Kynar Resins: Fabrication of Tubing

Extrusion

Smooth KYNAR PVDF products of all types - rod, sheet, film, pipe, tubing, monofilament, and wire insulation can be extruded at relatively high rates. No extrusion aids, lubricants, or heat stabilizers are needed. Equipment with materials of construction similar to those used for processing PVC and polyolefins is adequate. Gradual transition screws having L/D ratios of at least 20:1 and ample metering sections are recommended. Special die designs facilitate the proper distribution and homo-geneity of the melt. Extrusion temperatures vary from 232°- 288°C (450°- 550°F) depending upon the shape being extruded. For very thin sections such as fine caliper wire insulation, the temperature at the extreme tip of the die is held at 3l6°C (600°F) or higher as required for heat polishing. Water quenching is used for wire insulation, tubing and pipe; whereas, sheet and film extruded from slit dies are frequently crystallized on polished steel rolls operating between 66°- l40°C (150°- 284°F). Extruded, blown, or flat film can be uniaxially and/or biaxially oriented to sub-mil thickness. Monofilament is extrusion spun into a water bath and then oriented and heat-set at elevated temperatures.

Molding

KYNAR resins are readily molded in conventional compression, transfer, and injection molding equipment. In compression and transfer operations, pellets preheated in an oven or dielectric preheater to a range from 215° - 232°C (420° - 450°F) are charged to a heated mold between 188° and 199°C (370° and 390°F) with sufficient pressure. Time is then allowed for complete flow and fusion. For heavy sections, the part must be cooled to 93°C (200°F) under pressure to prevent vacuum voids and distortion. Annealing cycles will provide for increased dimensional stability by relieving residual stresses. The relatively high melt viscosity of KYNAR resins makes proper mold design an important factor in injection molding. Large diameter nozzle openings, sprues, and gates are recommended. Although operating conditions will vary depending upon the part, typical temperatures for the cylinder, nozzle, and mold are 221°, 221°, and 66°C (430°, 430°, and 150°F), respectively. In both molding and extrusion operations, care must be taken to eliminate "hang-up" areas where molten resin can collect and thermally decompose if residence time is excessive. Drying of resin before processing should not be necessary. Recommended processing temperatures are included in Table X. Shrinkage varies with mold design and geometry. Approximate shrinkage estimates are 0.015" per inch in sections up to 1/16" (1.58 mm) to greater than 0.030" per inch in sections up to 1/4" (6.35 mm), and as high as 0.080" per inch in heavier sections.

Kynar recommended injection molding temperatures.

Welding

Several methods of fusion welding KYNAR PVDF components are possible. The most common and successful approaches to welding KYNAR PVDF components are heat contact welding and hot gas (or air) using a welding rod. Ultrasonic, hot lamination, resistance heating, spin welding, and radio frequency are all methods usable to bond KYNAR PVDF to itself with limited success.

Contact welding is performed by holding two sections, which have each been heated to create a molten layer of resin, in position with adequate pressure until the polymer has congealed and cooled. Depending on the cross-sectional thickness of parts assembled in this way, the depth of the molten polymer will vary. Common terminology for forms of contact welding are lap joint, socket fusion, and butt joint. Tools, fixtures, or some form of equipment must be provided to hold parts welded in this manner in position. Commercial welding equipment is available for this purpose.
Heating temperatures required to adequately produce a molten condition of the conforming KYNAR PVDF surfaces will range from 220°- 288°C (425°- 550°F). Heating time cannot be easily predicted since this will depend on watt density of heating elements and size, as well as the geometry of the parts to be joined.

Normally some molten polymer will exude from the open edges of the joint formed in this way to create a bead. This bead can be carefully removed by machining if it interferes with the normal operation of the welded assembly
Welding in this manner should be performed when ambient temperatures are in the vicinity of 20° - 30°C (70° - 100°F). Higher temperatures could be beneficial but lower could be disappointing. Slow cooling of welded assemblies is preferred.

Hot gas welding, using a welding rod, is a common way to construct KYNAR PVDF lined components such as metals and FRP. Typically the stock shape supplier has a qualifying program due to the skills needed to accomplish this method of welding. Elf Atochem suppliers can recommend fabrication experts that have experience in this method of welding  KYNAR fluoropolymers.

Melt Behavior

As shown in Table XI, KYNAR homopolymer resin grades cover a broad range of melt viscosities to satisfy the various needs of the fabricator. KYNAR resin grade selection is important and depends upon the type of fabrication and the specific design of the tooling and finished part. In virtually all cases it is recommended that the fabricator consult with the local Elf Atochem sales representative.

Kynar resin apparent viscosity vs. shear rate chart.

Kynar viscosity and melt flow rate properties table.