Die and knives are one of the areas in extrusion that has many possibilities and the choices define the results in the final product appearance. To start for safety dies should be bolted on with a minimum of grade 8 bolts. The combination of the die strength and the use of grade 8 bolts is the design which hold the pressure behind the die and keeps it together under high pressures. Note there are differences in how these grade 8 bolts are made and quenched and tempered designs hold higher pressures. As far as dies go there are considerations with regards on how many openings in a die, their spacing and still have the needed strength to hold the pressure.
Hole placement can change the final product shape. Wedged products for example, thicker on one side then the other in a kibble is typically caused by the velocity of the material flowing out the die. It just so happens in this case that one side of the hole has higher flow resulting in non-uniform kibble thickness or a wedged shape. Looking at the area or the bore behind the die which is flowing and full of product towards the back side of the die, this flow is fairly constant except for the friction force seen near the sides inside this area where steel meets the product. The die openings need to be fed evenly which is basically an even area of this mass flowing forwards. To each opening. If a die opening or part of a hole receives more flow then the others then the product will most likely be uneven in shape. This can be seen in variations of product thickness off the die. Capacity of the extruder has an effect; typically, single screw extruders can run at different capacities or what is referred to as extruder capacity turn down. The designs usually allow for a three to one turn down which means as an example, nine tonne per hour extruder should be able to run at 3 tons per hour and make the same product. However, the die needs to be modified. The number of openings used at nine tonnes per hour will not operate correctly at three tonnes per hour to achieve the same product specs. Yes, you guessed it, you need 1/3 of the die openings equally spaced for three tonnes per hour. On a more scientific basis, typical expanded product dies have a total open area in the die of about 200 to 250 mm sq. per tonne per hour. Heavier density products have more open area per ton which reduces expansion. The total open area is adjusted to get the density at a given capacity.
Ok, a die can be designed where the holes are exactly evenly spaced but at lower rates produce an acceptable product. When pushed for maximum rate then you can potentially see different product densities out of various holes as these increased rates can show these slight die imperfections. The question then becomes do we run at lower rates or improve the die for the increased capacity control the product quality? The higher capacities require more exacting dies for exacting product specifications. All of these factors do relate to the final product parameters defined. Having said this, you can have different hole designs making multiple shapes at the same time as long at the holes have the same basic open area. If the open area of each shape is different then you will see a difference in each shapes length or size or expansion rate. The cutting blade design needs to be such that each die hole is not completely covered on a microsecond basis. Knife blades normally have angles so the product can grow in size behind the blade at it rotates over every hole. Slow motion videos show this effect and confirm correct design reduces fines at the die due to tails which can be caused by not taking these angles into consideration. An adjustable strobe light can freeze frame what is happening in the die area to define the holes which are not flowing correctly or evenly allowing for corrections so as to achieve maximum capacity.
Published in the February 2024 issue of International Pet Food.
