Impact Testing for Glass Packaging and How to Prevent Breakage on Impact

Alper Karairmak, Aegg's Technical Engineering Manager for glass packaging gives the lowdown in this technical article...

Glass containers can hit each other or an object during filling lines and afterwards during use. In order to prevent breakage due to impact, an impact test is applied during production. This test is used to predict the glass container's resistance to impact breakage in service, by multiple-point impacts on empty samples, commonly carried out with a pendulum impact tester.

The container is usually supported in an upright position, and a swinging pendulum is set to strike the selected point at right angles to the surface. The weight of the pendulum hammer multiplied by the vertical height through which it descends before impact is a direct measure of the impact energy.

A specific impact-testing device produced in the United States (AGR pendulum impact tester) has, in practice, become a reference in the glass industry. For a regular weight and size of hammer, such as when a standard AGR impact tester is used, it is accepted practice to express the results in terms of the speed of travel of the hammer, e.g. inches per second IPS or cm/s.

As can be seen in the photo above, the speed of the hammer can be set in cm/s or inches per second (IPS) on the pendulum impact tester. 1 IPS = 2.54cm/s

It is generally expected to withstand an impact force of min 30ips (76.2cm/s) for glass containers. The impact test is applied to the shoulder and heel contact points of the glass container. It is common to start from the mould seam and is applied from 4 points at 90° intervals

Impact resistance

Impact resistance is related to the design of the glass container. Points to be considered during design;

• Impact resistance increases as the height of the heel contact point increases. The impact strength of jar A will be lower than jar B.
• If the shape of the shoulder area is sharp, the impact strength decreases. Shoulder design B is better than A.
• The impact strength of round products is also higher than square products.

Glass Container Defects

Defects in the production of glass containers affect the impact resistance. For example:

• Checks in the stippling or heel engravings will reduce the impact resistance of the heel contact point.
• Checks in the shoulder or finish will reduce the impact strength at the shoulder contact point.
• Uneven glass distribution in the horizontal cross-section, one side being thin and the other side being thick, will reduce the impact strength.

Impact forces

A force is developed between a glass container and the object that hits it when the container is impacted. Impact forces increase with:

• Increasing striking velocity, as the speed of the filling line increases, the impact force will increase.
• Increasing mass, since the filled product has more mass, the impact force will increase compared to the empty product.
• Increasing stiffness, stiffness is a measure of rigidity at the impact point. Dependent on impact location increases with decreasing diameter, increases with increasing wall thickness

Tensile Stresses 

Three tensile stresses act on a round glass container in the event of an impact. 

1. Contact Stress

Contact stresses are the highest magnitude stresses generated during an impact and result from local deformation of the glass surface which is in immediate contact with the impacting bottle

2. Flexure Stress

Flexure stresses are generated on the inside surface of the glass container due to inward deformation of the glass wall when impacted. Depressing the glass wall inward, causes the inside surface to stretch and develop tension stress. Flexure stresses are the second highest stresses from impacts and are localized opposite of the impact location.

3. Hinge Stress

Hinge stresses are located on the outside surface and are the lowest in magnitude in comparison to contact stresses and flexure stresses. Where contact and flexure stresses were really localized at the impact point, hinge stresses cover a much larger area.

Prevention of Impact Fractures on Filling Lines

It should be remembered that repeated impacts on the container, though not causing immediate breakage, may progressively damage the glass surface,  therefore reducing the strength and shortening the life. Close cooperation between the machinery supplier, container manufacturer and packer will assist greatly in achieving optimum trouble-free service from the container. Taking proper precautions during processing not only improves safety but also raises filling line performance.

It is strongly recommended that the likely ‘impact’ points on the filling line are carefully investigated when filling lines are updated or sped up to ensure that increased impact loads on containers are avoided.

To avoid impact and abrasion, guide rails and line dividers should be made of, or be protected by, hard-wearing plastic or similar material, which should be checked at intervals to see that the covering has not worn away. Rails of semi-circular cross-section offer less resistance to the flow of bottles than those of rectangular cross-section.

When opening shrink-wrapped packs with a knife, care should be taken not to damage the containers.

Damaged, cracked or chipped containers should be separated from sound containers. Such containers should be disposed of carefully. Gloves should be worn whenever broken glass is handled.

Synchronise conveyor speeds, filling machine speeds (including input and output worm feeds), and star-wheels for smooth flow. If the filling line conveyor runs too fast, excessive impacts may occur when the line stops or when the flow changes direction.

For more information on this article, please get in touch

This article is part of Aegg's Technical Articles series.  Click the link to read more articles from the series.

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