Wrinkles, baggy edges, weaving or wandering, tension issues, guiding or tracking difficulties, an increase in rewind scrap – any one, or all, of these issues can indicate misalignment in your blown film line. Precision alignment of the line is essential to producing high quality film. Including alignment inspections as part of your scheduled maintenance plans, allows for early detection – and early correction – of alignment issues, resulting in a reduction of unplanned downtime and waste. In this post, we provide an overview of the causes of blown film line misalignment and some real world examples of alignment issues. Additionally we offer our approach to film line alignment and the benefits to plastics manufacturers of an aligned process.
Causes of Misalignment
Like most web based production lines, the components of a blown film line will become misaligned over time due to normal events such as roll and component repair or replacement, equipment wear and corrosion, vibration, and building and/or frame movement. There are additional factors that can cause misalignment to occur as well, such as improper alignment at initial installation, and a lack of alignment support during equipment modifications and rebuilds.
It is highly recommended that alignment be included as part of any equipment installation project, especially due to the tighter tolerances required of most films lines – often in the range of 0.002 to 0.005”. Establishing the machine’s centerline and installing a baseline during line installation creates a permanent reference to which all future alignment inspections are measured. Additionally, a verification survey completed during installation produces an ‘as built’ drawing of installed alignment conditions that can be used as a reference for future inspections. This allows for quick identification of issues and faster turnaround for corrective alignment adjustments, resulting in a reduction of costly unplanned downtime.
Examples of Misalignment
There are several areas within your blown film line that when misaligned, can cause poor product quality. Misalignment of the die with the fixed roll in the haul-off can cause the film to shrink or stretch in multiple directions, causing non-uniform product. If the rolls in any given section are out of alignment, the results are often wrinkling, baggy edges and tension problems.
In Image (1), you see an example of transverse wrinkles. This most often indicates that several rolls are out of alignment and are likely out of level or out of square to the process centerline in the direction of the sheet travel.
Image (2) also shows an example of wrinkles due to misalignment. Typically in a case such as this, there is significant scrap at the rewind of either slitter. Often, one slitter is in worse alignment condition than the other with the rewind unit operation not sitting square to the process centerline since installation.
The recommended approach in these situations would be to complete a full alignment survey of the line from the haul off through the rewind in order to determine the relationship of each unit to the best fit process centerline as well as, determine the level and square attributes of critical rolls in each part of the process. That level of detail in the alignment survey, allows for the overall alignment condition of the blown film line to be determined. Recommendations for alignment adjustments would then be made to the plant operations team.
The benefits of a properly aligned film line are many and include:
- The ability to run process at higher efficiency
- Achieve maximum productivity
- Reduce or eliminate
- Product problems associated with misalignment
- Equipment problems associated with misalignment
- Return of operational process control capabilities
If you would like a site visit to discuss the alignment of your film line, or would like additional information on the benefits of alignment please contact us. You may also be interested in our blog post on extruder alignment – if so, visit Extruder Barrel Alignment is Essential to Plastics Extrusion Process.