Machining PVC: Tips & Techniques

Available in both flexible and rigid sheets, PVC offers versatility and durability for a wide range of applications.

PVC is one of the most versatile thermoplastics in use today. It is used extensively in appliances, automobiles, computers, building and medical products, and many other end use applications.

First synthesized in 1835, PVC resin was not commercially produced until the late 1920s and grew quickly during World War II as a replacement for rubber. PVC is unique because with the addition of plasticizer it can be made to be flexible, and without plasticizer it is very rigid.

The total PVC market in the United States is approximately 10 billion pounds per year. Worldwide consumption is about 30 billion pounds per year. Of the totals, 85 percent of the resin is used in building products. Type I and Type II PVC sheets, usually listed in the miscellaneous columns, are a very small percent of the overall total. The focus of this article will be on Types I, II and specialty grades of rigid PVC sheet.

Physical Properties
Type I and Type II from different manufacturers have different properties. Also, there is a vast difference of properties between Type I and Type II. The American Society for Testing Materials (ASTM) has tried to address this difference by replacing the older "Type I, Grade 1" and "Type II" specifications with a cell classification. In accordance with ASTM-D-1784, there is a five digit system that helps classify the sheet. The first number of the five clarifies the base resin, the second refers to the impact strength, the third to tensile strength, the fourth to modulus of elasticity and the fifth to the deflection temperature under load.

After the five digits there is an "A," "B," "C" or "D." This suffix is for chemical resistance. This format allows one to evaluate physical properties from various sources to insure that materials supplied meet certain minimum standards. It further allows the proper selection of material for the application.

Specifically, Type I, Grade 1 PVC sheet is defined by the cell classification 12454-B and Type II PVC sheet is defined as 14333-D. These are the minimum requirements per ASTM. Higher numbers in digits two through five would denote increased physical properties.

Type I is a high corrosion resistant material with normal impact properties and is used where chemical attack is the major concern. Type II is an impact-modified formula which increases the ability of the material to withstand shock or impact, but has lower chemical resistance relative to Type I.

Other micro-ingredients can be added to the PVC to affect physical properties. The most useful are ultra-violet inhibitors to improve outdoor weatherability as well as additives to improve hot melt strength for stretchability for vacuum forming. Other micro-ingredients can be added to affect smoke emissions, heat deflection temperatures and electrical conductivity.

Applications
PVC sheet is most generally produced by the extrusion process. The end use applications for extruded sheets are many. PVC in sizes 0.040 inch through 0.093 inch are used for stampings, signs and mounting. Sizes 0.125 inch through 0.250 inch are used for guards, machine covers, dual laminate fabrication, ducts, fans and thermoformed components. And PVC in 0.375 inch through 1 inch are used for vessels, tanks, caps, electrical boxes, panels, bracings, pump plates and spacers.

The other process for making PVC sheet is laminating. This process is used to manufacture sheets in thicknesses 1.25 inches through 6 inches. These sheets are used when extensive machining is necessary like blind flanges, support rings, dutchman discs, pump impellers and valves.

PVC is also an ideal sheet for manufacturing process equipment. This product offers the following advantages: good weight to strength ratio, low cost of raw materials, ability to pigment, good mechanical strength, self-extinguishing burn characteristics, low coefficient of expansion, excellent chemical resistance, good electrical insulation and high rigidity. The sheet can be solvent, hot gas, fusion or ultrasonic welded. It can be riveted, nailed or screwed together. It can also be heat bent, pressure vacuumed or draped formed.

The Marketplace
Other market areas and industries for PVC sheets are:

• Plating tanks, automated plating lines, precious metal recovery systems, hoists, scrubbers for purifying exhaust emissions, exhaust hoods.
• Electronics industry -- etching, developing and drying equipment for printed circuit boards, containers for metering acids, holding tanks for spent acids.
• Wall cladding -- rooms for photographic development, plating, operating, pharmaceutical, wafer process and restaurants.
• Laboratories -- benches, trays, baskets, dispensing systems, booths, laminar flow units and electrical panels.
• Photographic -- film developing machines, sinks, small tankage for acids, vacuum formed covers, trays and sight windows.
• Chemical process -- pressure vessels, purification and drying of gases, odor control equipment, tankages, guards, sight shields and automatic processing equipment for dispensing of acids.
• Marine industry -- seat backs, bait tanks, hatch covers, cabinets and shelving.
• Waste treatment -- holding tanks separators, diverters and odor control equipment.
• Textile -- dyeing, washing and drying equipment.
• Transportation -- seat backs, head panels and trays.
• Food processing -- railings, conveyors, gears, canning machines.
• Home furnishings -- chairs, love seats, tables.
• Building products -- corrugated roofing for carports, soffits, cooling tower profiles and window flashing.

On the Horizon
In the years to come, disposal and recycling of PVC sheet will be of major concern. As technology for sorting dissimilar plastics improves, PVC will see even greater acceptance as a recycled material. It has already found success in plastic lumber, molding compounds for floor mats, non-critical profiles, sound insulation, garden and landscaping products, fencing, caps, plugs and sheets.

With respect to disposal, PVC's calorific value is midway between coal and wood, and can be a valuable ingredient in reducing the demand for external fuel in the incineration process. New developments in PVC will continue to extend the uses of this extraordinary material.

Bruce Merklinghaus is the manager of marketing & sales at Vycom Inc., in Moosic, PA. For more information, he can be reached at (717) 346-8254 or (800) 235-8320. Merklinghaus' story originally appeared in the IAPD Magazine. Reprinted by permission.

John Raynor, marketing manager at Piedmont Plastics, also helped with the coordination of the article. For more information, he can be reached at (704) 597-8200.

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