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).

Feeding Zone

Compression Zone

Metering Zone

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.

Single Screw

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

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.

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

Similarities

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.