PT ZONE: Equipment
|Secondary Process Equipment|
Profile Extrusion Equipment
Profiles may consist of a single material or multiple materials. Single material profiles utilize one extruder while multiple material profiles require more than one extruder to feed material into a central die. These processes can utilize single screw or twin-screw extruders. Single screw machines are focused on pelletized materials or applications requiring less true mixing. Twin-screw extruders are usually focused on powdered materials or where more intensive mixing is required. These extruders can be of a variety of sizes from a few pounds per hour of output up to thousands of pounds per hour output depending on the application.
Secondary extruders are called “co-extruders” and can also be a variety of sizes but normally fall toward the smaller end of the spectrum and are normally smaller than the main processing extruders. The number of co-extruders used is limited only by the space in the processing line. Some processes can cheat by utilizing the same co-extruder for multiple application requirements (as long as the material required is identical the co-extruder can be utilized for more than one location. (An example of this is stripes on the outside of drinking straws. If the strips are the same color, one machine can be used for all the strips). Co-extruders can be mounted on the floor or suspended over the die head as required.
Die tooling is application specific and can be a simple metal plate with a hole in it for the material to flow through, or a sophisticated multiple components affair designed for advanced flow characteristics and precision sizing. Dies are mounted either to the main extruder directly with the co-extruders connecting into it, or the die is on a floor stand with extruders piped to it. The Die tooling is oriented in one of two directions is relation to the main extruder. The directions are “inline” and “crosshead”. The inline process utilizes a flow that follows the extruder direction and the die is connected with flow in the process direction. The crosshead process utilizes a flow that is at an angle to the flow of the extruder. For profile extrusion this would be at a ninety degree angle to the flow. (Crosshead processes are required for any product requiring a component to be inserted into the extrudate for coating. Such as wire, pul-truded products, etc.
Downstream equipment is separated into three areas of the process: 1) cooling/sizing, 2) pulling, 3) cut/coil/takeoff. Below are the main styles of equipment to achieve these process requirements.
The part must be cooled to maintain the desired shape. This equipment falls into a few sub categories. Some processes require minimal impact to work. Others require extensive interaction at this stage to achieve the desired product. Cooling/sizing equipment falls into a few types:
The use of air to cool parts has been around for a long time. Today, it is rarely used because it is inefficient and normally requires slower process speeds. But this style of process is usually very inexpensive so for shorter runs and for quick setup it is still effectively utilized. It does not require expensive tooling to work effectively either. Some processors utilize simple clamps and bent wire to hold the product in the correct shape. The Air Cooling Rack is a simple frame, usually with cooling fans, and simple plates and fixtures to mount jigs and fingers to push on the part and hold it in the desired shape.
Water Cooling Tank
The Cooling Tank requires a supply of cooling water. This can be done in a few ways. Either directly dispensed to the tank from the plant water or a chilled water system. This could utilize two types of cooling systems: 1) A simple inlet/outlet of the water through the process with no recirculation in the tank, 2) A recirculation system to reduce water consumption from the plant. The recirculation system can utilize a heat exchanger to create separation of water from the central system or no heat exchanger and the process water co-mingling with the chilling water. The cooling water is then washed across the product as it is pulled through the tank. Some type of support is usually mounted inside the tank to hold the part while it is being cooled by the water. The water is either sprayed on the part from multiple directions to help the uniformity of the cooling or the tank is flooded with water so that the part is immersed in the water as it passes through the tank. Spray tanks generally offer more efficient cooling with the added cost and maintenance of the spray nozzles. Commonly, more than one cooling tank is required to ensure that the part is cooled sufficiently and maintains dimensional stability once it leaves the last tank in the line. The cooling requirements of a line may require 5 feet to 250 feet or more of cooling to achieve a good product.
Water Cooling/Vacuum Sizing Tank
A Sizing Tank is essentially a Cooling Tank with the ability to size the product in the process. This sizing is done with the help of a vacuum process that submits the product to a change in atmosphere in the tank to help maintain hollows, roundness, wall thickness, and true sizing. The sizing is done in an enclosed tank body that is strong enough not to collapse under high vacuum or via directly connecting the vacuum source to the tooling through a manifold with multiple connection points. With the addition of a vacuum pump and required plumbing, a constant atmosphere is maintained in the tank. Sizing is done with associated calibration tooling that is mounted in or prior to the tank. Maintaining vacuum to a constant level requires good seals via the calibration tooling, discharge end seal, lid seals, etc. Sometimes this is aided with a water seal chamber on either or both ends of the tank. Sizing tank lengths are based on the processing requirements and the vacuum length may be a small portion of the tank length or could require multiple tanks in line to achieve the time under vacuum for the process.
Water Cooling Calibration Table
An alternate water cooling sizing configuration is a Sizing/Calibration Table.
A Sizing or Calibration Table is a piece of equipment without a water tank body. Vacuum and water components are present and utilize manifolds with valved ports to connect to a tooling set and tank body that is placed on top of the table. These connections allow for a wide variety of tooling and tank configurations to be utilized. This type of process is sometimes called “wet/dry” as it allows you to use or not use water in the tooling configurations. Large window profiles are commonly run on this type of equipment as the time required to set/remove the tooling from a tank for each part is very extensive. If the tooling is mounted in a dedicated tank that can be placed on the table top quickly, it is far more efficient. This type of equipment is usually far more expensive than a tank so it is not a good option for the general processor.
Most extrusion processes require a puller to effectively process a product. The exceptions are pul-truded products. Pul-truded products actually push the product through the downstream components. The balance however do not have the material structural integrity to do this. If you think about holding a rope in your hand and trying to give someone else the end of the rope you will pull it out of your hand and send it toward the other person. If that person does not react, what happens? The rope bends quickly toward the earth and piles up at your feet. Extrusions are the same way. As they are typically not rigid enough in a molten state to push their own weight, a helping hand is needed. The other person would reach out and grasp the rope and pull it toward themselves. To keep the rope off the ground, the person needs to maintain tension on the rope. If they move farther from you, the force needed to pull the rope while keeping it off the ground requires more force. So to keep the process moving, the extrudate requires a bit of help from a puller to maintain tension in the process. The product goes through the cooling stage until it is cool enough to pull without breaking the web. The longer the cooling length and the longer the calibration tooling length, the higher the pull force required to maintain the process. Accuracy of the pulling speed is critical also. The product leaves the extruder at a given pounds per hour. That equates to a given linear speed based on product weight. That speed needs to be accurately maintained or the product weight and/or quality will vary and may cause out of specification product.
Pullers are constructed in a variety of types to fit the process requirements. Some of these are: 1) Belt – general applications and low to medium force requirements, 2) cleated – higher pull force and parts with shapes that would be distorted in a normal belt. Cleats can be contoured to fit the product protrusions, 3) wheel – Inexpensive and easy to use but accuracy may be questioned due to slippage in the wheels and the motor drive type used.
After the product is at the puller it should be cooled enough to work with to finalize it. Some products are now cut to length. Some are coiled lose. Some are spooled on reels of some sort. Each product has its own requirements for finishing.
Some products can be cut with a simple blade. The part is typically run through a bushing set to keep it oriented and to help with the cut quality and tolerances. As the product moves through the bushings, the blade is activated. This is usually a rotary arm or wheel that the blade is mounted to. The blade is brought through the product to cut it as required. The blade type, configuration, angle of attack, bushing size, space between the bushings for the blade to move through, and much more will determine the quality and tolerance of the cut product. Many cutters will operate in multiple modes to allow the processor to utilize them for a variety of products. There are two types of cutting modes – On demand – (this means the cutter is activated from a signal and each time the cut is needed, the cutter blade arm will rotate through the product and product one cut. The actuation can be via timer, optic sensor, physical flag switch, encoder wheel, etc). Continuous – (this cut type utilizes the blade arm rotating at a continuous given speed and the product is moved through the bushings at a consistent rate to achieve a cut length. This is usually used for very short parts or very high line speeds. Single and multiple blades can be used on the arm/wheel to achieve even higher cutting rate if needed).
These cutters can be operated in a variety of ways. Some old style cutters will operate with a clutch that flips the blade through the part. Some utilize direct AC motors to spin the blade arm. Modern cutters normally are using servo motors with or without gear reducers to cut product.
Other cutting machinery will utilize a guillotine style blade connected to air cylinders or motors. This is used for products that requires more than a blade style cutter described above and require chip-less cutting.
Some products will not cut with blade style cutters described above and require a saw with rotating blades. The saw is normally required for thicker, rigid products (heavy wall shapes, pipe, rigid materials such as RPVC, or materials that are prone to cracking with blade cutting). Saws are available in a few different configurations. For use in line with the process, the saws require a traveling table. This allows the cut cycle to not interrupt the forward motion of the product during the process.
There are two styles of traveling saws:
1) Pneumatic – this requires the use of pneumatic cylinders to “assist” in the table movement via the product being trapped between the table and the clamp.
2) Servo – this is the most popular style today and utilizes a servo motor and a mechanism to move the table (this mechanism can be a belt, ball screw, etc). The servo table movement is more accurate as it does not require the product to be clamped to move the table.
Several styles of saw are seen in extrusion operations:
1) Up-cut – these are the most widely used saws. The blade moved from under the product up through it during the cut. The blade them returns under the product. This style requires a clamp mechanism to operate correctly. Prior to the start of the cut cycle, the clamp traps the product against the table. This action is required as the blade would otherwise lift the product during the cut which could cause poor cut quality and accuracy. Upcut saws are limited only by the blade diameter that will fit under the table.
2) Crosscut – This style is used typically when the product is wider than a normal upcut saw can handle. (Would require a blade so large in diameter that it would not fit under the table at the required centerline height of the line).There are a few types of crosscut saws in use. Some utilize a blade that starts under the table surface and the blade moves upward and then across. This style can either have the blade come up and pass through the product and then return across the product OR the blade can cross the product and move down under the table to return. Some utilize a blade that is over the table surface and simply go across the product.
3) Downcut/Chop – These saws are usually using smaller blades and have smaller capacity.
Some round products can utilize planetary style cutters/saws. These utilize a cutting tool that rotates around the outside surface of the product and cuts with a variety of cutting tools: 1) Lathe style tool- A small cutting bit is mounted in the rotating head and removes material as it cuts similar to a lathe cut. 2) Saw blade – some will have a saw motor mounted on the rotating head with a toothed blade. This also removes material as it cuts. 3) Parting wheel – this style utilizes a cutting wheel that is similar to a pizza cutter and is plunged into the product and acts to part the product as it moves deeper into the product. No chips are created with this style and are the preferred type for this lack of chips to deal with. The downside of using these types of saws is the machine cost and the requirement for constant attention and “tweaking” to keep them cutting well.
It is typical to have some type of conveyor to take the parts after they are cut by the saw or cutter to where they are packaged. Otherwise, since the parts are no longer being pushed or pulled, they would back up at the cutter or saw and cause problems. These are used normally on more flexible products. This is generally a very simple conveyor that is running slightly faster than the line speed of the part.
For small or flexible parts it is often desirable to wind or coil the part for storage until a secondary operation can be done. Some products are sold in this configuration for the customer to apply the secondary operation. There are two types of machines utilized here. Coilers and winders: 1) Coilers – Coiling utilizes a machine with a mandrel set that typically is expandable and the product is wrapped directly on these mandrels. During the coiling process the mandrels are placed in their completely expanded configuration to coil on. When the correct length is coiled, the mandrels collapse to allow removal of the product. Winders – Winding is similar to coiling except there is a carrier spool/reel mounted on the winder over a single shaft that is connected to drive the spool/reel.
Winders/Coilers are available in a variety of configurations from very simple configurations with one station to fully automatic dual station with auto cut and transfer and wrapping or banding. The single station machines normally would be manually actuated machines (operator would mount the spool or connect the product on the mandrels to coil. When it was done wrapping the given length required, the operator would cut the product, remove the coils or spool and start the process again. The dual station machines can be manually operated as the single shaft, or have a variety of levels of automation up to full auto where the operator only has to mount fresh spools and remove the finished product.