Irish Food Packaging Information Service
Properties of glass when used as a packaging material for foodstuffs
|Advantages of Using Glass Containers as Food Packaging Materials|
|Disadvantages of Using Glass Containers as Food Packaging Materials|
|Glass containers strength|
|Classifications of Glass Defects|
Advantages of Using Glass Containers as Food Packaging Materials
- No odour
- Versatility in shape and colour
- Suitable for use in the microwave
- Excellent clarity
- Glass containers are distinctive, convenient and practical
- Glass containers are used for premium quality foods
Disadvantages of Using Glass Containers as Food Packaging Materials
- Weaknesses in glass containers are the single biggest factor limiting the use of glass as a food packaging material.
- Expensive food packaging material
The strength of glass containers is monitored continuously during the manufacturing process.
On-line Inspection of Glass Bottles includes:
Following the annealing operation, the bottles undergo an inspection process.
- Bottle Spacer. This machine is pre-set to create a space between the bottles on the conveyer to avoid bottle to bottle contact.
- Squeeze Tester. Each bottle is passed between discs that exert a force to the body of the container. Any obvious weakness or crack in the bottle will cause it to fail completely with the resulting cullet being collected by a return conveyor running underneath.
- Bore Gauger. The internal and external diameter at the neck finish entrance to the bottle and the bottle height are measured. Bottles outside specification are automatically rejected by means of a pusher positioned downstream from the gauger.
- Check Detector. Focuses a beam of light onto areas of the container where defects are known to occur from previous visual examinations, any crack will reflect the light to a detector, which will trigger a mechanism to reject the bottle.
- Wall Thickness Detector. This test uses dielectric properties of the glass, the wall thickness can be determined by means of a sensitive head which traverses the body section of the container. A trace of the wall thickness is then obtained and bottles falling below a specified minimum will be automatically rejected.
- Hydraulic Pressure Tester. A test carried out on bottles which will be filled with carbonated beverages and gauges the internal pressure of every bottle before it is packed.
- Visual Check. Bottles are passed in front of a viewing screen as a final inspection.
Glass failure is usually as a result of thermal shock or impact stresses. Each glass container has a maximum thermal expansion threshold and a maximum vertical load stress, which it can withstand without cracking. These values should be known before it is used for a particular application.
The shape of the container will influence its strength, smooth edges result in the formation of a stronger container than one with rectangular or sharp edges
There are 6 broad classifications of glass defects
- Non-glass inclusions
- Dirt, dope, adhering particles or oil parks
- Freaks and malformations, and
Defects are classified as
- Critical, those that are hazardous to the user and those that make the container completely unusable.
- Major, those that materially reduce the usability of the container or its contents
- Minor, those that do not affect the usability of the container, but detract from its appearance or acceptability to the customer.
Critical Defects in Glass Bottles or Containers
- Stuck Plug. A piece of glass, usually very sharp, projecting inwards just inside the neck bore
- Overpress. Is a defect where a small ridge of glass has been formed on the sealing surface of the finish
- Split. An open crack starting at the top of the finish and extending downward.
- Check. A small, shallow surface crack, usually at the bore of the container
- Freaks. Odd shapes and conditions that render the container completely unusable. Bent or cocked necks are a common defect of this type.
- Poor Distribution. Thin shoulder, slug neck, choke neck, heavy bottom are terms used to describe the uneven distribution of glass.
- Soft Blister. A thin blister, usually found on or near the sealing surface. It can however show up anywhere on the glass container.
- Choked Bore. Here excess of glass has been distributed to the inside of the finish or opening
- Cracks. Partial fractures, usually found in the heel area.
- Pinhole. Any opening causing leakage. It occurs most often in bottles with pointed corners.
- Filament. A hair-like string inside the bottle.
- Spike. Spikes are glass projections inside the bottle.
- Bird Swing. Is a glass thread joining the two walls of the container
Some Major Defects Commonly Found in Glass Containers
- Chipped Finish. Pieces broken out of the top edge in the manufacturing process.
- Stone. Small inclusion of any non-glass material
- Rocker Bottom. A sunken centre portion on in base of the container
- Flanged Bottom. A rim of glass around the bottom at the parting line
Some Minor Defects Commonly Found in Glass Containers
- Suncker Shoulder. Not fully blown, or sagged after blowing
- Tear. Similar to a check, but opened up. A tear will not break when tapped, a check will.
- Washboard. A wavy condition of horizontal lines in the body of the bottle.
- Hard Blister. A deeply embedded blister that is not easily broken.
- Dirt. Scaly or granular nonglass material.
- Heel Tap. A manufacturing defect where excess glass has been distributed into the heel
- Mark. A brush mark is composed of fine vertical laps, e.g. oil marks from moulds.
- Wavy bottle. A wavy surface on the inside of the bottle.
- Seeds. Small bubbles in the glass
- Neck ring seam. A bulge at the parting line between the neck and the body.
Tolerances for variation in any given characteristic will vary depending on size and container design. The British Glass Manufacturers Confederation has agreed tolerances on glass containers. Tolerances are laid out in their handbook, Tec 9 booklet "General Guidelines for the Use of Glass Containers", Tec 4 booklet "Glass container Tolerances" or Tec 6 booklet "Accurate Determination of Glass Container Capacity"
The following are examples of some permitted tolerances:
Vertical load control values
|Glass bottle||Vertical load|
|Nominal Capacity (ml)
up to and including
|Nominal capacity (ml)
up to and including
These tolerances are in accordance with EC directive 75/107.
Body and Height Dimensions
|Body/Diameter Tolerances||Height Tolerances|
|D (mm) up to and including||TD (mm)
|H (mm) up to and including||TH (mm)
Control Values for Verticality
|Height H (mm) up to and including||Tv (mm)
Minimum glass thickness values
Minimum glass thickness (mm)
|Non-refillable bottles||Refillable bottles||Surface protected non-refillable bottles|
|Up to 60||1.1||1.5||0.8|
|>61 £ 71||1.4||1.8||0.9|
|>71 £ 81||1.5||1.9||1.0|
|>81 £ 96||1.7||2.0||1.1|
|>96 £ 110||1.8||2.2||1.3|
Design headspace for products not pasteurised in bottle
|Nominal Capacity (ml)||Maximum design headspace (ml)|
The headspace specified allows for temperature rises of up to 50° C from 5° C. The increased headspace for pasteurised products, where temperature rises of up to 80° C may be experienced should be kept as small as possible and should not exceed 1.66 times the appropriate values listed above. Bottles for carbonated soft drinks and carbonated water bottles are required to be designed to more restricted values than those shown above.
Representative glass weights for round and irregular bottles
Impact strength is measured on-line and each bottle is tested individually. The object of the test is to pass the bottle through 2 vertical plates/pads, which squeeze/stress the bottle to a pre-determined level. Any bottle, which can not tolerate the predetermined stress level will break and is then discarded/recycled.
Major Suppliers of Glass Packaging Materials
|Unit 1B, Three Rock Road,|
|Sandyford Industrial Estate|
|Sheffield, S10 2UA, UK|
|The Irish Glass Bottle Co. Ltd.,|
|South Bank Road|
|Fox and Geese House,|
|Glass bottles and jars|
|Monk Bretton, Barnsley,|
|South Yorkshire, S71 2QG|
|Glass containers, sleeving, coating|
|United Glass Ltd.,|
|St. Albans, Hertfordshire|
|AL3 6NY, UK.|
|Glass containers and bottles|