How to Configure Twin Screw Extruder丨Melt Section Screw Specs
The melting mechanism of a co-rotating twin-screw extruder is special. The single screw process relies on heat from the barrel and friction between the polymer transported between the barrel wall and the screw threads. And the twin screw extruder mechanically process the polymer with a kneading block to stretch, shear, and fold the polymer to produce heat.
In this article, we will discuss how the kneading block melt polymer and how the design of the melting section affects the material melting method, the temperature and quality.
The melting section of plastic extruder
All of the blending operations take place in the molten state. But configure the screw to melt the polymer is not easy. An aggressive melt section will result in a high melt temperature, which may exceed the degradation temperature of the polymer. And that maybe cause the polymer degradation or scorching.
Or, too mild melt passes may not adequately melt the polymer. The type of polymer, particle size distribution and melting point of the material all influence the mechanical effects that occur during the melting process. A mild melting zone may allow unmelted particles to pass through, and produce a non-uniform product.
Pic 1 shows an assembled melting section. The solid material is conveyed from the inlet (from the right side of the picture). The solids are pushed from the conveying element onto the kneading block, the shearing action of the discs stretches and pulls the material. The incoming solid particles are mixed while the work applied to the polymer causes the temperature of the material itself to rise. And it will cause the polymer to soften and then melt.
Pic 1 Melting section in a Leistritz 40-mm MAXX extruder. (Photo: Polymers Centre)
Kneader Block of Extrusion Machine
The kneading block consists of a series of discs or paddles, each of which is offset from the previous disc by a fixed angle. This staggering angle and the width of each disc determine the strength of the work applied by the kneading block. The direction of staggering also determines whether the kneading block conveys the material downstream (forward) or upstream (backward).
Kneader blocks not only used for melting but also for mixing. Mixing and melting actions occur simultaneously in the melting zone of the extruder. In this article we will discuss the melting mechanism.
Pic 2 shows a forward conveying pinch block. Looking via the end of the pinch block, each disc on the element is offset 60°. We must pay attention on that the general movement from one disc to the next is clockwise. This is the same direction of rotation as the conveyor screw.
When this pinch block rotates in the screw, its rotation will push the polymer forward-deliver the pinch block forward.
The angle of staggering affects the energy transferred to the polymer, kind like depending on the amount of material transported by the polymer. For the delivery element, the rotation of the screw pushes the polymer advancing. Similarly, the forward conveying pinch block pushes most of the polymer. Some of the polymer passes over the end of the disc cutting on the barrel wall. Some polymer is transferred to the adjacent kneading block of the other screw, and a small flows back upstream of the barrel. The combination of these three elements causes the polymer temperature to rise and transports the polymer forward.
Pic 2 60° forward conveying pinch block. (Photo: ENTEK)
The method to mark the pinch block can be seen in the etching on the side of the pinch block in Pic 2-4. The following code is used widely:
KB6 – means a pinch block with six discs.
2 – indicates that the unit is a two-lobed unit. The shape is oval. This is similar to a conveyor element where the double lobe element has two separate threads at 180° to each other.
## – The first number is the length of the element (in millimetres). The length of pinch blocks shown is 60mm long.
XX – The second number is the staggering angle. In Pic 2, the angle is 60°. And Pic 3, the angle is 90°.
There are the other named way, like:
KB – the pinch block
## – the angle between discs
X – the number of discs
%% – the length of the element (mm).
A pinch block with a 30° angle between the discs achieves minimal mixing and delivers the material forward with the best way. 45° pinch blocks provide more energy to the polymer while increasing mixing, but deliver slightly less . The delivery ability of 60° pinch blocks is worse, but mix more efficiently and transport more energy to the material.
With the staggering angle increasing, the forward pumping ability of the pinch block decreases until the staggering angle reaches 90°.Then the pinch block is neutral and the polymer is mixed but it does not move forward. Pic 3 shows neutral blocks with discs aligned at 90° to each other.
With transport capacity decreasing, more polymer passes through the end of each disc, resulting in more energy being transferred to the polymer. With that, plastic polymer will be mixed better.
Pic 3 90° neutral pinch block. (Photo: ENTEK)
Disc Width of Twin Screw Extruder
The factor that greatly affects melting is the width of the disc. Kneader blocks can consist of 3-7 discs, but typically each block has 4-5 discs. Usually, the number of discs is constant, except that the length of the kneader block is adjusted to increase or decrease the width of the discs.
The wider the discs, the more energy is transferred to the polymer. The pinch block rotates in the extruder. The polymer can only flow in one of two directions: over the tip of each disc or around one disc to the next.
Increasing the width of the discs results in more polymer flowing over the tips of the discs. A narrow disc pinch block will essentially cut through the polymer, whereas a wide disc acts more like a plough.
The space between the tip of the disc and the barrel wall is where the shear rate is greatest. Polymer passing through this area is subjected to the highest shear in the extruder. A wide disc kneading block will impart more energy to the polymer than a narrow disc kneading block. This increased energy input will result in a higher melt temperature.
Reverse Feed Kneader
Pic 4 Reverse transport kneader. (Photo: ENTEK)
The last type of block used in the melting is the reverse transport pinch block. This type is the most severe, because it push the molten polymer back upstream. In this case, the smaller the staggering angle, the more drastic the effect on the polymer.
Pic 4 shows the reverse pinch block. For many manufacturers, the forward conveying element rotates to the right. The direction of rotation from one disc of the pinch block to the next disc is clockwise. Thus, the reverse conveying element has a scraper or disc that rotates counterclockwise. These manufacturers use the same nomenclature as the forward conveying elements to designate the reverse conveying elements with “LH”. It means left-handed, because they are left-handed elements, whereas the forward conveying elements are right-handed.
Other manufacturers use the letters “RE” to dedicate reverse delivery elements, which means Reverse Element. The reason is that these screw elements rotate in the opposite direction to their neighbouring elements.
he effects of staggered angles are shown in this form:
Combined melting section
The design of the melting section depends on the type of material being processed. We must take all of these factors into account when designing the melt section for the screw configuration, such as whether the polymer is crystalline or amorphous, has a high or low melt viscosity, or is being processed near its decomposition temperature.
The condition of the feed is also critical. Only one polymer or several polymers be fed through the feeder? What kinds of filler and additive are added with plastic? Is the melting temperature of additives significantly lower than that of the polymer? These factors affect the suitability of kneader block.
I always repeat that, while we can discuss the science behind the action of screw elements, screw design is an art. Engineers will design different screws to achieve the same function. This does not mean that one design is wrong and the other is right. They may just be different, but both can be acceptable if the product quality meets the requirements.
Figure 1 shows the melting section of a 40 mm twin-screw extruder. The kneading block consist of four discs. The solid material is fed by a transfer unit from the right side of the photo. The first unit in the melting section is a 30° pinch block, then is three 60°blocks. Finally, three 90° neutral pinch blocks complete the melting section.
When the feed enters the melt zone, it transfers from the conveying element to the 30° pinch block. This will help to pull the feeding material into the melt zone and transition from the conveying unit to the kneading block. Besides, the 60°block provides a stronger power input to start the melting process while still feeding the material. Most likely, the polymer starts melting between the end of the 30°block and the middle of the first 60°block.
There are two functions of the 90°block. The first is to apply higher energy to the polymer in order to melt the polymer and raise the melting temperature to the suitable temperature.The second is to act as a limiter, preventing the material from travelling through the melt zone with the speed too quickly. This ensures that the residence time of the polymer in the melt zone is sufficient to make sure that all of the polymer passing through the melt zone is molten.
Some alternative designs include the following:
– Reverse Pinch Block: We can use left-handed pinch block to increase the amount of restriction at the end of the melt section. By increasing the amount of time the kneading block processes the polymer, pumping material upstream improves the melt, especially for crystalline and high temperature polymers.
– Reverse Conveying Element: The left hand conveying element is more severe than the left hand kneading block. This provides more reflux for better mixing, increases melt homogeneity and gives higher melt temperatures. Some formulation even require the use of a left-handed kneading block followed by a left-handed conveying unit.
– Separate the Kneader Block from the Conveying unit: For some polymers, a single long series of Kneader Blocks is not the best configuration. Some of the problems that can result include high melt temperatures and extruder chatter due to uneven flow or melting. One approach is to design several short melting sections separated by conveying elements. We can configured as, for example, two 45° pinch blocks, a 90° pinch block, repeated once or twice, followed by a reverse 45° or 60° pinch block to ensure complete melting.