Staggered variable relief: A predetermined number of holes are relieved in “step
fashion” across the die face. Usually one third of the die holes are the true effective
thickness, one third at about 1 cm below this level, and the final third a centimeter
below this. The center third of the die is at the full effective thickness; one sixth of the
holes on each side are relieved 8 mm and the remaining one sixth on each side are
relieved 12 mm.
Straight hole die (no relief): This is a die with a common hole size the full thickness
of the die.
Die material composition
Alloy steel: This type of steel has one or more elements added to provide improved
strength and toughness over plain carbon steel. This metal is satisfactory for
pelleting most materials. If corrosive problems occur, stainless steel dies may need
to be used.
Standard stainless steel: Stainless steel is an iron carbon alloy in which chromium
has been added in order to provide superior corrosion resistance, as well as
resistance to pitting, scoring and corrosion. With stainless steel, less enlargement of
the die hole occurs compared to alloy dies.
Chrome-plus stainless steel: A higher quality stainless steel with higher percentages
of chromium and nickel to improve the corrosion resistance over stainless steel. This
metal may be needed for very abrasive or corrosive feedstock.
Other key components
Hopper
The hopper is a container designed to receive feedstock and serve as a reservoir
that maintains enough raw material for a sustainable flow throughout the pellet mill’s
operation. The hopper should be designed to store at least enough material to run
two times the capacity of the mixer. It is usually placed above other components in
order to take advantage of the force of gravity.
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