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By Design: Polyvinyl chloride part design | plasticstoday.com

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By Design: Polyvinyl chloride part design

In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on issues important to design engineers and the molding industry. During World War II, Germany could miraculously restore fresh water in a matter of hours to cities that had been demolished by bombs. Their secret was easy-to-assemble plastic pipe. Plumbing, along with film and electrical insulation, were the first uses for polyvinyl chloride (PVC) with commercial production starting in Germany in 1931.

In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on issues important to design engineers and the molding industry.

During World War II, Germany could miraculously restore fresh water in a matter of hours to cities that had been demolished by bombs. Their secret was easy-to-assemble plastic pipe. Plumbing, along with film and electrical insulation, were the first uses for polyvinyl chloride (PVC) with commercial production starting in Germany in 1931.

Throughout the war, PVC was treated as a military secret. These early PVCs were rigid, brittle, heat-sensitive materials that could be compression molded, calendered, and extruded. They were, however, extremely difficult to mold using the rudimentary ram injection molding machines available at the time.

Commercial applications for PVC in the United States were limited until 1926 when B.F. Goodrich’s Waldo Semon discovered that the material could be softened with a plasticizer. These softened PVCs found large markets in the plumbing, construction, and electrical industries. Film, blowmolded, and thermoformed packaging applications were developed and the B.F. Goodrich Co. prospered. (Waldo Semon also invented bubble gum, which enjoyed faster customer acceptance than PVC.)

Today PVC is the third-largest-volume material after polyethylene and polypropylene. Approximately 15 percent of all plastic used in the U.S. is PVC. Large quantities of general purpose PVC now sell for $.32/lb.

The material’s large volume and low cost classify it as a commodity plastic. This is a misconception, however, as PVC has mechanical properties that rival ABS. There are three different types of PVC with many special grades within each type.

Plastisols are liquid suspensions of PVC powder in a plasticizer. The plastisols are not suitable for IM, but they are widely used for casting, coatings, slush, and rotational molding.

Flexible PVCs find their widest usage in pliable film, electrical insulation, flexible garden hose, and medical tubing. Large quantities of these materials are injection molded into medical, electrical, and special commercial products. Flexible PVC is a low-cost alternative for thermoplastic elastomers. These materials can be tricky to injection mold. However, some processors have prospered by specializing in this area.

Rigid PVCs are the most interesting injection moldable members of this family of materials. The first PVCs were rigid but their brittleness limited their use. Their high viscosity and heat sensitivity made them difficult to injection mold.

Bad Reputation
Many uninformed processors treated PVC the same as the polystyrene, acrylic, and nylon materials with which they were already familiar. In some cases they thermally degraded the PVC, producing a corrosive gas that attacked molds and other bare metal surfaces. As a result, PVC developed an unjustified reputation as a difficult and dangerous material to injection mold.

The remnant of that bad reputation lingers on even today. I spent 10 of my early years in the medical device industry, where injection molding of PVC was a routine occurrence. I have never considered PVC difficult or dangerous. It was just a material that had to be processed differently.

In the ensuing years polymer chemists have made impressive improvements in PVC plasticizers and stabilizers. The advent of the inline reciprocating screw injection molding machines in the 1950s and improvements in screw design and temperature controls have rendered today’s PVCs much easier to mold than in the early years.

Defining Characteristics
Rigid PVC’s good resistance to water and weathering, plus its self-extinguishing characteristics, account for its wide use in the electrical and construction industries. Most rigid PVCs contain fillers that make them opaque; however, a few transparent grades are now available.

PVC is the lowest-cost plastic with a tensile strength of up to 7400 and a flexural modulus of 540,000 psi, with a minimum notched Izod impact strength of -1.0 and a maximum of 17.0 ft-lb/in. Fiber-reinforced grades have a tensile strength in the range of 15,000 psi.

Rigid PVC is specified for many load-bearing applications, but its maximum heat deflection temperature of only 180F at a 264-psi loading limits its use. There are always exceptions and some stress-free injection molded parts have withstood steam autoclaving at 285F.

Designing with PVC
The rigid PVCs are low-mold-shrinkage, amorphous materials with a high molecular weight.

  • Wall thickness on small injection molded parts can be less, but .050 inch is a good minimum for PVC. Some PVCs have flow lengths of 40 inches with thicknesses in the range of .150 to .200 inch. The maximum thickness can be whatever is required, but .250 inch is a good upper limit.
  • Radiusing inside corners less than 40 percent of the wall thickness produces high levels of molded-in stress. Larger radiuses are easier to mold into stronger parts with improved impact strength and reduced warp.
  • Draft angles of ½° can be specified on draws up to 1 inch. Larger draws require a minimum of 1° per side. These are rigid materials and a smooth polish is desirable.
  • Projections of all types can be specified. Sink marks can be eliminated and molded-in stress minimized if the thickness of projections is 50 percent of the adjoining wall. The maximum thickness should be limited to 75 percent of the thickness of the wall to which they are attached.
  • Depressions, or holes, can have good-appearing, strong weldlines with proper molding conditions. These are rigid materials and holes require draft angles and smoothly contoured surfaces. Since rigid PVC is a hard-flow material, the depth of holes must be limited to two to three times the thickness of the core to avoid the bending of small, unsupported core pins.
  • Tolerances on a 1-inch-long, .125-inch-thick PVC part can be ±.008 inch. Rigid PVCs have mold shrinkage of only .001 to .005 in/in. With some grades a fine tolerance can be ±.004 inch.

    Rigid PVC is an underused material. Engineers searching for a less-costly alternative for a low-temperature ABS, PPO, or polycarbonate application should consider PVC.

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