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        Kynar? Resin Processing Guidelines

        Watch the PROCESSING episode from our video series

         

        EXTRUSION

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        Smooth Kynar? PVDF products of all types can be extruded at high rates without extrusion aids, lubricants or heat stabilizers. Kynar? resins can be processed using standard equipment with materials of construction similar to those used to process PVC or polypropylene. Drying of Kynar? resin is usually not required; however, it has been shown to reduce some surface blemishes in film, sheet and pipe extrusion. High viscosity polyethylene (PE) can be used as a purge compound at the end of a production run. However, if the extrusion equipment is not properly cleaned after purging, PE will act as a contaminant in subsequent runs. Cast acrylic resin can also be used as a purge compound, and unfilled Kynar? grades can be used to purge flame-retardant Kynar? grades.

        PIPE EXTRUSION

        Kynar? 740 resin and Kynar? 1000 resin are the primary grades used in chemical pipe applications. Kynar? 740-02 resin contains a smoke suppressant package and is primarily used for waste drainage pipe and fittings. Kynar Flex? 2850 copolymer can be used for applications requiring higher impact and stress crack resistance.

         

        Standard metering screws with an L/D ratio of 24/1 and a compression ratio of 3/1 are commonly used. The screw should have an even flight distribution between the feed, transition and metering zones. Better temperature and output stability can be achieved by using a barrier-type screw. Maddox and spiral mixers are also acceptable, but pin type mixers are not recommended. Good temperature control of the extruder and tooling is required for optimal processing.

         

        It is important to eliminate areas of slow-flow, or hang-up points, to prevent Kynar? resin from discoloring. Common areas of melt accumulation include behind the breaker plate, at undercuts and in any other stagnant areas. In-line spider dies are commonly used for producing Kynar? pipe. The design should minimize material inventory in the head and be streamlined to eliminate material hang-up. Spiral dies should be designed similarly and can provide better weld line strength.

         

        The draw down ratio (DDR) for tip and die selection generally ranges from 1.3-2.1 (area DDR), or 1.05-1.5 (OD DDR), but varies based on the dimensions of the finished pipe. The optimum land length will vary based on pipe size, process conditions and material grade. Extrusion temperatures range between 200°-240°C (392°-464°F), but vary depending on material grade, as well as pipe and tooling size. Lower melt temperatures provide a “stronger” melt and are used when product whiteness is of primary concern.

         

        Pipe is typically sized using a vacuum calibration system equipped with either a solid brass or a brass disk caliper, which is oversized to accommodate material shrinkage. See Table XII for common pipe processing temperatures.

         

         

        PIPE EXTRUSION BARREL TEMPERATURES - TABLE 1

        GRADE

        BARREL TEMPERATURE °C

         

        REAR

        MIDDLE

        FRONT

        HEAD

        DIE

        Kynar? 460

        200 - 230

        220 – 240

        230 - 250

        230 - 250

        230 - 260

        Kynar? 740

        190 - 220

        200 – 230

        210 - 240

        210 - 240

        210 - 250

        Kynar? 1000

        190 - 220

        200 – 230

        210 - 240

        210 - 240

        210 - 250

        Kynar Flex? 2850

        190 - 220

        200 – 230

        210 - 240

        210 - 240

        210 - 250

        TUBE EXTRUSION

        The technical information mentioned for pipe extrusion (equipment, processing temperatures, etc.) also applies to tube extrusion. Kynar? homopolymers can typically be calibrated using standard contact sizing equipment similar to that used to produce pipe. The OD DDR used for contact sizing of tubing can range from 1.2 - 1.5. All other Kynar Flex? PVDF resin grades, as well as thin-walled tubing, are best processed using non-contact sizing calibrators. See Table XIII for common tube processing temperatures. Please contact our technical group for more information on this process.

         

         

        TUBE EXTRUSION BARREL TEMPERATURES - TABLE 2

        GRADE

        BARREL TEMPERATURE °C

         

        REAR

        MIDDLE

        FRONT

        HEAD

        DIE

        Kynar? Homopolymer

        195 - 220

        210 - 240

        210 - 240

        210 - 240

        210 - 250

        Kynar Flex? 3120

        195 - 220

        210 - 240

        210 - 240

        210 - 240

        210 - 250

        Kynar Flex? 2850

        195 - 220

        210 - 240

        210 - 240

        210 - 240

        210 - 250

        Kynar Flex? 2800

        195 - 220

        210 - 240

        210 - 240

        210 - 240

        210 - 250

        Kynar Flex? 2750

        195 - 220

        200 - 240

        200 - 240

        210 - 240

        210 - 250

        Kynar Superflex? 2500

        195 - 220

        200 - 240

        200 - 240

        210 - 240

        210 - 250

         

        INJECTION MOLDING

        Standard injection molding equipment and tooling can be used to process Kynar? resin. No specialty materials of construction are required, but chrome or nickel plating of polymer contact surfaces is recommended to prevent pitting.

         

         

        MOLD SHRINKAGE DATA

        Melt temperatures will vary based on the part geometry, tooling and resin grade. In general, lower melt and mold temperatures can be used effectively with low viscosity Kynar? copolymer grades. Kynar? resin is best processed with a large sprue or edge gates. To produce the best quality parts, fill the sprue, runners and gates slowly and then ramp up the injection speed until the screw reaches its transfer position. Small pin or subgates can be used for smaller parts and will require faster injection speeds and higher melt temperatures to fill the part. If a process calls for use of a hot runner system, please contact a technical representative before committing to this practice.

         

        Kynar? PVDF resin requires generous venting at the end of the filling process or a burning phenomenon known as dieseling can occur. Kynar? PVDF resin is a highly crystalline material and will exhibit shrinkage. Shrinkage rate is a function of part thickness, flow direction (which is a function of gate type and location) and processing conditions (See Table 1 below). Voiding can be a problem when molding Kynar? resin due to the polymer’s high crystallinity. Good part design practices are required to prevent voiding in thick sections of the part. See Table 2 below for common injection molding temperatures.

         

        Arkema technical service is available to discuss tooling and processing of Kynar? PVDF.

         

         

        KYNAR? PVDF MOLD SHRINKAGE RATE - TABLE 3

        GRADE

        % SHRINKAGE*

        IN FLOW DIRECTION

         

        CROSS FLOW DIRECTION

        Kynar? 370

        1.2 - 3.5

        0.8 - 3.0

        Kynar? 710

        1.9 - 3.5

        1.6 - 3.0

        Kynar? 720

        2.0 - 3.5

        1.6 - 3.0

        Kynar? 740

        2.8 - 3.5

        1.9 - 3.0

        Kynar Flex? 2850-04

        1.9 - 3.5

        1.6 - 3.0

        Kynar Flex? 2800-00

        2.5 - 3.5

        1.6 - 3.0

         

        *Measurements taken after 24 hours at ambient conditions.

        Actual shrinkage percent is related to size of the part and its geometry.

         

         

        INJECTION MOLDING BARREL TEMPERATURES - TABLE 4

        GRADE

        BARREL TEMPERATURE °C

         

        REAR

        MIDDLE

        FRONT

        HEAD

        DIE

        Kynar? 460

        200 - 230

        210 - 240

        220 - 250

        230 - 255

        50 - 90

        Kynar? 710

        190 - 210

        200 - 220

        200 - 240

        200 - 240

        50 - 90

        Kynar? 720

        190 - 210

        200 - 220

        200 - 240

        200 - 240

        50 - 90

        Kynar? 740

        200 - 220

        210 - 230

        210 - 245

        210 - 245

        50 - 90

        Kynar? 1000

        200 - 220

        210 - 230

        210 - 245

        210 - 245

        50 - 90

        Kynar? 6000

        190 - 210

        200 - 220

        200 - 240

        200 - 240

        50 - 90

        Kynar? 9000

        190 - 210

        200 - 220

        200 - 240

        200 - 240

        50 - 90

        Kynar? 370

        190 - 210

        200 - 220

        200 - 240

        200 - 240

        50 - 90

        Kynar SuperFlex?   2500

        170 - 220

        170 - 220

        170 - 245

        170 - 245

        50 - 90

        Kynar Flex?   2750-01

        200 - 220

        210 - 230

        210 - 245

        210 - 245

        50 - 90

        Kynar Flex? 2800-20

        200 - 220

        210 - 230

        210 - 245

        210 - 245

        50 - 90

        Kynar Flex? 2850-04

        190 - 210

        200 - 220

        200 - 240

        200 - 240

        50 - 90

        Kynar Flex? 3120-10

        190 - 210

        200 - 220

        200 - 240

        200 - 240

        50 - 90

         

        RECOMMENDED SAFETY PRECAUTIONS FOR MELT PROCESSING

        Kynar? resins are relatively nontoxic and non-hazardous under typical handling conditions. Mechanical malfunctions or human error, however, may lead to thermal decomposition with evolution of hydrogen fluoride (HF). Precautions must be taken to prevent excessive inhalation and physical contact with hydrogen fluoride should decomposition take place. Unlike PVC, Kynar? resins will stop decomposing when the heat source is removed and the temperature of the melt is allowed to fall to normal processing temperature.

         

        Additives, such as mica, asbestos, glass fibers, certain formulations of titanium dioxide, and very finely divided metals, may catalyze thermal de- composition rates during processing and should be used with caution. It is strongly recommended that the fabricator consult with the local Fluoropolymer Sales Representative before using any additives.

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