Sheet and Film Dies Information
Sheet and film dies transform extruded polymer into thin, malleable masses after extrusion. These dies process polymers to determine the shape of the plastic.
After leaving an extruder molten polymer is pressured into a sheet or film die to create a flat, conformable expanse of plastic. The die resembles the shape of a clothing hanger, as this allows the polymer to flow uniformly across the die width. Continuous sheets or films can be created with a constant flow of material. Cooling and surface texture is often imparted by calendaring. Completed sheets can be cut or sectioned for packaging and sale. Sheets may be layered for added integrity.
A blown-film extrusion mixes virgin resin and additives to produce sheets with the desired properties. Additives allow for calibrated regrind rates, if needed, to limit costs. The mixture passes through the extruder and is pumped through a rotating circular die. This produces a tube comprised of thin polymer. The extruded matter is then raised upwards by a nip roller. The tube expands as air enters and builds a bubble for three main purposes.
Air blown into the tube stretches the substance and causes the bubble to swell to its overall size and decides sheet width.
Blowing also determines the final film thickness, as does the speed at which the bubble moves away from the die.
The air lowers the temperature of the bubble as it travels upwards toward the nip rollers. When it enters the nip rollers, it collapses into the shape of a flat tube, cools further, and proceeds down the outer surface of the tower into another set of rollers.
Once this procedure concludes, the film is moved to the slitter station. The flat tubular film is slit along the sides to separate the flat tube into dual layers; slitting just one edge allows winding the flat tube onto one core as a double-wound layer, or slitting both edges produces two single-wound layers. The final product is wrapped onto plastic or corrugated cores, and packaged for shipping.
Benefits of blown-film extrusion include:
- Relatively lower costs. Blown film sheet and film die extrusion are cheaper than other methods when working with films 1-10 millimeters thick.
- Enhanced utility when carriers or interleafs are not required.
- Advantages when consistent flat roll put-ups are crucial. Employing a rotating die for extruders maintains a uniform distribution in gauge over the roll's width.
- Where high-slip treatment is preferable for a particular product requirement.
As with blown-die systems, flat-die extrusion starts with mixing substances such as virgin resin with additives and regrinding at controlled rates. The blended resources are melted and pressure pushes the mixture through a flat die slit oriented horizontally.
Adjustments at intervals along the die's width control the film gauge. As the film passes the chilled nip rolls, it cools by the time it reaches the die exit.
It is then trimmed to a preferred sized before getting wrapped onto corrugated or plastic cores and wrapped prior to shipment. Interleafs, including polyester film, paper, or double polyethylene film (DPE), are sometimes inserted before final winding to ease handling or to inhibit blocking. Using interleafs depends on any secondary fabrication needs, physical characteristics, or planned film uses.
A version of the flat-die technique, known as cast-on-carrier, is deployed in the following circumstances:
- A carrier is needed for secondary fabrication.
- The film is exceptionally delicate, requiring handling support.
- A requirement exists to preserve the product's surface finish.
The cast-on-carrier procedure starts much like flat-die extrusion. The primary difference is that the carrier is added at the die rather than at wind-up. As the film remains warm when exiting the die, it assumes the carrier's surface finish. The film-on-carrier is sent through chilled rollers in a manner akin to flat-die film extrusion.
Units manufactured by employing the flat die and cast-on-carrier techniques are considered to be of superior quality compared to those crafted via the blown-film approach, given that:
- In-line gauge adjustment is possible
- The finish of a surface is controllable
- Films contain less gel
- Producing thick films is feasible
The procedures do not require manufacturing aids or lubricants. Such additions impinge on adhesive wet-out. Lubricants interfere with downstream usage, including lamination and printing.
Sheet and film dies serve a comprehensive spectrum of functions, including:
- Extrusion coating
- Barrier film and sheet
- Cast film
- Compounding and palletizing
- Flexible packaging
- Fluid coating
- Lithium-ion batteries
- Optical film
- Hot melt
- Oriented film
- Solar power
- Thermoformed sheet
XPS foam board
In many fields, flat-film dies, including carrier-on-film, are given preference for their exceptional quality. However, economical blown-film extrusion is recommended in certain situations, including:
- When no carrier or interleaf is needed
- When costs are to be mitigated
- Where smooth surfaces are acceptable
- Where gloss or matte finishes are not required
Where even roll put-up is essential