Extruder with variable restriction element

Details Extruder with variable restriction element Extruder with variable restriction element

1999-07-15

2001-07-17

Nguyen, Nam (Department: 1722)

Plastic article or earthenware shaping or treating: apparatus

Stock pressurizing means operably associated with downstream...

Including restriction upstream of shaping orifice and...

C425S191000, C425S19200R, C425S461000

Reexamination Certificate

active

06261081

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention provides a method and an apparatus for extruding an extrudate with a controlled bulk density. In particular, the present invention provides an extruder with a variable restriction element which can be used to control the bulk density of the extrudate as the extrudate is extruded from the extruder, and a process of controlling the bulk density of an extrudate with a variable restriction element.
Extruders are commonly used to extrudate thermoplastic materials such as plastics and moisturized feedstuffs formed of farinaceous and proteinaceous materials. The extruder heats and pressurizes the thermoplastic material and forces the material through an extruder die, whereupon the extruded material is cut into a desired shape as it exits the extruder die. For example, in the formation of dry kibbled pet foods a mixture of farinaceous and proteinaceous material is mixed with water, plasticized, and cooked as it is pushed through the extruder under pressure, and is subsequently cut into kibbles as it is extruded through the extruder die.
Maintenance of an extrudate having a constant bulk density is important to maintain consistent product size and package weights in the production of extruded foods. A common method to maintain the bulk density of extruded foodstuffs at constant levels is to adjust the moisture content of the material being extruded, which varies the inherent lubricity of the extrudate. The bulk density of the extrudate may be increased by increasing the moisture content of the material being extruded since higher levels of moisture reduce the energy imparted to the extrudate by the extruder screw, reducing expansion of the extrudate. Inversely, decreasing the moisture content of the material being extruded increases the expansion of the extrudate which decreases the bulk density of the extrudate. Moisture levels in the material can be controlled by adding steam or water to the material prior to extrusion, or injecting steam or water into the material as is passes through the extruder.
Use of the moisture content of the extruded material to control the bulk density of the extrudate subsequent to extrusion introduces variability into the quality of the final food product. Typically, an extrudate is dried in a dryer after being extruded though the die and cut to the desired size. Extrudate containing different levels of moisture will dry to an extent dependent on the amount of moisture in the extrudate when the extrudate is dried for a set period of time in a dryer. Extrudates containing relatively little moisture will tend to be burndt by the drying process, and extrudates containing excessive moisture will not completely dry, leading to a food product susceptible to mold.
In large scale commercial applications several extruders are often serviced by one dryer, and the variations in moisture levels of different extrudates entering the dryer can be large, resulting in great variation in the product exiting the dryer. The ultimate quality of the product may be adversely affected by these variations. For example, a burndt pet food kibble may not be palatable to the pet for which it is intended, and a moist pet food kibble may be unacceptable because of mold spoilage.
A recent published article authored by S. Shonauer and R. Moreira entitled
A Variable Restrictive Valve as an Extra Independent Control Variable For Food Extrusion Processes
(Food Science and Technology International, Vol. 2, pp. 241-48 (1996)) discloses the use of a restriction valve located in the die of an extruder extending across the die outlet orifice as a method of controlling the bulk density of a food extrudate without changing the moisture content of the extrudate. At relatively closed restriction valve positions the bulk density of the extrudate is low since the extrudate expands significantly as it exits the restricted die orifice under pressure from the extruder screw. At relatively open restriction valve positions the bulk density of the extrudate is high since little expansion occurs as the extrudate exits the die orifice.
The die restriction valve of Shonauer and Moreira, while permitting control of the bulk density of an extruded product, causes variations in the extruded product as the valve is positioned at various levels of restriction. Throttling the die restriction valve changes the die geometry and produces a thinner extrudate. For production of extruded foods having uniform characteristics, use of a die restriction valve is not particularly desirable since the die restriction valve causes variations in the thickness of the extrudate.
Further, Shonauer and Moreira do not suggest that a die restriction valve can be adjusted to continuously control bulk density while the extruder is in operation. In a commercial food extruding process the ability to continuously adjust the bulk density without altering the moisture level of the food in the extruder while the extruder is in operation is desirable to produce a uniform product.
SUMMARY OF THE INVENTION
The present invention is an extruder which can mechanically control the bulk density of an extrudate without altering the moisture level of the components to be extruded in the extruder, and without causing variations in the form of the extruded product. The extruder has an elongated extruder barrel which extends about and defines an extrusion chamber. An extruder die is secured at an outlet end of the extruder adjoining the extrusion chamber. The extruder die has an orifice extending therethrough through which material can be extruded which is in communication with the extrusion chamber. An adjustable flow restriction element is secured within the extruder barrel proximate to the extruder die. The adjustable flow restriction element is structured and arranged to adopt a plurality of positions restricting the downstream flow of material through the extruder, where each position of the plurality of positions is uniquely restrictive. The adjustable flow restriction element is further structured and arranged to be adjustable between each of the plurality of positions as material flows through the extruder to alter the extent which the flow restriction element restricts flow of material through the extruder. The bulk density of an extrudate extruded through the extruder is controlled by controlling the extent which the flow restriction element restricts the flow of material through the extruder.
In another aspect, the invention is a method for mechanically controlling the bulk density of an extrudate of a continuously extruded food material. A food material is continuously extruded through an extruder to form an extrudate. An overall area within the extruder through which the food material can flow and which is proximate to the extruder outlet is mechanically controlled to control the bulk density of the extrudate. In one embodiment of the invention, the bulk density of the extrudate is controlled by maintaining the bulk density at a predetermined bulk density. In another embodiment of the invention, the bulk density of the extrudate is controlled by altering the bulk density of the extrudate from a first bulk density to a second bulk density.
In still another aspect, the invention is an adjustable flow restriction element apparatus for location in an extruder proximate to the die of the extruder which is useful for mechanically controlling the bulk density level of a material continuously extruded through the extruder. The flow restriction element apparatus has a fixed component, a movable component, and means for moving the movable component relative to the fixed component. The fixed component is structured and arranged to be fixedly secured in an extruder proximate to the extruder die. The movable component is located proximate to the fixed component so the movable component and the fixed component are structured and arranged to be jointly located in the extruder across the downstream flow of material through the extruder. The movable component is movable relative to the fixed component so the

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