Screw Barrel Assembly for Plastic Machine
Screw and barrel are the very main and essential parts of injection molding machine, Extrusion blow molding machine, Blown film and many others. In the start of plastic machine were consists of plunger type machine. There were lot of limitations of plunger type plastic machines like poor mixing of material and color and other fillers and additives. Other limitations were all the plastic has not even heating.
A screw is introduced to plastic machine it made plastic processing more convenient, easy, fast, and reliable method. Even now a day plastic machines are sold on the bases of screw type and single, double or multi screw used.
Types of Screw
Now a days different types of screw are available in market. Every screw is divided into three sections (Zones).
Some screws also have decompression zone, and mixing zones.
Injection Molding Machine
Injection Molding machines have single screw which is different from extrusion machine from tip.
Simple screw (Old one not used)
Screw with mixing zones
Screw with different zones
Screw with needing
Screw with pins
Screw with double flight
Screw with single compression and metering zone
Screw with two compression and metering zones
Screw with venting
Double screw used for high though put. Double screw have following configuration.
Based on rotation
Double screw rotating in same direction (Co-rotating)
Double screw rotating in opposite direction (Counter rotating)
Based on intermeshing
Fully intermeshing flight
Partially intermeshing flight
Fully separate flights
Barrel of Plastic Machine
Barrel is made from high strength material. Inside of barrel have grooved channels that increase its efficiency. Mostly bimetallic barrels are used for high performance. Heat is provided along the barrel by electric heaters by conduction.
Difference between screw used in injection molding and Extrusion Molding
In injection molding machines screw and extrusion machine screw many thing are common and much id different. When someone see the both screw cannot differentiate between two but there are some differences.
Injection molding machines screw has NRV (Non return valve)
LD (Length to Diameter) ratio is less than that of extrusion
Extrusion machines screw have mixing zones
Both have three zones (Feeding zone, Metering zone, Compression zone)
When to Repair an Extrusion Screw
As screws wear the clearance between the screw flight and barrel wall increases, especially in the high pressure areas of the barrel.
For example, a new 2.5” diameter screw will have 0.005” to 0.007″ diameter clearance or approximately 0.003” on the radius.