World Money Fair Technical Forum 2022 – a Review (Part 2)

In last month’s issue of Coin & Mint News, we covered five of the presentations made at the World Technical Forum 2022, which was held online over two days and hosted by Dieter Merkle of Schuler and Thomas Hogenkamp of Spaleck.

In this issue, we cover the five presentations made in the second half of the Technical Forum.

Innovation for Green Refining

Xianyao Li, Chief Technology Officer at the Royal Canadian Mint’s (RCM) R&D Centre of Excellence, stated that the RCM is always looking for ways to decrease its impact on the environment, reducing the risk of releasing pollutants into the air, water and the ground.

His presentation focused on how the RCM has implemented a green process in its refinery that has helped reduce the use of chlorine gas by 52%.

The RCM is sent gold by a variety of customers for refining to increase its purity. Although this doré gold normally has a high percentage of gold, it also has impurities such as silver, copper and other base metals. In the refining process the impurities are separated, producing gold up to 99.99% pure or better.

In a typical refining circuit, first the gold doré is analysed to determine the amount of gold content. It is melted in a furnace to a high temperature, after which chlorine gas is introduced into the molten metals. This process, known as Miller Chlorination, produces gold up to 99.5% purity, which is cast into shapes for further refining by electrolysis, increasing its purity to 99.99%.

In the Miller Chlorination process, chlorine gas is blown through molten, slightly impure, gold. The contaminants form chlorides, but at these high temperatures (1100°C) gold does not because gold chloride is unstable above 400°C. Some other metals volatise (lead, iron) and are recovered from the exhaust gases; others (silver and copper) form a low-density slag on top of the molten gold which is skimmed off. Some platinum and palladium may remain with the gold and are separated from the gold in subsequent electrolysis.

Miller Chlorination is a very effective and fast process, as the chlorination reaction continues the level of the molten metal rises inside the furnace crucible. Then, using a special tool, the operator bales out the slag from the surface of the crucible. This fluid slag is poured into a chute and granulated into water.

However, the Miller process is not environmentally friendly. The industry is making efforts to phase it out. The chlorine poses a health safety hazard, requires a high temperature and manual operator skills.

Many alternatives have been tried but have not proved suitable.

The RCM’s solution is based on metals having different vapour pressures. Gold doré is placed in a crucible in a vacuum furnace and heated up, above which is a cold plate. The vacuum pump lowers the pressure in the vacuum chamber as air is removed. When the temperature reaches the gold doré’s melting temperature, most of the impurities, including silver, start to vaporise and deposit on the cold service, leaving gold on the crucible. The impurities are collected separately from the purified gold in the crucible. Also, the system can be programmed to separate the various metallic impurities at different condensers depending on their different vapour pressures.

The name given to the process is Acidless Separation in Vacuum Furnace. It is a clean process with no chemicals.

Although gold bars can be refined using the vacuum furnace, the gold billets from this vacuum method would still go through the Miller Chlorination process due to copper contamination. However, it significantly reduces the overall need for chlorination and use of chlorine gas.

Looking for the Last Few Nano Meters

Felss Systems is a precision machine builder that builds four different types of rolling mills – strip rolling, flat and profile rolling mills, wire reduction rolling mills and ultra-precision rolling mills. Felss Systems’ presentation – Looking for the Last Few Nano Meters – was given by its Head of Sales, Thomas Marcol.

Rolling is the first process in the whole production chain, with the main characteristics defined as surface quality, microstructure and most importantly, dimensional tolerances, which is particularly important for coins. As an example, in a gold coin of diameter 30mm, a 4µm tolerance could result in around a €2.80 loss per coin (gold price of €50,655/kg). So a tighter tolerance is desirable.

However, it is the function of the product where re-work is not possible, which is more important with regard to tolerances. This led to the company rethinking the way of building rolling mills.

Three characteristics leading to thickness issues were identified – elongation of the whole roll stand, the accuracy and responsiveness of the screw-down system, and the roll set itself. The solution was to change to a 4-column roll stand with a rod and sleeve design, providing a pre-stressed solution and reducing elongation by 75%.

The second important aspect was the roll sets. The existing method was a combination of internal and bought parts all with their own tolerances, some of which were outside the tolerance range to meet the new requirement. The solution was to find new partners who could meet tighter tolerances, replace the roller bearings by specially designed bushes and most importantly, the final grinding was done in assembled conditions. The result is a roll set with a running tolerance of 0.1µm.

The last issue was to increase the accuracy and responsiveness of the screw-down system in order to achieve rolling in the nano range. The solution was piezo power. This provided the highest resolution, accuracy, stiffness, and responsiveness, plus no backlash.

The only problem was the stroke at 30nm. This issue was solved by using a combination of a high precision servomotoric screw-down covering material thickness range and, below, a piezo screw-down correcting defects in the nano range, enabling no backlash through the fast, strong, and rigid system.

In testing, the maximum range achieved was +/- 50 nm on 1,300 metres of rolled flat wire. Using the gold coin example earlier, with the tolerance reduced from 4µm to only 0.5µm, the loss per coin would reduce from €2.80 to €0.35.

Surface Conditioning of Minting Dies

Rösler Oberflächentechnik is an equipment manufacturer and process developer for mass finishing equipment and shotblasting. In addition, the company produces special industrial washing machines and provides solutions for post processing of manufactured parts.

Mass finishing is a technology for refining surfaces. It is a mechanical-chemical process in which an accelerated force is transferred to work pieces in grinding or polishing media. Supported by water and compound, the mass and product are set in a relative movement to each other. The relative movement is a very important topic, three of which – vibration, drag motion and rotation – could well be suited to minting dies, said David Soldan, Rösler’s Export Sales Manager, in his presentation, which covered both raw and minting dies.

Raw Minting Dies. The requirements here are to remove the turning lines, achieve surface smoothing and polishing, to prepare a surface for embossing and reduce the surface roughness from approx Ra 0.5µm to Ra loss than 0.1µm. This could be a customised two-step process; the first, surface smoothing, using vibration or drag motion with a wet process with media, followed by polishing using vibration or a dry polishing process.

For vibration the rotary machine has a fixturing system mounted on the working bowl and up to 20 dies could be fixed on the jig, with the media working around the dies to finish them.

The second machine uses drag finishing, where the dies are mounted on a jig system and those are dragged through a medium, the advantage being that a much higher intensity is achieved from the drag motion.

A third method utilises rotation, using only a dry polishing or dry cutting media. This system offers an even higher intensity from the bowl rotating in the opposite direction to the dies which are immerged in the media.

Minting Dies. The objective for dies after embossing and hardening is to mimimise the defects, to polish the table, motifs, and letters, and to prepare the surface for coating and to round the edges at the border between the table and reliefs. Two processes are applicable. The two-step process uses vibration with a wet process to fine grind, and a dry process to polish the dies. An alternative is a chemical accelerated process where the chemical is working on the surface and the media to remove the oxide layer created by the chemistry.

A case study was used to demonstrate how the company provided a bespoke system for the FNMT-RCM. The objectives were a reduction in manual work, improved surface, the elimination of oxidation, repeatability, and the development of a simple clamping concept.

The advantages achieved were increased repeatability, increased quality due to eliminating manual polishing, and increased speed, with the dies taking only six minutes to finish, producing an output of 12 dies in 30 minutes including loading and unloading.

Importantly, a 20-30% increase in the lifetime of the dies was achieved. Another important point was the surface improvement of the dies and coins.

New and Innovative, High Strength, Long Lasing PVD Coatings

In their presentation, Romain Waidelich, CEO of INORCOAT, and Dr Ing Martin Andritschky, Professor at the University of Minho in Portugal, reviewed the galvanic chrome plating and de-coating process against PVD chrome plating, de-coating and machine cleaning.

The former is a chemical process, known to generate carcinogenic fumes, Chromium VI, which is harmful to humans. PVD chrome plating is a vacuum deposition process which is not harmful and produces no hazardous waste or gases. However, the de-coating and machine cleaning utilises hazardous chemicals that are complicated to handle, carcinogenic genetically harmful vapours are produced, and Chromium VI is present.

INORCOAT has been developing chromium free, harm free PVD solutions for the coin and banknote industry with comparable or better properties than chromium-based coatings. The basis of these PVD coatings are ceramics, which are both hard and brittle. The hardness of ceramics is typically 4 times that of hardened steel. To overcome the brittleness of the ceramics, soft metal phases are introduced into the ceramic coating in the form of nano crystals or nano layers which stop it from cracking. This makes the whole coating very tough, and it also protects the substrate.

There are three variants of this new form of coating:

• CiruCoat® H(igh) Speed, which is ideal for heavy duty at highest pressing speeds and has strong elasticity of the lower layers to protect the coining die.

• CircuCoat® N(umismatic), which is ideal for high detailed coin designs and a specific coating design for soft striking materials.

• CircuCoat®M(ulti)M(aterial), which is ideal for high-speed striking and multi-material blanks.

A new machine, the MS 700, has been designed and built specifically for the minting industry to use these new technology coatings. The challenge was to provide a machine that had a compact footprint, an all-in-one design, plug and play, was user friendly, highly customisable, had a flexible tool holder, multiple cathodes for multilayer PVD coatings, integration of linear ion sources as an option, and that it should be able to coat not only dies but also coins and various tools.

The machine is divided into four segments – vacuum coatings, vacuum pumping, process gas control and the electric section. The machine has a compact footprint of just 1,600 x 1,600 x 2,250 mm high, an all-in-one frame and is fully automated on Siemens SCADA system.

If the computer control and the HMI is considered as the brain of the machine, then the cathode (or what INORCOAT calls magnetrons), are the heart of the machine. Atoms are liberated from the magnetrons and directed by the magnetrons’ magnetic field towards the substrate, enabling a ceramic coating with metal inclusions to form. The surface of the substrate is activated prior to coating with PVD to ensure good adhesion, which is achieved by plasma coating or a linear ion source. The flexible tool holder allows different sizes to be coated in one batch.

Pushing Coin Design to the Next Level

Carveco is the design software used by engravers. In its recent work, the company has established that any new software should provide higher resolution coins with more detail, greater visual security features, and which can be more quickly designed.

Carveco’s Commercial Director, Robert Newman, stressed that such software developments cannot be achieved overnight – they are achieved by incremental steps. The company has already completed some steps, others are a work in progress.

The first objective was to enable high resolution coins to be designed faster. Targets for development were sculpting tools and the pen, modified algorithms for higher resolution, vector handling, more intuitive models, and eventually higher resolution. There was also a focus on tool packing and file output.

A priority was the latent imaging tool – the question was how to produce a better surface finish without it taking so long to calculate. Carveco, working with partners ACSYS Lasertechnik and the Royal Canadian Mint, discovered that a 4000 x 4000 latent image calculation took 37 minutes. The Carveco development team managed to reduce this to six seconds!

Next, surface quality was reviewed. How could it be improved?  An experimental piece, a six-side latent image, was set up in conjunction with ACSYS lasers to allow testing. The resolution was expanded to 8000 x 8000 using ACSTS 64bit software and the new Carveco algorithm. This took 1 hour 48 minutes to calculate even on a very fast machine.

After a complete redesign of the algorithm involving new shapes that did not have surface walls around the gaps, the calculation time was 9 seconds. This development work has enabled latent images to be produced in seconds rather than hours and minutes.

Sculpting, and the power of the pen, was a main priority envisaged for this programme of improving coin design. The idea was for the development team to bring natural sculpting processes to the software. Some examples shown were texturing on skin, creating fur, and other repeating patterns. This was all done with a new functionality on the pen. Carveco has already added compatibility with the XP-Pen, HUION pen, and Wacom Dth Lins pens with additions to the latter pen of pressure, stroke direction, azimuth, tilt rotation and fade.

Both the presentations described here, and those covered in Part 1 in the October issue, can be viewed at https://technicalforum.de/virtual-technical-forum/technical-forum-re-watch/.

The next Technical Forum will take place in-person immediately before the World Money Fair, which – barring any further pandemic restrictions – will be held 3-5 February 2023 in Berlin.