Film Bubble and Scratch Inspection: What to Prepare

What shows up against a fluorescent lamp

Hold a piece of transparent film up to a fluorescent lamp. Bubbles and foreign matter show up right away. Lay the same piece flat on a desk and look straight down, and little is visible; lower your head and look at an angle, and only then do fine scratches glint.

A camera works the same way. Usually, under transmitted light passed through from behind, bubbles and foreign matter appear like shadows, while surface scratches show up well under light from an angle. If you need to find both, you may need two lights as well. That is why film inspection machines sometimes carry two sets of cameras and lights.

Dents and scratches on very glossy film are sometimes caught well by coaxial lighting, which shines from the same direction as the camera. In the Optics Lab, enter transparent film, the defect you are worried about, and the width and speed, and you can compare predicted images made with twelve lighting and optics configurations. These are calculated predictions.

So in a consultation we first ask whether the film is transparent, translucent or opaque, whether it is glossy, and whether it is printed or coated. If you know the color and thickness, write those down too.

Four defects, four different shapes

A bubble is air trapped inside the film or coating layer and looks round; a scratch remains as a line on the surface. Foreign matter is dust or specks that have got inside or stuck to the outside. Wrinkles are a little different. They are not dots or lines but the film folding or buckling, so the conversation also turns to tension and winding.

Even under the same name, the allowed size and position differ from company to company. Write down how many mm makes a defect, and whether the criteria differ between the edges and the center. These criteria set the resolution discussed next. Note also that all four are defects visible on the surface. Items that surface imaging does not reveal, such as thickness or defects between layers, need other measurement methods considered alongside.

Four numbers

• Value: Film width; Unit to write: mm; What it decides: The range to cover at once, the number of cameras
• Value: Line speed; Unit to write: m/min (min to max); What it decides: Lines to capture per second
• Value: Smallest defect to find; Unit to write: mm or µm; What it decides: The size of one pixel
• Value: How often and where defects occur; Unit to write: Count per roll, position; What it decides: Test samples and decision criteria

Moving film is imaged with a line-scan camera. It captures one line across the width, captures the next line as the film passes, and stitches them together. IntScan Pro is inspection software dedicated to line scan for such continuous materials: linked to the trigger and encoder, it images according to the distance the film has moved and decides OK/NG according to the inspection recipe. If the width is too great for the pixels of one camera, cameras are placed side by side.

How width, speed and smallest defect size lead to a camera configuration is worked through with example calculations in the paper line-scan guide. Put film in place of paper and the calculation is the same.

Look wide, then close in on anything suspect

IntCloud Optic is an optical module that captures a wide area in one shot with a wide-angle, low-magnification view, then re-aims at higher magnification at suspect areas to confirm even fine defects. Its published applications include film foreign matter and bubble inspection, and the INTELLIZ track record table also lists "Film foreign matter and bubble inspection · rated suitable in an external evaluation for intelligent processes in Korea's root industries".

Finding the location and number of foreign particles and bubbles in the captured images, marked with boxes, is the job of the detection engine (IntDetection) in IntLearn Pro. Results, logs and images are kept together in IntScan Pro, and results during operation are viewed on the IntViewer Pro screen.

Roll samples, do not fold them

If a sample is folded into an envelope, the crease images like a wrinkle. Please roll it and send it that way. A sample with both normal and defective sections is best, and nothing should be marked on the defect itself. Write the number and defect position on the edge of the sample or on the envelope.

Also mark the direction the roll was running with an arrow. How visible a scratch is can depend on direction, so without the direction it gets imaged differently from the real line. If you show us photos first, take them at the same distance under the same light, with a ruler beside the sample.

Where to put the camera

The position differs by process. Placed after coating, the camera catches defects from the coating right away; placed before winding, it sees the state just before the film becomes a roll. For both positions, write down the space and height available for the camera and lighting. Also tell us how the speed changes when rolls are changed or the line stops. Even when imaging follows the encoder, long acceleration and deceleration sections need a separate discussion.

Decide in advance, too, what happens after a defect is found. The equipment to link depends on whether you mark the defect's position, only record its position within the roll, or sound an alarm and stop the line. The summary of the film manufacturing defect inspection use case describes checking defects in real time to sort out defective rolls.

When you receive sample test results, start with the four questions in how to check missed defects, false rejects and throughput.