How Packaging Materials Are Made: Glass, Metal Cans, and Plastics

Every food product you pick up from a store shelf sits inside a container that went through a precise, multi-step manufacturing process. Whether it’s a glass jar of pickles, a metal can of beans, or a plastic bottle of juice, the packaging is engineered to protect food, extend shelf life, and maintain quality. Understanding how these packaging materials – glass, metal cans, and plastics – are manufactured is essential for anyone involved in food processing and engineering. Let’s break down each process step by step.

Glass container manufacturing 🔗

Glass has been used to store food for thousands of years, and it remains one of the most trusted packaging materials. It is chemically inert, meaning nothing passes from the glass into the product and nothing penetrates from the outside. It is also 100% recyclable without any loss in quality. Modern glass container manufacturing is a continuous, highly controlled industrial process.

Raw materials and batch preparation 🔗

Glass production starts with raw material selection. The primary ingredients are silica sand (about 70% of the mixture), soda ash (sodium carbonate), and limestone (calcium carbonate). To these, manufacturers add recycled crushed glass known as cullet, which helps lower the melting temperature and saves energy. Colourisers or decolourisers may also be added depending on the desired glass colour – green, amber, or flint (clear).

These raw materials are weighed in precise proportions, mixed thoroughly in a grinding machine, and stored in large silos. This mixture is called a batch or frit. The batch is then fed at a controlled rate into the furnace.

Melting in the furnace 🔗

The batch enters a furnace that operates at temperatures up to 1,575°C (2,867°F) and runs continuously, 24 hours a day, 7 days a week. The furnace has a typical service life of about 10-12 years. Inside, the raw materials melt and fuse into a homogeneous molten glass. This molten glass then flows through conditioning zones called forehearths, where its temperature is gradually reduced to an optimal level for forming – typically between 1,050°C and 1,200°C.

Gob formation and container shaping 🔗

At the end of the forehearth, the molten glass is cut into precise portions called gobs using shear blades. Each gob has a predetermined weight – just enough to form one container. The gobs are then gravity-fed through troughs and deflectors into the forming machine, known as an IS (Individual Section) machine.

There are two primary methods of forming glass containers:

Blow-and-blow method: Used mainly for narrow-neck containers like bottles. The gob drops into a blank mould, where compressed air first forms the neck finish and then creates a partially shaped hollow form called a parison. The parison is then transferred to a final blow mould, where air inflates it to the exact shape of the container.

Press-and-blow method: Preferred for wide-mouth containers like food jars. Instead of air, a metal plunger presses the gob into the blank mould to create the parison. This method allows faster production speeds and better glass distribution. A variation called NNPB (Narrow Neck Press and Blow) is also used for making bottles.

Annealing and surface treatment 🔗

Freshly formed glass containers are extremely hot and have internal stresses that could cause them to crack. To address this, containers pass through an annealing lehr – a tunnel-like oven that reheats the glass to about 580°C, then gradually cools it over 20 to 60 minutes depending on wall thickness. This controlled cooling process equalises internal stresses and strengthens the container.

After annealing, the containers receive surface coatings. A hot-end coating of tin oxide is applied before annealing to improve adhesion, and a cold-end coating of polyethylene wax is applied after annealing to reduce scratching and prevent containers from sticking together on the conveyor line. The containers are then inspected for defects and packaged for shipment.

Metal can manufacturing 🔗

Metal cans are among the most reliable food packaging solutions available. They provide an excellent barrier against light, oxygen, and moisture, and can withstand the high temperatures required for sterilisation. The most commonly used metals for food cans are tinplate, tin-free steel (TFS), and aluminium.

What is tinplate? 🔗

Tinplate is essentially a thin sheet of low-carbon steel coated with a microscopic layer of tin. The steel provides structural strength, while the tin coating prevents corrosion and protects the food from reacting with the metal. The tin layer is typically applied by electroplating at high speed, with a thickness of about 0.4 to 1.5 micrometres.

The manufacturing process begins with producing low-carbon steel sheets through a rolling process, where temperature and pressure are carefully controlled to achieve the correct thickness and mechanical properties. After rolling, the steel sheets undergo electrolytic tinning, where electric current deposits a uniform, dense layer of tin on both surfaces. The tinned sheets are then treated with a chromate solution for added corrosion resistance and oiled to prevent damage during handling.

Tin-free steel (TFS) is an alternative that uses chromium or polymer coatings instead of tin. It is lighter and more cost-effective but cannot be soldered and must be welded or cemented.

Three-piece can manufacturing 🔗

The three-piece method is the traditional approach and still accounts for a large share of food cans. The three pieces are the body, top end, and bottom end.

The process begins with flat tinplate sheets that are cut to size. If decoration is needed, the exterior is printed or coated at this stage. The sheet is then rolled into a cylindrical body, and the edges are joined – in modern production, this is done through electric resistance welding rather than soldering, which has been phased out due to lead contamination concerns.

After welding, the internal and external surfaces are coated with protective lacquers. Internal lacquers are critical – they prevent direct contact between food and metal, avoiding chemical reactions that could affect taste, colour, or safety. The coated can body is dried, then flanged (the edges are curled outward) so that the ends can be attached. The bottom end is seamed on by the manufacturer, while the top end is seamed on after the food processor fills the can.

Two-piece can manufacturing 🔗

Two-piece cans consist of a seamless body-with-bottom piece and a separate top end. These are manufactured using either draw and redraw (DRD) or draw and wall ironing (DWI) processes.

In the DRD process, a flat disc of tinplate or aluminium is pressed into a shallow cup shape and then redrawn into a taller, narrower container through one or more additional pressing stages. This method is common for shallow food cans.

In the DWI process – widely used for beverage cans made from aluminium – the drawn cup passes through a series of ironing rings that progressively thin and elongate the walls while keeping the base thick. This creates a lightweight, strong container with very uniform walls. The can is then trimmed, washed, coated internally, and printed externally before the top end is seamed on after filling.

Double seaming and quality control 🔗

The process of attaching ends to the can body is called double seaming. It involves interlocking and compressing five layers of metal (three from the body flange and two from the end curl) to create an airtight, hermetic seal. A sealing compound applied inside the end curl ensures a leak-proof joint. Seam integrity is critical – even microscopic leaks can compromise food safety and shelf life, so manufacturers use rigorous inspection protocols including visual checks, teardown measurements, and pressure testing.

Plastic packaging manufacturing 🔗

Plastics dominate food packaging today because of their versatility, light weight, and low cost. Different types of polymers – including polyethylene terephthalate (PET), high-density polyethylene (HDPE), polypropylene (PP), and low-density polyethylene (LDPE) – are used depending on the product requirements. The manufacturing of plastic containers relies on several key processes.

Extrusion: the starting point 🔗

Most plastic packaging begins with extrusion. In this process, polymer pellets (also called resin) are fed into a heated barrel containing a rotating screw. The screw mixes and pushes the pellets forward while the heat melts them into a uniform molten mass. This molten plastic is then forced through a shaped opening called a die.

For flat film production, the molten plastic flows through a flat die to create thin sheets that are wound into rolls – used for wrapping, pouches, and labels. For blown film production, the melt exits a circular die and is inflated with air to form a bubble or tube of film. This method is commonly used for producing bags, wraps, and liners. Precise temperature control throughout the barrel, die, and cooling zones is essential for consistent quality.

Blow moulding 🔗

Blow moulding is the primary method for producing hollow plastic containers like bottles and jars. It comes in three main types:

Extrusion blow moulding (EBM): The machine first melts the plastic and extrudes it through a die to form a hollow tube called a parison. The parison is captured inside a water-cooled mould, and compressed air is blown in, expanding the plastic against the mould walls. After cooling, the mould opens and the finished container is ejected. EBM is commonly used for HDPE and PP containers such as milk jugs, detergent bottles, and large food jars.

Injection blow moulding (IBM): This two-step process first creates a small, thick-walled tube called a preform by injecting molten plastic into a mould around a core rod. The preform is then transferred to a blow mould where it is inflated into its final shape. IBM produces containers with very consistent dimensions and is ideal for small, precise containers used in pharmaceuticals and personal care.

Stretch blow moulding (SBM): This is the preferred method for PET bottles – the clear bottles you see for water, soft drinks, and juices. A preform is heated to the correct temperature, then mechanically stretched by a rod while compressed air simultaneously blows it outward into the mould shape. The stretching step orients the polymer molecules both axially and radially, giving PET bottles their excellent clarity, mechanical strength, and reduced weight.

Thermoforming 🔗

Thermoforming is used to manufacture trays, cups, lids, and blister packs – flat or shallow containers rather than bottles. The process begins by heating a plastic sheet until it becomes soft and pliable. Two main techniques are used:

Vacuum forming: Atmospheric pressure draws the heated sheet against a mould as air is removed from beneath it.

Pressure forming: Compressed air pushes the heated sheet into more complex mould shapes, allowing finer detail and sharper corners.

Thermoforming is favoured for its versatility and relatively low tooling costs, making it suitable for producing everything from simple disposable cups to multi-compartment meal trays. After forming, the containers are trimmed, inspected for defects, and packaged for shipment.

Injection moulding 🔗

Injection moulding is another major process, though it is more commonly used for closures (caps, lids), tubs, and rigid containers rather than bottles. Molten plastic is injected under high pressure into a precisely machined mould cavity. Once cooled, the solid part is ejected. This process delivers extremely accurate dimensions and smooth finishes, making it ideal for threaded caps that need to seal tightly on bottles.

How material choice affects food packaging performance 🔗

Each packaging material brings specific strengths to the table. Glass is unbeatable for chemical inertness and product purity – it doesn’t interact with food at all, making it ideal for acidic products, baby food, and premium items. Metal cans provide the best barrier properties against gases, moisture, and light, and their ability to withstand retort processing makes them the standard for shelf-stable canned foods. Plastics offer the greatest design flexibility, are lightweight (reducing transport costs), and can be engineered with multi-layer structures that combine different polymers for customised barrier performance.

Modern food manufacturers often select packaging materials based on a combination of factors: the nature of the food product, required shelf life, processing conditions (such as retort or hot-fill), cost, and increasingly, environmental sustainability and recyclability.

Sustainability in packaging manufacturing 🔗

Environmental impact is reshaping how packaging materials are manufactured. Glass manufacturers now use up to 90% recycled cullet in some facilities, which significantly reduces energy consumption since cullet melts at lower temperatures than raw materials. Metal cans, especially aluminium, are highly recyclable – steel cans can always be recycled, and the steel industry actively uses them in producing new steel products. In the plastics sector, there is growing adoption of post-consumer recycled (PCR) content, bioplastics, and lightweighting strategies that reduce the amount of plastic used per container.

Advances like electric melting furnaces in glass manufacturing, thinner tin coatings achieved through precision electroplating, and energy-efficient servo-driven blow moulding machines are all contributing to a lower environmental footprint across the packaging industry.

What do you think? Given the trade-offs between glass, metal, and plastic packaging in terms of food safety, cost, and sustainability – which material do you think will dominate food packaging in the next decade? And how might emerging technologies like bio-based plastics or ultra-lightweight metal cans reshape the industry?

References

1. https://www.packagingtechtoday.com/materials/rigid-containers/the-evolution-of-glass-packaging-and-inspection-2/
2. https://en.wikipedia.org/wiki/Glass_production
3. https://www.aegg.co.uk/blog/news/item/the-glass-manufacturing-process
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC7270472/
5. https://www.sciencedirect.com/topics/chemistry/tinplate
6. https://www.sciencedirect.com/topics/materials-science/tin-plate
7. https://en.wikipedia.org/wiki/Steel_and_tin_cans
8. https://www.inlineplastics.com/blog/how-plastic-food-containers-are-manufactured/
9. https://www.plasticsengineering.org/2025/06/what-is-blow-molding-main-types-and-technologies-explained-009131/