Innovative Blow Molds Reduce Compressed Air Pressure in PET Blow Molding
Technological trends in plastics manufacturing are driving the costs of production down. In industrial PET blow molding specifically, two innovative techniques have had major impacts over the last 15 years: “light weighting” the plastic bottles, and recirculating high-pressure compressed air. Both have helped to improve the energy efficiency of PET blow molding by reducing compressed air requirements dramatically.
In the past, the pieces of PET used to manufacture 16-oz water and carbonated soft drink (CSD) bottles weighed anywhere from 12 to 14 grams. The compressed air pressure required to blow those PET bottles was between 365 and 590 psi (25 to 40 bar), depending on the container shape. Light weighting, a method for reducing the amount of PET needed to produce a bottle, has cut the weight of PET to about 8 grams for bottles of the same size. Consequently, the compressed air pressure requirement decreased, enabling tremendous energy savings.
Since the early 2000s, compressed air recovery systems have also added to the energy savings by recycling high-pressure compressed air for other processes. Recirculated compressed air can be used to produce pre-blow air and service air for the blow wheel, or it can be returned to the plant’s lower pressure compressed air network. In a traditional machine from Sidel, for example, up to 60 percent of the exhausted compressed air can be reused in the blow-molding machine’s low-pressure circuit. While there were struggles initially to achieve the advertised savings (there are a lot of controls required for the system), air recirculation technology is now used frequently, and is saving a lot of money for end users.
The molding section of the blow-molding machine presents the most recent opportunity for achieving substantial energy savings. Blow-molding machines can retrofit or incorporate new blow mold technologies requiring lower compressed air pressures.
Correct Compressed Air System Configuration for PET Blow Molding
Industrial PET blow molding requires a stable supply of compressed air, but maintaining the constancy of the supply is challenging. PET bottle-blowing applications are designed to be fast moving, with the capability to blow up to 2400 bottles per cavity per hour. This can cause large pressure drops in the headers and filters delivering compressed air to the blow-molding machine. In order to compensate, air compressors will operate at much higher levels than necessary. In some situations, manufacturers will have to turn a blow-molding machine down to keep enough pressure for the other machines.
Because of the system dynamics of a PET bottle-blowing plant, the design of the compressed air system is paramount. Typically, plastics manufacturers will design two separate compressed air systems, one for low pressure and the other for high pressure, so that the PET blow molders do not impact other processes downstream. This setup is also ideal for compressed air recirculation, as the manufacturer can take the high-pressure compressed air used to blow bottles and recycle it back into the plant’s low-pressure system.
In addition to separating the high- and low-pressure compressed air systems, localized air receivers and appropriately sized pneumatic components can help reduce pressure drop. The receiver tank should be positioned as close as possible to the point of use (the PET blow molder), and the piping should be as large as possible to handle instantaneous spikes in demand. Under-sized pneumatic components can be retrofitted for higher flow to optimize flow from the storage receiver all the way to the mold. An expert should be consulted for this process, as identifying and measuring pressure drop within a blow-molding machine can be very challenging.
Compressed Air Treatment and Filtration
Most problems caused by compressed air in PET blow molding are related to cleanliness, be it water vapor, oil or particulate. During production, high-pressure compressed air comes into contact with beverage containers—meaning quality is paramount. Compressed air dryers are standard, and it is common to place two sub-micronic filters where the high-pressure compressed air enters the blow-molding machine.
PET blow-molding machines use pneumatic valves to operate, and those valves need to work to the millisecond. Commonly, there are six valves found in a blow molder, including the pre-blow, high-blow, recovery, exhaust, air sweep (balayage), and a valve on the stretch cylinder. If the compressed air supply is even slightly contaminated with oil, water, or particulate, then any number of those valves will not operate as designed, and you will have trouble. Particulates, for example, can impact the flow of compressed air moving through the valve to the detriment of the final product’s shape.
Finding and Fixing Compressed Air Leaks
Whether they are caused by contaminated compressed air moving through the valving, or a more innocent reason, leaks in the compressed air system are inevitable. From our experience—and depending on the plant—many operations do not perform the maintenance required to keep their system running efficiently. As compressed air leaks accumulate, the costs can escalate quickly. And worse still, there might be seven or eight lines of blow molders running all day, making leaks incredibly difficult to detect audibly.
In PET blow molding, leaks regularly occur inside the machine itself—commonly in the tooling, pneumatic circuit and the valves. For instance, an application called “compensation” can be a culprit of leaking high-pressure compressed air. During compensation, a cavity next to the mold is supplied with compressed air through an O-ring, and it presses the two sides of the mold together. Because of a lack of maintenance, these O-rings can leak. Not only does the leak waste high-pressure compressed air, but the bottles produced at the machine are not uniform with the others in size, causing product defects.
Given the potential risk of leaks, a comprehensive leak management program is highly recommended for PET blow-molding facilities. Flow meters positioned at each machine can help identify leaks by showing users average flow rates. If the compressed air flow rate jumps, maintenance can be alerted to find and fix a leak. Vigilant maintenance of the pneumatic circuitry and valving on the blow molders themselves can also help prevent compressed air leaks from springing up.
Blow Mold Bases Designed for Better Venting
COMEP’s Low Pressure Base® can be used to lower the compressed air pressure requirements of blow-molding machines.
The patented design of the Low Pressure Base® can provide tremendous ROIs for customers.
Once system design and best practices are addressed, PET bottle manufacturers can focus on continuous process improvement. The molding section of the blow-molding machine presents another opportunity for achieving substantial energy savings. COMEP’s Low Pressure Base® has been tremendously successful for industrial clients in Europe, Japan, The Middle East, Central America and the U.S., with more than 2500 mold bases sold and installed since 2013. Currently, 0.5-liter CSD and water bottles can be blown at approximately 334 psi (23 bar), and 2-liter CSD bottles can be blown at 295 psi (21 bar), representing a large reduction in standard pressure requirements.
While the Low Pressure Base is approximately two times more expensive than standard mold bases, the savings can be huge, with a typical ROI of six months or less. In addition, the technology completely respects the customer’s original mold base shape, and original production speed. In most cases, the Low Pressure Base definitively limits the risks of stress cracking (better PET “naturally” stretch in the base, hence there is less amorphous material on and around the preform crystallized injection point). Finally, the technology can increase the general mechanical performances of the container.
Super Vented Molds® Present Opportunity for More Energy Savings
Developed in 2013, the Super Vented Mold is COMEP’s latest innovation. Blow mold machines need to vent the excess compressed air used during the bottle-blowing process. Traditional blow molds mainly vent compressed air through the parting line—or the seam—of the mold itself. In the Super Vented Mold, additional vents have been engineered into the ribs of the mold. The design ensures the PET is in contact with the mold surface much faster, making the process especially efficient when at high production speed, as the contact time is increased.
COMEP’s Super Vented Mold® is designed with additional vents in the ribs of the mold.
The quicker venting also reduces the required compressed air pressure it takes to blow the bottle. A 1.5-liter water bottle weighing 20.5 grams, for example, needs only 218 psi (15 bar) of compressed air pressure for full expansion. In addition, the process speed remains the same, as does the bottle’s shape. The molds can be added to blow-molding machines, such as those made by KHS, Krones, and Sidel, but they require extra machining for integration. COMEP sells the blow molds to both OEMs making blow-molding equipment, and to end users looking to retrofit their equipment for energy savings.
In order to take advantage of newer, more energy-efficient technologies, however, the compressed air system at the plant needs to be configured correctly. This means having the proper compressed air treatment equipment in place, along with an appropriate control strategy for the supply system. It also means implementing a rigorous preventative maintenance program for the PET bottle-blowing machines, because air leaks can cause major problems for system efficiency and process integrity.
About SKV Services
Founded in 2014 by PET blow molding industry veterans, SKV Services provides service, rebuild, and rigging for blow-molding machines. As a consultancy for plants with blow-molding applications, SKV can help optimize processes by retrofitting machines and training staff on how to properly maintain equipment for efficiency and reliability.
Company President Stephane Larcade has worked in the blow-molding industry for nearly 20 years, primarily in North America. Vincent Duvernois has 30 years of experience in the industry, and worked for Sidel previous to SKV. To date, the company has grown to 12 people, and its base of operations is located in Virginia. SKV partners with COMEP to bring energy-efficient solutions to industrial PET blow-molding applications.
About COMEP SA
Founded in 1998, Comep is an independent company based in Cognac (France), producing PET blow molds for all OEM machines regardless of the technologies and standards, and providing the related services as bottle design, prototyping, worldwide technical assistance and R&D for process improvement. Its own continuous production workshop supplies molds all around the world.
For more information, contact Vincent Duvernois, tel: (404) 513-3318, email: Vincent@skvservices.com, or visit www.skvservices.com. Additional information about COMEP can be found at www.sa-comep.com.
To read more about the Plastics Industry, please visit www.airbestpractices.com/industries/plastics.