Opto switch for detecting translucent polypropylene
For an industrial application the normal approach would be to submit a sample of your product to an industrial sensor vendor and let them propose a solution. A through-beam slot sensor should be able to give very good resolution. You would program the machine to move the part across the sensor, stop when the beam is broken and then make your move relative to that position.



Figure 1. An opto slot sensor has an infrared LED on one side and photo-transistor on the other.

You might be able to find one with a logic output which would simplify your setup rather than have to amplify the photo-transistor output current.



Figure 2. Using polarized light to detect translucent part. Image by LEDnique.

Another approach would be to use a vision system to report the position. Since your part is translucent you could use polarisation of light to your advantage. Illuminate the part from the rear using an LED backlight covered with a polarizing filter. Add a polarizing filter to the camera and rotate to 90&deg; so that the backlight appears black. Now when a part is positioned in between the filters it will randomize the polarization of the light passing through it and the part will show up as a light image on a dark background.