Method and apparatus for extrusion of expanding water holding products such as foodstuff particles or feeding stuff pellets

Abstract

In the extrusion of feed-stuff pellets and relevant consumer products, the extruded products are subject to expansion as a result of the boiling of their water content when the products, after being exposed to high extrusion pressure and a correspondingly high temperature, are discharged to the atmosphere through the extrusion nozzles. Such an expansion is directly aimed at, but it is desirable to be able to control it, which is difficult by adjustment of the ordinary process parameters. With the invention it has been found that the expansion which arises can be controlled in a highly constant manner, i.e. by carrying out the extrusion into a closed chamber in which an over-pressure of a few bar is established in relation to the atmospheric pressure, and which can be regulated. The extrusion products can be continuously discharged from this chamber, or they can be discharged intermittently in portions. The over-pressure moderates the momentary development of steam in the extruded product, and the product is given time to stabilise before being discharged to atmospheric pressure, so that the subsequent further development of steam influences the structure of the product only to a minor degree.

Description

[0001]This invention relates to a method of product extrusion where the extruded product material is formed into discrete products which stabilize after extrusion of the product material.[0002]This applies in the production of “solid” unexpanded products where it is desired to avoid expansion, and especially in the production of expanded products where the dimensions of the product are greater than the nozzle aperture(s), where it is aimed to achieve a specific dimension increase in relation to the nozzle dimension.[0003]The problem presents itself mostly in the manufacture of expanded products where the expansion is normally aimed at in order to ensure specific characteristics, such as viscosity (fish-foods), sinking characteristics (specifically slow sinking speed of fish-foods), liquid absorption characteristics (fish-foods, pet-food, foodstuffs), constant low specific density (pet-food, snacks, breakfast products), constant crispness (fish-foods, pet-food; foodstuffs), constant ...

AI Technical Summary

Benefits of technology

[0020]In its passage through the extruder's nozzle arrangement, the product will move from process pressure (2-80 bar) to atmospheric pressure, at which the cutting-off of the product in the desired length normally takes place. The fall in pressure across the nozzle plate and the energy content of the product will result in a momentary evaporation of a part of the product's water content in the opening of the extruder nozzles. The evaporation of the water forms steam pockets in the plastic / elastic mass, whereby the product expands. The fall in temperature brought about by the momentary evaporation results in a hardening of the product into a permanently firm and physical shape.

[0026]To overcome or minimize the expansion control problems specified above, it has been found by the present invention that the extrusion with advantage can be effected into a pressure chamber at the outlet side of the nozzle plate, in which the product is shaped, cut-off and stabilised to a permanent form (expansion degree) before being discharged to atmospheric pressure. By regulation of the working pressure of the pressure chamber, it is possible to regulate the boiling point for the mass in the mouth of the nozzle arrangement, whereby the development of steam brought about by the extrusion can be controlled. This provides a corresponding possibility of controlling the driving force which brings about the expansion of the product.

[0028]The invention will make it possible to use raw materials in the extrusion process which have hitherto been avoided because of control problems related with the raw materials.

[0030]In accordance with the invention, the method will make it possible to increase the capacity of extruders which are mounted with such equipment, since the practical exploitable capacity of many plants is today restricted by the limit of controllability of the product's physical shape.

Problems solved by technology

The ensuring of the above-mentioned constancy / controllability of the expansion gives rise to great problems with all extrusion systems.

Especially the variations in the raw material and the wear state of the extruder's worm conveyor and linings give rise to variations in production which are difficult to predict.

Depending on the conditions which give rise to the product variations, it can be particularly difficult to ensure (via automatic process control and regulation, or manually), that the finished products correspond to the product specifications with reasonable tolerances.

The fall in pressure across the nozzle plate and the energy content of the product will result in a momentary evaporation of a part of the product's water content in the opening of the extruder nozzles.

Images