Design of PVC pipe extrusion

       Design of PVC pipe extrusion 

1-Introduction

This article aims at giving a bird's eye view on PVC extrusion with particular emphasis on screw design.

PVC compounds are extruded in single as well as in twin screw extruders.

Whereas co rotating twin screws are mainly used for compounding operations, counter rotating screws are used for the extrusion of profiles, tubes and rods.

Twin screws are divided in parallel and conical configurations.

PVC is of low thermal stability and high melt viscosity. Therefore, it is combined with a number of additives to vary properties to suit different end-use applications. PVC formulation is key to processing success.

Extruded PVC can be 100% recycled. However hot spots in the extruder lead to burnt polymer which has to be rejected otherwise it will contribute to the degradation of the virgin polymer.

2-Twin screw extrusion

A co-rotating extruder with interchangeable elements is extremely versatile and can be used for most PVC compounds. Conical twins are best for rigid PVC. The production of rigid PVC pipes is dominated by counter-rotating twin screw extruders. This plasticizing system matches perfectly the demands of processing powder shaped, shear sensitive polymers. Compared to single screw extruders the twin screw extruders provide a constant feeding of powder as well as a gentle and uniform plasticizing at low screw speeds allowing for high output rates.
The counter-rotating twin screw extruders can be divided into two types with conical and parallel screw systems.

The lower the required output rate the more the conical twin screw extruder is preferred. The application range of this system goes up to 150 kg/h but as outputs increase so do the benefits of the parallel system. At output rates above 300 kg/h parallel twin screw extruders are dominant.

Conical extruders have been successful at the lower throughput end of the extruder market. In the feed zone, the surface area in contact with the material is large this improves the heat transfer to the material.

The conical shape of the screw and the continuous reduction of the channel volume result in very gentle plasticizing. Shear friction in the metering zone is very low, due to the very small screw diameter at this point.

Conical screw convey the melt smoothly and steadily even at high die resistance, although pellet rigid PVC compound can be run in twin screw extruders (starve feeding required to avoid high amps & torque), twins are ideally suited for powder dry blend.

Twin-screw extruders were designed to extrude powder by gravity feed. They normally have smaller horsepower motors, but are able to achieve higher production rates than an equivalent size single-screw extruder. The recipe should be designed to allow the powder to run flood feed. This takes advantage of the twin screw extruder's constant pump machine and achieves better dimensional control of the finished parts.

3-Single screw extrusion

Single screw extruders have been traditionally utilized for sheet extrusion because of their low initial cost.

A general-purpose screw design for rigid PVC extrusion consists of a constant flight and pitch (lead). The pitch is equal to the diameter, and the length is equal to 24D. It should be bored for screw cooling. Screw flights should be hardened. Regardless of the screw size, a 2.4:1 compression ratio is recommended for rigid PVC. Higher compression screws, such as those designed for flexible PVC, can cause over-heating and degradation of the material. (ref: PolyOne technical report). Around 14 metering flights and a metering depth of 0.3inches are required.

Four rows of pins in the metering zone at 3, 6, 9 and 12 flights from the screw tip contribute to the homogenization of the melt

Screw designs, such as double flighted screws, can also offer improved performance such as a higher rate at the same melt quality and temperature.

At recommended processing temperatures (190ºC to 216ºC for a smooth extruding), rigid PVC is typically higher in viscosity than many other materials. To prevent overloading the motor, a 150 to 250 horsepower motor for a 4 1/2" extruder is used. Screw speed ranges of 40 to 80 RPM are used. This means a gear ratio should be used to give a maximum speed of about 80 but not more than 100 RPM. Higher viscosities also mean higher temperatures are reached by shear heating or friction. Thus, screw cooling and efficient barrel cooling (i.e. water cooling), are critical to rigid PVC extrusion. An extruder length of 24 to 1 should be considered minimum while the longer 30 to 1 or 32 to 1 is desirable, especially if a vented barrel and two stage screw is used.

Typically pelletized compound is used with single screw extruders. Powder compounds are available for single screw machines but they require special handling and are more prone to air and moisture entrapment in the melt. One of two methods is used to address this issue. For pelletized or powder compounds, a two stage screw can be used with either an open vent or an applied vacuum at the vent. A two stage screw and vent combination must be carefully balanced for specific rates, dies and compounds.

A second technique used for powder extrusion is the vacuum hopper. This allows the use of single stage screws with a special vacuum seal at the screw shank. A two hopper system is used to maintain a uniform vacuum on the feed hopper. With either system, a crammer feed is recommended to assure uniform feed of the powder.

Screw cooling is mandatory for PVC sheet extrusion. The system should be set up so that the oil flows to the screw tip first so as to cool the tip and thus prevent hang up and burning on the tip. The oil should then return between the screw and the feed tube. The oil temperature should be controlled, with a recommended starting point of about 93-121°C.

The two popular methods of feeding an extruder are starve-feeding and force-feeding. During force-feeding, a reserve of material is maintained in the hopper of the extruder and material is forced in the extruder.

In starve-feeding the extruder is fed at a rate less than the capacity of the screw. The hopper remains empty and functions as a conduit to avoid material from spilling. Starve feeding is the more popular method for feeding.

How to Make Plastic Pipes

             How to Make Plastic Pipes

Plastic has largely become the material of choice for pipes. Its flexibility, ease of bonding, lighter weight than iron and steel and lower cost than copper has made it a common choice. Pipe is generally produced by an extrusion process. Although this process may vary in actual operating conditions (temperatures, pressures, extrusion rate) based on the type of plastic used, diameter of pipe and wall thickness, the fundamental process of extrusion is reasonably consistent throughout the industry. 

• Add the thermoplastic resin to the feed hopper. Although this can be done manually, it is generally transferred to the hopper via a vacuum feeder due to the steady state nature of the process. This helps minimize the chance of running out of resin during the process.
• Turn on machine screw and barrel heaters per specific resin requirements. The heaters add heat to the plastic while it is in the barrel to melt the plastic. The rotating screw adds shear heat to the plastic for melting as well as building up the pressure to force the plastic through the die.
• Extrude plastic through the die. The die is designed and built based on the dimensions desired in the pipe and the shrink rate of the type of plastic being used.

• Cut the pipe at the desired length. The extruded pipe will exit the die onto a roller system to allow the pipe to cool without deforming under its own weight. Once it has passed a certain length, it will trip a sensor (electric eye) triggering a cutting operation on the pipe. The cut is made by a cutter that moves forward at the rate of pipe extrusion to offset the motion of the pipe moving forward so that the end of the pipe will remain perpendicular to the pipe wall after it is cut.
• Continue the process in a steady-state environment until the desired quantity of plastic is produced.

     Ingredients of PVC Pipe Manufacturing

Polyvinyl Chloride (PVC) is a thermoplastic material that is used in a myriad of products including water pipe and electrical conduit. It is a polymer whose long chain molecules are made from the building blocks of vinyl chloride. Additives are used to enhance the properties of PVC for specific applications. Finished products come in a number of colors, are tolerant to ultraviolet light, and have various degrees of flexibility.

Raw Materials

• The two raw materials used in the manufacture of PVC are sea water and oil. Salt derived from sea water is used to make chlorine gas and petroleum oil is the original source for ethylene gas. Chlorine and ethylene are the two main ingredients needed to make PVC.

Chlorine

• Chlorine is produced from a saline solution by the process of electrolysis. A solution of sodium chlorine, or common table salt, is placed in a container with electrodes. When electric current is applied, the positive electrode attracts chloride ions in the solution where they combine to form chlorine gas.

Ethylene

• Ethylene is a colorless and odorless gas with the chemical formula C2H4. It is highly flammable and can cause explosions if mishandled. It is produced commercially from the refining of petroleum.

Making PVC

• The chlorine and ethylene gases are combined to produce ethylene dichloride which is converted at high temperatures to vinyl chloride (CH2=CHCl). The vinyl chloride molecules are then polymerized to form the PVC resin. Other compounds are added to improve its appearance and physical and chemical properties. The finished product is formed into pipe or other products where it hardens as it cools.

Additives

• A number of compounds can be added to raw PVC. The most common additives are pigments to add color, UV inhibitors to protect the material from being degraded by prolonged exposure to sunlight, and plasticizers to adjust the degree of flexibility of the specific product. Most plasticizers come from a chemical group called phthalates.

Toxicity of Materials

• Chlorine in the gaseous form is dangerous because it is a severe irritant to the skin, eyes, and respiratory system. Ethylene chloride compounds that are made in the manufacturing process and not converted into polymers are known to be carcinogenic. There are remnants of these compounds that are not combined in the finished PVC which makes water pipe and other products potentially hazardous as well. The phthalates that are used as to increase plasticity are also very toxic.

                  How is PVC Pipe Made?

Chemical Reaction

• PVC has its origins in the chemical gas referred to as vinyl chloride. When vinyl chloride is exposed to sunlight a chemical reaction occurs. The reaction is known as polymerization, which transmutes into a whitish solid material. To achieve the shape and solidity of a PVC pipe, a series of chemicals are introduced to one another. Natural gas is heated to create ethylene. The process is referred to as cracking. Later, sodium chloride (found in the form of rock salt) is spliced using electrolysis. As a result chlorine and lye (sodium hydroxide) is produced.

Molecular bonding

• Chlorine and ethylene (natural gas heated under pressure) are introduced to make vinyl chloride monomer (VCM). The molecules are bonded from each molecule's end. The result is a long chain of polyvinyl chloride polymer. In essence, plastic is created. The polymerized plastic, called thermoplastic PVC powder (which is compounded), melted and molded into piping. The result is a tube of PVC plastic. As a result of the chemical process (PVC becomes very solid and rigid), PVC is less likely to break during earthquakes. It can withstand pressures that many metals (such as copper piping) cannot tolerate. This is why PVC is the preferred material for plumbing and underground wiring.

Manufacturing

• As small as 16 mm and as large as 630 mm tubing/pipes are manufactured using a machine called an extruder. PVC plastic is routed through a double screw stem extruder (conical twin screw) or a parallel double screw extruder. Molding determines the wall thickness of the PVC hose. The diameter of the pipe is made by the PVC pipe extruder (The standard is 1/2 to 24 inches in diameter). The production speed of the standard PVC pipe extruder is about 20 meters per minute. The PVC hose runs through a vacuum pump. A ring cutting machine is employed at the end of the assembly line to divide the PVC tubing into sections of individual pipes. The individual pipes are cooled and racked. After inspection, the completed PVC pipes are sent to the warehouse for final inspection, labeling and shipping.

The Differences Between UPVC & PVC Pipes

The Differences Between UPVC & PVC Pipes

To the casual observer, there's little difference between PVC pipe and uPVC pipe. Both are plastic pipe used extensively in building. Beyond the superficial similarities, the two types of pipe are manufactured differently and thus have different properties and slightly different applications in building and other industrial processes and most repair-work exposure to plastic pipe is to PVC rather than uPVC.

Manufacture

• PVC and uPVC are largely made of the same material. Polyvinylchloride is a polymer that can be heated and molded to create very hard, strong compounds such as piping. Because of its rigid properties once it's formed, manufacturers frequently blend additional plasticizing polymers into PVC. These polymers make PVC pipe more bendable and, generally, easier to work with than if it remains unplasticized. Those plasticizing agents are left out when uPVC is manufactured---the name is short for unplasticized polyvinylchloride---which is nearly as rigid as cast iron pipe.

Handling

• For installation purposes, PVC and uPVC pipe are generally handled in the same fashion. Both can be easily cut with plastic-cutting hack saw blades or power tools designed to cut PVC pipe and both are joined using gluing compounds rather than through soldering. Because uPVC pipe doesn't contain the plasticizing polymers that make PVC slightly flexible, it must be cut perfectly to size because it doesn't allow for give.

Applications

• PVC pipe is used as a replacement for copper and aluminum piping on non-potable water, replacing metal piping in waste lines, irrigation systems and pool circulation systems. Because it resists corrosion and degradation from biological sources, it's a durable product to use in plumbing systems. It's easily cut and its joints don't require soldering, fastening with glue instead, and offers a little amount of give when pipes aren't sized perfectly, so PVC pipe is frequently chosen by handymen as an easier-to-use alternative to metal piping.
  The use of uPVC isn't quite as widespread in plumbing in America, though its durability has helped it to become the material of choice for plumbing sewage lines, replacing cast-iron pipe. It's also frequently used in manufacturing exterior drainage systems such as rain gutter downspouts.
  The only type of plastic pipe that should be used for transmission of drinking water is cPVC pipe.

Types of PVC Pipes

                   Types of PVC Pipes

Polyvinyl chloride, or PVC, is a synthetic material that has been used to make pipes since the early 20th century. It has become an increasingly popular substance for builders and plumbers because of its resistance to corrosion. In addition to varying by size, thickness and chemical composition, PVC pipes can be color-coded by function (e.g., blue for water main, red for fire main)

Plain PVC

• Cheap, light and resistant to corrosion, PVC became an obvious solution to many of the problems faced by plumbers, builders and city planners. It gained popularity in the United States in the 1940s, and has since become a mainstay for many uses, including irrigation, sanitary sewer collection and electrical communications. Because it is a thermostatic plastic, PVC can only be melted and molded once. A second melting to re-shape PVC pipes will cause them to lose some of their integrity.

CPVC and UPVC

• CPVC stands for chlorinated PVC pipe. Suitable for a variety of applications, CPVC has a higher chlorine content than regular PVC, which means it can withstand a broad spectrum of temperatures. CPVC is ideal for use with hot water heaters. UPVC stands for unplasticized PVC pipe. This type of material is more rigid than traditional PVC material, and is commonly used for water waste transport. It can also be used to make siding materials for construction and remodel purposes.

PVC-U, PVC-M and PVC-O

• If you see PVC with a dash after it, the subsequent character has something to do with the thickness of the pipe walls. PVC-U pipes have thicker walls than regular PVC pipes. This modification enables them to withstand more internal pressure. PVC-M and PVC-O pipes are newer versions of the PVC-U. Though they have thinner walls than the traditional PVC-U, PVC-M and PVC-O pipes are specifically designed for high levels of internal pressure as well.

Lead Reinforcement

• Outside the United States, where the use of lead pipes for potable water has been banned since 1986, PVC pipes are often reinforced with lead, a process that makes the pipes stronger and less susceptible to bursting under pressure. In China, more than 90 percent of PVC pipes are manufactured with lead reinforcement. The Environmental Protection Agency has advised that exposure to lead can cause sundry adverse health effects. Globally, other governments have succumbed to public pressure and are in the process of phasing out lead in drinking water pipe manufacture