DIES AND DIE SELECTION by Andre Eksteen
As a large die ring supplier in Southern Africa we come across so many false ideas about pellet dies that I decided to write this short production to the pellet mill die ring.
For many years, pelleting was considered an art, a process involving imprecise measurement, uncertain results, and that undefinable quality of feel. This so-called “art” of pelleting came about in an environment lacking the understanding of the effects occurring when the differing characteristics of feed ingredients are subjected to the pelleting processes of heat, moisture, and pressure. No excuse, however, exists today for the uninformed pellet mill operator.
The science of grain processing has now reached a point that, by knowing the characteristics of the feed (moisture, content, fiber, etc.) and using machinery with accurate measurements, the guesswork has been removed from pelleting. As a result, much more is required of the pellet mill operator in the way of knowledge and ability. The skill of the pellet mill operator, through his ability or mistakes, influences plant profitability.
Pellet mill operators have a vitally important role in the manufacture of animal feed. They should recognize this responsibility and the great contribution they make to an efficient feed plant.
The Purpose of Pelleting
Pelleted feeds have been defined as “agglomerated feeds formed by extruding individual ingredients or mixtures by compacting and forcing through die openings by any mechanical process”. Basically, the purpose of pelleting is to take a finely divided, sometimes dusty, unpalatable and difficult-to-handle feed material and, by using heat, moisture and pressure, form it into larger particles. These larger particles are easier to handle, more palatable and usually result in improved feeding results when compared to the unpelleted feed.
Pellets are generally formed with diameters from 2.2mm to 18mm and will be somewhat longer than the diameter. A small part of the production of large pellets, 12mm and above in diameter, is produced in other than cylindrical shapes; they may be triangular, square or oval and, in some cases, may exceed the maximum dimension indicated above. The largest diameter usually found is rarely greater than 25mm. In most cases where particle sizes smaller than 4,0mm are desired, it has been found to be more satisfactory from the standpoint of economics to produce a 4,0mm or 5,0mm pellet and reduce it into the desired particle size by means of crumbling.
Almost all livestock feeders agree that animals make better gains on pelleted feed than a meal ration. The most logical reasons are that (a) the heat generated in conditioning and pelleting make the feedstuffs more digestible by breaking down the starches, (b) the pellet simply puts the feed in a concentrated form, and (c) pelleting minimizes waste during the eating process. When pelleted feed is fed, each animal receives a well-balanced diet by preventing the animal from picking and choosing between ingredients. Tests have shown that most animals, if given the choice between the same feed in pellet or mash form will prefer the pellets.
By combining moisture, heat and pressure on feed ingredients, a degree of gelatinization is produced which allows animals and poultry to better utilize the nutrients in these ingredients. Feed conversion will be improved. These advantages are particularly noticeable in the broiler industry.
The feeding merits of pelleted feeds over the mash form have been repeatedly demonstrated in the feeding of swine. One state college reported the results of an eight week swine feeding test in which pelleted feed performance was compared against the same feed in mash form.
This test gave the following results:
All animals, on the average, consumed the same amount of feed (2.30kg. per day of pellets vs. 2.20kg. per day of mash), yet the pellet fed pigs gained a 11 grams per day more weight than did the mash fed animals. Since the pellet fed hogs gained more while eating the same amount, it is evident that pelleting causes the feed to be utilized more efficiently by these animals. This is shown in the comparison of the average amount of feed required for each kilogram of gain. The pellet fed hogs consumed 600 grams of feed per kilogram of gain while the mash fed hogs needed 1,47kg to make one kilogram of weight gain.
Pellet fed hogs not only gain faster but they do it with less feed for each kilogram of weight increase. Pelleting prevents the segregation of ingredients in a mixing, handling or feeding process. By feeding a pelleted feed, the animal is more apt to receive a totally mixed ration than one that has separated through these processes. It also prevents waste. Bulk density is increased, which enhances storage capabilities of most bulk facilities. Shipping facilities are also increased, thereby reducing transportation costs. This is particularly evident in such fibrous ingredients as alfalfa, gluten feed, oat hulls, rice, bran, etc. A better flow and handling characteristic of pellets is one of the least mentioned advantages but probably the most important, particularly as it relates to dairy farmers.
In 1978, there were 9,977 feed mills registered with FDA producing 78.2 million tons of feed annually; about 60% is pelleted. Not all feed mills, of course, are equipped to pellet feeds. In 1958, these mills produced 40 million tons of feed and about 55% was pelleted. In 1968, about 70% of all commercial poultry feed produced in the United States was pelleted. In the Midwest, almost 80% of all manufactured feed is pelleted, crumblized or cubed.
Short description of the term Pelleting
The process of producing feed pellets can roughly be described as a plastic moulding operation of the extrusion type. Feed ingredients are made up of various compounds such as proteins, acids, sugars, fibres, and minerals. These products can be softened (conditioned) by the addition of heat and water. When sufficiently controlled compression is applied to the “conditioned” feed ingredients, they will form a dense mass, shaped to conform to the die against which they are pressed. When the heat and moisture is again withdrawn (dried and cooled) as to withstand moderately rough handling without excessive breakage and has retained or enhanced its nutritive value.
In modern feed mills, the ingredients are usually stored in bins above a weighing system composed of one or more scales. Those ingredients which have coarse texture, such as whole grains and other fibrous materials, are ground into a fine meal to facilitate the pelleting and mixing process. Weighted quantities of each ingredient (either as a batch or continuously) are thoroughly mixed (either in a batch mixer of a continuous flow mixing unit) and then conveyed to a bin above the pellet mill. Some products require systems to grind all of the premixed materials prior to entering the pellet mill to allow for aeration.
The die ring
The die ring is simply put a cylindrical drum with thousands of small Dies (holes) that will form the material.
Similar to the Dies found any type of industrial pressing system a Pellet Die must be specifically designed for a certain product recipe.
A good example can be found in the motor industry. A die designed to press the bonnet skin of your car from 0,8mm A107 mild steel will produce hundreds of perfect skins per day. If someone should change the steel properties to A109, which is also a mild steel but with a different carbon content, this die will not produce a perfect bonnet skin. The bonnet skin will be flatter than the design spec. Why? Well the carbon in the A109 will react differently from the A108 and that will make the A109 steel more springy resulting in a flatter bonnet.
In the pellet industry most clients will cry wolf and blame the machine or the die, but in reality the die simply did what it was designed to do. The problem lies in the change in material properties where a die was designed to pelletize recipe 1 and the client now tries to pelletize recipe 2.
If only I could make people understand this basic reality, life would have been so much easier and cheaper for them!!!! But, life goes on and everyday I get more and more callouts to solve “faulty die rings & machines” only to find that there is nothing wrong with the machine, the problem lies with the feed material or rather the machine owner’s ignorance.
A Pellet Die is a small hole that forms the product it was designed for into a cylindrical shape.
A Die ring is a cylindrical or flat steel forging containing thousands of Pellet Dies.
Die Ring Metallurgy
The basic physical properties of the die materials are controlled by the heat treating process and the composition of the die’s steel.
Carbon and Chromium
The two key elements in our dies are carbon and chromium. The chromium content of the steel affects the corrosion resistance and the wear resistance of the die. A free-chromium content above 12% classifies a steel as stainless. We use a steel mixture called X46Cr13 which can be described as Stainless steel dies, but we prefer to call them Chrome steel dies for the simple reason that there is no such thing as a rust free “stainless steel” die ring.
Chromium carbides are formed during the heat treat process through a combination of chromium and carbon atoms, which increases wear resistance, but that also increased the oxygen atoms left behind after quenching. These oxygen atoms are sitting there waiting for the right conditions to start a reaction called oxidation, RUST!
Heat Treating: carburizing vs. vacuum hardening
There are two different ways in which dies are commonly hardened; carburizing or vacuum hardening and the method used depend on the type of material used to manufacture the die.
Case carburizing dies can be done in a pit furnace and vacuum hardening is done in a vacuum furnace by the addition of a carbon rich gas, such as propane. Alloy Die are processed in a pit furnace and chrome steel Dies are vacuum hardened.
When carbon is added into the atmosphere of either of these furnaces, it soaks into the steel to form a hard outer case. Case carburized dies tend to create more friction in the pelleting chamber, meaning they will usually provide better quality pellets at the expense of production capacity.
Vacuum hardening is a process that gives the same relative hardness throughout the thickness of the die, not only the outer case like you would find in carborized dies. Due to its homogeneous hardness throughout, Chrome steel offers excellent die lifespans and are much easier to break in.
There are three common types of dies being used in the industry today:
1. Alloy is a medium-grade carbon steel which is case carburized for a hard outer case of about 6mm thick to 57 HRC and a very soft core.
3. Chrome Steel (the most common of all) is vacuum hardened to 52 HRC through the thickness of the die.
5. Mor-Ton is a stainless steel which is carburized in a vacuum for a hard case 61 HRC and a soft core.
Die Material Application
Each material has characteristics that may make it more desirable than another for an application.
Alloy is the most breakage and crack resistant die material currently offered, which means that it is best suited for heavy tramp metal situations. Alloy has been used in heavily abrasive situations where die life with other die materials is not significantly longer in order to keep cost/ton ratios low. Alloy is also used extensively with high mineral content applications.
Mor-Ton dies can be used in mildly corrosive applications. Because of its case carburization and hole erosion characteristics, Mor-Ton should only be used with customers whose primary concern is pellet quality. It works well with moderately to highly abrasive materials which tend to keep the pellet hole inlets open. The best wear characteristics and throughput seem to be achieved with closer hole patterns, especially with smaller hole diameters of 4,00mm and below. Reducing ligament thickness lessens the occurrence and severity of rollover. Thicker die blanks should be used to offset the brittleness of these dies. Even though Mor-Ton is a stainless steel, it will rust or corrode.
Chrome Steel allows high throughput and die life. It should be used in pure feed material applications, high throughput applications and extended die life applications. Chrome Steel needs more effective thickness to achieve a pellet quality similar to carburized dies and pellet quality can be devastated by changes in thickness of as little as 2mm. These dies are the kings of industry because of their low running cost, high throughput and anti seize properties, but they will not tolerate low plasticity, frictional and high shear properties to name but a few changes in input material properties. These dies MUST be expertly matched to a certain mixture or pellet quality will suffer. When changing from a carburized die to Chrome steel always increase the blank thickness to avoid cracking.
Die Design Features
The physical characteristics of a die can determine its performance through the blank and effective thickness, the reliefs and the hole pattern. Perhaps the most important physical characteristics of a die are the blank thickness and the effective thickness.
The blank thickness determines the overall strength of the die. The thicker the blank, the more it resists deflection caused by the rollers. Blank thickness should be increased instead of ligament thickness, especially in cases of repeated circumferential breakage.
Effective thickness is the length of the pellet chamber that will perform the pelleting. Effective thickness governs the amount of work the die will perform on a material, increasing pellet quality. It also controls the mount of stress added to a die and the mill:
More thickness = more stress + more wear + less throughput + more consumed power.
Stupidity and bull headedness is one of the biggest causes why businesses go under. In my profession 4 out of 5 clients will go against all these odds and fit thicker and thicker dies to compensate for bad mixture design or pre-conditioning practices. The result is a much higher running cost reduced or non existent profit margins leading to bankruptcy. If you are reading this, I beg you to seriously consider spending your money once on proper laboratory development instead of gradually bankrupting yourself with the wrong combinations.
Changes in material necessitate changes in effective thickness due to changes in the materials coefficient
of friction, plasticity and shear strength properties. Reliefs were developed so that increased blank thickness could be used with applications that needed thinner effective thicknesses. Currently, there are two major types of reliefs: Straight hole and tapered.
Straight holes can be oversized, while tapered holes can be at different angles. Tapered reliefs over 25,00mm are extended by adding a straight relief to it. These two reliefs can be drilled in three methods: variable, non-variable, and special variable.
The hole pattern is the major factor in determining a die’s wear pattern. Each of the types of pattern can have one of the three types of ligaments. This is determined by the ECJK value.
Close is used for easy running, high throughput feeds. Benefits of the close pattern include increased throughput, better die face wear, and easier startup. Spreading the feed over more holes in the close pattern also retains pellet quality over life of the die. Increasing the amount of steam to the feed means more heat and moisture. Conditioning improves pellet quality increases, die life increases and operating costs decrease (lower operating amperages, power savings).
Standard is the normal pattern and Heavy is used in high pressure, hard running feeds. These patterns, when applied to various materials, will give you different results. Every opportunity should be taken to get maximum hole count in a die as it increases throughput and die life. If you have a limitless supply of dies or money, you can develop a die/product by trial and error, but if you are serious about your business you will have this development done by a pellet development lab.
Keeping accurate and complete tonnage reports allows the study of individual dies and the compilation of this information enhances the proper review of specifications.
Review of the various die categories:
1. The most important result is tonnage. Tonnage is the basis for evaluating your costs and productivity.
2. Hours run is an equally important statistic because it details the rate at which the product flowed through the die.
3. Tramp metal is a source of potential damage to a die so you are well advised to take all actions possible to prevent tramp.
4. Depth of wear has significant impact on both quality and throughput of pellets.
5. Honeycombing is indicative of a section of the die that has worn well, maintaining a steady throughput.
6. Rollover is a peening action that closes the hole entrance, decreasing throughput and increasing the stress on the die.
7. Pitting is a common result in dies which are not corrosive resistant. Corrosion occurs in the pelleting chamber, slowing the production.
8. Scoring is caused by highly abrasive material eroding a groove in the pelleting chamber.
9. Clamping surfaces show the state of the mating surfaces with the die.
10. Breakages occur in three major categories.
a. Circumferential is related to the strength of the die blank.
b. Blowouts are when part of the pattern releases from the die face and tramp is usually the main cause.
c. Flange failures are the result of a snapping or stretching motion. These are common where the die is not retained properly in the mill.
Depth of Wear
The most noticeable and important trait of any worn die is the depth of the surface wear experienced by the die face. The depth of surface wear is defined as the perpendicular distance to the die face from the horizontal plane that marked the original die face surface. This information should be recorded in three sections:
and the quill.
Cone readings are taken in the front third of the die face, while center and quill readings are taken in the thirds of the die face bounding their respective areas. A good standard practice is to take these measurements in the exact center of the die and three rows in from each flange. This helps bring consistency to the measurements. The only exception to these locations would be if there was an exceedingly deep band of wear in an area, then the measurement should be taken in the deepest spot.
To take readings, simply remove the pellets from the respective holes. Make sure the outer surface is clean by scraping away excess fat, sediment, and other pellets that may hinder a proper measurement. Using micrometers or a small diameter rod, insert from the O.D. of the die the depth gauge of the micrometer or the rod until it appears on the I.D. of the die face. Mark the rod on the IO.D. or check the measurement of the dial micrometers. You have just measured the remaining blank thickness. This measurement can be subtracted from the original blank thickness to arrive at the depth of wear. These measurements should be given in millimetre.
The depth of wear measurement gives important information concerning feed distribution by evaluating which portion of the die is worn the deepest. This measurement identifies worn scrapers and deflectors as well as unevenly worn roller shells.
If all the wipers and deflectors are within specification the Die designer will use this information to design a variable relief die for you that will eliminate the problem of uneven wear and improve pellet quality. He may also suggest a different track width or roller pattern. The key is proper data gathering by you, the client. Engineers are good at what they do if they have the data they need to work with, without data they can only guess and guesswork will rarely result in success.
Rollover is the condition of the die face when the hole inlets start to peen closed. This peening action will have dramatic effects on both the pellet quality and the throughput of the die, usually lowering both
Rollover is caused when the force exerted on the face of the die exceeds the toughness of the die material used. Roller adjustment, and or patterns and certain types of feed exert excessive stresses on the die face, initiating rollover. In the case of Mor-Ton and Alloy dies, the likelihood of rollover occurring increases as the
depth of wear increases, exposing greater amounts of softer ligament material.
Honeycombing is the result of the abrasive wear of the pelleted material enlarging the hole entrance on the die face. This action at its most severe may result in a serious reduction of the ligament thicknesses between holes. It is characteristic of a honeycombing condition for the ligaments to round at the tops, allowing material to flow into the holes. Honeycombing in its mild state is an indication of a good producing die and is characteristic of Chrome Steel material.
Pitting is a condition caused by corrosion, which is the result of the effects of moisture and heat combining with the feed in a die. Pits appear as small places of micro corrosion that grow as the die continues to wear. Pitting slows down a die by reducing the throughput and often can reduce pellet quality due to the rough surface created inside the hole. This is especially true for cube dies, which are more prone to the effects of pitting.
The condition can be observed by removing feed from a hole and shining a light from the die I.D. so that a hole is illuminated. When looking down the hole from the outside, a contrast can be caused on the hole walls so that early pitting will appear as little pin pricks in the sides of the hole wall.
More severe pitting will appear as larger blotches. WARNING: Make sure that all the sediment has been
removed from the sides of the holes with either a test tube cleaner or a pipe cleaner. Pitting is a more common cause of loss in production in the case of Mor-Ton and Alloy dies.
Scoring is the appearance of longitudinal lines down the hole wall. These marks are caused by severely abrasive pelleting materials scratching the hole wall as they pass through. Very commonly, scoring will occur as the result of earlier pitting in the shape of a comet. The origin is the pitting and the tail is caused by scoring being passed through the pitted area. Severe scoring will slow die throughput and disrupt pellet quality.
The condition can be observed by removing feed from a hole and shining a light from the die I.D. so that a hole is illuminated. When looking down the hole from the outside, a contrast can be caused on the hole walls so that early scoring will appear as little scratch marks in the sides of the hole wall. More severe scoring will appear as larger grooves. WARNING: Make sure that all the sediment has been removed from the sides of the holes with either a test tube cleaner or a pipe cleaner.
WARNING 2: Scoring can also be caused by incorrect manufacturing processes where twist drilling is used instead of gun drilling methods. Twist drilled Dies are cheap as dirt, but they are so expensive to run that you can effectively buy proper gun drilled Dies for the money lost in production.
An important part of any die inspection is the inspection of the quill flange clamping surfaces. These areas can commonly be cause of a broken die if the proper maintenance has not been performed on the adjacent wear parts, such as clamps and wear inserts. Thus, the overall goal of examining the clamping surfaces is to gauge damage to other parts of the machine. There are four surfaces that are inspected: the loose clamp surface, the clamp bottoming surface, the wear ring surface and the keyway.
Loose Clamp Surface
The loose clamp surface is the front of the die quill flange that mates against the clamping surfaces. This surface can suffer wear for various reasons. Items that can affect the wear include undersized quill flanges, worn pellet mill quills, worn die clamps, worn wear rings, and worn keyways. Finding the cause of the wear will necessitate gauging the rest of the surfaces.
A worn quill can cause wear on the loose clamp surface and the butt surface that mates against the quill directly opposite the loose clamp surface. Wear on both of these surfaces could indicate a quill rebuild is needed. Certainly the quill should be inspected carefully and the next opportunity.
Usual wear on the loose clamp surface is the result of worn die clamps. The clamps should be checked with the appropriate wear gauge provided for that die. The condition can be observed by cleaning off any anti-seize material or rust from the surface. Wear will appear as polished metal. A good item to determine is whether the surface was ground or hard turned during manufacture. By determining the type of finish, the wear can be gauged.
Clamp Bottoming Surface
Clamp bottoming occurs when wear to the die, quill, clamps, or any combination of the three becomes so severe that the inside of the clamp rubs against the top of the quill and the top of the die quill flange (the clamp bottoming surface).
The condition can be observed by cleaning off any anti-seize material or rust from the surface. Wear will appear as polished metal. A good item to determine is whether the surface was ground or hard turned during manufacture. By determining the type of finish, the wear can be gauged.
Wear Ring Surface
The wear ring surface is commonly called the die pilot surface. This surface mate up against the wear ring insert in the quill. Again, the types of grinding and turning are similar to those previously mentioned, depending on the place of manufacture. Wear on this surface can be caused by installation with an already worn wear ring or an undersized pilot diameter.
Undersized pilots are a common occurrence. Most wear on these surfaces is caused by installation of the new die with an old wear insert. Wear on this surface indicates that wear ring inserts should be changed immediately. Wear can also indicate that a die is loose and may often be the cause of wear on other surfaces within the mill. The condition can be observed by cleaning off any anti-seize material or rust from the surface. Wear will appear as polished metal.
The keyway is the area recessed into the die pilot that fits the key and provides the drive for turning the die. This surface will tend to experience wear only if the die is loose or the key was undersize and worn to begin with at installation. The battering or “wallering” effect experienced by the keyway is caused by the loose key. As the key pivots in the keyway, it causes a doming effect on the sides of the keyway.
Common Problem Solving
Die throughput loss has several causes: conditioning, grind, die face rollover, and die hole condition (pitting and scoring) and so on.
Pellet quality losses are the other common complaint. Conditioning, grind, uneven die face wear, excessive honeycombing, die hole condition (pitting and scoring), relief pelleting and insufficient effective thickness are all culprits of pellet quality loss.
Good Die and Roller Maintenance Procedures
Inspect dies carefully when they are on and off the mill, looking for key items.
1. Rollover and pitting/scoring – the most common cause for a slowdown in production.
2. Inspecting clamping surfaces for wear when removing old dies can prevent breakage.
3. Check the face wear pattern for indications of poor feed pad distribution.
4. Inspect your wipers and deflectors.
Roller maintenance is the key to getting maximum life out of your die.
1. Check your rollers routinely for unusual wear, chipping and/or scuffing.
2. Probably the most important factor in die life is the setting and adjusting of the rollers.
3. Roller lubrication is an important factor.
4. Depending on the amount and frequency of your die changes, always try to start new rollers with new dies.
5. Try to even out the wear on the roller shells by rotating them frequently on machines that allows this.
What you need to know about rollers and how to adjust them.
Two or three rollers are mounted inside of the die cavity on eccentric shafts so their outer faces can be adjusted to contact the inner surface of the die. This is the most important adjustment on your pellet mill. Correct adjustment will result in maximum capacity, minimum wear on both rollers and die, and eliminate undue stresses in the pellet mill. When properly adjusted, the rollers will contact the die just enough to cause them to rotate. Damage can be done by excessively tightening the rolls.
1. Do not adjust rollers while die is turning.
2. Pellet mill start switch should be locked out before adjusting rollers.
3. Adjusting any machine while parts are in motion is extremely hazardous and failure to
comply with this warning may result in injury or death.
Roller adjustment should be made whenever required. The die should not be run without feed any more than is absolutely necessary. Operating the pellet mill with rolls too tight will result in peening closed the entrances to the holes in the die and excessive wear of the die and rolls. See below for proper roller adjustment procedures.
Different types of roller shell surfaces are available to meet varied conditions experienced in pelleting different material and material combinations. Before ordering other configurations, discuss with your pelletmill supplier or even better have a lab test done to confirm your configuration choice.
1. Rollers must be moved away from the die surface before changing dies. Use special roller wrench, if available, to rapidly rotate the rollers to their full back position.
2. Wear occurs on the surface of die and rollers as the pellets are produced. Check these surfaces periodically for wear and adjust rollers according to the above instructions when necessary.
Common Die Problems
I have compiled a small list of common die related problems and their causes. Please note that these are DIE RELATED problems with the assumption that the product being fed into the die was developed by a pellet lab for that specific die. In my experience the cause of a problem can be traced back to improper product/die ring development or changes in the products physical properties in more than 98% of the cases.
Loss of Production Rate: •Die face rollover
•Pellet chamber pitting/scoring
•Pelleting in reliefs
•Rollers need adjustment
Loss of Pellet Quality: •Die face rollover
•Loss of effective thickness
Poor Die Face Wear: •Badly worn rollers
•Worn wipers and deflectors
•Die face rollover
Roller Maintenance: •Follow proper lubrication procedures
•Proper roller adjustment, avoid hitting the die face
•Look for unusual wear, especially chipping or sluffing
•Start new dies with new rollers
•Rotate rollers to evenly disperse wear
Conclusion on die rings
People are quick to blame the die or the machine for their own incompetents or bull headedness. A Die ring can and will only perform to its designed characteristics. Untested, undeveloped products should NOT be introduced to a production die. A production die is just that, a production die.
If you have the time, money, equipment and expertise to experiment with different die ring combinations, designs and products you should use a special experimental die that can be scraped after each trial. This method ensures that you do not get skewed results because of a damaged test die. For one hole size you should have at least 14 test dies. More on this in the product development chapter…