Atmospheric pressure plasma: Interface modification of tool steel

Atmospheric pressure plasma processes enable targeted modification of steel surfaces, whereby high cleaning effects and adhesion properties can be achieved, for example in relation to adhesives or coatings.

Plasma technology

Anti-adhesive coatings are applied to the mold to reduce adhesion between the plastic and the mold during injection molding. In order to achieve good adhesion between the coating and the tool steel, the steel must be pre-treated. Plasma technology can be used for this purpose. This can be roughly divided into high, low and atmospheric pressure plasma. With atomic pressure plasma, the plasma is ignited under ambient pressure.

A significant advantage of this method is its inline capability, which reduces costs and process time. In addition, the different discharge techniques and gas compositions enable a wide range of surface modification options. However, the interactions with the surrounding particles can impair the process stability and have an unfavorable effect on the treatment, which is why constant ambient conditions are necessary. The plasma jet process variant was selected for the investigations carried out. This is characterized by the use of a central electrode and an overlying ring electrode, which also serves as a housing. A high voltage at the electrode leads to an arc discharge between the electrode and the grounded housing.

Apart from the primary electrons of the background radiation, the strong thermal load on the cathode causes further electrons to be emitted. These are moved to the anode by the influence of the field strength. The resulting arc between the anode and cathode transfers the process gas flowing through to the plasma state.

Based on the tests and the screening model, further tests were carried out. The voltage range was increased to 260, 280, 300 and 320 V. The flow rate range was reduced to 15, 20, 25 and 30 l/min, the nozzle spacing to 2 and 5 mm and the nozzle speed to 25, 50, 75 and 100 mm/s. Due to transient plasma ignition of the 6 mm nozzle, this was not considered further, but only the nozzles with a diameter of 4 and 5 mm. The test series was carried out full factorial. As in the model mentioned above, the comparison between the test points was based on the mean value of the maximum values of the peel resistances.

It was found that lower velocities tend to lead to higher resistances. However, an increase in the volume flow at higher speeds also leads to an increase in the peeling resistance. At a voltage of 260 V, a speed of 25 mm/s was identified as optimal for achieving the highest peel resistance. It was found that a speed of 100 mm/s in combination with a low volume flow leads to a decrease in peel resistance.

A comparable trend manifests itself at a voltage of 300 V, whereby a speed of 75 mm/s leads to lower peel resistances than 25 mm/s. Increasing the nozzle diameter to 5 mm results in a shift of the optimum, so that a value of 50 mm/s achieves the highest peel resistances. This underlines the relevance of this parameter. At a speed of 25 mm/s and 50 mm/s, there are differences in the peel resistance at different voltages, while the influence of the voltage is less clear at higher speeds. The present results indicate that low speeds in combination with a high volume flow and a higher voltage lead to maximum peel resistances.

In addition to the travel speed, the nozzle diameter is also of decisive relevance. A nozzle with a diameter of 5 mm in combination with a larger nozzle spacing of 5 mm, compared to 2 mm nozzle spacing, exhibits higher overall peel resistances, especially at higher travel speeds. Reducing the voltage from 300 V to 260 V results in increased effectiveness at lower speeds. Changing the nozzle distance between 2 mm and 5 mm tends to have little effect with a nozzle diameter of 4 mm. In most cases, however, a nozzle distance of 5 mm leads to higher peeling resistances. If the nozzle distance is too small, a larger stagnation point can occur, which reduces the effectiveness of the plasma treatment.

In addition, the influence of the voltage as a function of the volume flow is investigated. The highest peel resistances are observed at a voltage of 300 V and a volume flow of 30 l/min. A low volume flow of 15 l/min in combination with low voltages can also generate high peel resistances, which could indicate a higher degree of ionization than with high volume flows. Increasing the nozzle size to 5 mm results in an increase in the peeling resistances. In particular, the voltages of 280 V and 300 V are responsible for the highest peeling resistances. A flow rate of 20 l/min or 30 l/min remains optimal in both cases. However, the results show a greater scatter with a larger nozzle, which indicates the existence of other influencing factors.

Finally, the influence of the nozzle distance in relation to the volume flow is considered. It can be seen that a volume flow of 30 l/min achieves the highest peel resistance, regardless of the nozzle configuration. Furthermore, the peeling resistance in this series of tests also tends to increase with a nozzle distance of 5 mm. This could be due to a less pronounced stagnation point flow. The selected voltage of 300 V confirms its positive influence on the peeling resistance, especially in combination with a larger nozzle diameter. The results of the investigation show that both the volume flow and the distance between the nozzles are significant influencing factors on the peeling resistances.

Results of the fluorescence analysis

The present study deals with the application of fluorescence analysis to evaluate the cleaning effect of plasma treatment of metals where contamination by organic compounds such as abrasives (oils and greases) plays a significant role. The fluorescence scanner is a device that scans the surface point by point with a UV laser. At a wavelength of approx. 405 nm, a large number of organic materials, such as greases, oils, adhesives and release agents, show significantly increased fluorescence activity.

These materials are able to convert part of the UV light into visible light. This process is known as luminescence and is caused by the emission of light by atoms that have previously been put into an energetically excited state. In contrast, most inorganic materials, especially metals, do not exhibit this behavior. These substances can be measured using spectral filtering, as the fluorescence of the substances enables a high-contrast and unambiguous measurement. As a result, it is possible to identify even small quantities of organic substances per square meter, regardless of whether it is a contamination or a desired coating, such as oiling.

The selection of the plasma process parameters for the various examination points of the fluorescence analysis was made with the aid of the determined peel resistances. In this context, parameter combinations that resulted in high, medium and low peel resistances were analyzed in order to establish a link between surface cleaning and adhesion.

A peel resistance with average values was determined as part of the investigation. The measurement was carried out under the following parameters: a voltage of 280 V, a volume flow of 15 l/min, a traversing speed of 50 mm/s, a distance between the nozzles of 2 mm and a nozzle diameter of 5 mm. The evaluation revealed an intensive cleaning effect. The results show that plasma treatment of steel surfaces produces both a cleaning effect and an increase in strength in terms of peel resistance.

Summary and outlook

It has been demonstrated that atomic-sphere pressure plasma treatment has an influence on the tool steel surface and on the interface properties between steel and adhesive. The increased adhesion can be seen as an indicator of these effects. Another aspect that contributes to this increased adhesion is the cleaning effect and thus the removal of surface contamination. When selecting the process parameters for plasma generation, the voltage, travel speed and nozzle size are particularly relevant. However, it should be noted that the other parameters must also be adapted to the selected parameters in order to ensure successful cleaning and activation of the surface. The fluorescence analysis demonstrates that setting parameters that generate an average peel resistance between the metal surface and the adhesive are sufficient to effectively clean the surface of organic contaminants.

Author: Dennis Rauen