I am often asked how can a company verify, in a production environment, that a part has been treated successfully with plasma? How can they verify post-treatment surface activation? I will answer these two questions separately as they have two very distinct answers.
Most plasma applications utilize some form of automation to treat a part and some form of fixture to hold the part in a known location while it is being treated. These systems also run, sometimes for months at a time, without human supervision.
Let’s break down the three key parameters that will ensure that the part has been successfully treated using plasma.
1- The Plasma
2 – The Robot Path
3 – The Part Fixture
As discussed in my last blog, Plasmatreat equipment has the ability to monitor the voltage, current, frequency and duty cycle of the plasma as well as the incoming and internal jet (optional) air pressure. In addition to the control and monitoring of all of these parameters, the LCM option allows for the independent verification of the presence of plasma allowing for QS9000 rating. If all of these feedback signals are present then it can be assured that plasma is present, with the correct operating parameters, at the plasma jet.
The Robot Path
Once a robot has been programmed with a treatment path, it cannot vary from this path without triggering some sort of error. It is safe to assume that a robot will follow the proscribed path exactly each time and if for some reason it does not, then an error will be indicated.
The Part Fixture
A part fixture must hold the part in an exact location to ensure that the plasma jet, when being moved by the robot, will maintain a fixed and repeatable distance from the part that is to be treated. The part fixture usually includes some form of mechanical clamping or vacuum to ensure that the part sits properly in its nest. Through the use of proximity or optical sensors, it can be verified that the correct part is seated in the correct position within the part fixture.
With the abovementioned three parameters verified; the correct plasma, the correct robot path at the correct distance from the desired part, it can be safely assumed that the part is receiving the proper plasma pre-treatment.
Post Treatment Surface Energy Verification
The above steps ensure that the part has been successfully treated with plasma but does not verify that the plasma has been successful in raising the surface energy to the level expected. One of the largest varibles in pre-treatment that has not been mentioned so far is the substrate itself.
Even if you have very tight control over your resin manufacturer and supplier, there may be variations in the molded part due to molding machine parameters, mold release agents and/or the environmental conditions present at the time of molding or during part storage after molding. If you don’t have tight control over the resin you are using then the variation in the properties of the finished part can be even more significant.
These variations due to the substrate can have a negative overall effect on the surface energy of the product before and after plasma treatment.
In order to verify that the post treatment surface energy is sufficient, the only practical method that I have seen, for use on the plant floor, is to use dyne test inks. The frequency of this testing is usually dictated by the Quality Control department. A part is taken off of the line, its dyne level is measured and checked against the level required and, assuming that the level is sufficient, the part is usually re-introduced into the assembly line for further processing.
To recap, by verifying that the plasma treatment is correct and by further verifying that the resultant surface energy is as expected, a manufacturer can rest assured that their process is performing properly.