GLASS TECHNICAL TRAINING CENTER

Glass Containers Manufacturing Process

What is glass?

           Glass is made by melting a mixture of minerals and compounds, and then cooled down without crystallization. Glass can be classified into several categories depending on materials used and the properties of different applications. Containers made of glass are either transparent or translucent. Glass containers are healthy safe because they are chemically inert and do not react with products inside. In addition, glass containers help to preserve quality and taste of products. Flint, amber and green glasses are widely used for producing glass containers. Amber and green glasses are unique because they are able to filter or prevent ultra-violet light from reacting with products inside the containers. Products, for examples, beer, drink tonic and medicine having ingredients susceptible to react with ultra-violet light are usually packaged in amber or green glass containers.

Glass containers can be categorized into 4 types according to Industrial Standard TIS 501-2527 as following.

1. Type I glass container: borosilicate glass or “hard glass” with boron oxide more than 5 percent by weight. This type of glass container exhibits good resistance to impact force and heat. It also can tolerate to thermal shock (rapid change in temperature). In addition, type I glass container exhibits excellent corrosion resistance especially against alkaline solution such as sodium hydroxide (NaOH). Type I glass containers can be found in daily uses such as the lid of an oven, a glass lid for Sukiyaki pod, lighting cover for indoor and outdoor uses and headlamp cover for automobiles. For laboratory use, type I glassware is common.
   Although type I glass container exhibits good resistance to many corrosive chemicals, it can be dissolved in hydrofluoric acid (HF), hot phosphoric acid (H₃PO₄) and concentrated alkaline solution (conc.NaOH).
2. Type II glass container: or "soda lime glass”. Type II glass is typically made from limestone, soda ash and silica sand as major ingredients. Surface layer approximately 0.1 to 0.2 micron thick of type II glass container is pH neutralized through a special treatment. Type II glass container is ideal packaging for acidic or pH-neutral injectable drug. However, it can be used as container for basic injectable drug if proven to stabilize in the container. Type II glass container is also used for alcoholic drinks, beers, soft drinks, pharmaceutical products, food and beverages. Tableware is also made of Type II glass as well.
3. Type III glass container: or "soda lime glass” with no surface treatment. Without surface treatment, type III glass containers are not used for injectable drugs.
4. Type NP glass container: or "soda lime glass" for externally use drugs or edible medicine. 

Glass containers can also be categorized according to their characteristics and applications as following.

1. Returnable or reusable glass container: glass containers such as soft drink bottles and water bottles that can be reused several times.
2. Non-returnable or one-way glass container: glass containers typically lightweight that can only be used one time and crushed as cullet for remelting process. 
3. Narrow mouthed glass container: glass containers with finish diameter smaller than 38 mm. such as beer bottles and sauce bottles.
4. Wide mouthed glass container: glass containers with finish diameter larger than 38 mm. such as food jars.
5. Pressurized glass container: glass containers for pressurized or carbonated products such as soft drinks, beer and sparkling water.
6. Non-pressurized glass container: glass containers those are not subjected to internal pressure or gaseous products.

Among these types of glass containers, they can be color printed for decoration or used as plain glass containers as requested by customers. 

Suggestions on how to use glass containers

1. Thermal shock : glass container can withstand thermal shock on the average of 42 degree Celsius difference in a short time. However, if subjected to change in temperature, gradual increase or decrease in temperature is recommended.
    
2.  Blooming or weathering : it is not unusual to observe white stain on glass containers. White stain is known as weathering or blooming. It results from a chemical reaction between the surface of glass containers with carbon dioxide, sulfur dioxide and moisture in the air resulting in sodium carbonate and calcium carbonate on the surface. White stain can be easily removed using hot water. If extensively spread on the surface, cleaning glass containers with 5%-vol hydrochloric acid or washing them with 5%-vol caustic soda at 70 to 80 degrees Celsius is recommended.
   
   It is also recommended to use glass containers after receiving from manufactures. If necessary to store them for 3 to 4 months, the containers should be kept in the shade and well ventilated area. If necessary to store more than 4 months, customers should inform manufacturers to apply  surface coating during the manufacturing process to slow down blooming or weathering processes.
    
3. Leakage at finish : it is possible to find that liquid or product inside the glass container leaks out from the container around the finish after hot sterilization process. It is possible that there is not enough head space for liquid or product to expand once the containers exposed to the heat. In this case, leaving enough head space allowing for expansion of liquid or product is necessary.
    
4. Transportation of glass containers : glass containers can survive considerable level of compression. However, they are susceptible to impact force. It is necessary to avoid severe impact of glass containers during transportation or filling process. Coating outside surface of glass containers with commercially available material can reduce surface damage during transportation and hence decrease a chance of fracture due to impact. However, the coating material can be washed away upon several uses.

 Capping for glass containers. 

Caps are indispensable for glass containers. Depending on applications and uses, caps can be categorized as following.

1. Crown Cap: used for soft drink, beer and sparkling water bottles.
    
2. Maxi Cap: used for drinking water bottles. 
    
3. Twist - Off Cap: used for food jars with sterilization process.
    
4. Screw Cap: used for food jars that do not need for sterilization.
    
5. ROPP Cap: used for energy drink bottles, pharmaceutical bottles and carbonated bottles.
    
6. Cork Cap: used for wine bottles.

Glass containers and the environment. 

          Glass containers can be totally recycled and remelted endlessly compared to other packaging materials such as PET bottles or steel cans. Recycled glass containers are crushed into cullet. On average, glass container manufacturers use cullet in conjunction with other raw materials. Ten percent of cullet in batch recipe can save approximately 2% in melting energy. Use of cullet can also help to reduce the use of natural resources and minimize waste hundreds of thousands tons per year. 

Raw materials for glass container manufacturing. 

Raw materials for container glass manufacturing come from various sources; both domestic and imported. On arrival to container glass plants, raw materials are kept as stock piles or directly loaded into silos nearby batch houses. Raw materials can be divided into major and minor raw materials. Major raw materials including glass sand, soda ash, limestone, feldspar and cullet are converted by the melting process into the main glass structure. Minor raw materials such as sodium sulphate, coke dust, selenium, iron oxide, iron chromite and selenium are used to adjust reactions and color during the melting process.

1. Glass sand : or silica sand contains more than 99.5% of silicon dioxide (SiO₂), which is a primary structure of glass container. Glass sand can be found in Rayong and Chumphon provinces of Thailand. Glass sand with high level of iron oxide (Fe₂O₃) is used for the manufacturing of amber and green glass containers. On the other hands, glass sand with low level of iron oxide is preferred to the manufacturing of flint glass containers.
    
2. Soda ash : or sodium carbonate (Na₂CO₃) is a type of salt found in soil or can be synthesized from sodium chloride (NaCl). Soda ash helps to reduce melting point of silica sand. It is necessary to import soda ash from foreign sources such as Lake Magardi in Kenya and China. Glass with soda ash as an ingredient is generally called soda lime glass. 
    
3. Limestone : or calcium carbonate (CaCO₃) is a natural raw material. It helps to improve the strength and chemical resistance of glass. Sources of limestone can be found in Ratchaburi and Saraburi provinces of Thailand.
    
4. Feldspar : is a natural raw material providing aluminum oxide (Al₂O₃) for glass. It helps to increase the durability of glass. Feldspar can be found in Ratchaburi and Kanchanaburi provinces of Thailand. 
    
5. Cullet : recycled glass containers are collected from commercial and residential sectors. They are initially separated by colors. Contaminants such as papers, caps, metallic rings etc. will be separated before recycled glass containers are crushed into cullet. Use of cullet as a supplementary raw material will help to save melting energy. Cullet ratio in the batch recipe can be varied depending on availability, glass colors and variation in cullet composition. Cullet ratio for Thai Glass Industries is generally between 65 and 85%.

Besides major raw materials listed previous, following minor raw materials are also indispensable to glass container manufacturing.

Sodium Sulphate : used for eliminating gas bubbles inside molten glass.

• Coke dust : used as reacting agent for melting, refining and coloring reactions. 
• Selenium : used for eliminating unwanted coloring effect in flint glass. 
• Iron oxide : used for providing amber color in glass. 
• Iron chromite : used for providing green color in glass.

 Production of glass containers. 

1. Incoming raw material inspection and storage: raw materials arrive at glass container plants are sampled and inspected according to their specifications. Once passed the inspection, they will be stored properly for subsequent uses.

2. Weighing and mixing: each raw material is weighed according to a batch recipe using a computerized system and will be transferred to a mixer. Mixed raw material or batch will be stored in a silo and ready to be loaded into a furnace. Batch is continuously fed into the glass furnace operated using natural gas, oil and electricity. Batch is generally melted at 1,500 degrees Celsius and at this state gas bubbles such as CO₂ and SO₃ generated by melting reactions slowly evaporate away from the molten glass surface. Refined molten glass delivered through working end and forehearth is then ready for forming operation. 

3. Forming: At the end of forehearth, molten glass is cut into a lump of glass called gob to proper size, shape and temperature as needed by forming operation. Gob is then delivered into a blank side to form an initial shape called parison. At this stage, complete bottle finish is formed. Parison is inverted into a mould side to form a complete bottle shape. According to modern glass container manufacturing, there are three common forming processes as following.

     3.1 Blow & Blow (BB): in this process, parison and bottle cavity are formed by compressed air. This technique is used for manufacturing narrow mouthed glass containers such as soft drink, beer and liquor bottles. 

     3.2 Press & Blow (PB): in this process, only parison cavity is formed using a plunger. Final cavity on the mould side is formed by compressed air as in the BB technique. This technique is used for manufacturing wide mouthed containers such as food jars, chicken essence bottles and coffee jars. 

     3.3 Narrow Neck Press & Blow (NNPB): this technique is similar to the PB technique. The plunger used in this process is generally long compared to the BB process. Precise parison cavity and glass distribution is easily obtained at this stage. Parison is subsequently blown to form a final bottle cavity with better glass distribution compared to the BB technique.  This technique is used to manufacture modern lightweight and non-returnable narrow mouthed containers.

Bottles obtained from the forming machine are carefully monitored and inspected for weight, shape and appearance defects by forming staffs to ensure that minimum defective bottles will enter to the next process.

4. Annealing: hot glass containers from the forming machines will be transported down the conveyor into the annealing Lehr. The purpose of this process is to slowly decrease bottles temperature to minimize residual stress in containers. Residual stress in containers can result in immediate breakage or weakening of containers strength for subsequent uses. Containers exiting the annealing lehr are also sprayed on outer surface to improve surface finish and durability of glass containers.

5. Inspection and Quality Assurance. 

5.1 Automatic finish product inspection: all containers from the annealing lehr are physically screened by series of automatic finish product inspecting machines. Appearance defects and inclusions are detected throughout a container. Defective bottles are then automatically rejected for remelting process.

5.2 Quality Assurance - Lab testers: besides automatic inspecting machines, glass containers are sampled and tested for other properties such as thermal shock, hydrostatic pressure and impact resistances. Glass containers with unsatisfactory test results will be quarantined for repeating test or scraping. This is to insure that the quality of every glass container is satisfactory and meet customer satisfaction.

6. Labeling and decoration: upon requests from customers, label and decoration can be applied to glass containers using applied ceramic labeling  (ACL) technique. Ceramic colors are applied to glass containers and heat treated at the temperature approximately 600 degrees Celsius.  

7. Packing: glass containers ready for delivery to customers will be palletized and shrink wrapped using automatic machines. Glass container pellets are stored in well managed and hygienic warehouse and ready to be delivered to customers.

Packaging design.