Definition: pellet: 1. Granules or tablets of uniform size, consisting of resins or mixtures of resins with compounding additives that have been prepared for molding or extrusion in a pelletizing machine or extrusion into strands that are cut. Size: the dimensions, proportions, Shape: the flat character of a form, or object as defined by its contour.
In defining the pellet size/ shape we must consider whether we are discussing virgin and or regrind materials, and or reprocessed materials.
In virgin and or reprocessed materials the shape and size of a pellet is defined by the extrusion system and cutting system used to generate said pellet and material. These shapes can be tubular in shape, sometimes referred to as cylindrical in shape. They can be round, oval, and or spherical.
In the case of cylindrical these are usually extruded in a strand, (multistrand mostly) which is than feed through a water trough to cool and than ran through a cutter which now cuts the pellets to length. What is found in these types of pellets is that length can be adjusted, by cutter rotation speed and that diameter of cylindrical part is by draw down and or feed and pulling rate within the system. The diameter of the die which is forming the strand is also an issue when over time these start to wear or break.
With the spherical cut it is cut hot from the face of the die in a wiping motion and is typically done under water so as to cool it as it cuts its. The diameter of the sphere is created by the die diameter and the thickness is by the speed of the cutter.
An additional cut if you run PVC is what is called a cube cut. This looks almost as if it were square in shape.
In both of the above slight variation well occur but this is nothing to be concerned about. If in fact different lines are used there could be major differences in product size and shape as the dies used could be different. This is where it is noticed the most in size/shape of pellets in that a product was produced on a different line or comparing from another material manufacture.
In 90% of the cases there is no issue when you run one type versus another. But in those 10% there is an issue that can be slight, and or detrimental depending on the process conditions, throat size and screw being used.
1- Size in a volumetric sense is what should be considered. Since we are heating the pellets to melt or soften if the pellet is larger and or smaller than previously used we may see issues with following:
a. Melting / uniformity
b. Material pick-up in feed section
c. Screw recovery time
d. Blocked gates due to unmelted pellets
e. Unmelt in parts
2- Shape is key for how the material flows in our hopper, throat and feed section
a. Material pick up
b. Throat blockage
c. Screw recovery time
d. Overall through put
e. Amperage used in extrusion
3- Example:
a. Client was processing a glass filled Nylon for many years and never had an issue with part quality and or filling the parts. Upon receiving a new lot of material they found they could not fill said parts on a continuous basis. It should be stated that their process was locked; we were not able to change anything due to their standardization. What was found was that the new lot of material was a larger size both in diameter and length than the size they previously had been using. This resulted in unmelt blocking the gates, and the short shots. A temporarily fix was to raise melt temperature, which resulted in 100% good parts, but than resulted in recertification of process, which was un acceptable. The final solution was to specify out a pellet size.
Regrind due to it being an irregular cut or grind varies greatly in size, shape and volume of material per pellet. The shape is based on part or runner system being ground up and screen size also being used. Regrind due to it form can have all the issues previously described and also impede the flow of material in the hopper by itself. If in fact the material is of a soft hardness it is particularly a problem if the wrong shape exists in the hopper and or there is a small throat size.
From a processing and quality issue it is important to note what one is using to produce parts from. In most cases it is not necessary but on those occasions when something goes wrong it is important to know what changed. A small retained sample bag showing the pellets used may be helpful in sorting out that issue when and if the time comes.
From a time and temperature standpoint if in fact the pellet size has increase we may need more heat given that our time is the same.
In processing we must monitor the variables and pellet size is one of those, though in most cases it doesn't matter, it is something to be aware of.
SL Silvey
