Specks and Discoloration

Discolored specks are a common problem in extrusion and molding. This problem is similar to another common defect, which is the problem of gels. Like gels, discolored specks are formed not only in extrusion and molding at the processor but also in polymerization at the resin producer. As a result, in order to get a handle on the problem, we need to know how many specks are in the incoming raw material. We will distinguish between a speck formed in polymerization and located inside the pellet (P-speck) and a speck formed in extrusion (E-speck). 

One of the challenges in testing pellets produced at the resin supplier is that pellets are quite small. A typical plastic pellet may have a diameter and length of about 3 mm with a volume of about 20 mmT If we assume a density of 1 gr/cm, the mass of a typical pellet will be about 20 milligram. This means 1 kg of resin will have about 50,000 pellets. If we run an extruder at 300 kg/hr, approximately 1,500,000 pellets will pass through the extruder every hour. 

If we have one pellet with a discolored speck for every N pellets, we can determine the average incidence of specks in the extruded product. If the cross-sectional area of the extruded product is and the volume of the pellet or powder particle is Vp, the average length over which a P-speck will occur in the extruded product is  

As an example, we will consider the extrusion of tubing with an inside diameter of 10.0 mm with a wall thickness of 1.0 mm. The cross-sectional area of this product is  

The volume of the pellet is 20 mm We will consider a plastic raw material that contains one pellet with a discolored P-speck for every 10,000 pellets this corresponds to 100 ppm. With this input data, we find the average length over which a P-speck will occur is 5806 mm or 5.8 m. If we generate specks in the extrusion process, the average length over which a speck occurs in the extruded product will be less than 5.8 meter. 

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