Stringing Up The Part
With plastic profile extrusion, the stiffer the melt coming out of the die the easier it is to string up and make the desired part. The “perfect” material with a “perfectly” balanced die will come out of the die so stiff and so straight that the part will be pushed by the extruder all the way down the line to the puller. But in the real world almost every part must be strung up by the operator. In order to string up the part, the plastic melt coming out of the die must be pulled down the line to the puller. There are many techniques used to accomplish this.
Generally the calibration or sizing tools for pipe, tubing or other simple closed shapes are not split so the part has to be strung up through the sizing tools. Usually, it is easy to first string a leader of wire, cord, cable, or rope through the sizing tool openings, fold the plastic melt over a loop in the leader and harden it with water spray. Be sure that the loop and the folded-over plastic will fit through the sizing tools. Then the end of the leader can be led into the puller to complete the stringing operation. Often a separate wire or plastic strapping loop will be used between a loop the rope and the hot plastic so that it can be easily cut off after the part is strung up.
Although this method of stringing is required for tooling that is not split, it is often used on any profile. This technique is also commonly used when the extrusion is large and therefore heavy and hard to pull, or when the line is so long or line speed is so slow that it takes a very long time to string the part from the die to the puller.
The sizing tools for more complex profiles is built to be split open to allow the part to be strung down the line with the sizing tool open. Obviously, the tops of the sizing tools should be opened and set aside to allow for easy stringing of the part. In addition, other equipment, tools, hoses, and obstructions should be removed to allow easy, free, and unobstructed path for both the part and the operator to pull the part from the die to the puller. Using the rope or wire can be used with the open sizers; however, stringing up the part by hand is also possible.
To string up the part by hand, the hot plastic melt must be grabbed in order to pull it down the line. Obviously, grabbing hot plastic at 300°F to 500°F with bare fingers can cause severe burns so this is not recommended. While most plastics used in profile extrusion are relatively high in viscosity and not very sticky, some can be sticky enough to stick to bare skin and cause severe burns. Using gloves or a tool is the recommended method. However, because plastic has a relative low heat content and low heat transfer rate, handling the part with bare fingers can be achieved if certain precautions are observed as discussed below. Whenever hot plastic is touched, the fingers should be placed in cold water for a few seconds to cool the surface of the fingers and more importantly to get them wet. This thin layer of water will cool the surface of the plastic enough to prevent an immediate burn and prevent the material from sticking to the skin. However, heat will begin transferring from the hot plastic into the skin so that this will only give a second or two before the material must be released.
To handle the hot plastic right out of the die, a simple tool like a pair of pliers or a metal wire with a hook on the end can be used to pull the part into the cooling tank. An alternative is to put a loop into a rope or attach a loop of wire or plastic banding to hook the hot plastic and hold the part as it is pulled all the way down to the puller. Most operators use gloves to pull the part down the line. However, most water resistant gloves are not thermally insulated and most thermally insulated gloves are not water proof. Thermally insulated gloves, like cotton or leather, that are not water proof can actually cause more severe burns than no gloves at all because the heat builds up in the glove and the skin does not sense it until it is hot enough to burn. Then the heat contained in the water soaked glove continues to transfer into the skin and cause a burn. The normal reaction to burning sensation on the skin is to pull away from the heat, but in this case the glove is surrounding the hand and the only way to prevent the burn is to pull off the glove. When pulling a part down the extrusion line, this means dropping the part in order to pull off the hot glove. The better way is to use no glove so that when the fingers sense the heat you can switch hands to get the hot fingers away from the heat immediately and into the cooling water. Some melts are stiff enough to push themselves into the front of the cooling tank. Any others need to be helped or pulled into the tank with pliers or other tool. But once the surface of the part has been cooled by water and the part is either under water or water can be continually splashed onto the section that is being held it is generally possible to hold and pull the part with bare hands if they are continuously switched as each gets hot. Since the heat transfer is increased with the amount of surface contact and the pressure being applied, it is best to hold the part as lightly as possible and still pull the required amount.
It is important when stringing the part that the material is pulled away from the die and down the line with a uniform speed. The puller achieves this when the above rope technique is used. When the part is pulled down the line by hand the operator has to develop a feel to pull at a uniform speed. The best method while string up is to always watch the material coming out of the die so that it is apparent if the part is drawing down and getting smaller, the pull speed is too fast and if it is bunching up or getting too big, the pull speed is too slow. Walking backward toward the puller while watching the material at the die is another safety reason why the area must be kept unobstructed. If or when it is not possible to see the material at the die, it is important to maintain a uniform pulling speed. The normal human instinct is to pull faster if the pull force is reduced and to pull slower when the pull force or drag is increased. This is opposite of what is normally required. If the resistance to pulling the part is going down, this usually means that the part is being pulled too fast already and is being stretched out of the die. The normal instinct to speed up will stretch the part too much and break it off. If the drag goes up, this usually means that the part is catching on something or is sometimes too big to go through some part of the sizing. The normal instinct to slow down will make the part bigger and more likely to catch and break off. When stringing up the part by hand, vary the force needed to maintain a uniform pulling speed.
It is not unusual for the part to form a twist as it goes down the line. Although it seems logical that it should be possible to untwist the part by simply reversing the twist on the part by hand at the puller, this usually doesn’t work. The reason for this is that the part is rigid for most of the length of the line but the material is still soft and malleable between the die and where it hardens. By untwisting the part at the puller, the rigid part is simply turning and putting a counter twist in the soft material at the die. The solution to this is to grab the part as it hardens and hold it without the twist and in the proper orientation while walking all the way down the line to the puller.