Stress analysis simulation to determine glass bottles failure
That the glass thickness and distributions are paramount for a glass container is a statement that comes without question. But most probably we’ve found situations where a container is within the specifications and still breakages occur.
This article will explore a real case situation, where it was requested to Empakglass to evaluate the design of a 750ml bottle, which has a consistent glass thickness and distribution and nevertheless, breakages did occur when internal pressure load was applied.
For the simulations presented on this article, the Empakglass Forming Software was used (Empaktor Suite).
The method applied by Empakglass consisted on the following:
The client provided the current bottles in order to make cuts to determine the glass distribution;
The client provided the glass composition and gob temperature;
The current client’s parison design was simulated using Empakglass’s Forming Software and based on the physical properties of the used glass and the client’s IS machine timing;
A comparison between the real current glass thickness distribution and the one achieved by the simulation was performed. Although there was still a deviation on the settle wave line position It was verified that the achieved simulation profile was quite similar to the actual bottles,
Using the achieved wall thickness profile, a stress analysis for internal pressure load was performed on the model to assess the resistance of the current bottle design.
Following the principle that failure only occurs when generated stresses exceeds the glass strength, all the maximum surface stress values achieved on the above shown Internal pressure simulations are below the glass strength limits, meaningthis design is theoretically acceptable for beverage usage with a Carbonatation of 4Vol-CO and for pasteurization at 70°C.
Nevertheless and although the simulation results theoretically pass the bottle design, a pattern between the breakage origin and the simulation can be determined.
The sidewall area of a bottle is one of the identified critical areas of a container in what regards its resistance to internal pressure. The type of breakage shown on the above pictures has undoubtedly a sidewall origin (settle wave area).
On this particular bottle the settle wave line (associated with lower wall thickness values) is placed on the contact area of the bottle.
Due to the production process limitations (B&B), to have an absolute control on the thickness values achieved on the settle wave area during production is statistically low and therefore will generate bottles that will fail under the tests performed by QC.
The parison design is without any doubt one of the main tools where a glass producer can try to compensate the lack of glass on the settle wave area.
In order to prove the relation between the wall thickness values and the internal pressure resistance, a new parison was developed and again simulated. This parison used the same glass weight. As well, the new parison was also shorter in order to compensate the fast rundown seen on the forming simulations
The forming simulation results show that on the new developed parison, a higher wall thickness on the settle wave area has allowed to shift the weakest area on the container from a glass contact area to a non-contact area on the shoulder.
By increasing 0.55mm on the settle wave area (see figure 4), there was an increase of extra 15% resistance to the internal pressure load even by keeping the same glass weight.
Also, complementing the proper mould design and having in mind that it is critical to protect the surface areas against surface abuse, especially in the contact points, where the probability is higher of having any kind of abuse.
By achieving this is by having good Hot End Coating (HEC) and Cold End Coating (CEC) application. The combination of these two coatings protects the glass surface against friction damage. This damage typically occurs when a blunt hard object slid across a glass surface – for example – two bottles slid against each other in manufacturing line and filling line. Resistance to scratching must be achieved so as to keep the inherent bottle strength high.
For HEC – for carbonated bottles – the level should between 30 to 40 CTU and 25 CTU should be the absolute minimum. It should be guaranteed an even distribution of the coating throughout the glass surface.
For CEC the recommend values are between 9 to 12 degrees of slip angle (determined with AGR Tilt Table). Again, it should be guaranteed an even distribution of the coating throughout the glass surface.
So, summoning this article:
We can have a bottle with a reasonable glass thickness and distribution;
The glass distribution is within Quality AQL’s for that particular container;
Nevertheless, the container fails under internal pressure loads;
By developing a new parison and keeping the same glass weight on this container, it is possible to shift the weakest area on the container from the labeling area (glass contact area) to the shoulder (non contact area).