How Does A Breathable Film Casting Machine Create Microporous Structures?
How the Process Works
1. Material and Filler Preparation
It begins with a resin blend (often a polyolefin such as Polyethylene or Polypropylene) combined with inorganic fillers (for example, calcium carbonate) that will later help form the micropores.
The fillers act as “void-initiators” in the polymer matrix—when the film is stretched later, the polymer separates around these filler particles to create cavities/pores.
2. Extrusion & Casting
The resulting blend is melted and extruded through a flat die (in a cast film machine) onto a chill roll (or casting roll). This produces a thin “quenched” film sheet. For example, one method described is: extrude molten web → cast/press against chill roll → obtain quenched film.
In this casting step, controls like a vacuum box, air-knife or air-blanket may be used to regulate cooling and adhesion to the chill roll surface.
3. Stretching to Create Micropores
After the cast film is formed, the key step is stretching (either machine-direction (MD), cross-direction (CD), or biaxial) which pulls apart the polymer around the filler particles, forming elongated voids and micropores.
According to one source, “The film is then stretched in one or two directions. This stretching process creates micropores in the film, which allow gases to pass through while blocking liquids.”
Another patent describes: “casting the molten web … and stretching the quenched film to form the microporous breathable film.”
4. Annealing / Stabilization
After stretching, often an annealing or heat-setting step follows to stabilise the film structure, lock in the pore network and relieve internal stresses. For example the patent mentions annealing at a specified temperature range.
5. Winding / Finishing
Finally the microporous film is cooled fully, wound into rolls, and can then be laminated or further processed (e.g., for hygiene back-sheets, medical garments, breathable packaging) depending on application.
Why the Microporous Structure Matters
The micropores allow moisture vapor (e.g., water vapor from a human body or package contents) to escape, maintaining breathability. For example: “These films are composed of a polymer matrix that contains microscopic pores … The pores … are created … adding fillers … and then stretching the film.”
At the same time, because the pores are very small and the film structure is controlled, they prevent liquid water or other larger particles/droplets from passing through. For example: “After longitudinal stretching, the film has a unique microporous structure. These special micropores with high density distribution can not only block the leakage of liquid, but also allow gas molecules such as water vapor to pass through.”
This dual function (breathable yet liquid-resistant) is critical in applications such as hygiene (diapers, sanitary pads), medical gowns, protective packaging, and house-wraps.
Role of the Breathable Film Casting Machine
A casting machine designed for this purpose will typically include:
A high-precision extruder and die for polymer-filler blend
A chill roll or casting roller for quick solidification
Mechanisms (vacuum box, air knife, etc.) for casting control
Stretching units (MD/CD or biaxial) capable of controlling tension and strain
Annealing stations to stabilise the microporous film
Winding units with tension control for uniform rolls
Such machines enable manufacturers to produce consistent, high-quality microporous films with controlled pore size, distribution, and film properties.
Key Process Control Variables
Filler content & dispersion: The amount and distribution of inorganic filler strongly affect pore formation and breathability.
Stretching ratio and strain rate: How much the film is stretched in MD/CD determines pore size, orientation, film thickness and mechanical properties.
Cooling rate & casting conditions: The rate at which the film is cooled on the chill roll influences crystallinity and how the filler interacts with polymer.
Annealing temperature/time: Fixes the structure and can improve mechanical strength, reduce shrinkage and ensure stability.
Film thickness and uniformity: Uniform gauge ensures consistent performance and avoids weak spots.
Tension control and substrate handling: To avoid defects (e.g., tearing, non-uniform pores) the machine must precisely control web tension during stretching and winding.
Benefits & Applications
Because of the microporous structure, such breathable films offer:
High moisture vapor transmission rate (MVTR) — improves comfort in hygiene/medical applications
Liquid barrier properties — prevents leaks
Lightweight design — lower basis weight but good performance
Possibility of lamination with nonwovens or other substrates for composite materials
Applications include: baby diapers, adult incontinence products, sanitary pads, medical gowns and drapes, protective apparel, breathable packaging, house wraps, and more.
Summary
In essence: a breathable film casting machine creates microporous structures by:
Extruding a polymer/filler blend
Casting it into a thin film via chill roll
Stretching the film to generate controlled pore networks
Stabilising via annealing
Winding the finished microporous film
This process delivers films that allow vapor out but block liquids — a key feature in hygiene, medical and specialized packaging applications.