Examples of the appearance of melt fracture, a primary cause of extrusion instabilities.

Plastic Extrusion Tooling Specialists ph: 800.272.8878 Optimize Your Polymer Extrusion Process By Eliminating Instabilities Strategies to Reduce or Eliminate Melt Fracture

At B&H Tool Company, we are • Increase the melt temperature reducing shear stress, facilitating flow committed to helping customers in the die region. A small tempera- through the die. Chemical compatibility optimize their extrusion process. A ture increase may sufficiently reduce is a must. Typically, small amounts of major barrier to optimal performance the polymer’s viscosity, lowering the additive will not cause adverse affects, is polymer extrusion instability, and shear stress imparted in the extrudate. but they can alter the mechanical or the most common cause is a phenom- • Reduce the land lengths of the cosmetic properties of the final product. enon known as melt fracture. die, pin or mandrel. This will decrease • Reduce slip-stick phenomena in Instabilities can also result from the shear stress in the extrudate, but the die region with coatings or improperly prepared feedstock, solids it will increase the extrudate swell/die impregnations. Using materials such conveying issues, insufficient melting swell and decrease the molecular as nickel, TeflonTM or diamond can or mixing capacity, barrel temperature alignment along the direction of reduce the coefficient of friction and fluctuations, lack of uniformity in material flow.1 the imparted shear in the polymer. melt temperature or viscosity in the • Reduce the entrance angle of Coatings may alter the physical die region, improper screw design, the die and the angle of the pin or dimensions of the treated parts and and suboptimally designed or improp- mandrel. Larger length to diameter must be chosen to withstand the erly operating ancillary equipment ratios (L/D) increase the uniformity of operating temperature and chemical downstream. flow and decrease melt fracture. affects of the polymer. Melt fracture is often signaled by • Increase the Draw Down Ratio. • Apply medium frequency (kHz) surface roughness and/or surface This will reduce the affect of melt ultrasonics to the die region. irregularities in the extruded plastic fracture on product appearance but Using externally mounted transducers involving a severe distortion of the too large an increase in DDR can facilitates molecular alignment and extrudate. This occurs when shear cause draw resonance, which is a reduces the degree of extrudate swell.1 stress imparted into the polymer cyclic pulsation in the product diameter.2 exceeds the critical shear stress value • Use a polymer grade with a By: Doug Hennies, Ph.D. of that polymer or slip-stick flow higher Melt Flow Index. If it does Design Engineer, conditions occur in the die. not adversely affect the mechanical properties of the final product, switch- B&H Tool Company The following can help to reduce ing to a higher MFI material allows or eliminate melt fracture: higher throughput rates before melt • Reduce the haul-off rate or line fracture occurs.1 References: speed. Where high line speeds are • Decrease the elasticity of the 1. Hans Kramer: not essential, small reductions in line resin. This results in a higher critical “Polymer Extrusion Instabilities”, speed may sufficiently reduce the shear stress value for the polymer.2 Applied Polymer Research, 1999. amount of imparted shear into the • Incorporate low molecular extrudate and reduce surface defects. weight additives into the base 2. Paul M. Hendess: “The Effect of Die Design on Extruded Product Quality”, • Streamline the die head. Flow can resin. A small amount (5-10%) of a StreamLine Extrusion, Inc., 2002. often be improved by reducing the compatible low molecular weight addi- entrance and exit angles. tive will act as an internal lubricant,

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