As far as multi-disciplinary simulation challenges go, predicting how a molten lump of glass is formed into a structurally rigid (although still technically liquid) glass bottle is perhaps one of the most complex things to predict. Manufacturing a glass container involves all modes of heat transfer and structural & fluid mechanics of a material whose viscosity changes by seven orders of magnitude as it is moulded, formed and blown into a bottle or jar.
In a deeply traditional industry, one company is revolutionising bottle manufacturing by deploying multidisciplinary simulation to understand exactly what happens inside the bottle making process and using that information to build better bottle making machines.
Creating a quality product
Bottero Spa is an Italian company that specialises in making machinery for the manufacture of various types of high-quality glass products, including a ‘hollow glass’ division that designs and manufactures bottle and container making machinery. Bottero’s aim is to allow their customers to develop innovative new lightweight glass products that are structurally superior to previous designs (and therefore more durable), but can be manufactured using less raw material and less energy, both to melt the glass and to transport the final container. Ultimately, this creates a quality product at a lower overall cost.
Put simply, Bottero is using multi-disciplinary simulation to discover how to make better bottles, faster than ever before. Marcello Ostorero, Head – R&D, Bottero, who pioneered the use of engineering simulation at Bottero, and Simone Ferrari, who performs many of the STAR-CCM+® software calculations at the heart of the simulation process, put some light on this process.
The challenge: making stronger, lighter bottles
Although we think of glass as a solid, it is actually a supercooled liquid, whose viscosity is so great that its molecules do not move freely enough to form crystals. Managing the way that glass flows and is cooled to its (near) solid state is critical in ensuring the strength of the final container.
In simple terms, a glass bottle is formed by moulding a glob of molten glass (enough to make a single container) into a preliminary bottle shape known as a parison. This parison is then carefully cooled, while being blown into the final bottle shape by a stream of compressed air before the bottle is subjected to a number of downstream processes.
“Our aim is to make lighter bottles, that use less raw materials, less energy to melt and therefore cost less to manufacture. However, since glass is a very sensitive material, we also have to ensure that the bottles are very strong,” says Simone and adds, “Not breaking is the most important thing that a bottle has to do.”
In the past, the robustness of glass containers was ensured by over-engineering them to some extent, thickening the walls of the containers by adding more glass. However, this resulted in heavier products that were less consumer-friendly and more expensive to manufacture. In the past 20 years, thanks to developments in manufacturing technology and to the combined influence of consumer preference and economic necessity, the weight of a typical glass bottle has reduced by over 40% without any loss in structural rigidity or increase in fragility. However, modern lightweight bottles are often much stronger than their older, heavier counterparts.
Stages in the bottle-making process
“In order to make a structurally strong bottle, there are two critical stages. In the first step, a glob of molten glass is moulded into a parison, which is a preliminary bottle shape. After this, the final bottle is formed in another mould,” says Marcello and further shares, “Getting this first shape right is extremely important in ensuring the structural strength of the final bottle; it has to be very precise otherwise the bottle will break during normal usage conditions.”
During the manufacturing process, the glass is cooled from over 1,000 oC to ambient temperature, during which time, the viscosity of the glass increases by seven orders of magnitude (from 100P to 1e9P). If the bottle is cooled too rapidly or unequally, then internal stresses are generated in the walls of the container that reduce its overall durability.
Employing ‘trial and error’ analysis
The significant problem, in this regard, is that it is impossible to understand what actually happens to the molten glass during the moulding process, which happens unseen inside the bottle making machine. Historically, the only way to judge the effectiveness of an extremely complex physical process was to look at the quality of the final product, its glass distribution and try to imagine what might have gone wrong inside the mold.
“The strength of the final product depends to a great extent on how the glass is cooled during the manufacturing process,” says Simone and elaborates, “Although we can measure the temperature of the mould in the glass plant, without simulation, we have little or no insight into the actual temperature of the glass itself. The standard approach in the industry is one of trial and error. Stopping the manufacturing process for months at a time, so that trials can be performed, costs both time and money and doesn’t really give much insight into any problem in the process itself.”
A large bottle manufacturing plant can produce more than two million containers a day, or 25 bottles per second. The cost of these ‘trial and error’ investigations or unresolved problems in the manufacturing process is huge. For this reason, Bottero decided to deploy engineering simulation as a way of gaining detailed insight into the bottle making process, performing simulations that improve both the process itself and the quality of the glass containers produced.
Multidisciplinary simulation using STAR-CCM+ software
“To make a good glass container, you need to actually study the physics of the glass,” says Marcello, relating the key insight that is at the heart of Bottero’s simulation philosophy. “Rather than thinking only of the mold, we oriented our view on the glass itself. The big advantage of the simulation is that it allows you to really understand what is actually happening inside the mould. You cannot see that from physical experiments because the mould is closed, it’s made from cast iron, so you can’t see what is happening inside.”
The glass forming process is also extremely sensitive to changes in machine timing, glass composition and environmental conditions. Simone explains, “As it is nearly impossible to physically visualise what really happens inside the moulds during the different phases, numerical simulation is the only tool available to help better understand the details of the physics as they occur during the process.”
“We have very complex physics,” explains Marcello. “If we look just at the machine production, structural and fluid dynamic aspects can be separated. If we look at the product we have to manage, they can’t. They must be treated together. They are very, very coupled. We produce machines to make containers. The container is made by the cooling down of the molten glass, but it has very hard structural requirements. Understanding the actual temperature of the glass is by far the most important factor in ensuring the strength and quality of the final container. Multidisciplinary simulation using a tool like STAR-CCM+ (software) is the only way that we can achieve that,” he said.
Marcello continued, “To improve a glass container using trial and error alone can involve many weeks of lost production. We can achieve the same thing using simulation in less than a day.”
However, solving the engineering problem is not the only challenge that Marcello and his team had to face. The glass making industry is deeply conservative, sometimes relying on experience gathered over decades and passed down over centuries. Although this experience-based knowledge is always valuable as a starting point for developing new products, it is unsuitable for the sort of intelligent design exploration required to facilitate true innovation.
“Many people from glass plants tell us that it is impossible to simulate the structural resistance of the glass or to simulate the forming process of the glass,” says Marcello and adds, “Maybe at the beginning, we were far-fetched. But now we are consistently obtaining good results and simulation is allowing us to discover important new markets in which we have no competition at the moment.”
The payoff: stronger, lighter bottles
How much better are the products designed through simulation? “Well, for example, not many people realise that the bottle for a sparkling beverage must resist upto at least 13 times atmospheric pressure,” says Simone and further elaborates, “At the same time, we are trying to reduce the weight of the container, save money, and increase the structural performance of the vessel. We can only do that by controlling the glass distribution in the container so that it is as close as possible to the ideal, and thereby avoid defects in the glass.”
“Recently, we helped one of our customers to reduce the weight of a bottle that holds carbonated beverages using simulation,” continues Marcello. “When he tested the bottle that we helped him to produce, he was unable to make the bottle explode. This is an incredible achievement - simulation has pushed the structural performance of the bottle beyond the capability of the testing machines. This would have been impossible without simulation.”
“None of our competitors make extensive use of simulation,” continues Simone. “So this has been really good for our customers. They are starting to pay us to show them how to improve their manufacturing, for example, how to design better moulds and ultimately, produce better bottles. So, we are actually acquiring knowledge from simulation that is superior to that previously gained by experience alone.” In this way, by using simulation in the manufacturing process, Bottero satisfies consumer demand for a unique product by producing a bottle that is well made, lightweight and leaves a lasting impression.