How planchets are made at the RCM
By Patrick Glassford | Sunday, 14 February 2010
The following content comes from Patrick Glassford website which no longer exist. To keep this information available to the public and for a perpetuity reason, we reproduced it here.
The process of creating planchets involves several operational sections in the Royal Canadian Mint (RCM).
Melting House: - Alloy Mix
In this section the necessary raw materials are combined to form the alloys required for coining purposes. RCM furnaces were not compatible with the melting and casting of pure nickel. Since 1922 all nickel blanks have been purchased from private suppliers. The steel strip used for war time five cent pieces was also produced by the private sector. At present, bronze for the one cent coin and copper-nickel for the five cent coin are prepared for Canadian coinage. Bronze, copper-nickel, tombac, various brasses and other alloys are prepared for the fulfillment of foreign contracts.
Metals are measured out by weight in the correct proportions and placed into a crucible in a high frequency induction type water-cooled furnace. Electrical current melts and thoroughly mixes the metals. At this point, the furnace is tilted and the molten metal is poured into an upright water-cooled book-type mould. In a very short time the metal solidifies and becomes a bar. The mould is then opened, the bar is removed by an overhead crane and then cooled in a tank of water. The bar is then placed on a roller table, rough edges are ground off and the defective top end is sheared off by a large power shear.
Planchet Varieties associated with the Melting House include:
- Laminations: Trapped impurities in the alloy associated with Copper content, may appear as flaking, peeling, crack or as a hole in a planchet.
- Occluded Gas: Trapped gas (in planchet) expands after strike, giving coin a bulge that may or not appear on both sides.
- Split Planchets: A planchet that, due to trapped impurities, has split in half parallel to the face of the planchet. Typically appear with striations that run straight across one side of the coin.
- Hinged Split Planchet: Similar to the split planchet, except that there is not total separation giving the coin a "clamshell" effect.
Rolling Room : - Strip Rolling
Within this area, the cast bars, which come from the melting house, are rolled down to the required gauge (thickness) for the intended coin. RCM rolling mills were built to handle copper and silver alloys and similar metals, and are not capable of rolling pure nickel, which must be processed while hot. Nickel is either melted or cast, or the strip is produced by direct rolling of pure nickel powder (Sherritt Mint). The control of the rolling mills is all done by push button.
The various conveyors are so interlocked that nothing can move unless the conveyor ahead is clear. The operator always has in view indicators, or gauges, showing the setting of the rolls, the temperature and the pressure of the lubricating oil. Any deviation from a pre-set value will stop the mills. A flying micrometer permits reading the thickness of the metal as it is rolled.
Both mills are "two high" type with 18 inch rolls and are equipped with power driven conveyors to handle the bars, or slabs, which weigh approximately 175 pounds each and are 4 feet long, one and a half inches thick and seven and a half to eight inches wide. They produce approximately seven tons of coil per shift.
The rundown, or roughing mill operates at about 100 feet per minute and reduces the bars to approximately one fifth of an inch in 10 to 12 passes depending on the material. The finishing mill operates at about 200 feet per minute and reduces the slab to the desired gauge. The number of passes needed depends on the final gauge but usually five to eight times will accomplish the desired results.
When this mill receives the metal from the rundown mill it is in slab form, but once through the finishing mill it takes a coiled form about 20 inches in diameter. This is done by the upcoiler on the exit side of the mill. Each mill requires a 200 horsepower electric motor for its rolls, as well as smaller auxiliary motors for the screwdown roll adjustments and the upcoiler.
A large furnace mounted on stilts above part of the rolling mill equipment is an annealing furnace which is used to anneal bars and coils at certain steps during the rolling process.
Planchet Varieties associated with the Rolling Room include:
- Rolled Thin: Cut from portions of strip that, due to malfunctions during rolling (e.g. roller misalignment), and are short of prescribed thickness. These differ from Split Planchets, as there are no striation marks evident.
Cutting Room: - Blanking
It is within this area that the metal in the process takes on the first vague resemblance to a coin, round flat discs are punched out of the coils of strip received either from the rolling room or from outside suppliers. An eight inch wide strip provides the blanks from the one cent to one dollar and is processed in two 50 ton blanking presses.
Payoff reels, holding two coils at one time, are provided for each blanking press. Coils are placed onto reels by jib crane and then swung into position for feeding to presses.
Strip levelers, with an automatically controlled feed, flatten the strip prior to it being fed into the blanking press. Perfectly flat blanks are crucial. The flattened strip emerging from the strip leveler is gripped by the blanking press feed rolls and passed through the machine where the blanks are cut by punches moving up and down synchronous with the forward movement of the strip. Up to 7,000 blanks, in the case of the one cent, can be cut each minute.
The residue or webbing from the blanking operations is called scissel and is recycled. Pure nickel is recoiled and returned to suppliers. Bronze and copper-nickel is chopped by scrap choppers, remelted in the melting house, and cast in to bars again.
As the blanks drop from the presses they are vibrated over a screen made of holes slightly smaller than the blanks. Any blank, therefore, not being fully round or undersize, drops through into a scrap box.
Planchet Varieties associated with the the Cutting Room include:
- Curved clips: Due to overlapping cuts in strip during blanking. Improper feeding of strip, either by moving to far ahead or lagging. Curved Clips of over 60% are known as "Crescent Clips."
- Incomplete clips: Result of the blanking press partially cutting into strip, may occur during set-up of equipment.
- Elliptical clips: May be the result of an incomplete clip separating before or after the strike.
- Straight-edge clips: Blanks cut from the sides or ends of strip.
- Ragged-end clips: Blanks cut from ragged portions of strip.
Edge Marking: - Upsetting
The screened blanks are then put through a machine which rolls them on their edge in grooves set in a stationary circular segment and a revolving wheel. As the blank passes through these parts it is under pressure from both sides which pushes the metal on the edge back and raises it-slightly. This action gives a uniform edge and size to all blanks and also helps in the striking of the coin designs and extends die life. It is called edge marking or upsetting. Each machine can process up to 3,600 blanks per minute.
Planchet Varieties associated with the Edge Marking process include:
- Type I and II Planchets: Planchets that have not been upset are known as "Type I", if upset then they are referred to as "Type II."
Annealing Room
Within this area all planchets are softened (annealed) and thoroughly cleansed, preparatory to the striking of a design into the metal. Previous operations, such as rolling and blanking, have hardened the metal. Also an accumulation of oil, acids, oxidation, etc. has taken place. Blanks are softened by heat and cleansed by detergents, soaps, or chemically with acids. Planchets are fed into a slowly rotating electrically heated spiral furnace, by the time the planchets reach the exit end they are red-hot and soft. They are then quenched by being dropped directly into water. Nickel planchets are dropped into a weak solution of sodium nitrite.
Cleansing and rinsing takes place in a large rotating drum. Bronze and Silver, in a 2% solution of sulfuric acid, followed by a rinse in a water mix of soap bark, cream of tartar and ammonia. Nickel, in water with sodium nitrite, oakite and ammonia and copper nickel in 6% sulfuric acid with sodium bichromate, rinsed and washed with soap bark and cream of tartar in water.
Following final rinsing, blanks are fed into a spiral rotating dryer, hot air-heated and when they finally emerge they are glistening bright, thoroughly clean, and soft. All operations are automatically controlled, temperatures, timings and cycles are all pre selected. Production is approximately 100,000 blanks per hour.
Planchets intended for numismatic strikes are "burnished" by being tumbled in large drums with small metal pellets to produce lustre on planchet surfaces prior to striking.
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