extrusion screw with barrel on blow molding machine
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Extrusion screw spindle with extrudor barrel on blow molding machine


Length-to-diameter ratio
We express the length of the system as length-to-diameter ratio (L/D). The most common L/D is around 24:1; some are longer at 30:1 or even more, and a few are as short as 20:1. More length may mean more output if heating, melting, or mixing are output limits

A standard screw has three zones:

Feed zone — depth is constant and deep enough to take in the plastic particles.
Compression zone — depth gets shallower, as if the walls were closing in on the plastic, and that builds pressure preventing the air between the particles from continuing down the barrel.
Metering zone — as the plastic enters this zone, it is nearly all melted and ready to be mixed and pushed out through the die. This zone has a constant depth, but it is much shallower than the feed.
Many screws are square pitch, which means that the distance from one flight to the next is the same as the diameter. This makes it easy to get L/D just by counting turns. The portion under the feed opening should not be included in L/D but many people do count as it makes the screw appear longer.

Compression ratio
The compression ratio of a screw is the ratio of the volume of the first flight to the volume of the last one, usually between two and four. It is often taken as the ratio of first to last channel depths in a constant-pitch screw. Compression ratio is useful, but it is an indefinite number and cannot properly describe a screw unless at least one channel depth is known.

Flight width (thickness) is around 10% of barrel diameter. Wider flights waste screw length and develop too much heat in the clearances to the barrel wall, while narrow flights may allow too much flow (leakage) in those clearances. To avoid stagnation where the flight meets the root, corners are rounded

Screws are usually machinable steel, but the flight surfaces closest to the barrel are further treated to delay wear. For light use, flame-hardening is enough. The entire screw surface can be hardened by nitriding, but the usual treatment is a cap of hard alloy on these flight surfaces.

Barrels are steel cylinders usually lined with a wear-resistant alloy.

The clearance between the screw flights and the barrel on new screws is between 0.005 and 0.010 in. (0.125 to 0.25 mm), less for very small screws and more for very big ones. A tighter fit would be more costly to make and develop too much heat. Some wear beyond these values is usually harmless, and may even be helpful, so be sure there is a real problem before rebuilding or replacing (such as overheating because the screw must run faster for the same output).

Screws can be designed by computer if we know resistance (pressure at screw tip), desired output rate, and material viscosities, but it is still a good idea to “season” the computer with some experience before cutting metal.

Chrome-plating a screw may increase slip on the root (which is good) and prevents corrosion, especially when out of the machine, but is unnecessary for most plastics. For highly abrasive materials, the entire screw surface can be hardened. Finally, PVDC and some fluoroplastics need special metals, as iron-based materials corrode and plating doesn’t last long enough.

Some screws are bored with a central passage. Water-cooling the entire length improves mixing in the last flights. Oil is used with rigid PVC to hold the screw tip around 300°F (150°C), so the PVC doesn’t degrade there. Screw cooling only partway down the barrel is done with some plastics to prevent sticking to the screw root in the feed zone.

The Maddock section
A Maddock section is a length of screw around two diameters long, normally found just a few turns before the end, with large grooves (called flutes) in pairs instead of flights.

Each inlet flute has a corresponding outlet, with a barrier ridge between them (see drawing below). The clearance to the barrel over this ridge is around 0.020 to 0.030 in. (0.50 to 0.75 mm). The first Maddocks had flutes parallel to the screw axis but now more are helical.Inlet and outlet flutes in Maddock section. Sketch courtesy Allan Griff.
The melt enters the inlet flutes, flows over the barrier ridge, and leaves through the outlets. Unmelted pellets can’t pass over whole, but are sheared and flattened and finally pass over as melt. Also, the cooler melt stays in the high-shear area longer than hotter melt, providing more thermal uniformity. It is often called a Maddock mixing head, but is seldom at the end (head) of the screw, and is more a strainer than a mixer.

The barrier screw
A barrier screw has a section that occupies most of the compression zone, with an extra flight that forms two parallel channels — one for melt and one for pellets. Clearance between the new flight and barrel is big enough that melt formed in the pellet channel can flow backward into the melt channel, but small enough to block the pellets, which are about 0.060 in. (1.5 mm). The pellets stay in their main channel, but are drained of excess melt and, thus, generate more frictional heat as they rub against one another. Consequently, melting is more efficient per rpm. As material moves down the screw, more melt is produced so the melt channel grows in volume. The pellet channel, however, gets smaller as fewer pellets remain unmelted, until the section finally ends, the pellets are gone, and a single flight carries the melt away through the metering zone. It is common to combine such a barrier with a Maddock section in the metering zone, or another special mixing device.

The barrier section in the drawing is only 4 diameters long, but this is shortened for clarity; the usual length is at least 10 diameters.

Mixing pins are rings of studs projecting from the screw root to disrupt streamlined flow like rocks in a stream, thus improving mixing. They usually are put in the last quarter of the screw.

Grooved barrels have axial or helical grooves in the barrel, in a water-cooled separate feed zone, to improve intake of slippery, hard plastics like high-density polyethylene. A screw with a shallower feed and deeper metering zone is needed, often with no compression at all. Because a deep metering zone gives poor mixing, further hardware is needed, either an intensive mixing section at the output end of the screw or a static mixer in the head.

Vented extrusion
For vented (two-stage) extrusion, a very long screw is used, as all material must be melted before the vent, which is typically at around 70% of total length. This first portion is a normal three-zone screw, but then it suddenly gets deep again, reducing melt pressure so a vacuum can be applied through a hole in the barrel (the vent) to draw off air, moisture, or other volatiles. Melt continues downstream, is recompressed, goes through a final metering and mixing section, and then leaves through the die.

Materials can be added through the vent hole, such as foaming gases and agents, scrap, blending resin, and micro-additives. Even non-thermoplastics can be inserted, such as glass fiber, which doesn’t have to melt and is much less abrasive when added to molten material rather than mixed with hard, solid feed particles.

In a vented screw, the second stage must take away what the first (rear) stage puts into the vent zone, and must also work against the head resistance. Therefore, the pumping capacity of the second stage must be greater than the capacity of the first stage, which works against zero resistance — or else the feed must be separately controlled — to avoid melted plastic coming out of the vent. The usual ratio of front:rear metering depths is between 1.5 and 2.0. However, deep channels can’t pump well against high pressure, so a typical vented screw can only work against a maximum resistance (including screens) of around 2500 psi (17 MPa). For higher head resistance, either controlled feed or a gear pump is needed to allow venting.

Double-flighted and wave screws
Double-flighted screws have two parallel paths in part or all of the screw. In the metering zone, this helps heat transfer, so it is sometimes used where very high temperatures are needed, such as extrusion coating. A double-flighted feed zone is believed to give smoother feed (less pulsation) but is seldom seen today. All barrier screws are double-flighted in their barrier sections, but the two paths are not equal, as explained above. In a wave screw, the two (or three) paths are equal width, but there is a barrier between them low enough for melt to flow over. The channel depths have wave-like increases and decreases, out of phase with each other, so when one path is shallow, the path across the barrier is deep and melt flows from the shallow to the deep. Half a turn later, the depths are the reverse. Melt still flows from the shallow to the deep, so it moves back and forth across the barrier as it goes downstream, which is good for mixing and stabilizes the flow.

Features & Application

The extruder is a device consisting of mechanical and electrical components connected together. Residing within this combination of mechanical and electrical components is the screw or the heart of the extruder. These connected components support the heart. The extruder control is like our brain, it tells the rest of the extruder what to do using feedback from its various components. The extruder motor and gearbox can be compared to our legs and arms, they help get things done. However, without a heart, none of these bodily functions would work. If the extruder did not have the screw it couldn’t achieve its objective of melting a solid plastic into a flowable mass. This is not to discount the importance of the other components, but a poor performing screw or heart makes it difficult for the extruder to perform as expected. So what is the screw’s objective?

Extrusion screws on blow molding machine

Extrusion screws on blow molding machine

The screw is connected to the gearbox which is connected to an electric motor. The screw resides inside a barrel or cylinder which is encapsulated with heaters. As the screw rotates inside the barrel it takes energy from the electric motor and imparts into the plastic pellets causing them to melt. The screw has various geometric features which causes the plastic to transform. The plastic enters the extruder at a given temperature and it must be raised to a certain temperature to flow. This is achieved through shear and exposing the plastic to the heated barrel surface; the screw does all the work to the plastic.

All plastics have certain characteristics, a melting or softening point, specific heat, specific gravity to name a few. Each of these characteristics plays a part in the amount of energy required to process the plastic; but also the shape or geometry of the screw and extruder design. Let’s focus on the screw geometry or what we call the screw design.

Base on this reason, different extrusion screw if different plastic material. so there are special designed extrudion spindle (screw) as below:

HDPE extrusion screw

PVC extrusion screw

PC extrusion screw

PET extrusion screw

ABS extrusion screw

EVA extrusion screw

PA  nylon extrusion screw

PMMA extrusion screw

1. Polycarbonate material
Features: Amorphous plastic with no obvious melting point, glass transition temperature 140 ~ 150, melting point 215 ~ 225, molding temperature 250 ~ 320.
High viscosity, sensitive to temperature, good thermal stability in the normal processing temperature range, basically does not decompose when staying at 300 for a long time, and begins to decompose when it exceeds 340, the viscosity is less affected by the shear rate.
Strong water absorption
In view of its good thermal stability and high viscosity, a. L/D try to choose a large aspect ratio to improve the plasticizing effect, the factory takes 26. Due to the wide melting temperature range and long compression time, the gradual change screw is used. L1=30% full length, L2=46% full length.
B. The compression ratio needs to be adapted to the melting rate from gradient A, but the melting rate cannot currently be calculated. According to the machinability of PC from 225 to 320, the gradient A value can be relatively upper middle. When L2 is large, ordinary gradient screw = 2 ~ 3, our factory = 2.6.
C. Due to its high viscosity and strong water absorption, a mixing structure is added on the screw before the homogenization section and after the compression section to strengthen the disintegration of the solid bed, and at the same time the entrained water can be converted into gas and escape.
D. Other parameters such as E, S, and the gap with the cylinder can be the same as other ordinary screws.
Ii. Plexiglass
Features: The glass transition temperature is 105, the melting temperature is above 160, the decomposition temperature is 270, and the molding temperature range is wide.
High viscosity, poor fluidity, good thermal stability.
Strong water absorption.
A.L/D, choose a gradient screw with an aspect ratio of 20 ~ 22. According to the molding precision requirements of its products, L1=40%, L2=40%.
B. Compression ratio, generally 2.3 ~ 2.6.
c. Due to its certain hydrophilicity, the front end of the screw adopts a mixed ring structure.
d Other parameters can generally be designed according to the general screw, and the gap with the cylinder should not be too small.
3. PA (Nylon)
Features: There are many kinds of crystalline plastics with different melting points and narrow range of melting points. Generally, the melting point of PA66 is 260~265.
Low viscosity, good fluidity, obvious melting point, poor thermal stability.
Average water absorption.
a, l/d, select a mutant screw with an aspect ratio of 18-20.
B. Compression ratio, generally 3 ~ 3.5, where H3=0.07 ~ 0.08 d is to prevent overheating and decomposition. Due to its low viscosity, the gap between the non-return ring and the barrel should be as small as possible, about 0.05, and the gap between the screw and the barrel should be about 0.08. If necessary, according to its material, the front end can be equipped with a non-return ring, and the nozzle should be self-locking.
d Other parameters can be designed according to the general screw.
4. Polyester fiber
Features: The melting point is 250 ~ 260, while the molding temperature of blown PET is wide, about 255 ~ 290.
Blow-molded PET has high viscosity, temperature has a great influence on viscosity, and poor thermal stability.
l/d is generally taken as 20, L1=50%-55% and L2=20% in the three-stage distribution. The screw with low shear compression ratio is generally used, and the compression ratio is generally 1.8 ~ 2. Overheating due to shearing will cause discoloration or opacity H3=0.09 d
There is no mixing ring at the front end of the screw to prevent overheating and hiding materials.
Because this material is sensitive to temperature, manufacturers often use recycled materials. In order to increase production, our factory uses low-shear screws, so the motor speed can be appropriately increased to achieve the goal. At the same time, in the use of recycled materials (mostly plates), according to the actual situation, in order to increase the conveying capacity of the feeding section, the factory also adopted the method of increasing the diameter of the blanking material and opening grooves in the barrel, and achieved good results.
Heat-sensitive materials are generally divided into hard materials and soft materials. The difference lies in the amount of plasticizer added to the raw material, the hard one is less than 10%, and the soft one is more than 30%.
Features: no obvious melting point, softening at 60, viscoelasticity at 100 ~ 150, melting at 140, rapid decomposition at 170, softening point close to the decomposition point, decomposed and released in HC1 gas.
Poor thermal stability, temperature and time will cause decomposition, poor fluidity.
Design principles a. Strict temperature control, screw design as low shear as possible to prevent overheating.
B. The screw and barrel should be corrosion resistant.
C. The injection molding machine process should be strictly controlled.

Assuming the extruder is properly designed a significant amount of due diligence is required to properly specify and design a screw. An improperly specified or designed screw can make a really good machine design perform poorly.


Application: extrusion screw on blow molding machine