plastic 2 shot injection molding
Two-shot silicone-thermoplastic injection molding is a method to create a silicone-and-thermoplastic part in one process. These parts are traditionally plastic molding individually and assembled as one 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 performance and consistent quality. The process offers the ability to produce complex mouldings from two different polymers at the same time during one machine cycle. Separate, but compatible, materials or colours are chemically bonded or mechanically interweaved during the forming process when the second material is injected into the mould, which avoids the need for further assembly or postprocessing. Alternatively, incompatible materials can be mechanically connected by two-shot moulding without them fusing to meet specific product design and assembly needs such as articulation or movement. Some of the benefits of the new moulding technology over conventional injection moulding machines with screw and barrel are:
Shot-to-shot consistency: There is no check ring in the newer design, which means that all the strokes of the metering unit result in material transfer into the mould cavity to yield tighter tolerances and greater shot-to-shot consistency.
Less degradation: Degradation is reduced because of the shorter residence time of the heated material in the machine by virtue of the smaller volume of material it holds. This prevents material deterioration and discoloration such as heat induced yellowing of clear plastics. In addition, wastage of expensive high performance polymers is reduced on purging.
Quick plastic mold changeovers: Results from the machine’s platens and tie bars acting as the bolster for the mould tool. To manufacture each time requires just a tool insert, which also saves money in tooling costs.
Low energy consumption
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. Since 2005, manufacturers have 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 after sterilization. Different grades have been developed to bond polycarbonate, polyester, polyamide and PEEK with silicones.
Two-shot processing requires knowledge of silicone-thermoplastic chemistry and adhesion characteristics. The materials must maintain appropriate processing temperatures to adhere to one another. In the material selection process, it is critical to first determine requirements for sterilization, clarity and biocompatibility. Selection of 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 160deg. C. Materials that offer high-heat stability allow for less differentiation in thermal dynamics of the silicone and thermoplastic molds. Higher temperatures permit faster curing and shorter cycle.
The thermoplastic process needs to be defined and optimized before any silicone is injected into the tool. 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 180deg. and the silicone is injected into the mold. The mold does not open until the silicone curing process inside the mold is complete. Silicone can be used in applications that require high-temperature, low compression set, and purity, but for which thermoplastic materials are not suitable. 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 160deg. C to avoid distortion. Materials with high heat-distortion temperatures are recommended including PC, polyamide, PEEK and polyester.
For thermoplastics, sharp corners not only negatively affect the filling of the mold, but also affect the final properties of the molded product. Sharp corners in the material flow path can cause stresses in the material, creating uneven flow. The uneven flow can lead to many defects such as 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. However, as an exception, a sharp corner may be acceptable in either a thermoplastic or a silicone part at the parting line, because it provides a much better shutoff of material flow and it is easier to machine.
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, gradual transitions need to be employed. The thermoplastic gate should be located at a thicker section to help eliminate sink marks and voids. 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. 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 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.
The two main disadvantages of two-shot molding that are of interest to medical device OEMs are 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.
Few tips useful when designing a product that might use two-shot molding:
Know your materials
Material compatibility is key in the two-shot process because not all materials bond well. The first injection places the thermoplastic material often around an insert such as the shaft of a surgical tool. It is followed by an injection of silicone, which requires high heat to cure. So the thermoplastic material must be able to withstand temperatures of 300F or more. Candidate thermoplastics that can handle the heat and bond to silicone include polycarbonate, polyester, nylon, PEEK, and ABS alloys. Optimize the bond between the two materials. Creating opportunities for the silicone and thermoplastic parts to bond generates the best covalent-bond strength.
Gate locations and wall thickness
The gates for thermoplastic parts should be located at their thicker sections. This avoids sink marks and voids. It is often useful to visualize the flow path before selecting gate locations on the thermoplastic part. The thermoplastic and silicone use separate runners. Silicone can then be used to aesthetically improve the look of gates or hide them all together. The gate location for thermoplastic must allow for silicone to flow. Variations to wall thickness in the thermoplastic part should be limited. Thickness variations on a thermoplastic part increase the possibility of warping and sink marks. Silicone, on the other hand, allows for more varied but still gradual changes to wall thickness.
Know the material’s shrink rate
Mold temperature, cavity pressure, cure time, and flow direction all affect shrinkage. It is important to understand the shrinkage of materials and account for it in the tooling and part design.
Get an expert second opinion of the design work
It is critical to work with an experienced two shot, silicone-thermoplastic manufacturer from the design stage. The right manufacturer can assist in a design that will function as a device component and lend itself to two shot, silicone-thermoplastic molding. The manufacturer should also be experienced in silicone molding and material compatibility.
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.
Shot-to-shot consistency: There is no check ring in the newer design, which means that all the strokes of the metering unit result in material transfer into the mould cavity to yield tighter tolerances and greater shot-to-shot consistency.
Less degradation: Degradation is reduced because of the shorter residence time of the heated material in the machine by virtue of the smaller volume of material it holds. This prevents material deterioration and discoloration such as heat induced yellowing of clear plastics. In addition, wastage of expensive high performance polymers is reduced on purging.
Quick plastic mold changeovers: Results from the machine’s platens and tie bars acting as the bolster for the mould tool. To manufacture each time requires just a tool insert, which also saves money in tooling costs.
Low energy consumption
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. Since 2005, manufacturers have 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 after sterilization. Different grades have been developed to bond polycarbonate, polyester, polyamide and PEEK with silicones.
Two-shot processing requires knowledge of silicone-thermoplastic chemistry and adhesion characteristics. The materials must maintain appropriate processing temperatures to adhere to one another. In the material selection process, it is critical to first determine requirements for sterilization, clarity and biocompatibility. Selection of 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 160deg. C. Materials that offer high-heat stability allow for less differentiation in thermal dynamics of the silicone and thermoplastic molds. Higher temperatures permit faster curing and shorter cycle.
The thermoplastic process needs to be defined and optimized before any silicone is injected into the tool. 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 180deg. and the silicone is injected into the mold. The mold does not open until the silicone curing process inside the mold is complete. Silicone can be used in applications that require high-temperature, low compression set, and purity, but for which thermoplastic materials are not suitable. 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 160deg. C to avoid distortion. Materials with high heat-distortion temperatures are recommended including PC, polyamide, PEEK and polyester.
For thermoplastics, sharp corners not only negatively affect the filling of the mold, but also affect the final properties of the molded product. Sharp corners in the material flow path can cause stresses in the material, creating uneven flow. The uneven flow can lead to many defects such as 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. However, as an exception, a sharp corner may be acceptable in either a thermoplastic or a silicone part at the parting line, because it provides a much better shutoff of material flow and it is easier to machine.
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, gradual transitions need to be employed. The thermoplastic gate should be located at a thicker section to help eliminate sink marks and voids. 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. 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 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.
The two main disadvantages of two-shot molding that are of interest to medical device OEMs are 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.
Few tips useful when designing a product that might use two-shot molding:
Know your materials
Material compatibility is key in the two-shot process because not all materials bond well. The first injection places the thermoplastic material often around an insert such as the shaft of a surgical tool. It is followed by an injection of silicone, which requires high heat to cure. So the thermoplastic material must be able to withstand temperatures of 300F or more. Candidate thermoplastics that can handle the heat and bond to silicone include polycarbonate, polyester, nylon, PEEK, and ABS alloys. Optimize the bond between the two materials. Creating opportunities for the silicone and thermoplastic parts to bond generates the best covalent-bond strength.
Gate locations and wall thickness
The gates for thermoplastic parts should be located at their thicker sections. This avoids sink marks and voids. It is often useful to visualize the flow path before selecting gate locations on the thermoplastic part. The thermoplastic and silicone use separate runners. Silicone can then be used to aesthetically improve the look of gates or hide them all together. The gate location for thermoplastic must allow for silicone to flow. Variations to wall thickness in the thermoplastic part should be limited. Thickness variations on a thermoplastic part increase the possibility of warping and sink marks. Silicone, on the other hand, allows for more varied but still gradual changes to wall thickness.
Know the material’s shrink rate
Mold temperature, cavity pressure, cure time, and flow direction all affect shrinkage. It is important to understand the shrinkage of materials and account for it in the tooling and part design.
Get an expert second opinion of the design work
It is critical to work with an experienced two shot, silicone-thermoplastic manufacturer from the design stage. The right manufacturer can assist in a design that will function as a device component and lend itself to two shot, silicone-thermoplastic molding. The manufacturer should also be experienced in silicone molding and material compatibility.
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.
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