How to Describe Defects in Underwater Pelletized Pellets

During normal underwater pelletizing of products in properly designed equipment, the pellets produced should be uniform in size, shape, weight or other characteristics for subsequent processing and/or aesthetic reasons. But, can you describe to others pellet defects that may be caused during start-up, when processing or operating conditions are not correct, when the pelletizer components are worn beyond specification, or other problems exist?

This Black Clawson Converting Machinery Tech Tip will pictorially define common pellet defects and give the typical name or names associated with each. When common nomenclature is used, interested parties can better define the defect and work to eliminate these from the process to provide high quality pellets.

Uniform Pellets

Non-Uniform Pellets- Length & Size

Non-Uniform Pellets- shape

Internal Voids

Longs or Elbows

Longs

Dog Bones or Dumb Bells

Dimples

Tails

Twins

Triplets

Chains

Clusters

Agglomeration

Agglomeration and Large Clustering

Agglomeration

Fines or Angel Hair

Fines

Trash or Shavings

Melt Fracture

Upon determining your actual pellet quality or performance, changes to process conditions or equipment designs can be made to improve your process. After all, a correctly designed underwater pelletizer operating under the proper process set-up conditions can produce high quality pellets with greater efficiency.

If you have a competitor’s machine or an older Black Clawson Converting Machinery underwater pelletizer, take the challenge and compare the results to the performance of Black Clawson Converting Machinery’s new pelletizer designs. You’ll be surprised at what you find.

For further information on how to improve your pelletizer performance or for a free Black Clawson Converting Machinery Pelletizer Performance Calculator, contact Charles H. Crumb, Business Group Manager of Extrusion Pelletizing Systems, at 315-598-7121.

Questions? Call Black Clawson Converting Machinery at 315-598-7121 or e-mail us at bc@bc-egan.com.

BC Publications is a technical newsletter designed to provide operational and maintenance information to improve the performance of your Black Clawson Converting Machinery equipment.

Pellet Defect Causes

"A Troubleshooting Guide" By Charles H. Crumb

Pellet Defect	Typical Cause	Typical Solution
Internal Voids	Moisture, entrained gas, or volitiles in polymer to the pelletizer Quench rate too fast	Dry polymer Degas polymer Strip polymer solvents Increase quench water temperature
Longs or Elbows	Knife speed too slow Extrusion rate too high Frozen die holes Low melt temperature Too few knives	Do pellet efficiency calculation Increase knife speed Reduce extrusion rate Increase die temperature Increase water temperature Restart Pelletizer Increase number of knives
Dog Bones or Dumb Bells	Low viscosity product Elastic stretchy product Blades worn beyond limit	Make knife clearance adjustment Lower quench water temperature Insure blade leading edge is sharp Replace knife and hub
Dimples	Shrinkage due to fast quenching	Increase quench water temperature
Tails	Chipped or cracked orifice tips Knife to die gap too large Die surface irregular Shaft not aligned Knife tracking	Plug chipped holes Make knife adjustment Realign Pelletizer Regrind die face Replace and Reface die plate Check ID/OD knife sweep coverage on die holes
Twins, Triplets or Chains	Product too hot Quench water too hot Low quench water flow Product inherently tacky Knife speed too high Die holes too close	Lower product temperature Lower water temperature Check water flow rate Make knife adjustment Reduce knife speed

Clusters or agglomeration	Product too hot Quench water too hot Low quench water flow Product inherently tacky Die holes too close	Lower product temperature Lower water temperature Check water flow rate Make knife adjustment
Fines or Angle Hair	Chipped or cracked orifice tips Die holes partially blocked Shaft not aligned Knife tracking	Make knife to die clearance adjustment Plug chipped holes Purge die and check for product flow uniformity Check ID/OD knife sweep coverage on die holes Realign Pelletizer Regrind die face Reface die plate
Dust	Product is brittle or friable in solid state	Slow centrifugal dryer speed (not more than 10%)
Surface or Melt Fracture	High die hole shear rate Product too viscous Extrusion rate too high Frozen die holes Orifice hole final land scored	Reduce extrusion rate Increase product temperature Use larger hole diameter Use shorter orifice hole final land length Smooth final land surface Increase die temperature
Trash or Shavings	Knife to die gap too large Infrequent knife adjustment Die face build up Blades not ground properly Die surface worn irregular Product inherently stringy	Make knife adjustment Blades worn beyond limit Insure blade leading edge is sharp Replace knife and hub Reface die plate
Irregular size and shape	Temperature or viscosity variation in product Die sector temperature uniformity Die plate heating medium inlet / outlet manifold design	Add static or other mixer Check condensate traps or other heating medium restrictions at each die plate segment Reface die if insulation is lost

Please Note: In most cases defects can have multiple causes. It is therefore desirable to first make checks and changes to adjustments in an orderly fashion. Adequate time should also be allowed between incremental adjustments to insure full effect has been seen. Consult Black Clawson for additional assistance at Phone 315-598-7121, Fax 315-593-0396 or email bc@bc-egan.com.