Method for manufacturing product in membrane or film form

A method for forming a web that is permeable to liquids and designed to separate two environments and to allow liquid to flow in one direction from one of said environments to the other, said method including:

• providing a web of material,
• providing two contra-rotating rollers,
• providing on one roller projections with outer surfaces,
• providing a smooth surface on the other roller,
• pressing said rollers together,
• rotating said rollers at different speeds whereby to provide a slipping action between the smooth surface of the one said other roller and the outer surfaces of the said projections on the said one roller,
• adjusting peripheral speed of the outer surface of the projections of the one roller to be greater than peripheral surface of the smooth surface of the other roller,
• feeding said web between said rollers at a speed slower than the peripheral speed of the outer surfaces of said projections, wherein the projections penetrate the web material and create holes in the material and also create a thickening of the web material by pushing web material from said holes and piling the web material up at a side of the holes while said web is in contact with said smooth roller.