PE Pipe Extrusion Process (2)

Extrusion Basics

The function of the extruder is to heat, melt, mix, and convey the material to the die, where it is shaped into a pipe . The extruder screw design is critical to the performance of the extruder and the quality of the pipe. The mixing sections of the screw are important for producing a homogeneous mix when extruding blends. A typical extruder is shown in Figure 2.

There are many different types of screw designs , but they all have in common the features shown in Figure 3. Each screw is designed specifically for the type of material being extruded.

The extruder screw operates on the stick/slip principle. The polymer needs to stick

to the barrel so that, as the screw rotates, it forces the material in a forward direction.

In the course of doing this, the polymer is subjected to heat, pressure and shear (mechanical heating). The extent to which the material is subjected to these three conditions is the function of the screw speed, the barrel temperature settings and the screw design. The design of the screw is important for the production of high quality pipe.

If a natural resin and concentrate blend is used, the screw will also have to incorporate the colorant into the natural resin. Various mixing devices are used for this purpose as shown in Figure 4. They include mixing rings or pins, fluted or cavity transfer mixers, blister rings, and helix shaped mixers, which are an integral part of the screw.

The pipe extrusion line generally consists of the extruder, die, cooling systems, puller, printer, saw and take-off equipment. Each of these items will be addressed in the following section.

Advantages of HDPE Pipe

                    Advantages of HDPE Pipe

High Density Polyethylene Pipe (HDPE) is a thermoplastic pipe made from material that can be melted and reformed. It is rugged, flexible, and durable.  It has outstanding chemical and environmental stress crack resistance.

Relative to existing infrastructure, like ductile iron, concrete or PVC, HDPE seems like a new product. In reality, it has been successfully used in a wide variety of piping applications for over 50 years.

The outstanding physical and performance benefits of HDPE pipe make it the perfect choice for your piping systems.

Corrosion Resistance

Corrosion is one of the most costly problems associated with metal piping systems. It occurs both inside and outside the pipe and affects hydraulic efficiency. Many cities treat their water to help slow rust and pitting that is inevitable with metal pipes.  Others choose costly cathodic protection, plastic coating, or sleeving to try and extend the service life of the pipe.

Unlike traditional metal infrastructure products, HDPE pipe does not rust, rot or corrode.  It is resistant to biological growth. This means an extended service life and long term cost savings.

Fatigue Resistance

HDPE pipe is flexible and ductile, not rigid. It has outstanding resistance to fatigue.  Unlike other plastic pipes, it is designed and pressure rated to handle the kind of occasional and recurring surge events that are common in water distribution systems.

In many instances, this will enable you to utilize a thinner wall HDPE pipe as compared to other types of plastic piping.

Extended Service Life

HDPE pipe is a safe and durable product ideal for your piping infrastructure. The service life of HDPE is estimated to be between 50 to 100 years, depending on application, design and installation.

Leak-Free Joints

An independent study reports that municipalities in 43 states average a water loss of 16% due to leaking joints. Some report water losses as high as 50%.

Traditional infrastructure piping is joined with bell and spigot or mechanical type joints and all acknowledge a specified leakage factor. Not only is our most precious resource being lost, but leaking pipes are costing our cities money. HDPE piping systems can be joined with heat fusion to produce permanent leak free joints.

Fusion Joints

HDPE piping systems can be joined with heat fusion welds. Heat fusion involves the heating of two HDPE surfaces then bringing them together to form a permanent, monolithic, leak-free system.

Unlike the fusion process developed for other plastics pipes, the fusion process for HDPE is proven and has been used by the natural gas industry for over 40 years. Approximately 95% of all gas distribution piping in the United States is polyethylene pipe joined by heat fusion.

Fusing HDPE pipe is not difficult and personnel can be trained in the process.

Adaptability

In addition to joining HDPE with heat fusion, HDPE pipe can also be joined with Stab or Mechanical Fittings.

There are a wide range of these fittings available, specific to your pipe size and application.

HDPE pipe can easily be transitioned to and from non-HDPE piping systems utilizing Mechanical Joint adapters (MJ’s), Stab fittings and Mechanical and Flanged Connections.

Trenchless Installation

Traditional piping systems are installed by open cut (digging a ditch), resulting in traffic and environmental disruption. HDPE can be installed using this traditional open-cut method or by utilizing eco-friendly trenchless technology.

For trenchless installation, a horizontal directional machine bores a continuous hole beneath the ground. When the drilling head reaches the end of the bore, the pipe is attached and pulled back through the hole.

The flexibility of HDPE, combined with its outstanding tensile strength and abrasion resistance, make it the preferred and proven choice for trenchless installation technology.

HDPE pipe can be installed utilizing trenchless technology under creeks, rivers, lakes, roads, or right-of-ways with minimal environmental and public disruption.

When compared to a non-plastic pipe installed using the open-cut method, a leak-free HDPE system installed utilizing less invasive trenchless technology is more cost effective.

Pipeline Rehabilitation

Trenchless technologies are also used to rehabilitate old, failing pipelines with HDPE. There are several technologies to choose from when rehabilitating old pipelines.  These technologies include slip lining and pipe bursting.  Both are excellent techniques for cities to revitalize or replace and upsize older existing infrastructure.

Eco-Friendly

In addition to its outstanding physical characteristics, HDPE is recognized for its minimal impact on the environment:

• It takes less energy to manufacture HDPE than non-plastic pipes.

• HDPE is lightweight and is often more cost effective to transport than metal pipes.

• The flexibility of HDPE, combined with the use of heat fusion to join the pipe, means fewer fittings are required.

• In trenchless installations, the physical characteristics of HDPE pipe enables you to use a smaller pipe, resulting in less ground disruption than when installing other fusible products.

• HDPE pipe joined with heat fusion provides leak free connections.

• HDPE does not emit potentially hazardous levels of toxins into the air during production, during fusion or into the ground or water during use.

• HDPE pipe can be recycled back into non-pressure piping applications.

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.

Advantages of Single Screw Extruder

     Advantages of Single Screw Extruder

The significant advantages the Single Screw Extruder with barrier according to the present invention has the advantage that the pressure build-up capability is improved. Compared to the known Twin Screw Extruder it is not necessary any more to build up a very high pressure in the so-called feed zone as to enable a predetermined lower pressure at the end of thegravimetric feeder With the according to the present invention it is possible to substantially reduce the pressure between the feed zone and the melting zone. This in turn results in a reduction of wear of the gear pump in the transition area between the feed zone and the melting zone since it is operated with lower pressures.

(1)In a preferred embodiment the barrel comprises several grooves being equally spaced apart in circumferential direction which grooves extend preferably parallel to the longitudinal axis of the barrel. This has the advantage that the process of matching the grooves is simplified compared with the groove helically arranged.

(2)Due to the pressure reduction in the area of the feed zone it does not have to be constructed any more as complicated as in prior solutions and does not have to be provided with cooling means and a “heat separation” towards the heated melting zone. Advantageously, the feed zone and the melting zone may therefore be formed individually.

(3)In a further preferred embodiment the width and/or the depth of the groove varies in a longitudinal direction, preferably the grooves depth decreases towards the downstream end of the melting zone section, preferably to zero.

This has the advantage sheet production extruderthat the groove extends continuously without Single Screw Extruderany break along the feed zone and melting zone thereby further improving the output and the pressure build-up. 

In a further preferred embodiment of the invention at least one groove is provided also in the barrel inner surface (innerwall) in the area of the feed zone section, the groove extending parallel or helically relative to the longitudinal axis. Preferably, the groove in the area of the feed zone section leads into the groove in the area of the melting zone without transition. Preferably both grooves have the same lead angle.

What are the key differences between PVC and PE pipes and fittings?

      What are differences between PVC and PE pipes ?

PE  benefits

• Chemically inert, no off gassing or leaching.
• Flexible, can be pulled to around corners to eliminate fittings.  This give a better water flow, and saves time and money.
• Can freeze without bursting
• Is rated to last at least 200 years
• No glue to add poisonous vapors or soldering to reduce risk of fire.

Negatives

• Pipe and fittings are reduced in diameter, may need to up size for good flow.
• Pipe has no rigidity.  Must be supported it's entire length.
• Need special tools to install.

PVC and CPVC "for hot" benefits

• Any can do it, all you need is a cutter to assemble.
• Very cheap and you can get it anywhere.
• PVC is full bore, but only for cold water.

Negatives

• All sorts of leaching flavors and chemicals into water.
• Rated for 20 years, this is an over statement.
• Glue needs to cure for 24 hours before charging with pressure.  Most do not do this, but that is what is instructed.
• Freezes and bursts quite quickly.
• Some plumbers will not attach to it for it has a high failure rate.

Difference between single screw and double screw Extruder

              Difference between single screw and double screw Extruder

Compared to single screw extruder, double screw extruder has below features:

1) Material flow is stable, uneasy happen cutoff or billow, productive process is reliable.2) Most heat of double screw extruder mainly come from mechanical transformation during operation, small amount of heat comes from the heating jacket. The single screw extruder often require additional equipment preheat quenched materials.3) Time distribution is relatively narrow range of material retained in the machine, which is easier to control the temperature of the material, sufficient energy utilization, yield and quality are very stable.4) Double screw extruder has a screw surface self-cleaning effect, so that the transportation of materials is stable, little residual material in cavity when finish working, and no need stop to cleaning if change the production material.5) Double screw extruder has large productivity, is appropriate for processing of materials containing high oil (> 17%) and high humidity of materials (more than 30% moisture content) Double screw extruder has complex structure, request high precision machining, therefore,  investment for production facilities is much greater if select this type, so whether select it according to the actual situation.