Stronger, lighter glass bottles

In a deeply traditional industry, one company is revolutionizing bottle manufacturing by deploying multidisciplinary engineering simulation software to understand exactly what happens inside the bottle making process and using that information to build better bottle making machines.

Bottero Spa is an Italian company that specializes 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, but can be manufactured using less raw material and less energy.

Put simply, Bottero is using multidisciplinary simulation to discover how to make better bottles, faster than ever before. Heading this effort are Bottero’s Head of R&D, Marcello Ostorero, who pioneered the use of engineering simulation at Bottero, and Simone Ferrari, who performs many of the STAR-CCM+ calculations at the heart of the simulation process.

Making stronger, lighter bottles

Although glass seems a solid, in reality it’s a supercooled liquid, whose viscosity is so great that its molecules do not move freely enough to form crystals. Managing the way 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 molding 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,” says Ferrari. “However, since glass is a very sensitive material, we also have to ensure that the bottles are very strong. Not breaking is the most important thing 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 percent without any loss in structural rigidity or increase in fragility.

“In order to make a structurally strong bottle, there are two critical stages. In the first step, a glob of molten glass is molded into a parison, which is a preliminary bottle shape. After this, the final bottle is formed in another mold,” says Ostorero. “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°C 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.

The significant problem in this regard is that it is impossible to understand what actually happens to the molten glass during the molding 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 Ferrari. “Although we can measure the temperature of the mold 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 you can perform trials, costs both time and money but doesn’t really give much insight into any problems 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 to gain detailed insight into the bottle making process.

Multidisciplinary Simulation

“To make a good glass container, you need to actually study the physics of the glass,” says Ostorero, 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 mold.”

The glass forming process is also extremely sensitive to changes in machine timing, glass composition and environmental conditions. Ferrari explains: “As it is nearly impossible to physically visualize what really happens inside the molds 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 Ostorero. “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+ is the only way that we can achieve that.”

Complete simulation with STAR-CCM+ of the glass forming process, starting from the initial glass gob to the final shape.

However, solving the engineering problem is not the only challenge that Ostorero 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, many, many people from glass plants tell us, ‘Oh, it is impossible to simulate the structural resistance of the glass. Oh, it’s impossible to simulate the forming process of the glass. You are crazy,’” says Ostorero. “Maybe at the beginning, we were crazy. But now we are consistently obtaining good results and simulation is allowing us to discover important new markets in which we have no competition for the moment.”

Exceeding Customer Expectations

So, how much better are the products designed through simulation?

“Recently we helped one of our customers reduce the weight of a bottle that holds carbonated beverages using simulation,” continues Ostorero. “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 Ferrari with a smile on his face. “So this has been really, 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 molds and ultimately produce better bottles. So we are actually acquiring knowledge from simulation that is superior to that previously gained by experience alone.”

An eye-catching bottle that sets itself apart from the crowd can be a conversation starter at a gathering or just something that is fun to look at. Bottero understands this and realizes the importance of a creative bottle design to the consumer. 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.

www.cd-adapco.com

Stephen Ferguson is the Marketing Director CD-adapco/Siemens PLM Software, who holds a BEng in Aeronautical Engineering and an MSc in CFD. He spent the early part of his career working as a consultant engineer for WS Atkins and as a member of the BMW Rover Powertrain Design Analysis team.