The blow-molding process represents a relatively small segment of the plastics industry as a whole – compared to injection molding or extrusion, at least – but when it comes to the manufacture of rigid packaging, it is extremely important. The projected growth rates for the overall packaging market are all positive no matter who you listen to and blow-molded containers and bottles are sharing in that expansion.
Like extrusion processors, blow-molding companies tend to use large quantities of a relatively small number of different resins. There are two primary blow-molding technologies: injection blow molding is used almost exclusively to produce polyethylene terephthalate (PET) bottles, with the ubiquitous soda-pop bottle being the most recognizable example. Extrusion blow molding processes primarily polyolefin materials, especially high-density polyethylene (HDPE), to make such containers as milk and juice jugs. Extrusion blow molding (EBM) is also used to make large consumer and industrial products ranging from children’s’ toys and play structures, to oversized drums, gasoline tanks and air manifolds. These markets don’t represent the same volume as packaging, but many of the production issues and equipment requirements are similar, if only on a larger scale.
Handling large volumes (i.e.: millions of pounds per year) poses some important handling problems. Material is likely to be delivered by rail or truck and it will need to be unloaded into storage silos. This entails moving thousands of pounds of pellets over significant distances and lifting them high to receivers on top of the silos. Then resin needs to be further conveyed – sometimes over 1000 feet – to in-plant surge bins, blow-molding machines or auxiliary equipment like dryers or blenders. Accomplishing this feat efficiently and without damage to the material or equipment is not easy. Tips on conveying large-volumes of material – as well as other equipment-related suggestions — can be found in the Conair article entitled “Extrusion Processing: The Basic Guide to Using Auxiliary Equipment.”
TRENDS IN BLOW MOLDING
We see two significant trends that influence the selection and use of auxiliary equipment in the blow-molding sector.
The first arises from initiatives to reduce the amount of virgin plastics being used in packaging and it is leading to thinner wall sections and the increased use of recycled material – either post-industrial scrap generated in-house or post-consumer recycled resin (PCR). The second major trend is seen in efforts to increase manufacturing efficiency and reduce energy consumption.
REDUCING THE USE OF VIRGIN PLASTICS
Let’s look first at the increased use in-process scrap or regrind. In extrusion blow molding, a certain level of scrap is expected as a by-product of the process. So-called “tops and tails” must be trimmed after a bottle is blown, and then trimmed bottles are leak tested to identify unacceptable products. This scrap will be conveyed either to a holding bin for batch granulation or directly to a granulator. The current labor shortage, not to mention overall plant efficiency, argues strongly in favor of an automated systems approach. Once scrap has been conveyed and fed into a granulator, regrind can be vacuum conveyed from the grinder catch bin or directly back to the blow-molding machine where it can be metered by a gravimetric feeder in a precise ratio to virgin material and other ingredients.
For maximum control – and additional production advantages — regrind can be conveyed to a gravimetric blender for delivery to the blow-molding machine. The controls on Conair gravimetric blenders utilize a unique feed-forward dosing algorithm. The algorithm incrementally doses toward a required set percentage amount entered by the operator. Each material is dosed multiple times so that the amount dispensed gradually approaches the target weight for each component with very little over-or under-shooting. In fact, Conair uses this approach for every component within the recipe – virgin, regrind, color or any other ingredient — achieving an accuracy of +/- 0.5% of the set gram amount for each ingredient. The control also gives you the ability to track blend performance and material consumption over time for improved quality and inventory control.
PET injection-blow-molding operations don’t have the same scrap issues that extrusion blow molders do. Preforms (unblown tubes with the threaded ‘finish’ molded-in) are injection molded, with very little scrap, assuming processing conditions are well maintained. Then the preforms are reheated and blown in a separate operation, again with very little scrap. Scrap preforms and bottles can be granulated and reintroduced to the preform-molding process, but processing considerations limit the amount that can be used to less than 10% unless the regrind is re-crystallized. Handling in-process PET regrind involves the same auxiliary equipment as extrusion blow molding, with the possible addition of a crystallizer (if more than 10% regrind is to be used) and, of course, regrind needs to be dried just like virgin pellets.
INCREASING USE OF PCR
The other way to reduce the use of virgin material is to use post-consumer-recycled resin. In fact, environmental consciousness (and government mandates) is driving many brand owners to specify that a certain percentage of PCR must be incorporated into their bottles and other blown containers. Additionally, a number of states, including California, Washington, and New Jersey, have passed legislation requiring that consumer bottles contain a minimum amount of PCR – initiatives start at the 10-15% level with the intention to increase PCR content over time.
PCR will arrive at your plant the same way virgin materials do, although probably in smaller volumes than virgin materials, and likely packaged in sacks or Gaylords. You’ll want to keep these separate from virgin resin, so a separate silo or surge bin may be required, although material-handling requirements are substantially the same. Then the PCR pellets get blended with virgin material before being fed to the processing machine.
Most PET bottles have just a single layer of plastic, but in HDPE blow molding applications it is quite common to see multilayer structures. The multiple layers ensure that the regrind or PCR is isolated from the packaged product by layers of virgin material. This approach is especially important in food and pharmaceutical packaging because it eliminates any concern about contaminants that may be in the recyclate. The different layers will have to be processed in separate extruders, with the two (or more) materials coming together in the blow-molding die. The blending and feeding equipment will probably be the same as discussed above, and tracking functions available make it easy to precisely control and track usage of different materials. Another approach to reducing the amount of virgin plastics used in packaging involves ‘thin-walling’ – reducing the thickness of bottle walls to an absolute minimum without affecting package integrity. This is very evident in PET blow molding, where wall thicknesses, particularly in water bottles, have been reduced to a few hundredths of an inch. Achieving these thin sections is mainly a process-control issue, but auxiliary equipment becomes important in the drying of PET prior to processing. That’s because PET is hygroscopic. It will absorb moisture from the air and, if it is processed without proper drying, the entrained moisture can cause unacceptable degradation and structural as well as cosmetic flaws. You want a desiccant dryer that offers you complete control over the four fundamental drying parameters: drying temperature, dew point, time and air flow. For more information, check out the on-demand Conair webinar The 4 Fundamentals of Drying and How to Choose the Right System.
Another concern for PET blow molders using PCR relates to higher levels of volatile compounds in the air returning from the hopper to the dryer. Ignoring the presence of these fine oils and condensates can lead to desiccant contamination and end-product quality issues. There is a simple solution, however. A condensing filtration system called a ‘demister’ is an optional feature that should be installed in the dryer’s return air-circuit whenever PCR is incorporated in PET being dried before processing. Added to new or existing systems, a demister does a great job of removing the volatiles, increasing confidence that a quality product is being produced and the rest of the drying system is well maintained.
REDUCING ENERGY CONSUMPTION
The second important trend driving auxiliary equipment decisions is the increase in efforts to reduce energy consumption. This trend has been evident for quite some time, but that doesn’t mean that cutting energy use is any less important. In fact, quite the contrary is true. This objective, along with reducing the use of virgin plastics, is part of the broader goal of increased sustainability for plastic packaging.
Utilities, especially electrical power, are the second highest cost (just behind raw materials) in most blow-molding applications and drying, by far, is the major contributor. While drying is not normally required when HDPE and other polyolefins are processed in blow molding, it is, as noted above, critically important in the processing of PET.
The first step in minimizing energy consumption in drying should always be sizing the system. Many times dryers are oversized or selected based on maximum processing capacity requirements, which are only rarely needed. A properly sized drying system can save a significant amount of energy.
Another simple option is the use of gas instead of electrical energy. In some regions, gas can be less expensive and the payback on the equipment required to replace electric heating with gas heating can be measured in months. This will not reduce the total amount of energy used, nor will it increase sustainability, but it can cut costs.
Then, it is important to consider the design of the dryer. As noted, PET processing almost always requires a desiccant drying system – the gold standard in the drying world. And the gold standard in desiccant drying is desiccant wheel technology. The wheel has less mass to heat than twin-tower or canister dryers, which use much higher volumes of desiccant. The wheel also can be regenerated (purged of previously adsorbed moisture) at a much lower temperature, and has very few moving parts. Furthermore, Conair Optimizer dryer control software, coupled with the unique Conair Drying Monitor, automatically adjusts drying conditions so that energy consumption to the absolute minimum required for the application. Using a system like this, PET bottle producers have cut drying-energy requirements to levels below as 0.075 kW/kg of material processed.
But dryers are not the only energy consumers in a blow-molding plant. While vacuum-conveying pumps do not consume anywhere near the energy required to dry PET, all materials must be transported and, thus, any processor can save energy by optimizing conveying efficiency. The industry’s latest material-handling innovation – Wave Conveying™ — has been shown to be 15-30% more energy-efficient than conventional, high-speed, dilute-phase systems over many flow rates and distances.
The last area where a blow molder should look for energy savings – no matter what polymer they process — is in heating and cooling systems. Chiller systems can be tremendous energy hogs if not designed properly for the application and environment.
Today’s chiller designs can incorporate multiple energy-saving features. Variable-speed compressors can continually adjust the speed to match the cooling load in order to eliminate energy waste almost entirely. The variable-speed drive also uses soft-start technology to reduce peak energy demand and extend compressor-motor life. Outdoor air-cooled chillers can be equipped with variable-speed fan motors for energy efficiency as well as reduced noise levels.
Depending on your plant location, remote condensers, which can use low-temperature outdoor air for essentially free cooling during certain times of the year. In addition, when different equipment requires different coolant temperatures, a bridal-loop system can utilize one chiller for the multiple cooling temperatures required. And, finally, an adiabatic cooling tower can be used to reduce the chiller load, saving both energy and water. The adiabatic design keeps the process fluid separate from the water evaporated during the adiabatic process. And, by using ambient air only, or forced air (driven by a variable-speed fan), an adiabatic tower consumes energy only when it is absolutely required, such as when outdoor temperatures are high. The system also minimizes water consumption to a fraction what traditional systems use. This can be highly advantageous in regions where water is in short supply.
Sustainability and the circular economy are watchwords in today’s plastics-processing world and nowhere is this truer than in the blow-molding industry, where consumer and brand-owner environmental consciousness spawns trends toward reducing material use and energy consumption. Understanding the critical role that auxiliary equipment plays in a blow-molding plant and monitoring recent developments in technology and system design, blow molders take advantage of these trends to manage their operations for maximum efficiency and profitability.