ECP This is how Electrochemical Polishing ensures smooth mould surfaces

From Alexander Stark

The surface finish of moulds is crucial, because it has a major impact on the quality of the articles produced and on the lifetime of a tool. ECP is a unique method that ensures smooth surfaces. PMM Moldes from Portugal makes this technology available to other mould makers as well.

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(Source: PMM)

The mould maker PMM Moldes based in Maceira, close to the Portuguese mould making hub of Marinha Grande, specialises in the study, technical development and manufacture of moulds with high technical requirements. Their high-quality moulds are used for the injection of plastic, rubber and die-casting. Since its foundation in 1987, the family-run enterprise has been covering the whole value chain — from product design and tool engineering to production of parts. Its focus is on customers in the automotive industry, but in its 35 years of activity the company has gained experience in the production of parts for other markets as well, such as medical, electronics, packaging and houseware. “What sets us apart is our ability to simplify mould concepts in ways that make the tool more reliable and profitable for our customers,” says David Barbeiro, Business Developer and IT Director.

Adopting the latest technologies and recognising innovative applications, the company also offers a very unique process that delivers unprecedented surface qualities of moulds: Electrochemical Polishing (ECP). This highly rigorous process is able to remove the thermally affected layers left by the EDM process from moulds and dies. “Our CEO, Virgílio Barbeiro, met one of the scientists who developed the technology at a trade fair. At that time we had a very specific project that was suitable to test the potential of the technology and the results were quite impressive, which led us to the acquisition of the first machine,” Barbeiro recalls.

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Electrochemical polishing, also known as electropolishing, is a process that has different applications, depending on the actual technology used. “Our ECP machines — type Echode 53 — use a unique technology that is suitable to the mould industry, allowing us to use the same electrode used during the EDM process,” explains David
Barbeiro.

This figure shows the amount of time it takes to achieve a certain Ra using different processes. PMM demonstrates that to achieve 0.8 Ra using traditional processes would take 23 hours, while using their process it takes less than 5 hours.
This figure shows the amount of time it takes to achieve a certain Ra using different processes. PMM demonstrates that to achieve 0.8 Ra using traditional processes would take 23 hours, while using their process it takes less than 5 hours.
(Source: PMM)

ECP is based on the advantages of a homogeneous electrostatic field that is created in the gap between the tool and a mating electrode when a voltage is applied between them. If an electrolyte is present in the gap, electric current can flow. The current has a uniform density, if the gap is also uniform. Its density is typically 40-80 A/ cm2. This current density is distributed evenly over the entire tool surface. The size of the gap can vary between 0.1 to 0.5 mm. However, the gap must be uniform across the whole surface. The current forces negative ions to the positive pole, i.e. the tool. There, they react chemically with the material, dissociating immediately, since the pH of the electrolyte is around 8.5. As a result an insoluble sludge of hydroxides is formed, consisting of, for instance Fe(OH)2. In this way, the surface of the tool is dissolved gradually. After a layer of about 0.1 mm is removed, a clean, uniform and glossy metal finished is achieved. This surface is free of the recast layer, microcracks and mechanical stress caused by EDM or milling and grinding processes.

Removing unfavourable residues and microcracks

The process reduces surface roughness up to seven times and eliminates microcracks left by the EDM, increasing wear resistance and consequently, the lifetime of the part. By increasing the smoothness of surfaces, users can reduce injection pressure, increase injection speed and use smaller injection machines. This also decreases the likelihood of bacterial contamination, which makes this process especially suitable for medical and food products. This is added by the fact that unlike conventional electropolishing, it does not involve any hazardous chemicals such as phosphoric, perchloric and chromic acids. Therefore, the process does not produce gases or toxic chemical by-products.

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Gallery with 8 images

“We use the machine every time we can, as the process is much faster than manual polishing or fine EDM and leaves a more homogeneous and shiny (blind mirror) surface with no recast layer,” the Business Developer points out. “For example, parts with deep or thin ribs that are inaccessible by manual polishing are usually suitable for this process.” These ribs are commonly made by EDM and end up with roughness that hinders the flow of the material during injection, requiring more clamping force, bigger machines, and longer cycle time, with associated costs. Another example are complex geometries that would be deformed by manual polishing whereas with ECP, they can be polished leaving a perfectly defined 3D geometry.

One of the company’s projects was for a loudspeaker cover full of small holes made by EDM. The results achieved with ECP can be seen on the right side.
One of the company’s projects was for a loudspeaker cover full of small holes made by EDM. The results achieved with ECP can be seen on the right side.
(Source: PMM Moldes)

Visible differences of the moulded products

Barbeiro explains that there are, however, a number of factors that have to be taken into account in order to achieve the desired surface quality. The quality of the surface material has a decisive influence on the outcome of EPC, as does its heat treatment, its surface roughness (Rmax) preceding ECP. The surface finish of the electrode is particularly important, as its imperfections will be copied onto the mould surface, which is why they use a very precise 5-axis-CNC to machine it. “For aesthetic parts, we use cooper electrodes, which have significantly less wear than graphite after the EDM process. The workpiece can weight up to 500 kg and because of the 24,000 Ampere machine limitation, the area of electrode can go up to 650 cm2. To assure proper dimensions of the electrode we can inspect it using a probing cycle on the machine, CMM machine, or even GOM,” Barbeiro says.

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Advantages of Electrochemical Polishing

EDM machining can result in porous surfaces and microcracks. The surface treatment with ECP results in surfaces without imperfections that could result in fatigue marks. The process uniformly and completely removes recast layers caused during EDM. The result is a smooth, glossy, pure metallic surface. Its properties are identical to the base material and show no residual stress. ECP can be employed to create surface topologies that cannot be produced with conventional manufacturing methods. It reduces or even eliminates the need to perform medium or fine EDM or polishing. The process can therefore generate significant cost-savings for both the manufacturer and the user of the moulds. Additionally, the ECP process works with a saline solution instead of acid like conventional electropolishing.

One of the company’s projects was for a loudspeaker cover full of small holes made by EDM. The plastic material used didn’t have a high melting index and after the first trial, the parts had white marks due to plastic straining. This was caused by the roughness of the surface, which reduced the flow of the material. After applying ECP, the roughness was removed, decreasing injection pressure, increasing injection speed, and successfully removing the white marks (see image below), making the customer happy.

Another example was a tea strainer that needed a glossy finish, it had a lot of small holes that would be deformed if polished by hand. With ECP, the company could do a better job in a fraction of the time that would be needed with traditional methods.

“We decide whether we use ECP or not based on the surface finishing requirements that come in the RFQ and every time we can foresee the possibility of injection problems caused by deep thin ribs,” explains Barbeiro. The other big advantage is the fact that once the ECP process is finished, the recast layer and resulting micro cracking from EDM is gone, enabling a normal chemical texture with perfect result.

“As far as we know, we are the only company in the mould making industry that has this technology. However, we offer our services to other mould makers as well that approach us with problems that would otherwise be difficult or impossible to solve,” Barbeiro concludes.

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