What causes extrudate swell?

What causes extrudate swell?

Die swell, also known as extrudate swell or Barus effect, is a common phenomenon in polymer processing. Die swell occurs in instances of polymer extrusion, in which a stream of polymeric material is forced through a die, a specialized tool in manufacturing to shape or cut polymeric materials.

How do you reduce a die swell?

There is a decrease in die swell with an increasing length of the die for a given shear rate. Extrudate swell through a capillary die is somewhat less than through a slit die; extrudate swell through the slit die also increases more rapidly with an increase in shear rate.

What is the die swell ratio?

Die swell is a complex Theological phenomenon [1]. It can be observed as an extrudate with a cross-section (Dex) which is greater than the die cross-section (Do). This effect, also known as extrudate swell, Barus effect, or % memory, is defined as the ratio D ex /D 0 = B and is a feature of polymer melt flow.

What is an annular die used for?

1 Introduction. In polymer extrusion process, a device called annular die is used for most shaping of polymer products, such as in pipe extrusion process, extrusion blow moulding process, and blow film extrusion process.

What is Bambooing effect?

If the conditions causing shark skin becomes more intensive, eg. Pressure at the extruder becomes excessive or die temperature drops, the extrudate “snaps back” — “Bambooing effect”.

What is polymer melt?

Polymer melts and solutions are highly viscous liquids, possessing non-Newtonian and viscoelastic natures. The flow behaviors of molten polymers or solutions are becoming increasingly relevant to their engineering processing techniques.

On which factors die swell depends?

Die swell depends on three main factors; material, die geometry, and extrusion condition [4][5] [6] [7][8][9].

What causes die swell in the capillary rheometer?

It happens as a result of the molecular orientation that is generated by the flow in the die (with the greatest extension occurring near the wall) and recoiling after exiting the die (contracting in the flow direction and expanding in directions perpendicular to the flow).

What is extrusion die?

Extrusion dies are essentially thick, circular steel disks containing one or more openings to create the desired profile. They are normally constructed from H-13 die steel and heat-treated to withstand the pressure and heat of hot aluminum as it is pushed through the die.

What is die lip?

Die lip buildup, also called plate-out, die drool, die drip, or die peel, refers to the unwanted material accumulation on the open faces of extrusion dies. Since this buildup of material can interfere with extrusion, the processing line must be shut down periodically to clean the die. These shutdowns are costly.

What is Bambooing in extrusion?

This generates tensile stress and if the stress exceeds Tensile strength, the surface ruptures causing the visual defect – “shark skin”. If the conditions causing shark skin becomes more intensive, eg. Pressure at the extruder becomes excessive or die temperature drops, the extrudate “snaps back” — “Bambooing effect”.

What is blowup ratio?

The blow up ratio is simply the ratio of the final diameter of the tube relative to the die diameter. The amount the film is oriented in the machine direction is controlled by the draw down ratio (DDR), which is simply the ratio of the thickness of the extrudate exiting the die to the final film gauge.

What temperature melts polymers?

The value of Tg depends on the mobility of the polymer chain, and for most synthetic polymers lies between 170 K to 500 K. The transition from the glass to the rubber-like state is an important feature of polymer behavior, marking a region of dramatic changes in the physical properties, such as hardness and elasticity.

What is viscosity of polymer?

The measurement of the viscosity of polymer solutions in organic solvents gives us a value that is directly related to the molecular mass of the polymer. In other words, the higher the viscosity, the higher the molecular weight, although the correlation is not direct.

Is viscosity a rheological property?

The rheological parameters were viscosity (cp), torque%, shear stress (dyne/cm2) and shear rate (s-1).

What is die land?

The land length of the die is the dimension of the straight section within a die. For denser pellets, this dimension is longer and for lighter pellets, this dimension is shorter. During extrusion, the product is under several pounds of pressure within the barrel.

What is melt fracture?

Melt fracture is defined as the phenomenon caused by excessive shear stress exerted on the molten resin that leads to roughness in the extrudate.

What is die gap?

[′dī ‚gap] (engineering) In plastics and metals forming, the distance between the two opposing metal faces forming the opening of a die.

Why is extrusion used?

Extrusion is a metal forming process in which metal or work piece is forced to flow through a die to reduce its cross section or convert it into desire shape. This process is extensively used in pipes and steel rods manufacturing. The force used to extrude the work piece is compressive in nature.

What is sheet die?

Sheet/film extrusion A T-shaped die consists of two arms extending at right angles from the initial extrusion channel; these arms have a small slit along their length to allow the polymer melt to flow through. The melt is then further thinned by a short, flat approach before being pushed through the lips of the die.