Gels are inherent in the extrusion process. Often the best way to control your process is to use a vision system to count. This is becoming a common requirement for plastic film web and sheet producers. This article details the different applications that OneBoxVision offer for Gel counting.
Whast are Gels ?
Gels can come from several sources: HMW tails in a bimodal resin, crosslinked material caused by overheating, additives with poor thermal stability, fines from regrind, catalyst residue such as silica, and other organic or inorganic contamination. As a result of shear forces in extruders, gels usually end up as elongated ellipses. Those caused by contamination typically have a dot or "fisheye" in the center; gels caused by HMW material do not. When gels consist of HMW droplets, they are primarily a cosmetic problem. Gels caused by contamination, however, can produce a weak point in a tube or even start a hole in film.
If you have gels in an extruded product, it's important to know whether they originated in the incoming raw material or were created during extrusion. Gels formed during polymerization are called P-gels. They form in stagnating regions of the reactor where resin overheats. They may also be high-molecular-weight fractions created by a bimodal process or resin made when a reactor needs maintenance. P-gels are a common problem in polyolefin. Some non-olefins also have high gel problems. PVDF can pose severe gel problems from the manufacturer, especially if it's made shortly before a maintenance turnaround. So processors using a material that's prone to gels should be aware of reactor maintenance schedules.
Gels can also form during extrusion. These are called E-gels and tend to form as a result of high temperatures or long residence times. E-gels can be created by dead spots in the extruder e.g.) stagnating regions in the screw. Maddock mixing sections, for example, can create dead spots.
The best defense against P-gels is to test incoming material for them and communicate clearly to resin suppliers that you do so. When resin suppliers realize they're dealing with a sophisticated user whose product requires low gel counts, they will try to send good material. Incoming QC for gels is especially important when running multi-layer film because you have more materials and extruders to check for gels. Testing for P-gels requires a thin plaque of the material. Plaques can be prepared several ways, such as by compression molding a few pellets' worth of resin. It's important to control the pressing temperature to make sure you don't create more gels in the test process. The number of gels per unit area in the plaque can be counted using an overhead projector and polarized light to project an image on a screen. Make sure you standardize gel-test procedures within your company. They should be written and followed consistently.
To avoid making gels during extrusion, be sure the screw and die have a streamlined design. Also the screw, barrel, and die surfaces should be free of grooves, scratches, or gouges that might collect melt and cause degradation. It's also important to check resin feed tubes, blenders, feeders, hoppers, and other bulk-handling hardware for fines, streamers, or contamination from other plastics. Bulk-handling equipment should be
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.
Gel Counting - understand how it works
It's a key feature for any surface inspection system that operates on plastic film. If included as part of a machine vision system for plastic inspection, it will allow an extruder determine quality .
Three key features enable gel counting.
- Structured lighting
- Defect classification
- Density analysis
Why do you need it? Gels are transparent and plastic film is also transparent. It's key to direct the light so that a gel can be seen. The trade-off is that common defects can be difficult to distinguish from very large gels.
The next task is to break the gels down into different sizes and classes. These classes will then be used by the vision system to analyze the quality. The following classifications are common on a vision system being used for surface inspection.
- Gels - small
- Gels - medium
- Gels - large
- Carbon specks
- Fracture melts
The next task is to be able to classify defects and have the bandwidth to process particle heavy images as there can be many gels in a square meter. It's almost impossible to produce gel free plastic. The system must keep a moving average of how many gels of each class are within a given area. The user will have set limits on how many different types of gels are allowed. It may be OK to have 10 medium gels if alone, but if you add 100 small gels then the alarm may need to be triggered. A well-presented gel counting solution will allow the user to set alarm decisions on as many of these scenarios that the user deems necessary. If the limit is exceeded, an application alarm can be triggered. Application alarms can then be connected to physical I/0 outputs to drive field devices.
Gel counting is a desirable capability but it is important to understand that often the role of defect detection and monitoring process variations can become blurred. Vision systems used for surface inspection must have a system that allows alarms to be triggered only on key parameters.
OneBoxVision is a specialist in building solutions for the plastics industry. Contact us and we can provide you with a free consultation or just go ahead and access our vision library to learn more. We provide solutions to industry leaders worldwide and can deliver an out of the box solution that will meet your requirements in weeks.