- March 01, 2010, By Ricky L. Keller, Davis-Standard LLC
Blown film producers are challenged by the continual search for high-value-added products to bring to their portfolios. Often, these products are in small market segments that do not have enough volume to justify a complete new system.
Economics in the form of capital and operating costs drive the machine configuration to be able to make multiple, high-value products on one machine. Finding the common links or “threads” between dissimilar products and resolving the dissimilar needs can lead to machine designs for multiple, high-value-added products.
The issue then becomes resolving the varying extrusion needs and proposing a system with performance standards at a level consistent with dedicated, purpose-built machinery.
Film quality is driven by the quality of the materials and process stability achieved. While it is easy to change materials, changing screws, dies, and air rings is not an economically viable option. Therefore, these components need to be versatile and operate at a high performance level.
A focus is needed on extrusion stability. While we can develop machine design strategies that allow for optimum production of several products, we need to maintain quality levels.
Quality is largely affected by the stability of the extrusion process, and low process variations must be maintained.
However, a better understanding of process variables and the factors impacting blown film quality go a long way in achieving goals in a thoughtful, logical, and high-value fashion and should be applied when specifying new components.
Maintaining Extrudate & Film Quality
An absolute prerequisite for blown film quality is extrudate quality. If there is a melt temperature variation coming off your screw tip, the die and air ring will not correct it no matter how fancy or advanced they may be.
Measuring and understanding the extrudate variation is a must for any extrusion operation and one that many do not understand. It is vital that existing extruders have the proper instrumentation to measure melt temperature and extruder pressure in a manner that will optimize quality. This often is not the case. So how do we solve this problem?
First, we need to understand the proper method to measure melt temperature.
There are three types of melt temperature probes (see Figure 1):
- Fixed depth probe, which is flush mounted
- Fixed depth probe that is immersed at a fixed distance (¼ in., ½ in., ¾ in. or 6 mm, 13 mm, 19 mm)
- Variable depth probe, which can be screwed into the melt from flush to 1 in. (25 mm) with an exposed junction thermocouple
The flush mount is a poor measurement as it is heavily influenced by the adapter pipe temperature. This can be seen in the melt temperature reading when the adapter temperature is increased or decreased.
However, the flush mount probe is the most robust as it is not in the melt stream and is commonly used because of cost, no risk of damage, and it can hide melt temperature problems (Figure 1A).
The immersion or fixed depth type measures the melt temperature at one point across the melt stream (see Figure 1B). It is a one-piece design that resists cold starts, is more robust, and is a good compromise in accuracy between flush and variable depth.
The variable design can measure the cross-channel melt temperature gradient across the pipe (Figure 1C). It is a two-piece design that insulates the tip from the adapter pipe temperature.
It is also of a delicate nature that is susceptible to cold start damage. The exposed tip offers the most accurate melt temperature measurement, but high viscosity resins can shear heat the exposed junction at times providing erroneous information.
The key is to have an extrusion process in which the melt temperature and pressure are stable. Combining such stability with precision-machined die lips and air ring parts will help you produce a fine, flat blown film. The individual component performance goals should meet the following criteria:
- Extruder pressure variation | This should be less than 1%; more than 1% variation in pressure yields 2.5% in MD thickness.
- Extruder melt temperature variation (time dependent) | Design criteria should be less than 2 deg F; ±2 deg may result in ±1% in MD thickness.
- Extruder melt temperature variation (position dependent) | Design criteria to be less than 5 deg F. More serious effects, such as cold matter, tend to flow in the bottom of the spiral and emerge as “port lines.”
Should your measurements fall outside these parameters, the production of high quality films will be difficult to achieve. The variations in temperature will result in uneven cooling and stretching of the film that result in gauge variation.
Commonly, a couple degrees Fahrenheit variation can result in a couple percent gauge variation in a 1-mil film. As the melt temperature variation increases, the result is even higher levels of thickness variation.
Similarly, airflow distribution about the bubble is extremely critical for uniform films. We can measure these variations through the use of air velocity measurement devices or velometers. While these can be difficult measurements to take, the result of poor distribution is quickly seen in film variability. This variable is very important, and we can add a statement to our list of goals.
- Airflow abnormalities in distribution | Variation should be less than 1% about circumference.
While air velocities are generally measured in the range of 2,000-5,000 fpm, a variation of 100 fpm is very significant and will contribute percentages to the overall variability. Identifying the root cause of these variations and correcting them will lead to higher levels of film performance.
In conclusion, focusing on critical process variables, understanding the measurements taken, and then addressing the root causes of these variations will lead to higher levels of film quality and performance. The corollary of this concept also applies to a machine's ability to process broadly different materials while maintaining excellence. Tight temperature and pressure control must be maintained through the entire range of products and materials for a multipurpose machine to be successful.
Rick Keller is VP-blown film of Davis-Standard LLC, Bridgewater, NJ. He has more than 28 years of experience in the film industry on the processing and machinery sides of the business. Keller received a BS in polymer science from Penn State Univ. in 1979 and worked with film processors developing LLDPE films. He can be reached at 908-722-6000 ext. 2266; firstname.lastname@example.org.
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