All extrusion processes require the generation of pressure within the material to be extruded. When materials are put under pressure heat is generated. This cannot be avoided but different extrusion types generate different amounts of heat. There are basically three different types of extruder and the one chosen may have far reaching implications on production operations. It is important that the right type of extruder is chosen as this is a critical decision that could have long term implications.
Screw driven extruders (with radial or axial output)
Screw driven extruders move the product to the extrusion die head along an axis by the use of a screw thread. The product is worked hard for a significant time, high pressures are generated and this process generates heat. This cannot be avoided and all screw extruders do this. The generation of heat can be useful in some industries such as plastics or food but for pharmaceutical products or any other products that can be damaged by heat this can be a unwanted when high volumes and continuous extrusion are required.
It is possible to add cooling systems to screw driven extruders but these vary in efficiency at keeping the product cool during the process even though they may keep the equipment cool! It is recommended if you have restrictions on the amount of heat allowed, that any screw driven extruder is carefully considered. It is also advisable to measure the product temperature of the extrudate produced. This is particularly true if you need to extrude product through very small holes. You will need to do this if you need very small pellets because the diameter of the final pellets is largely determined by the diameter of the extrudate.
Basket or screen extruders
There are two main types of mechanisms. One type scrapes the product through the screen and the other type uses rollers that push the product through the screen. The type with the roller action is marginally gentler on the product but both types do generate some heat. The heat generated in screen extruders is often less that the amount of heat generated with screw driven extruders. Screen extruders are generally suitable for a great number of pharmaceutical formulations and they are widely used in the pharmaceutical industry. They are also the lowest cost extruder to make for any particular required capacity. These are the two main reasons is why they are widely used.
Gear extruders are less well known but have significant and real advantages when extrudate is required of products that are sensitive to heat. The principle of operation is quite different from other extruders. The product is allowed to fall between two rotating gears. The base of the gear form has holes so that the product is pushed through the base of each gear tooth into the hollow center of the gear and falls out through the front of the gear.
This process is the type of extrusion that generates the least possible amount of heat. This low generation of heat is principally for three reasons:
Very little work is done to the product before it enters the extrusion teeth and the actual moment when the product is pushed through the holes in the die.
The product moves through the extrusion holes very quickly and emerges into the center of the gear. Because the extrusion is very quick and the time the product is under pressure is short very little heat is generated.
Finally because the gears themselves are significantly large and solid lumps of steel any small amount of heat generated is easily transferred away. The point of extrusion in both screw fed and screen extruders is generally much less solid and therefor the transfer of heat away from the product is significantly slower.
Before any decision is made about which extruder type to choose then the points mentioned above should be given serious consideration. Caleva are leaders in this field and will be happy to talk to you and discuss your application in detail.