Packaging Film Compatibility: Questions Beyond Film Thickness

A film that looks right on a specification sheet can still run badly. It may drag on the forming shoulder, drift sideways on the reel, lose its registration mark, or seal inconsistently on a machine that handled the previous film without trouble. When that happens, the usual first question is about thickness, and thickness is rarely the cause.

The sections below follow the film in the order it travels through the machine. For each point they list what can go wrong, what to ask the film supplier and what to ask the machine supplier. The reader is a buyer or engineer who is specifying a new film, changing the film on an existing machine or preparing an inquiry. Every parameter in your answers should come from the specification or certificate for that specific film, not from a general rule.

The film path

Figure 1 shows the points where the film meets the machine. At each one, a property of the film meets a feature of the machine.

The same path applies, with variations, to vertical form-fill-seal (VFFS) machines (see the comparison of VFFS and premade pouch machines) and to horizontal flow wrappers (see flow wrapping vs. overwrapping). Stick-pack machines add a narrow web and often several lanes from one reel, which changes how reel width and tension matter (see stick packs vs. sachets).

Film reel and core

The reel is where the machine and the film supplier first have to agree. The core inner diameter, the maximum outer diameter, the reel width, the unwind direction (printed side in or out) and the reel weight all have to fit the unwind shaft and the machine’s reel handling. A reel wound with loose or uneven edges can telescope or wander as it unwinds, and no machine setting fully corrects that.

Brake and dancer (tension)

The unwind brake and the dancer arm keep tension on the film as the reel diameter and the machine’s draw speed change. Films differ in how much tension they tolerate before they stretch, neck down or distort the printed pattern. Thin or highly stretchable films, and laminates with a soft layer, can behave differently from stiffer ones. The film supplier should document the tension range the film can take, and the machine supplier should state the range the brake and dancer deliver.

Splice table

When one reel ends, the next is joined to it, either by a manual splice or by an automatic splicer. The splice has to pass the registration sensor, the forming shoulder and the sealing jaws. Ask which splice tape or method the film supplier supports. A splice that adds thickness or a different surface can seal poorly or catch on the shoulder.

Print registration mark and eye-mark sensor

On printed film, a sensor reads a printed mark to place the cut and seal in the right position relative to the artwork. The mark needs enough contrast against its background, the right position on the web and a clean area around it. Reverse-printed films, metallized layers and glossy surfaces can all change what the sensor sees. The film supplier controls the mark’s ink and position, and the machine supplier knows the contrast and size the sensor needs.

Forming shoulder (friction, COF)

On a VFFS machine the film is pulled over the forming shoulder and down the tube. The friction between the film surface and the metal, and between the film and the pulling belts, decides whether the film feeds smoothly or sticks, jerks or wanders. The coefficient of friction (COF) describes this, and the film supplier may specify a static and a kinetic value for each side of the film. ASTM D1894 is a commonly cited method for measuring it and covers static and kinetic coefficients of friction of plastic film and sheeting. Ask which method and conditions the supplier used, since friction depends on temperature, additives and the age of the film.

Sealing jaws (seal layer, sealing window)

At the sealing jaws, the sealant layer on the inside of the film is heated and pressed together. The sealing window is the range of conditions in which the film forms a good seal without being damaged. The film supplier should state the seal initiation temperature range and the sealing window for that film’s sealant layer, and the machine supplier should say what its jaws, heating system and dwell control can deliver. A peel-type test usually checks seal strength, and ASTM F88 describes a test method for the seal strength of flexible barrier materials. In an inquiry, cite the method and not a number. The acceptance value comes from your own product and shelf-life requirements.

Cut-off

The knife or the sealing-and-cutting jaws separate the finished bag. A film with a tough or stretchy structure, or a thickness or layer combination the cutting system has not run before, may tear, leave a stringy edge or cut unevenly. Check this in a trial, since specifications rarely describe it.

What can go wrong, and what to ask

The table collects the questions for each point. Treat it as a starting list and adapt it to your film and machine.

Point on the film path	What can go wrong	Question for the film supplier	Question for the machine supplier
Film reel and core	Core does not fit the unwind shaft; outer diameter or weight exceeds the machine’s capacity; reel telescopes or wanders	What are the core inner diameter, maximum outer diameter, reel width and reel weight for this film, and how is it wound?	What core sizes, outer diameters, reel widths and weights does the unwind accept?
Brake / dancer (tension)	Film stretches, necks down or distorts print; tension swings as the reel empties	What is the permitted tension range for this film, and does it differ by structure or thickness?	What tension range do the brake and dancer deliver across the reel, and how is it adjusted?
Splice table	Splice sticks at the shoulder, opens at the seal or loses registration	Which splice method and tape do you support for this film?	Which splice methods does the machine support, and how does it handle the splice at the sensor and the jaws?
Print registration mark and eye-mark sensor	Mark is missed or misread; cut and seal drift from the artwork	What are the mark size, position, color and contrast against its background, and how are they controlled?	What mark size, contrast and clear area does the sensor need, and can it be adjusted for this film?
Forming shoulder (friction, COF)	Film drags, jerks, sticks or tracks to one side; static builds up	What are the static and kinetic COF values for each surface, which method (for example ASTM D1894) and conditions were used?	What surface finish and coating does the shoulder, tube and pulling system have, and what friction range suits it?
Sealing jaws (seal layer, sealing window)	Weak seals, channel leaks, wrinkles, burn-through	What is the seal initiation temperature range and sealing window for the sealant layer, and how was seal strength tested (for example ASTM F88)?	What jaw type, heating, pressure control and dwell control are available, and how are they verified?
Cut-off	Torn, stringy or uneven cut; film sticks to the knife	Is there any known behavior of this structure with the cutting method used?	What cutting method is used, and has it been run with this film structure?

Information to request from suppliers

These documents let you verify the answers in the table above.

From the film supplier

• The current technical data sheet or material specification for the specific film grade, with the structure and which side is sealant and which is print.
• A batch certificate or certificate of analysis for the lot you will receive, showing the properties the supplier measures and the test methods used.
• Friction values (static and kinetic) for each surface, with method and conditions.
• Seal initiation temperature range and sealing window for the sealant layer, with the test method.
• Tension limits, recommended splice method and storage conditions.
• For food contact, a declaration or other document that addresses the relevant food contact material rules for your market (for example, the EU framework in Regulation (EC) No 1935/2004), as applicable.

From the machine supplier

• Unwind, brake and dancer capacities and adjustment ranges.
• Eye-mark sensor requirements.
• Forming shoulder and tube material, finish and available sizes for the film width and bag size.
• Sealing jaw type, temperature control, pressure control and the way each is verified.
• A statement of which films, in terms of structure and surface, the machine has been configured for, and any limits the supplier knows about.
• A trial offer using the film and product you plan to run, so compatibility is shown rather than assumed.

Record supplier specifications as the supplier’s statement, and keep your own trial results separate.

A pre-trial checklist

• The film’s technical data sheet is on file and matches the structure you intend to buy.
• The batch certificate for the trial reels is on file.
• Reel core, outer diameter, width and weight have been confirmed against the unwind.
• Tension range from the film supplier is compared with the machine’s range.
• Registration mark has been checked against the sensor requirement, on the actual trial reel.
• COF values and test method are documented, and the machine supplier has seen them.
• Sealing window from the film supplier has been shared with the machine supplier.
• Splice method has been agreed.
• Acceptance tests (for example seal strength by an agreed peel test, and leak testing where relevant) and their pass criteria are written down before the trial.
• The trial uses your product, your pack size and the speed in packs per minute you actually need.

If seals fail during or after the trial, record and test the evidence before changing anything, as described in the seal defect investigation checklist.

Limits and on-site verification

This guide gives questions, not values. The right tension, friction coefficient, sealing temperature and dwell time depend on the specific film structure, the product, the pack size, the machine and the environment. Numbers from a general source do not replace the film’s own certificate.

Even with complete documents, run a trial with the film, the product and the machine in the configuration you intend to use. A trial shows what paper cannot: how the film behaves at your speed, with your product in the seal area, at the humidity and temperature of your plant. ASTM D1894 and ASTM F88 are written for laboratory conditions on prepared specimens, so their results describe the specimens and not the full behavior on the machine.

Adjusting the unwind, the forming shoulder or the sealing jaws means working near moving reels, belts and heated jaws. Guarding for these areas should come from a risk assessment under a method such as ISO 12100, reviewed by qualified personnel. Keep guards and interlocks in place while adjusting, and isolate energy before any intervention.

Related decisions

When film compatibility is part of an equipment purchase, put the film specification, test methods and trial terms into the RFQ checklist so every supplier quotes against the same material.