 5, an induction-based temperature control system offers faster heating speed, higher temperature levels and improved controlled temperature distribution across the mold surface. This system uses induction coils to quickly heat the tool surface in seconds. Temperatures typically reach up to 180 degrees Celsius, however, up to 240 degrees Celsius is possible. The tool surface is cooled by conventional cooling lines. The induction coils’ location in the tool must be customized for each mold and embedded underneath the mold surface. Induction-based systems can result in higher tool costs due to the customization required in integrating the induction coils.

Medical demonstration tool shows how RHCM coupled with conformal cooling improves surface quality and appearance.
FIGURE 6: Medical demonstration tool shows how RHCM coupled with conformal cooling improves surface quality and appearance.

When designing a tool to be used with RHCM, it is critical to place the cooling lines as close as possible to the surface of the cavity, to improve process quality and efficiency. For complex-shaped parts, conformal cooling can be used to ensure the cooling channels remain close to the mold surface in visual regions of the part. Coupling RHCM with conformal cooling has been shown to reduce cycle times by 20 to 40 percent, while achieving a high-quality surface appearance. The uniform cooling of the mold also reduces rejects and molded-in stress as shown in Figure 6.

Standard vs. RHCM mold temperature.
FIGURE 7: Standard vs. RHCM mold temperature.

Processing experiments have shown that there is no significant difference in the mechanical properties of polycarbonates and PC+ABS blends using RHCM compared to CIM processes. Tensile bars were molded to compare the stress-strain curve properties between CIM and RHCM. Figure 7 shows that the maximum elongation is similar for both molding conditions.

Mold temperature and stress
FIGURE 8: Mold temperature and stress

Molded-in stress can be quantified via optical tests such as Strain-Optics. Different color-bands, spacing, and repeated colors represent different stress levels in the part. Figure 8 illustrates how during injection, mold temperature affects the amount of molded-in stress. In general, a reduction in stress level is shown in the center of the test bar for hotter molds achieved through RHCM.

Medical OEMs and their customers are continually looking to improve part aesthetics and optimize production to meet performance and cost challenges. With many advantages, injection molding has traditionally been the primary technology for mass production of polycarbonate parts for the medical industry. RHCM is an advancement in injection molding that brings a variety of benefits to the medical device market. This technology eliminates surface defects to deliver high-quality surfaces on thin-wall, complex designs – offering medical OEMs and molders countless possibilities and cost savings.

For more information, email us: plastics@covestro.com

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