Extrusion: The Importance of Proper Barrel Supports

Increased temperature causes thermal expansion of almost all materials. Thermal expansion results from an increase in the kinetic energy of the material&#;s molecules that causes an increase in their spacing. The degree of expansion is called the coefficient of thermal expansion. Solid materials tend to keep their exact shape during such expansion and simply change dimensions. Nothing can stop thermal expansion; because it occurs on a molecular level, trying to restrain it is fruitless. In an extruder, particularly a large one, this can be a significant issue.

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Although every part of the extruder changes dimensions with increasing temperature, barrels and screws are the most critical components since they generally have the highest degree of thermal change. Since the barrel and screw are made of essentially the same material, their expansions are closely matched and cause no problem. However, their expansion must be considered in the design and use of the extruder, and in any equipment that&#;s attached to it.

The accepted values for the expansion of steel range from 0. to 0. in./in.-°F due to varying molecular structure, but carbon steel is typically 0. in./in.-°F. That doesn&#;t seem like much&#;only 6.45 millionths of an inch per degree of temperature change.

Something is going to move about 5/8 in. to provide for that expansion ... and nothing can stop it.

But if you have an 8-in. barrel that is 240 in. long, with an average temperature of increase of 400° F that adds up to 0.619 in. (240 × 400 × 0.). Something is going to move about 5/8 in. to provide for that expansion ... and nothing can stop it. It could be the downstream equipment moving forward or the extruder moving backward or, even worse, the barrel bending to accommodate the change in length if everything is solidly fixed.

If the barrel cannot expand in a straight line it will bend. This is the cause of many severe wear problems. When the barrel bends, the screw will bend on each rotation to follow the barrel bore. This causes extreme pressure on the flights, resulting in galling of the screw and barrel material and rapid wear. Even very large screws will eventually break from the repeated reverse flexure, just like what happens when you bend a wire back and forth in your fingers.

The first thing to consider is whether thermal expansion can be accommodated by allowing the extruder to move backward, or configuring the downstream equipment to freely move forward. If you&#;ve done that and are still experiencing thermal-expansion problems, most likely it&#;s because the barrel supports are not allowing the barrel to slide forward and backward freely as the barrel is heated and cooled. When the barrel cannot slide freely, the expansion force is so powerful that it simply pushes the support over. This can result in varying amounts of misalignment depending on the length of the barrel, the operating temperature, and the design of the support.

The barrel support is simply a cradle to support the weight of the barrel, taking out the natural sag, and to correct any minor lateral deflection. It is not a device to straighten the barrel, which must be straight to begin with in order to put the screw in place.

The worst case is when the barrel is rigidly clamped into a ring or halo-like collar and then bolted to the support. This setup can prevent any movement. Even though there may be initial clearance between the ring and barrel, the combination of increased friction from corrosion and the weight of the barrel will often prevent sliding. Figure 1 shows a support that is working properly; the barrel remains straight. Figure 2 shows a support that prevents the barrel from sliding, resulting in the support being pushed over and the barrel bending.

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You can check to see whether your supports are working properly by putting a level on the support when the extruder is cold and then checking it again when it&#;s fully heated to see whether it tilts. Also, you can check the slide marks on the barrel at each support and see whether the movement over the support is proportional to the calculated expansion at that point (Fig. 3). The problems can be more severe when there is more than one support. If each support allows a different degree of sliding, that causes it to distort into a complex bend, which is worse than a gradual bend (Fig. 4). When the total cost of screw and barrel wear from an out-of-alignment barrel is computed, and other replacement parts are considered, downtime and reduced production efficiency can wind up being a bigger expense than what&#;s considered catastrophic such as loss of a drive or reducer. Maintenance of the supports is an important aspect of overall extruder maintenance but is seldom considered even by the most experience operators when corrective action is very inexpensive.

ABOUT THE AUTHOR: Jim Frankland is a mechanical engineer who has been involved in all types of extrusion processing for more than 40 years. He is now president of Frankland Plastics Consulting, LLC. Contact or (724)651-.

BLOW MOULDING MACHINE

Extrusion blow moulding (EBM) is a manufacturing process whereby plastic is melted and extruded into a hollow tube called a mould. The parison is held and sealed in the metal mould. Air is then blown into the mould, thereby shaping it into the desired shape of the bottle or container. Once the plastic has cooled, the mould is opened and the part is discharged.

IBM shares many technological characteristics with conventional injection moulding. Many of the techniques for reducing energy use in injection moulding can be simply and directly transferred to IBM. These include improved setting, barrel insulation, improved motors and drives, all-electric machines, improved control of ancillaries and improvements in tool design. The additional blowing step needs control but does not add much to the direct energy use of the machine, although it adds it through the increased use of services such as compressed air for blowing.

IBM is primarily a process used for packaging and articles with less weight & high accuracy.

The main difference between blow moulding and injection blow moulding is that blow moulding is used for making hollow parts like water bottles and liquid soap containers. One of the key benefits of blow moulding is that it can be used to manufacture complex shapes &#; something that can be of tremendous value for such things as product branding and identity.

The injection blow moulding (IBM) process uses an injection moulding machine rather than an extruder to produce the precursor. This precursor is called a preform rather than a parison as in extrusion blow blowing (EBM). A major advantage in IBM vs. EBM is that the preform shape can be designed to receive a more consistent or desired wall thickness when Blow Moulding. The process consists of blowing a molten thermoplastic against the inside walls of a female mould cavity and chilling it to a rigid solid product. The IBM machine has an integral injection unit and a multi-impression mould assembly in which the mould cores are usually mounted on a rotary table.

These techniques are used to create many disposable and recyclable plastic parts, including products used in medical and research applications. Both processes emphasise unique functions to produce different results, which is why it&#;s important to understand each process to determine which is suitable for manufacturing a particular product.

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