Film extrusion is the continuous process in which an extruder melts thermoplastic granulate and forms it through an annular or flat slot die into an endless film web that is then cooled, stretched where required and wound. Three routes are common: blown film from an annular die, cast film from the chill roll and biaxially oriented film. Blown film usually varies in thickness by ±15 to 20 percent (Kunststoffe 10/2004). Oriented PP film is typically stretched four to five times lengthwise and eight to nine times crosswise (patent EP0616885B1). This article compares the three production routes with process values and shows why the route decides the dielectric strength, shrinkage and thickness tolerance of an insulating film. We do not extrude film ourselves. We convert finished films, such as our Hostaphan polyester films, into slit rolls, cut sheets and die-cut parts.
Three production routes lead to three different films
The extruder and the melting stage are similar in every process. The difference lies in the die, the cooling and whether the film is stretched afterwards. The same material name, PP for example, can mean a soft sealing film or a capacitor film that holds more than ten times the voltage per millimetre of unstretched PP sheet, depending on the route.
| Feature | Blown film | Cast film | Biaxially oriented film |
|---|---|---|---|
| Die | Annular die, gap 0.5 to 2 mm | Flat slot die, up to 4,000 mm wide | Flat slot die, followed by a stretching line |
| Cooling | Cooling air, frost line 20 to 48 cm above the die | Chill roll, first roll 40 to 65 °C | Chill roll, then reheating for stretching |
| Orientation | Weakly biaxial through blow-up and haul-off | Largely unstretched, slightly in machine direction | Strongly biaxial, BOPP about 4.5 × 9, PET about 3 to 4 per direction |
| Typical thickness | 15 µm to 0.4 mm | 15 to 300 µm, as sheet up to 60 mm | BOPP 4 to 80 µm, PET 1 to 500 µm, capacitor film from 2 µm |
| Materials | LDPE, LLDPE, HDPE, PP, PA, EVOH | PP (CPP), PE, PET, PS, PC | PP, PET, PA, PE |
| Typical products | Bags, shrink and stretch film, agricultural film | Sealing film, thermoforming film, cast sheet for stretching | Electrical insulation and capacitor film, separators, packaging |
Table sources: IKV Aachen (IGF report 20100 N), LyondellBasell (guide to polyolefin film extrusion), Kuhne, BHL, JKU Linz, Mitsubishi Polyester Film and Brückner. Alongside them sit calendering, which rolls the melt out between heated rolls and is the standard for PVC, and extrusion coating, which applies plastic directly to paper or aluminium. For insulating parts, the right-hand column is almost the only one that counts. Polyester and PP insulating films come from the stretching line, while blown film serves at most as a protective or interleaving layer in insulation. Every film extrusion needs a plastic that melts when heated, in other words one of the thermoplastics.
Blown film extrusion: inflating, cooling, collapsing
Blown film is the most widely used process for thin PE film. The line stands vertically, can be more than 15 m tall and processes polyethylene at 150 °C (LDPE) to around 250 °C (HDPE) depending on the grade, in up to 11 layers (Wikipedia, Blasfolie). The process in six steps:
- A single-screw extruder melts the granulate, with around 600 bar ahead of the screw tip. Output ranges from 5 to 1,500 kg/h.
- A spiral mandrel distributor forms the melt into a ring with a 0.5 to 2 mm gap. In coextrusion, each extruder feeds its own layer.
- Internal air expands the tube. The blow-up ratio (BUR) is the bubble diameter divided by the die diameter, the take-up ratio is the haul-off speed divided by the exit speed. Together they stretch the film in two directions.
- An air ring blows cooling air onto the bubble. At the frost line the melt solidifies, and above it the orientation no longer changes.
- Nip rolls collapse the tube. The haul-off rotates or oscillates by up to 360° so that gauge variations do not always land in the same spot on the roll and build up piston rings there.
- The tube is edge trimmed and wound as lay-flat tubing, sheeting or single-wound film.
Final thickness follows from the die gap divided by blow-up and take-up ratio. In trials at IKV Aachen with LDPE, the blow-up ratio was between 2.7 and 3.7, the melt at 180 to 200 °C and the film at 130 to 160 µm (IGF final report 20100 N). Lay-flat width equals 1.57 × die diameter × BUR, and LyondellBasell recommends a frost line 20 to 48 cm above the die for tubes up to about 90 cm in diameter (A Guide to Polyolefin Film Extrusion). According to Reifenhäuser, a downstream MDO unit can additionally stretch blown film up to 1:10 in the machine direction.
The strength of blown film is its toughness, with similar tear propagation resistance lengthwise and crosswise. Its weakness is gauge. As a rule the thickness tolerance is ±15 to 20 percent, so a 20 µm film measures 16 to 24 µm in practice (Kunststoffe 10/2004). The German industry rule for PE film allows ±13 to ±22 percent individual deviation, depending on thickness (GKV test clause 2018). For a bag that is no problem. An insulating layer fails at its thinnest point, and at 20 percent tolerance a fifth of the insulation thickness is missing there.
Cast film extrusion: casting onto the chill roll
In cast film, also called chill roll film, a flat slot die spreads the melt across the full width. Cast dies run hotter than blown film dies, at 230 to 290 °C for polyolefins. The melt drops through a short air gap onto a chrome-plated chill roll. The first roll runs with 40 to 65 °C cooling water, held to ±2 °C across the width (LyondellBasell). Rapid quenching keeps crystallinity low and makes the film clear.
To keep the web flat on the roll, an air knife, vacuum box or electrostatic pinning presses it down. Between die and roll the web narrows, and this neck-in thickens the edges. The edges are therefore trimmed, ground and fed back into the extruder, while inline gauge measurement adjusts the die lip (JKU Linz, cast film defects). Multilayer cast films are built with a feedblock of 2 to 24 layers, and a typical three-layer PP film has 15 percent skin, 70 percent core and 15 percent skin. Cast film has a more even gauge profile than blown film (Materials 2023, 16, 1071).
For electrical insulation, cast film is mainly an intermediate product. Almost every polyester and PP insulating film starts as a thick, unstretched cast sheet on the chill roll and only becomes the material on the data sheet in the stretching line.
Biaxially oriented film: where insulating films are made
Stretching means pulling the solidified film lengthwise and crosswise above its glass transition temperature. The molecular chains align, the film becomes stronger and thinner, and semi-crystalline materials such as PET and PP become more crystalline. Sequential stretching, the most common route, runs in five steps:
- Extrusion and quenching. PET is extruded at 265 to 285 °C and frozen amorphous on the chill roll. The cast sheet is far thicker than the final film, up to 0.5 mm for 5 µm PP film.
- Machine direction orientation (MDO) over rolls that turn progressively faster. BOPP is stretched four to five times at 100 to 130 °C, PET around three to four times.
- Transverse direction orientation (TDO) in a tenter frame whose clips grip the edges. BOPP is drawn eight to nine times at 150 to 170 °C, PET again around three to four times.
- Heat setting under tension, above 200 °C for PET. Crystallinity rises to around 50 percent, and the film no longer creeps back towards its original dimensions when heated.
- Edge trimming, corona treatment where required, winding into a parent roll.
The BOPP values come from patent EP0616885B1, which makes 15 µm film at 1:4.5 lengthwise and 1:9 crosswise in its examples, the PET values from a technical article on Wevolver. At 4.5 × 9 the area grows around 40-fold and the film gets correspondingly thinner, so by calculation 0.6 mm of cast sheet becomes around 15 µm. The largest BOPP lines work at 10.4 m width, 700 m/min and 10 t of film per hour (Brückner, Chinaplas 2026).
In simultaneous stretching the tenter pulls lengthwise and crosswise at the same time. This allows an area stretch ratio of up to 1:100 for PP, against around 1:50 sequentially (patent DE19749121C2). For capacitor film down to 2 µm Brückner offers both variants, the simultaneous one under the name LISIM. There is also the double bubble process, a blown film that is biaxially stretched in a second bubble. Material values of the finished films are covered in our articles on polyester film and PP film.
Coextrusion builds layers in a single pass
In coextrusion, a feedblock or multi-channel blown film die combines the melts of several extruders before they leave the die. The layers bond in the melt, incompatible materials such as PE and EVOH through a tie layer. No laminating adhesive is needed. In packaging one layer carries the barrier and another the seal. In electrical insulation the three-layer polyester film is the familiar example: the RNK grade of Hostaphan is coextruded in three layers, high gloss and 90 percent transparent. We slit coextruded paint protection films with defined adhesion to size.
Coextrusion reaches its limit with materials that cannot be extruded together. Paper, aramid and metal foil can only be joined to the film by lamination. How that works and which adhesives hold the thermal class is described in our article on laminating film.
The production route sets the electrical values of an insulating film
A common objection in purchasing: PP is PP, so why should the production route matter? Because the same material delivers values an order of magnitude apart depending on how it is processed. Unstretched 1 mm PP sheet withstands 50 kV/mm, while biaxially oriented 10 µm PP capacitor film reaches 650 to 770 kV/mm in a laboratory DC test (Tampere University of Technology, BOPP baseline study). Part of the gap comes from the lower thickness. That stretching raises the value further is shown by a study from Tampere University of Technology: a higher biaxial stretch ratio improves breakdown performance, and compared with unstretched cast film of similar thickness, biaxial orientation proved to be the basis of the high strength of thin capacitor films (European Polymer Journal, 2017).
| Property | What production determines | Value |
|---|---|---|
| Dielectric strength | Degree of stretching, thickness, defects | BOPP 10 µm 650 to 770 kV/mm (DC, laboratory), 13 to 20 % lower at 100 °C; PET film up to 180 kV/mm (catalogue value, different test) |
| Dissipation factor tan δ at 50 Hz | Material and purity of the raw material | PP 0.0002, PET 0.002 |
| Shrinkage | Heat setting | PET 1 to 3 % MD, 0 to 2 % TD at 150 °C; BOPP 3 to 5 % MD at 120 °C |
| Tensile strength MD/TD | Ratio of MD to TD stretching | PET 200/200 N/mm², BOPP 150/250 N/mm² |
| Elongation at break MD/TD | Ratio of MD to TD stretching | PET 100/100 %, BOPP 150/50 % |
Dissipation factor, shrinkage, strength and elongation come from the general data sheet of Mitsubishi Polyester Film, the PP sheet value from our article on PP film, the PET value from Wikipedia. The anisotropy of BOPP stands out: it is stretched twice as much crosswise as lengthwise and is therefore stronger and less extensible crosswise. PET insulating film is stretched in a balanced way, and IEC 60674-3-2 carries that in its title (balanced biaxially oriented) and distinguishes five types, from standard to hydrolysis-resistant film. Capacitor film is made from highly isotactic PP with minimal catalyst residue and impurity content (Tampere University of Technology).
Our recommendation: order insulating film by standard type, thickness and shrinkage value. The polymer name alone only describes the granulate. How thickness and field strength relate is explained in our article on dielectric strength, why capacitors almost always use BOPP in the one on capacitor insulation.
Typical extrusion defects and what they do in insulating film
Most film defects originate in the extruder or already in the raw material. In packaging they are cosmetic, in insulating film they become weak spots. Tampere University of Technology found individual breakdowns at 100 to 400 kV/mm in non-metallised BOPP film alongside values around 700 kV/mm. The most common defect patterns:
- Gels are unmelted or cross-linked particles, clear, round and hard enough to feel. They do not stretch with the film, as LyondellBasell puts it, and remain as hard spots in the web.
- Fisheyes are elliptical, with their long axis in the machine direction.
- Black or yellow-brown specks are thermally oxidised material, released from deposits on screw and barrel or formed already at the resin producer.
- An uneven gauge profile builds up in the roll into piston rings or hard bands and costs flatness.
- Melt fracture shows as a rough, wavy surface, pinholes as fine holes.
The master's thesis from JKU Linz shows that gels often originate before the extruder: resins differ in their gel level, and cheap granulate can bring many times more defects. For insulating film, the film producer's choice of resin is therefore a quality feature that does not appear on the data sheet.
What we take over from extrusion as a converter
When slitting and die-cutting, every part inherits the properties of the parent roll. We slit parent rolls of up to 1,000 kg into slit rolls from 5 to 1,600 mm width with ±0.1 mm and die-cut material thicknesses from 0.023 to 3.0 mm. Five points from production decide whether the part fits your assembly:
- Machine direction. State on the drawing whether the long edge should run along or across the machine direction. PET shrinks more lengthwise than crosswise, and BOPP stretches only a third as far crosswise as lengthwise before it breaks.
- Shrinkage at your process temperature. One percent shrinkage turns a 300 mm slot wedge into a 297 mm part. Compare data sheets with care: Mitsubishi measures 15 minutes at 150 °C, DuPont Teijin for Mylar A 30 minutes at 150 °C with 1.1 to 1.2 percent lengthwise.
- Edge zone. The extrusion edges are trimmed, but the gauge profile towards the edge remains. For tight thickness requirements it pays to ask for the gauge profile across the parent roll width.
- Winding quality. IEC 60674-3-2 specifies windability for insulating film, including camber and sag. Camber in the parent roll carries into every slit roll. Web edge guiding keeps the web on track during slitting, but it does not straighten a curved film.
- Coated, coextruded or corona-treated side. For bonding and laminating it must be clear which side faces out. The effect of a corona discharge fades with storage time.
Details on widths, cores (25, 55, 76 or 152 mm) and speed are on our contract slitting page. For many contours the lower-cost steel rule die is enough, and where tolerance and tool life matter most we use solid steel tools, both described under stamping and forming. For a roll enquiry we need width, length, core diameter, core material and tolerances, for die-cut parts the drawing, quantity and thickness.
Is it worth buying film directly from the extruder?
For large consumers, yes. A BOPP line produces 10 t of film per hour, a new PET line in Wiesbaden 27,000 t a year. Anyone who needs 500 slot wedges or a few slit rolls is orders of magnitude below that. That is what converters are for. We are Europe's largest converter of Mitsubishi Hostaphan thick films and supply thicknesses from 7 µm to 3 mm from a single piece, within two weeks at most when the material is in stock. Our technical consulting helps you choose the film type and thermal class.
GOBA Takeaway
Insulating films come almost exclusively from the stretching line, where the stretch ratio and heat setting decide how much voltage the film holds and how much it shrinks when heated. Anyone ordering insulating parts should therefore state the standard type, machine direction and shrinkage as routinely as the thickness.
