Gels and black speck detection are a requirement for plastic film inspection. This article focuses on giving the reader more information about detecting and dealing with gels and black specks.
Gels and black specks can occur when PE Resin becomes degraded during the primary PE manufacturing process or the secondary conversion into a finished product. Such defects can seriously compromise the quality, aesthetics and performance of products manufactured from PE.
Where are Gels formed?
Fortunately it is relatively easy to minimize the chances of gels and black specks occurring through the application of the correct quality assurance (QA) processes and procedures. Gels and black specks are visual or structural defects that can occur when polyethylene resin has become degraded. Such defects typically occur when polyethylene is exposed to high temperatures for a prolonged period while oxygen from air is present and are often observed within the first hour of a new production run.
Gels are usually formed at one of three stages during production:
- In the polymer reaction process itself as the raw polyethylene is manufactured.
- In the granulation process when the product from the reactor is being mixed with additives and compounded into granules.
- In the conversion process where the polyethylene is extruded and molded into a finished article, for example during the process of blow molding.
There are two broad types of gels, with each type characterized by the manner in which it was formed. The first type is created during the reaction process and is caused by an accumulation of raw polyethylene particles growing within the reactor and producing material which has a significantly higher molecular weight than normal. These gels are colorless.
The second type is formed during the processing of the resin as it is run through equipment such as an extrusion or molding line. This type of gel occurs due to a degradation of the resin which is typically caused by exposure to high temperatures for a long time in the presence of oxygen. Such gels tend to be cross-linked and are identifiable as they have a different structure to the gels produced in the reactor. If a gel has remained at high temperature in the extruder for an extended period of time, it is possible for it to oxidize further and eventually discolour, creating brown or black specks within the finished product. This type of gel often arises from hang-up of resin somewhere in the extrusion system. Such gels are more often observed during the first hour of a new production run, but can occur at any time if the equipment or conditions are not optimum.
Gels and black specks can arise when polyethylene resin becomes degraded at any point during the conversion process. This type of degradation typically occurs when polyethylene is exposed to high temperatures for a long time while oxygen is present. Once formed, such defects can seriously compromise the quality, aesthetics and performance of articles manufactured from polyethylene. The solution to avoiding such issues lies in the control of equipment operating temperatures, the amount of oxygen that is present and the shut down and start-up procedures.
Gels and black specks are much more problematic in certain polyethylene applications than others. Blow molding is an application where gels and black specks can be more troubling, particularly in the production of thin walled bottles. In this application, the presence of gels and black specks can lead to the formation of holes and cause blow outs – a structural issue, rather than merely an aesthetic problem. Even if the defect doesn’t necessarily lead to holes, it can be visible on the surface and diminish the apparent quality of the article. This can be especially pronounced in products such as milk bottles where black specks will easily stand out.
Film extrusion is another application where gels and black specks can be problematic. Excessive defects in film affect the appearance of the film and can cause issues with printing. In applications such as the packaging of high quality goods, these defects can detract from the consumer’s perception of the product quality.
Avoiding gels and black specks
So what are the steps that can be taken to reduce the likelihood of them appearing?
Polyethylene producers typically use a range of controls to avoid producing resin with gels;
- Precise control of reactor conditions.
- Specific procedures for the maintenance of the reactor.
- Close control of compounder conditions.
- Strict procedures for shut down, start-up and purging the compounder.
- Detailed procedures for the maintenance of the compounder.
- Frequent and thorough quality testing of the product.
Sourcing resin from a high quality supplier will reduce the risk of the presence of gels in the raw material. Many polyethylene resins contain additives such as antioxidants which are added during the manufacturing process and serve to stabilise the resin and increase the resistance to degradation. Your resin supplier will advise you on the raw materials that are most appropriate for your process. In the conversion process, polyethylene resin can adhere to the metal surfaces of the machinery in areas of low flow and degrade over time. When the equipment is shut down, the metal surfaces cool down and the degraded material can dislodge. When the equipment is restarted the degraded polyethylene resin may emerge and present with the appearance of black specks. It is essential to follow the correct machinery operation and maintenance guidelines in order to minimize the occurrence of gels and blacks specks. The key is controlling the machine operating temperatures, the amount of oxygen that is present and the start-up and shut down procedures. Close management of machine start-up and shutdown times is critical. The times can be optimized to minimize the time the resin is heated and expose.
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