BOPP and PET often run on the same slitter rewinder, sometimes on the same day. Settings that work for one can wrinkle, stretch or scratch the other. This guide explains the differences that matter on the machine and the defects each film is prone to, so you can specify a line that handles both and diagnose problems when they appear.
Two films, two behaviours
| BOPP (biaxially oriented polypropylene) | PET (polyester) | |
|---|---|---|
| Stiffness | Lower. Stretches more easily under tension. | Higher. Holds its shape and tracks well. |
| Main risk | Stretching, necking and wrinkles from too much tension; static. | Scratches, edge dust and hard rolls; air trapped at speed. |
| Static | Charges readily, especially in dry air. | Also charges; dust attraction is a common quality complaint. |
| Common forms | Plain, heat-sealable, metallized, pearlized, printed. | Plain, metallized, coated, optical grade, printed. |
Both are usually slit thin: packaging grades are often well under 50 μm. Check that the machine's thickness range covers your thinnest grade with margin. The KINYUN SW4500, built for packaging film, is specified from 6 μm to 75 μm.
Choosing the slitting method
Razor slitting is widely used on thin BOPP and thin PET. It is simple, fast to set and inexpensive to run. Cutting over a grooved roll gives a more stable edge than cutting in free air, particularly at high speed.
Shear slitting gives a cleaner edge on thicker PET, metallized film and laminates, and generally produces less dust on stiff materials. It takes longer to set because knife overlap and side pressure must be adjusted for each slit.
If most of your volume is thin plain BOPP, razor slitting may be all you need. If you also run metallized PET or laminates, specify a machine that can take both razor and shear knives and ask how long it takes to change between them. Worn razor blades are one of the most common causes of dusty or ragged edges, so plan a blade-change routine from the start.
Setting tension
Machine tension is quoted per metre of web width (N/m). Your total web tension is that figure times your web width. The practical rules are:
- BOPP needs gentle, stable tension. Too much stretches the film, narrows the web and can show later as baggy lanes or wrinkles after the roll relaxes.
- PET tolerates higher tension and needs enough of it to track straight and wind a firm roll, but excessive tension makes hard rolls that can block or mark the film.
- Use taper tension on the rewind. Reducing tension as the roll grows keeps inner layers from being crushed and reduces starring and telescoping on large diameters.
- Separate the zones. Unwind, slitting and rewind tension should be controlled independently so a change on the rewind does not pull the web at the knives.
Start from the film supplier's recommendation if you have one, then adjust from test rolls. Save good settings as recipes per material and thickness so every shift starts from the same point.
Common winding defects and causes
| Defect | What you see | Usual causes |
|---|---|---|
| Wrinkles | Creases in the roll, often starting near the core | Too much tension on BOPP, poor roller alignment, air trapped between layers, uneven web thickness |
| Telescoping | Roll layers slide sideways into a cone | Roll wound too soft, too little lay-on pressure, tension dropping too fast with taper |
| Gauge bands | Raised ridges around the roll | Thickness variation in the film always landing in the same place; oscillation of the web or roll can spread it |
| Interleaving | Edges of neighbouring rolls weave into each other | Rolls too close together on the shaft, weaving web, insufficient separation after the knives |
| Hard and soft rolls on one shaft | Rolls from the same set wind to different firmness | Thickness varies across the web; a plain shaft cannot give each roll its own tension, where a differential shaft can |
| Air entrainment | Soft rolls at high speed, layers slipping | Air carried into the roll at speed; a lay-on or contact roll pressing against the rewind helps push it out |
Static, dust and surface damage
Both films build static as they unwind and pass over rollers. Static attracts dust to the film surface and can give operators shocks. Ionizing static eliminators at the unwind and near the rewind are standard practice; ask where they are positioned on the machine you are quoting.
For surface damage, the rollers matter as much as the knives. Scratches under strong light usually come from rollers that are not turning at web speed, from contamination on roller surfaces, or from a roller surface unsuited to the film. KINYUN applies customised roller surface treatment by material type, with the aim of a scratch-free result under strong light inspection after cutting. That is worth describing to any supplier if your customers inspect this way.
Edge dust from slitting is best controlled at the source: sharp blades, the right slitting method for the thickness, and dust extraction at the knives if the material is prone to it.
Metallized and printed film
Metallized BOPP and PET add a very thin metal layer that can crack or scratch. Keep the web path as simple as possible, avoid sharp wrap angles, and treat the metallized side as the critical surface when deciding which rollers it touches.
Printed film usually needs slit positions registered to the print. Ask whether the machine offers web guiding and print registration, how knives are positioned against the print repeat, and whether a defect-marking or inspection station can be added. See printed film applications for more.
Questions to ask about the machine
- What is the minimum and maximum thickness of BOPP and PET this machine is set up for?
- Which slitting methods are included, and how long is a change from razor to shear?
- What is the tension range, and are unwind, slitting and rewind tension controlled separately with taper on the rewind?
- Are the rewind shafts differential or plain, and is there a lay-on roll?
- Where are the static eliminators, and is dust extraction available at the knives?
- What roller surface treatment is proposed for my films?
- Can a trial be run with my own material before shipment, and will I receive the test rolls?
The answers belong in the written quotation, not only in an email. For flexible packaging lines, see the flexible packaging application page or the SW4500 specification.
Figures quoted for KINYUN machines come from the manufacturer's published specification. Everything else is general guidance; the configuration for your material is confirmed in writing in the formal quotation.
