Profile Extrusion Fundamentals
The extrusion process involves feeding a polymer or blend to the extruder where it is heated and forced through an orifice with a die that transforms the molten plastic into a continuous desired shape. This extruded shape is then pulled through a cooling process that forces the extrusion to hold its shape and dimensional stability while it hardens.
The most critical part of the extrusion process is the balance between the volume of material being pushed out of the die by the extruder and the volume of material being pulled away from the die, through the sizer tooling by the puller. Initially achieving this balance during startup and then maintaining the balance during the run is the most important job of the extruder operator. Like any continuous process, the consistency of the equipment and process is extremely important because variation takes the process away from the target or optimal. This variation means looser tolerances, greater chance for scrap, and probably higher costs. With profile extrusion, trying to put too much material through the sizer can cause the line to jamb up and break. Therefore, the operator needs to adjust the profile size small enough to remain below what will cause jamming and line to breakage.
accurate and consistent. The state of the technology in screw, tooling, and equipment design has improved to the point where these can operate with very little variability. However, there are still factors involved with extrusion that are difficult to control.
On the extruder side of the process, one point of variability is the bulk density of the material being fed into the extruder. Pellets are generally very consistent in bulk density but if different size pellets are blended they can segregate and cause variation. Even greater variation can occur with powders or regrind. Loss-in-weight feeders are a significant improvement in reducing this variability.
On the downstream side of the process there are more factors that can upset the balance. The most important factor is the puller. Again, with current technology the control and consistency of the speed of the puller is usually very good. However, there are many factors that can cause the part to slip in the puller creating inconsistencies in the pulling of the part even though the puller speed is very consistent.
The puller must transfer its consistent speed to the part by the friction of the belts or cleats against the surface of the part. This friction must be greater than the drag on the part going through the rest of the process. Friction is a function of the force applied by the belts and the surface contact area.
It is important to use a puller that is long enough to allow enough surface contact on the part so that it does not slip. It is also important to keep the belts or cleats in good shape because wear and damage decrease the available contact area. Choice of belt and cleat material is a compromise between the softer material that gives better gripping power and the harder materials that are better at resisting wear and damage. Friction between the belt and part is also affected by water that is carried by the part from the cooling tanks.
The puller must be able to apply enough force across the entire contact surface to grip the part. It is often desirable to reduce the force applied to the part to keep the belts from crushing or otherwise distorting the shape of the part. The ideal situation is to control the applied force so that it is always consistent and not so much that it is crushing the part but not too little to allow slippage.
The drag on the part comes primarily from the calibration tooling that is needed to hold the part in shape as it is being cooled (while in motion moving through the cooling process). The design of the calibration tooling is a compromise between providing enough control to keep the part within the required dimensional tolerances while not creating any more drag than necessary. The more critical the dimensional control of the part the more drag is created by the tighter calibration and the longer the tooling is in contact with the part.
Variation in the drag on the part can come from variation in contact between the part and the tooling. When using a vacuum tank, this is usually caused by variation in the vacuum that is pulling the part out against the tooling. Control of the vacuum is obviously important but so is the design of the tooling which controls how the vacuum is distributed to the part / tooling interface.
Subtle variation can also come from many other sources such as: the sag of the part if not properly supported along the entire line, interruption of the part during cutting, change of air pressure inside a hollow part when the opening is interrupted by the saw or cutter blade, water circulation within the cooling tanks, temperature variation from a variety of sources, and many other subtle variations.
All of these variations can affect the balance between the extruder pushing and the puller pulling the part. When variation causes the puller to pull faster than the extruder is pushing the part will get smaller. The only problem this creates is variation in the part dimensions but if the variation is smaller than the tolerance allowed in the part, there really is no problem.
When the extruder is pushing more than the puller is pulling, the part will get as large as the tooling will allow. If the variation is small or the tooling allows for large variation, the problem is the same as when the part gets small – only a variation in dimensions. It is not unusual for a slight increase in size to cause an increase in drag that may cause a slight slippage that exaggerates the problem. If there is too much material being pushed by the extruder to be pulled through the calibration tooling, then the material will bunch up between the die and calibrator and eventually break the line. This is a major concern of operators because it not only means they have to string the line up again, it creates wasted material and lost production time.
Like any continuous process it is important to limit and reduce variability in the extrusion process.