Coperion Ask the Expert

Why are my compounds fluctuating in quality?

How to optimize your process to solve most common quality issues with strand pelletized compounds?

A high-quality level of strand pelletized plastic compounds is usually a prerequisite for any further process steps. Generally, the compound quality is not only determined by the formula and process specifications itself but also by the pelletizing process and its equipment.
Compounding processes tend to be fairly complex which is why problems are not always easy to pinpoint. A slight adjustment in the process parameters may already cause a chain reaction and lead to a different pelletizing performance. Nevertheless, it is crucial to understand the most common problems that are directly visible during strand pelletizing. Possible causes and their solutions are described below.

What are possible causes for the most common issues with strand pelletized compounds?

Rough pellets
Rough pellets are characterized by an abrasive surface. Depending on the type of compound this roughness can be either completely normal or caused by a process related issue. While fillers are considered smooth, nylon glass fibers for example are known for being naturally “grippy”. If your compounds are not supposed to be rough or incur sudden surface changes, you should ensure that feeding and mixing is sufficient. In this case, it might also be necessary to change the screw design or the feeding configuration to optimize the product.

Another common problem in this connection is caused by the so-called “stick slip effect” which normally occurs when the mass touches the die head. In the first step you should try adjusting the temperature of the die to the actual melt temperature or slightly above. If this does not lead to the desired results, you should change to a higher number of die holes in order to decrease the speed in the die head.

Moist pellets
A high level of moisture is usually not desired and may also lead to undesired chemical reactions with the potential to change the properties of the pellets. To reduce moisture of the pellets, it is therefore recommended to either shorten the strand cooling length in the water, increase the water temperature or extend the cooling length in the air in order to achieve better moisture evaporation before the strand is cut. Only if the strands have enough time to dry it can be ensured that moisture is deprived from the product and that no unwanted effects during the cutting phase occur. At the same time, it is crucial to keep an eye on any changes in the environmental conditions. High humidity both in the processing and storing area does not work well with some products. In these cases, additional equipment might be necessary.

Particles in the strand
Particles in the strand can be another frustrating issue. Usually this is related to problems with the mixing, feeding or with the flow in the die head. Possible solutions you may consider are optimizing the process parameters, optimizing the screw design, unplugging holes, purging the die and checking flow uniformity.

Fines or high dust content in the product
A high percentage of fines is usually not desirable. Fines content mainly can occur when processing brittle materials. For such products, a high strand temperature during cutting is important. This can be achieved by shortening the cooling length in the water or increasing the water temperature. To ensure a clean cut the cutting gap and wear issues of the pelletizer blade also need to be checked.

Deformation of the pellets
If your pellets leave the process deformed and not how you want them, you are most likely confronted with chains, tails or even agglomeration. Depending on the form and the severity of deformation this may have different causes and thus various solutions. Often the product is either too hot (soft), the cutting gap is too big, wear on the pelletizer blade occurs, the die holes are too close, or the strands are not sufficiently separated.

Feed intake of the strands
In some cases, strands may not feed cleanly into the pelletizer. This behavior can be caused, among other things, by an unclean production start, an excessively high haul off speed, wear on the feed rolls, or an insufficient contact pressure of the feed roll.

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