Two-Shots: Molding for Silicone and ThermoplasticsTwo-Shots: Molding for Silicone and Thermoplastics

COVER STORY: MOLDING

Two-shot silicone-thermoplastic molding is a method to create a silicone-and-thermoplastic part in one press and in one process. These parts are traditionally molded individually and assembled as one completed medical device component. The two-shot process eliminates secondary operations and assembly, which are often the main contributors to increasingly higher part costs. By reducing the chance of misalignments seen in traditional inserts or in overmolding processes, the two-shot process enables improved part performance and consistent part quality. Two-shot also allows manufacturers to eliminate a tool and the associated tool validation costs.

Most importantly, the technology provides design engineers with increased freedom in part design. Two-shot eliminates the need to design for assembly. It may also enable medical OEMs to bring high-quality, low-cost medical device components to the market faster than other molding processes.

Two-shot is not a new manufacturing technology. Silicone-thermoplastic two-shot molding has been used extensively for more than 20 years in automotive and industrial applications, but has only recently been introduced into the medical device market. The medical market was slow to adopt this process because until recently, there were no commercially available USP Class VI self-bonding grades of silicone.

Over a two-year period, silicone manufacturers developed commercially available self-bonding silicones for the medical market that could chemically bond to rigid thermoplastic polymers during the two-shot molding process and could maintain bond strength poststerilization. Different grades have been developed to bond to polycarbonate, polyester, polyamide (nylon), and polyetheretherketone (PEEK).

How It Works

Two-shot processing requires knowledge of silicone-thermoplastic chemistry and adhesion characteristics. The materials must maintain appropriate processing temperatures in order to adhere to one another. In the material selection process, it is critical to first determine requirements for sterilization, clarity, and biocompatibility.

Selecting a self-bonding silicone and thermoplastic material that is appropriate for the requirements is crucial. It is also important to choose a thermoplastic material with a high softening temperature that meets or exceeds 300˚F. Materials that offer high-heat stability allow for less differentiation in thermal dynamics of the silicone and thermoplastic molds. The higher the softening point of the thermoplastic material, the higher the temperature at which the silicone can be cured during processing. Higher temperatures permit faster curing and shorter cycle times.

Once a thermoplastic material is selected, the thermal data can be used in running rheology tests on the silicone. Rheology testing determines the temperature at which the silicone begins to cure, as well as the cure profile value for various temperatures. This testing allows the processing temperatures to be chosen before molding.

After the materials and temperature processing range are determined, the two-shot process begins. It is critical to produce thermoplastic parts first. The thermoplastic process needs to be defined and optimized before any silicone is injected into the tool. As long as the temperatures selected were based on accurate analytical testing, the two-shot process should be set up as a standard thermoplastic process, and then as a standard silicone process.

Knowledge of thermodynamics is required for the two-shot silicone-thermoplastic molding process. In this process, the purpose is to solidify a thermoplastic melt and to try to heat and cure the liquid silicone rubber. A properly designed two-shot mold, based on thermodynamic principles, is the first step in being able to accurately process a two-shot part. The mold typically can be broken into four quadrants: three cold quadrants to cool the thermoplastic material and one hot quadrant for curing silicone.

The thermoplastic material is the first injection shot; the mold is then rotated 180˚ and the silicone is injected into the mold. The mold does not open until the silicone curing process inside the mold is complete. The final result is a completed silicone-thermoplastic medical device component.

The keys to successful processing are understanding the thermodynamics of the mold and choosing a material combination that is compatible at the required thermal conditions.

Design

Designing medical device components that combine silicone and thermoplastic materials follows basic design considerations for the silicone and thermoplastic parts. Silicone can be used in applications that require high-temperature use, low compression set, and purity, but for which thermoplastic materials are not suitable. The silicone enables designing for applications for which thermoplastic materials cannot meet the specifications. Understanding and considering two-shot silicone-thermoplastic processing at the design stage results in a successful two-shot part.

At the early stages in the design process, expectations of part functionality should be determined and clearly defined. The part should be examined for characteristics that are critical to function and design, including locations of parting lines and gates.

After the basic design is complete and main characteristics are defined, the process of designing the part for two-shot molding begins. Typical thermoplastic and silicone design considerations can be used as a baseline for the two-shot silicone-thermoplastic molding process.

Choose a Thermoplastic Material that Can Withstand the Processing Temperature of Silicone. The silicone is cured in the high-heat mold. As a result of the two-shot silicone-thermoplastic molding process, the thermoplastic material must withstand a high mold temperature of around 300˚F to avoid distortion. Materials with high heat-distortion temperatures are recommended. These materials include (but are not limited to) polycarbonate, nylon, PEEK, and polyester.

Avoid Sharp Corners. For thermoplastics, sharp corners not only negatively affect the filling of the mold, but also affect the final properties of the part. Sharp corners in the material flow path can cause stresses in the material, creating uneven flow. Depending on the location, the uneven flow can lead to many defects such as nonfills, trapped air, and flow lines.

In silicones, sharp corners create tears in the silicone during demolding. Silicone flows more easily into a rounded corner than a sharp corner, which optimizes the flow path and helps prevent any possible flow defects.

There is one exception to this rule: A sharp corner may be acceptable in either a thermoplastic or a silicone part at the parting line. A sharp corner at this location is desirable because it provides a much better shutoff of material flow and it is easier to machine.

Keep a Constant or Gradual Transition in the Wall Thickness. It is important to have uniform wall thickness in the thermoplastic. Uniformity helps mold filling and prevents warping and sink marks in the completed part. If a part design has thick sections in load-bearing areas, substitute by using uniformly thick ribs. Uniform wall thickness promotes more-uniform fills and faster cycle times, which ultimately result in a more consistent and reliable part. If thicker sections are necessary, employ gradual transitions. Figures 1 and 2 show the basic concept of a two-shot mold.

Unlike thermoplastics, silicone can have varying wall thicknesses. However, it is critical that the transition from a thin to a thick section is gradual. A gradual transition helps with mold filling. Keep in mind that the thicker the wall, the longer it will take to cure the silicone, which increases the cycle time and cost. Silicone can also be molded into thin membrane sections of 0.015 ± 0.0015 in. thickness.

Consider Material Gating Locations. Position the gate in an aesthetically pleasing location. But also think about the function and manufacturing of the part. Based on the critical areas that were previously defined, the gate needs to be located where it will not interfere with the functionality, such as a sealing surface or fitting location.

The thermoplastic gate should be located at a thicker section to help eliminate sink marks and voids. When choosing a gate location, consider he material's flow path. If there is a point in the part where the flow will split and then rejoin, causing a knit line (or weld line), reexamine the position. Consider whether this knit line is at a point of high stress. Knit lines are weak spots in the part and will be the first point of failure if located in a high-stress area. If a knit line is unavoidable, properly locate the gate where the resultant weld line is in a non-load-bearing area.

One key advantage of two-shot silicone-thermoplastic molding is the ability to design the silicone layer to conceal the thermoplastic gate, providing a completed-part look. However, it is important to know the location of the silicone gate on the part. And a part may have multiple silicone locations. In these cases, the injection location needs to be situated where the silicone can flow.

Make Sure All Silicone Features on the Part Can Be Filled. A two-shot part may contain multiple two-shot features, such as soft-grip and sealing features, and membranes. To save on cost, depending on the part size, design the thermoplastic section to include runner segments to connect all silicone locations. For example, consider a cylindrical part that contains a seal on the top and on the bottom. Instead of gating the part on both sides and using two cold-runner drops, the thermoplastic cylinder can be designed with recessed channels to allow silicone to flow down from one silicone sealing feature to the other sealing feature.

Research Materials to Define Shrinkage. Shrinkage of a thermoplastic part can vary significantly, depending on the base thermoplastic and additives or fillers. Typical shrinkage of thermoplastics varies from 2 to 5%. The optimal shrinkage value to reference is the one supplied directly from the manufacturer.

In a two-shot design, the thermoplastic base part is typically used to create shutoff locations. If shrinkage is miscalculated, the silicone does not fill properly. Therefore, understanding thermoplastic shrinkage and considering that shrinkage in the design is critical.

Silicone varies significantly from thermoplastic. The liquid silicone is maintained at room temperature during plastication and is injected into a hot mold. The silicone expands during molding and shrinks as it cools. Typical silicone shrinkage is 2–3%. Factors such as mold temperature, cavity pressure, flow direction, and postcure affect the amount of shrinkage.

Design the Part to Optimize Bonding. It is important to optimize the part design to enable the strongest bond. Allow large areas of contact between the silicone and thermoplastic to create a significant bond. Also, where possible, include mechanical interlocks. Doing so will provide chemical and mechanical forces that function to bond the silicone and thermoplastic.

Limitations

The two-shot technique is not necessarily the best option for every situation. Although the cost savings of two-shot molding can be significant, the lead time for tooling is longer, and tooling costs are higher compared with traditional molding.

The two main disadvantages of two-shot molding that are of interest to medical device OEMs are the longer lead times and higher tool costs. Justifying the initial investment is specific to each application. Production volume, lower piece price, and elimination of assembly are key economical justifications of two-shot molding.

Conclusion

Two-shot molding can provide significant benefits in part quality. Further, it provides a cost-efficient means of manufacturing medical device components comprised of adjoining silicone and thermoplastic parts. Two-shot molding eliminates costly secondary operations and assembly, the main contributors to increasingly higher part costs. It also eliminates the additional tooling and validation costs and improves part performance. Device OEMs can expect a process that offers consistent quality and allows freedom in component design.

Acknowledgments

The author would like to thank Mark Simon, Danny Ou, Adam Nadeau, and Chuck Klann for their contributions to the article.

Sarah Voss is the medical marketing product specialist at Saint-Gobain Performance Plastics (SGPPL; Portage, WI). She can be reached at [email protected].

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