14 Questions
you MUST ask before you buy a doming machine!
How often have you bought something, only to decide after a short period of ownership, that with knowledge you now have, you would have bought something different? This is particularly true if the equipment you areconsidering buying is something entirely new to you, and of which you have therefore little or no previous experience to fall back on. In this situation, it is ‘half the battle’ to at least know what questions to ask. With this in mind you may find the following useful, if you are considering buying doming
equipment.
What type of resin is best for doming?
Three types of resin are used in doming: Two part epoxy resin, One part UV-curable polyurethane resin, and Two-part polyurethane resin.
Two part epoxy resin
Epoxy is a forgiving material, which is easy to mix, and can be poured by hand. However, it is very slow to cure, taking several hours, even when heated, this makes volume production of domes very difficult. It is now also quite expensive, due to the recent reductions in the price of polyurethane, occasioned by the greater volume in production. However, epoxy is still widely used for promotional items, such as key fobs etc. which are not likely to be exposed to outdoor (UV) light. This is very important, as epoxy yellows very quickly when exposed to light, and this makes it unsuitable for most doming application.
One part UV-curable polyurethane resin.
Needs no mixing, and cures in a matter of seconds! The perfect material for doming? Maybe one day – but not yet! Expensive, typically up to five times the price of two-part polyurethanes. Also, its very low viscosity means domes of only half the height that can be achieved with standard doming grade polyurethane. Furthermore, the instant flash cure UV resins (where the benefits of this type of material are seen to best advantage) must be used in near darkroom conditions, this can make difficulties in production that offset the advantages of the instant cure.
Two-part polyurethane resin.
The dominant product used in the doming industry. Consists of an isocyanate (hardener) and a polyol (resin). What makes this product so popular is the quality of surface finish, resistance to yellowing or deterioration, and its overall durability. Surprisingly, even with all these essential advantages, it currently has the lowest cost of the three types of resin used in doming.
How is the resin packaged?
Some machines use cartridges and other forms of pre-packaged resins. The advantage of this is that the machine is simpler to build, and should be much cheaper to buy. The down side is that ‘one use’ packaging of this type makes the resin both more expensive, and less friendly to the environment. Also, these costs can, over time, amount to more than the initial saving on the price
of the machine. Also, some of the more obscure types of packaging (bag in a box etc.) are only available from the machine supplier, and you are therefore compelled to use his material, at his prices, whether you like it or not. The only way to ensure your independence from the supplier is to use industry standard metal screw cap drums, then you can buy on the open market at the best possible price, and with the freedom to choose the resin that works best for you.
What method of dispensing does the equipment use?
The most common methods of dispensing for the doming industry are; Piston
Pumps, Peristaltic Pumps or Gear Pumps. Each has advantages and
disadvantages, as follows;
Pistons.
The normal displacement for a single stroke of an average doming type piston pump is about 10 cc of resin. The pistons must then make a slower return stroke to draw resin from the storage vessels into the piston chambers, before being ready to again eject material onto the labels. This is fine if the row of labels to be domed needs 10 cc or less. However, as soon as the row of labels requires (say) 11 cc of material the pistons must then make two strokes, each of 5.5 cc per row of labels. And of course, for each forward stroke we then have the waiting time for the refill strokes. For large labels this can sometimes take several forward and recharge strokes per row. It is perhaps not surprising, that demonstrations of this type of machine usually involve doming rows of labels, which can be coated in one stroke of the pistons. Because the pistons are inside the machine they are often made of plastic. Many times machine owners are amazed to find that the solid metal machine they have bought has, as its vital ‘heart’ a piece of plastic. In truth, these plastic pistons work fine at first, but of course, are more prone to wear than metal ones. Some piston systems attempt to overcome the dead spot of the refill stroke by having a two sided piston, which is discharging on one side whilst simultaneously refilling on the other. This is better than the single stroke piston designs, but still suffers from a slight pause during the change of
direction at the end of its travel, and it is still limited to the fixed amount of resin dispensed by the given piston size, often necessitating several cycles of the machine to dome one row of labels.
Peristaltic
Peristaltic pumps work by squeezing a flexible pipe, which is attached to a resin supply. The pressure created by the squeezing ejects the resin contained in the section of pipe under pressure. The advantage of this system is that it is very cheap to build, its Achilles heal however, is that it is difficult - if not impossible - to regulate output accurately, or to guarantee repeatability. It is therefore limited, in most cases, to fixed shot size
applications, making it less than ideal for the doming industry, where different sizes of labels require different shots of resin to cover them.
Gear Pumps
Look at the top of the range doming machine in most makers lists, and you will find that they have opted for a Gear Pump design. This is because it is without question the most suitable method of operation for the doming
industry. The Gear Pump however has one major disadvantage, and that is - price! You will normally find that manufacturers offer either a Piston Pump or a Peristaltic Pump machine at an eye-catching price, whereas the same
manufacturers Gear Pump machine will cost nearly twice as much. The presumption is that having been attracted by the low price, the customer will opt to pay a great deal more once the advantages of the gear pump design are appreciated. The truth is, whilst gear pumps are expensive, they should not cost anything like twice as much as the piston or peristaltic designs. Gear Pumps are however, so ideal for the doming application, that customers are often induced to pay a lot more than they originally intended, to have this degree of suitability (this fact greatly enhances the machine makers ‘bottom line’). So, what is it that makes Gear Pumps so desirable for the doming application, and what is a Gear Pump anyway? The gear pump, as the name implies, looks on the inside like a gearbox, having several gears or cogs, which intermesh with each other. The liquid to be pumped is forced by the action of the gears through several small holes in the casing, in such a way that the amounts can be very finely controlled, making accuracy and
repeatability a built in feature. The whole gearbox design requires very
precise machining of the metal used in both the gears and the housing, and it is this that costs the money. Because the Gear Pump has a rotary action, it is free from the reciprocating ‘stop go’ of the other designs. This means that even the largest decals (such as those used on caravans or motor homes) or rows of several large or small labels, can be domed in one ‘hit’ ensuring greater speed and accuracy. Also, because the gears are lubricated by the solution being pumped, they are virtually free of wear, even when constantly in use, requiring only the occasional change of a set of wear plates, perhaps every 12 to 18 months, at the cost only a few dollars.
What is the maximum area that can be domed?
The maximum size of decal or sheet of labels that can be domed on a machine is artificially constrained by some manufacturers. To have the capacity to dome larger areas one must therefore buy a bigger and more expensive machine. If we think about it for a moment, it is clear that this is not necessary, since a well designed metering, mixing, and dispensing machine should be able to dome a large label area as easily as a small one. The only real limitation is the size of the doming bed on the machine, and the
manufacturer decides that.
Can the ratio be varied to enable the use of different makes of resin?
Most piston pump and peristaltic machines are made to a fixed ratio. This can inhibit you in the future if you wish to use alternative sources of supply for your polyurethane. Some machines are designed in such a way that a simple adjustment makes it possible to have the machine set to virtually any ratio required. A word of caution: some makers describe their machines as
‘variable ratio’ when in fact they are using the words to mean that the machine has the capability to be adjusted for wear in the system, so as to bring the machine back to the correct ratio when it has drifted off. The question to ask the salesman is “Can I change the ratio from 100/55 (normally referred to as 2
to 1) to 100/100 (1 to 1) ? Be cautious if the answer is anything other than a straightforward “Yes”.
Are any important working parts of the machine made from plastic?
Plastics have a useful place, but the inside of your new doming machine is not one of them! It is a sad fact that many a solid looking piece of equipment has at its heart moulded plastic components, a feature that the manufacturer can occasionally forget to mention. Most resins have natural affinity to other
plastics, and when cured are often stronger that the plastic to which it they are clinging. The implications of this are obvious if you have to clean plastic
machine parts that have become coated in cured resin. Also, of course, moving parts made of plastic wear out much quicker than metal. They are (or should be) very cheap, and if the benefit of the low cost is passed on to the machine buyer, then that is fine. Consider however the possibility of a
replacement part being required to be fitted in the middle of an urgent doming job. Statistically this is likely to happen more often with plastic components than with metal.
Which is best, a Disposable Mixer, or a Washable reusable one?
Mixing of two part component resins is normally achieved by means of a mixing tube containing an eccentric helix (much like a spiral where every revolution is out of synch to the next) with the result that the two liquids are repeatedly folded into each other, providing a thorough mixing of the resins. Once the two materials meet in the mixer they will start to set, irrespective of other factors. Therefore, if the machine is stopped for any length of time there is a risk that the material in the mixer will set hard. Before this happens the tube must be flushed clear, or disposed of, and a new one provided for each session. Machines, which are fitted with disposable mixers, are often
described as “solvent free” although in practice a small amount of solvent must usually be used to clean the non-disposable parts of the machine, such as the distributor or manifold. Nonetheless, they do in fact use very little solvent. The main advantage of this type of machine however, is that it is cheaper to build. By omitting the wash station, one saves on the cost of the solvent pressure vessel, and the waste collection tray, as well as the
additional valves and features on the doming head. Against this, we must consider the cost of replacing one or two disposable mixers for each day of using the machine, at a cost of several dollars each.
The advantage of the more expensive, washable (sometimes called flushable) mixer, is often stated as being that one mixer will last more than a week, with the resultant saving. In practice however, the reasons for having a washing system installed on the machine are that it retains all the manifold settings and pipe work at the end of each shift, or during unexpected halts in production. Also, very important, when doming small labels, where the
amount being dispensed is also small, the resin is moving very slowly through the pipe work. This can result in the inside walls of the pipes becoming coated
with congealed resin, reducing the internal bore and impeding the flow of resin. Whereas it takes only a matter of seconds to wash the system and be back in production, it can take twenty minutes or so to replace the mixer and re pipe the manifold.
Why do I need Vacuum Degassing and Bubble Removal?
When it is cured polyurethane is extremely durable and resilient, however liquid Polyurethane is easily damaged by moisture. All air carries moisture, and therefore the quick removal of air bubbles from the liquid polyurethane is essential. No bubbles also means no time lost in getting back into production, for example, after changing to a new drum of material. By reducing the air pressure in the space between the top of the liquid and the underside of the drum a situation is created where the air pressure inside any bubbles is greater that that of the surrounding atmosphere. This causes the bubbles to rush to the top of the drum in much the same way that the bubbles rush to the top of a can of beer when it is opened. The bubbles, and the moisture-laden air they contain, are then removed by the vacuum system.
What is a Nitrogen Blanket?
Once a drum of polyurethane is opened and moisture-laden air enters, the life of the liquid product is starting to tick away. At room temperature Nitrogen is heavier than air. If a small amount of the Nitrogen gas is applied to the storage container - after the vacuum process detailed above - it covers the surface of the polyurethane with a 'Blanket' of Nitrogen gas, which ensures the maximum shelf life for the liquid polyurethane.
What are the material handling facilities?
Is the polyurethane pumped to the measuring and mixing part of the equipment, or will the operator have to fill the hoppers on the machine by hand? This is more important than it might at first seem. Stopping production to fill the hoppers with liquid polyurethane slows down production as well as risking the contamination of the polyurethane with moisture laden air. No matter how carefully the filling operation is performed it is almost certain that air (and thus, bubbles) will be introduced into the material as it is poured into the hoppers. The possible damage to the liquid polyurethane can be
minimised by ensuring that only enough material is decanted into the hoppers to complete the work in hand for that session of doming. The bubbles can take time to clear however, preventing a quick return to doming production. A closed loop pumped supply system overcomes all these problems.
How long do the domes take to cure, and can this be speeded up?
One of the major production advantages of polyurethane for doming is that the domes can be cured very quickly. Ideally, the polyurethane should begin to gel (cure) within minutes of being applied to the surface of the labels and
cure to touch dry in about one hour at normal room temperature of 20 C (72 F) To thoroughly harden off the domed labels are best left overnight, or at least for 8 to 12 hours before being shipped. Sometimes it is necessary to ship a quantity of domed labels immediately, and in this case a heated cabinet can be used to reduce the cure time to about 15 to 20 minutes, depending on the size of the finished domed labels.
Do I need an XYZ Scripting machine?
It is a fact that there are some types of doming which can only be done on this very expensive type of machine. But consider that about 98% of the labels and decals that are domed around the world, can be produced on a single axis machine at a fraction of the cost of a three axis machine. As I travel around the world visiting doming companies I see so many three-axis
machines that are hardly ever used for anything other than single axis doming of rows of labels. When one considers that these machines are always priced in the tens (or hundreds) of thousands of dollars, it all amounts to a colossal waste of money. These XYZ axis machines are often called “Scripting” machines because of their ability to dome fine walled cut out script type writing, and at exhibitions one often sees impressive videos showing this application in progress. The question this always prompts with me is this; “Where are these cut out domed script type decals used?” I very rarely see a product with this type of domed logo or writing type lettering, do you? You would need a lot of this type of work to justify buying one of these machines. Most of the irregular shaped labels that are domed can easily be produced on a simple low cost machine using an acrylic template. Using this method the actual doming operation is instant, and usually about ten to twelve times faster than the time it takes an XYZ machine to trace the outline of the shape, and then go back to fill in the solid areas. Because of the time taken to
program the three axis machines, including test runs on actual labels etc. it is often the case that they are not used unless the run length is very long with the result that short runs of shaped labels are domed by hand using hand held doming kits or syringes, whereas to make a simple acrylic template takes only a few minutes, and requires no computing skills whatever.
Do I need a heated cabinet, and if so why?
Some makers of doming machines include an oven because the polyurethane they provide requires heat to make it set satisfactorily. Since one of the major qualities of polyurethane is its ability to be touch dry in an hour, or less, this may seem strange. By making the resin so slow the manufacturer is able to make his machine much simpler in construction, and is also able to obviate the need for the wash out station, which is necessary when using material that sets in minutes. This slow cure method of working is satisfactory, but can become a problem when it is required to produce large volumes of domes beyond the capacity of the oven supplied.
If you plan to use epoxy resin then a heated cabinet is vital, due to the slow curing time of this resin holding up doming production.
Is the system modular, and can it be expanded as your needs grow?
To be frank, this is not essential. But, if you are successful in your doming venture, the time will come when you will feel the need for better facilities on your machine. Perhaps you will want to add some of the features from the more expensive machines in your chosen manufacturers range. Is the
machine designed in such a way that you can add these features easily at a reasonable cost? Or, are you going to be faced with trading in your present machine at a loss, to buy a more sophisticated one? Preferably, you need a modular machine which can easily be upgraded as your business grows, without losing the value of your original purchase.
So, there you have it! If you get the answers to these questions right, before you make a purchase, you are less likely to be disappointed. Good Luck! Mike Mockridge.
