Extrusion force

The A-A extruders are equipped with a flat perforated plate or screen, combined with two flat extrusion heads. The wet mass is pressed axially through the screen parallel to the feed screws. The extrusion forces are the highest, compared to other low pressure extruders, resulting in a hard and dense extrudate. It is commonly used in the extrusion of food, thermoplastics, and other industries, where a large pellet diameter and thermosetting properties are desired. 

Figure 12 Influence of the amount of water on the extrusion forces (gravity feed extruder) and power consumption (twin screw extruder) on extrusion of MCC. , gravity feed extruder (N) , twin screw extruder (W). Source From Ref. 37.

Baert L, Remon JP, Knight P, Newton JM. A comparison between the extrusion forces and sphere quality of a gravity feed extruder and a ram extruder. Int J Pharm 1992 86 187-192. 

With the grain, ie, samples cut parallel to the molding plane or extrusion axis. Across the grain, ie, samples cut perpendicular to the molding or extrusion force. To convert GPa to psi, multiply by 145,000. 

Even when protein ingredients are added to food in relatively small amounts, they may significantly influence some of the physical properties of the food. Hermansson (1973) found that addition of 4 percent of a soybean protein isolate to processed meat significantly affected firmness, as measured by extrusion force, compression work, and sensory evaluation. 

The application of the theoretical treatment depends on the ability to measure the extrusion force and rate. Most commercial extruders do not allow for these types of measurement. Normal rheological equipment, such as cup-and-bob or cone-and-plate, do not have a suitable geometry or instrumentation to handle materials of the consistency normally used. A ram extruder is a suitable experimental design. 

Fig. 3 Steady-state extrusion force as a function of the length-to-radius ratio of the die for microcrystalline cellulose-lactose-water (5 5 6) at constant die diameter (1.5 mm) and extrusion rate (20cm/min).

Baert, L. Fanara, D. Remon, J.P. Massart, D. Correlation of Extrusion Forces, Raw Materials and Sphere Characteristics. J. Pharm. Pharmacol. 1992, 44, 676-678. 

Bacrt L. Fanara D. Remon JP. et al. Correlation of extrusion force, raw materials and sphere characteristics. J Pharm Pharmacol 1992 44 676-8. 

Baert L. Fanara D. Baets. De. et al. Instrumentation of a gravity feed extruder and the influence of the composition of binary and ternary mixtures on the extrusion forces. J Riarm Pharmacol 1991 43 745-9. 

Huang and Bourne (16) studied the rate of softening of green beans, dry white beans, carrots and three varieties of beets for times up to 120 minutes using the extrusion principle to measure firmness and found a similar pattern in the softening curves for every product. Figure 1 plots log extrusion force versus process time for diced beets processed at 220F. The solid circles and solid line show the experimental data. 

Figure 3. Texture loss due to freezing post blanch, calculated as the ratio of the loss In back extrusion force consequent upon freezing to the back extrusion force after blanching but prior to freezing.

Since the extrusion force must be larger than the frictional resistance, considerable pressures develop in front of the extrusion channels. Therefore, the die body thickness must be selected such that it will not break during operation even if, occasionally, overloading takes place. Also, the open area as defined in Section 8.4.1, Fig. 8.33 ( % free area ) is normally relatively small, also to achieve structural integrity that means that the land between the holes is large. 

Studies are carried out at three different storage conditions, that is, in refrigerated condition (RF 5°C 3°C), room temperature (RT 25°C 2°C/60% 5% RH), and elevated temperature (HT 40°C 2°C/75% 5% RH) for a period of 6 months. Active content assay, gel strength, PTT, viscosity, extrusion force, pH value, and organoleptic characters are measured at different predetermined time points (i.e twice in first month, and every month afterward) during the stability period of 6 months. Physical stability is noted by microscopy or by visual inspection at periodic time intervals (i.e., twice in first month, and every month afterwards) for any perceptible change in physical appearance, that is, odor, color, consistency, phase separation, etc. 

Stiff clays may set up in the tube and require larger extrusion forces if there is a delay. 

In indirect bar extrusion, the die at the front end of the hoUow ram moves relative to the container, but there is no relative displacement between the billet and the container as shown in Fig. 2. Therefore, indirect extrusirm is characterized by the absence of friction between the billet surface and the container. The absence of friction leads to reduced axial extrusion forces compared to direct extrusion. Therefore, indirect extmsion is used for the forming of heavy extradable alloys. But the major disadvantage of the method is the strong limited die design due to the required hollow ram geometry. 

Neglecting the shear forces at the die interface, the total extrusion force in direct bar extrusion can be calculated as ... 

A piston pushes the polymer through the fixed capillary at the end of a reservoir. The movement of the piston is realized by connection through a moving crosshead. The shear rate is adjusted through the crosshead speed. The extrusion force is recorded using a load cell connected to the upper part of the piston [284]. 

Table 3.86 shows the shear rate and extrusion force values for the usage of some capillaries with different diameters. 

In order to verify this theory, the calculation of the total pressure loss and of the end correction is used, on the basis of the extrusion force. The end effects are calculated using the method indicated by Brydson [747],... 

Table 3.86. The influence of the diameter of the capillary on the extrusion force and on the shear rate [167]...

Figure 3.247 shows graphs for the molten PET, representing the evolution of the extrusion force depending on the piston position. 

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