It is a zone which screw takes polymer from the hopper. Main purpose of this section is taking polymer from the hopper and conveys it into the screw channel. Mainly this section has stable pitch depth. Polymers are pushed forward by this pitch. Solid conveying directly depends on material slipping on the screw and holding the barrel with the friction forces.
** Melting or transition zone
Main purpose of this section is to melt the feeding plastic. Along the section polymer granules melt and are formed melting pool. Polymers compress with decreasing depth in conventional screw design and it provides to melt the polymer. Then compressed material, have a high pressure, is pushed forward to pumping section.
** Dosing Zone (Metering or pumping zone)
It is the most important section on the screw. Pumping section builds up enough pressure for exiting the material from the die. Volumetric quantity of feeding section over pumping section is known as Compression Ratio of the screw. In conventional 3-stage-screw, pressure reaches the maximum value at the end of the screw but it is sensitive to the counter pressure coming from the mold and output capacity can decreases. In grooved feeding system, pressure reaches a maximum value in front of the groove channels.
Best extrusion conditions are obtained by maximum molten and homogenous material pushing through the mould under stable pressure conditions.
Ekstrüzyon vidaları basınç dağılımı
Basınç, hammadde besleme işleminden, kalıp çıkışına kadar sürekli artan bir faktör olmaktadır.Vidada sıcaklık kalıba yaklaştıkça artar. Kalıba ulaştığında sıcaklık ve akışkanlık değerleri en üst seviyededir.
Pressure Distribution in Injection Screw Pressure is a factor of effects the material from feeding zone to materials exiting from the die.
Best injection conditions are gained; maximum homogeneous and melted polymer pumping with highest stable condition to the mold. Pressure increases with getting closer to the die.
Straight Pitch Single Screws
Barrier Pitched Single Screws
Parallel Twin Screws
Conical Twin Screws
Screw Geometry and Melt Phase Formation;
Vidanın dışarıdan aldığı tahrik ile hareketi sayesinde, hammadde hatve kanallarında taşınır, ısıtıcıların da etkisi ile eritilir karıştırılır ve kalıba doğru iletilir.
Polymer is conveyed to the screw channel with the moving force of the motor and the material is melted by effect of heat resistance then the material is mixed and pushed forward to the mold. Barrier Screw Technology With the developing of the material technology, difficult-shaped materials with high mechanical and thermal properties are used widely. In parallel with material technology design of the screw and barrel also develop. New designs were created for easy processing of this material. In 1959, new barrier type screw was introduced by Maillefer in Switzerland. With this design, solid and melt phase are divided different channels and design is not allowed to unmelted polymer pass through the barrier section. This compact design provides higher melting quality in a short length when compared with the conventional screw design.
Barr Type Screw Different from the conventional screw, Barr screws have secondary flight. This secondary flight has a constant width but depth of the flight increase along the barrier section. Molten plastic transfers to the secondary flight, while main flights depths decreases and applies compression to the solid material. With this combination, molten polymer conveys without any degradation on the other side solid material melts fast and getting better melting quality by the effect of compression causes by decreasing depth of main flight.
In Barr screw design melting channel starts with shallow depth. Along the screw depth of main flight decreases and compresses the solid phase. At the same time secondary flight depth increases. When all the solid phase melts, secondary flight closes the main flight and becomes one.
Maillefer Type Screws;
Maillefer type screw has also secondary flight and flights have constant depth. On the other hand, widths of the flights change. In the direction of flow, solid phase flights width getting smaller and secondary flight width increases. There is no unmelted plastic on the screw when the secondary flight closes the main flights in the Maillefer type of screw.
Advantage of Barrier Type Screws ;
- Better melting quality, lower melts temperature - Better dispersive mixing - Better processability of scrap and regenerated material - Minimum pressure and melt temperature changes -Higher screw speeds and outputs with using suitable barrel design
Mixing Equipments in Extrusion Screw ; Additional zone can be added to standard zones for better melting and mixing quality in injection molding screws. For example, adding mixing equipments on the pumping zone can provides better homogeneous melt. For the better mixing quality of the masterbatches or fillers, mixing sections can be added. Mixing equipments are divided two:
Dispersive Mixers • Mixing equipments which are providing different size and phase particles make the same size in the material. Molten polymer and filler particles are forced to pass through the narrow size section along the mixer and at this time biaxial forces are applied to the material. So homogenous melts is obtained.
Distributive Mixers • Mixing equipments which are providing homogenous melt by distributing the fillers, reinforcements etc. with same size and same concentration along with the material.
Parallel and the Conical Twin Screws and Characteristics;
In blends and hard to process powder PVC materials processing, twin screws are used for the high melting and output capacity requirements.
Twin screws are processed in the special CNC Screw Cutting Machine and screw pitches are positioned sensitively and they are geared one to another optimally. Flank clearances, lateral pitch angles, and the depth of the channels are the same through the screws. It provides to material flow uniformly.
Twin screws are positioned sensitively in the barrel. Screws can process with the 0.5 mm precision during processing.
Segmented Screws and Properties;
Special processed segmented screws generally using in the twin screws but it can be also used in the single screws. Main advantages of the segmented screws are, in a worn or breaking conditions, only segmented parts which are worn or broken can be changed, not the whole screw. It also provides the decreasing of the refurbishing and the renewable screw cost.
These types of screws provide to process different materials with the same screw depends on the changeable parts (mixers etc.).
Extrusion Barrel Types and Working Principles;
Barrels using in the extrusion process are different from barrels in the injection molding applications. Injection molding barrels are exposed to the high pressure, and stability is required. On the other hand, in extrusion applications, feeding different materials to the barrel and the wearing resistance properties are taken precaution. Barrels, which have not a suitable feeding design according to feeding material, never work healthily and provide good quality product.
Extruder barrels have different feeding designs according to process requirements. These are; • Smooth bore feeding • Soft groove feeding • Hard groove feeding
Smooth Bore Feeding; This system is used for the materials which are easy to feed to the extruder. If easy feed materials are using and there are no high capacity expectations, smooth bore feeding is cost effective process. Cooling process can be provided by classically or spirally systems.
Especially this system is used for the materials which are hard to feed to the extruder, having high molecular weight, regrind or recycled. At the beginning of the feeding there are grooved channels which provides better material feeding and spiral cooling system locates close to the barrel inner surface.
• Soft Groove Feeding In this system spiral cooling system is not essential. Because, temperature increasing at the feeding zone is not too high. Standard cooling system can be used. • Hard Groove Feeding Hard groove feeding is used for high capacity expectations and for feeding the hard to feed materials which have high molecular weights. With the deep grooves at the feeding, pressure and the temperature increases rapidly due to friction forces. With the increasing of the friction forces and the output capacity, temperature can reaches up to 350°C. For this condition standard cooling system cannot removes the excessive heat and using spiral cooling system is essential. If the spiral cooling system is not used, polymers melts at the deep grooved channels and stuck the system.
• Sığ Kanallı Besleme (Soft Groove)
Sığ kanallı beslemede spiral kanallı soğutma sistemi kullanımı şartı yoktur. Çünkü beslemede sıcaklık artışı çok yüksek değildir. Beslemede klasik soğutma kanalları kullanılmaktadır.
• Derin Kanallı Besleme (Hard Groove)
Hard Groove besleme, yüksek kapasite elde edebilmek ve yüksek molekül ağırlıklı hammaddelerin düzgün beslenebilmesi için kullanılan bir besleme tipidir. Derin yivlere sahip besleme bölgesi nedeniyle bu bölgede sürtünme ve kanal hacimleri maksimum derecede artar.
Artan kapasite ve sürtünme sebebi ile hard groove beslemede sıcaklık 350 ºC’lere kadar çıkabilir. Bu durumda klasik soğutma sistemi besleme bölgesindeki ısıyı uzaklaştırmaya yetmez. Bu sebeple besleme bölgesinde spiral kanallı soğutma sistemi kullanmak mecburidir.
Spiral kanallı soğutma sistemi kullanılmadığı taktirde hammadde besleme kanallarında eriyerek sistemi tıkar. Besleme bölgesindeki sıcaklık etkin bir soğutma ile uzaklaştırılarak bunun önüne geçilmesi gerekir. Bu ise spiral kanallı soğutma ile sağlanır