Substrate Secrets

In my post on embossing cast and oriented films during orientation, I commented on the impact of casting defects on the film. They never really disappear especially if they cause physical bumps or “dings” in the film surface. During stretching the dings are reduced in thickness as is the film surrounding it, but a thick spot in the cast sheet remains a thick spot in the final film and never really disappears (this is why we adjust die lips for gauge uniformity). It may not be a problem depending on the polymer and the end use, but if it is too large it can cause a cold spot in the MDO affecting MD orientation and cause breaks if the polymer remains too cold (bump causes film to lift off heat transfer roll surface and heating is less uniform causing a stress concentration during stretching) it may cause tearing during orientation giving splits in the TDO, or complete web breaks in the MDO. Typically bumps in the base sheet are produced from a low spot in the cast roll due to a surface scratch or a small hole from a dropped tool or die part. This creates a problem in that to properly repair the roll may have to be removed, sent to a manufacturer and then reground and plated.  This is a big deal, very time consuming and expensive. Worse, if there is no spare roll the line is out of service perhaps for an extended period of time. So what can you do?  If it is from a dropped tool or part, the defect will generally have a low spot surrounded by metal displaced and raised by the impact. The raised section in the roll surface you can stone out the small raised portion back to the average diameter of the roll and then hand polish back to the roll finish. However, stoning will generally leave a small hole remaining in the cast roll surface (it is easy to reduce a defect height but difficult to impossible to mechanically raise a defect in a metal roll surface). Hopefully the small hole is then not a problem. If however, the remaining surface defect is too large or cannot be ignored it is possible to electroplate the area and fill in the hole with new metal.  Once the defect is fill the area is stoned back to match the surrounding surface and repolished as described before. This works very well and the system I have used in the past to repair roll surfaces is the Dalic brush plating system (http://www.dalicworld.com/histogb.html). It may not give a completely perfect visual repair as the small differences in the original surrounding roll surface polish and the polish you achieve by hand may be visible and it may leave a noticeable, visual difference in the film surface, but if this does not detract from the films required quality, it can be a life saver and keep you in production until you can replace or repair the roll during a scheduled outage. In one extreme case with a hole about 1mm deep and 1mm wide in a cast roll surface, we filled it with an epoxy resin. This allowed the film to run well and in some cases the film would run without the patch (it fell out every year or so) leaving a large defect found in slitting and it would be replace. I discovered it while making a pigmented film because it would leave a clear ring in an otherwise opaque film. Repeat length was the roll diameter times the MD stretch ratio and in this case it was about every 43 meters. I think I still have the sample (It may still be in my research notebook left at my last employer) and if so I will try and take a picture for a future posting.