Foam Converting Applications

Where Does PTFE Fit in a Foam Line?

Foam plants rarely run one process. A converter might flame laminate in the morning, run an adhesive lamination line in the afternoon and press moulded parts on a third machine. The release requirement changes with each. The table maps the stage to the part.

Why Does Flame Lamination Need a Release Surface?

Flame lamination bonds foam to fabric with no adhesive at all. The foam web passes over an open flame at roughly 950°C, which melts only the top skin and turns it tacky, and it goes straight into a nip where an incoming fabric layer is pressed onto the melt. The bond forms as the polymer cools, so there is no solvent and no cure time.

That is elegant, and it is also the single stickiest moment on any foam line. Molten polyurethane will transfer to anything it touches before it sets, and the laminating nip typically runs around 165 to 180°C, hot enough to keep the transfer soft on the roller. A PTFE film or coated fabric layer on the nip and the guide surfaces after it is what stops the build-up that otherwise means stopping the line to scrape a roller. PTFE is rated to 260°C continuous, comfortably above the nip temperature, and the flame itself never touches the release surface.

What Does an Expansion or Crosslinking Oven Need?

Crosslinked EVA and polyolefin foam is made by carrying sheet through a tunnel oven whose zones step up gradually, commonly from about 120°C to 180°C. The gradient matters: heat the sheet too fast in one place and the blowing agent decomposes unevenly, which shows up as coarse or irregular cell structure. Through all of it the sheet is expanding, soft and in contact with the belt, so the belt has to release cleanly and hold its dimensions under tension at temperature rather than stretching zone by zone.

Both the temperature range and the dimensional requirement sit inside what PTFE coated glass does well. Where the oven needs air to reach the underside of the sheet, an open mesh belt is used instead of a solid one, at the cost of leaving a weave impression on the face in contact.

How Is a Double Belt Press Different?

In a double belt press the belt is not just carrying the product, it is the pressing face. Typical operation is continuous flat pressure in the region of 12 to 25 psi at 180 to 230°C with a dwell of several minutes, which puts sustained heat and load on the belt at the same time. The two common press surfaces are PTFE impregnated glass fabric and thin stainless steel.

PTFE coated glass is the cheaper and more forgiving of the two, it releases without dressing, and it is replaceable in-house. Steel gives a harder, flatter face and lasts longer where the product demands a mirror finish. Because the belt is under load as well as heat here, the join specification and the fabric tensile strength matter more than they do on a simple conveying run.

What About Rebond Foam and Carpet Underlay?

Rebond is made by spraying binder onto shredded foam crumb, blending it, then transferring the mix to a mould and compressing it to a set ratio for the density wanted. Steam is introduced to cure the binder while the block is held compressed, and the block then has to come out of the mould clean. PTFE coated glass is used on the mould and platen faces for exactly that release.

There is a useful second effect here. A thin PTFE coated fabric conducts heat well enough that it does not fight the process it sits inside, which is why a thin tape is preferred over a heavy section where heat has to pass through the release layer to melt a film or set a binder. On laminated underlay and film-faced product, that is the difference between a release layer and an insulator.

Which Surface Finish Should You Specify?

Worth knowing before you order, because it is the specification people most often leave to chance. Coated fabric is made in finishes from rough and semi-porous through to very smooth, and the choice is a trade-off rather than a quality ladder. A smoother face releases more easily and gives a better finish on the product. A rougher, more textured face grips enough to stop soft stock creeping or tracking off, and will impart its own texture on anything hot pressed against it.

The same logic decides between PTFE and silicone coatings on a belt. Silicone grips and is sometimes chosen deliberately to texture the product, while PTFE is the better release surface and stays inert where silicone would swell in solvent. Both run to the same 260°C, so temperature does not decide it. We coat fabric both ways, so tell us what the product face has to look like coming off the machine and we will match the finish to it rather than sending the smoothest thing in stock. PTFE vs silicone coated belts compared works through the choice stage by stage.

How Is a Foam Line Belt Specified?

To quote a belt we need the width, the developed length, solid or open mesh, the surface finish, the join and whether it runs guide strips. Measuring the belt coming off the machine is more reliable than working from a line drawing, which will often show the original specification rather than what has been fitted since.

On heated lines the belt usually fails at the join rather than across the face, because sustained oven heat works on the splice, and edge reinforcement and tracking is what keeps a wide belt from damaging its own edges over a long run. If you would rather configure it yourself, the custom belt builder covers material, thickness, join and tracking.

For press platens, moulds and benches rather than a belt, a non-adhesive Teflon sheet covers a whole surface more economically, the self-adhesive version stays put on a platen without taping, and PTFE film tape suits rollers and detailed tooling.

Frequently Asked Questions