Editor?s note: Chris Rauwendaal is a designer, consultant, and seminar instructor who has written extensively on process engineering and extrusion topics.
Part 1 of this article ( PM&A Nov./Dec. 2002) discussed how the size and shape of an extruded product changes from that of the die flow channel and how to control the size and shape of the product. Part 2 describes basic extrusion dies for sheet, flat film, tubing, and pipe dies.
Dies for Sheet and Flat Film
Dies for flat film extrusion are essentially the same as dies for sheet. The difference between sheet and film is primarily the thickness. Webs with a thickness of .5 mm or less are generally referred to as film; webs with a thickness of more than .5 mm are generally referred to as sheet. The simplest film die is the T-shaped manifold die.
This flow channel geometry is simple and easy to machine. The T-die is often used in extrusion coating applications. However, the distribution of the polymer melt is not very uniform and the flow channel geometry is not well streamlined. As a result, this die is not suitable for polymers with limited thermal stability.
|Even well-made dies produce variable thickness, since dies can make sheet 30 ft or more in width. Built-in adjustment capabilities eliminate variations.|
The most common sheet and film die is the coathanger die. The coathanger die is more difficult to machine and more expensive than the T-die, but the geometry of the coat hanger section can be designed to create a uniform distribution of the polymer melt. To streamline flow, the cross section of the manifold is often shaped like a teardrop or half a teardrop. This cross section reduces from the center of the die to the edge to take into account the reducing flow rate through the manifold.
Even when a sheet die is well designed, it is possible for thickness variations to occur in the extruded sheet or film. This is not surprising when you consider that some dies produce sheet with a width of 30 ft or more. For this reason most sheet and film dies are designed to allow adjustments so that thickness variations can be largely eliminated.
in Sheet and Film Dies
Most commonly, a mechanical flow adjustment changes the actual geometry of the flow channel. The flex lip and choker bar adjustments (Figure 1) are two common mechanical adjustments in sheet dies. The flex lip mechanism allows adjustment of the final die gap by turning die lip bolts, which are generally spaced about 25 mm apart. For example, a die 1m wide has 40 flex lip adjustment bolts. Some sheet dies have a sliding lip adjustment rather than a flex lip. The flex lip allows for fine adjustment of the thickness of the extruded sheet or film. In some automated extrusion lines the gap of the sheet die is adjusted automatically. This is done with heat-expandable die lip bolts, first developed by Welex (Blue Bell, PA) in the early 1970s.
The choker bar is located upstream of the