A plastic bag has become a routine part of everyday life. Whether you’re carrying groceries or packaging a medical waste, the various types of plastic bags: spout-bag, zip‑lock, trash‑bag, pouch-bags and biodegradable options – are widely used. Ever wondered how a plastic bag is produced? It is a complex process with lots of different professions and working hours involved. Let’s start from the moment when tiny plastic pellets are transformed into a finished product. The process is known as blown‑film extrusion. Herein we will take you through the major steps of making plastic bags.
Raw material preparation
The plastic bag starts with preparation of raw materials (polymers) derived from crude oil. Polymers are converted into plastic‑resin pellets undergoing multiple treatment processes. These polymers are called virgin raw materials. Sustainability codes and recycling opportunities brought us also recycled plastic pellets which serve as a raw material for manufacturing new products. They are created by collecting, washing, shredding, melting down, and filtering used plastic, then extruding it into a pellet form. These pellets are used in a wide variety of industries, including blown film, blow molding, injectin moulding etc.
Film blowing
Plastic pellets are fed into an extruder, where a rotating screw melts and homogenizes them using external heat and internal friction. The molten material is then pushed under high pressure through a die head, shaping it into a continuous tube of a thin plastic film. As the tube leaves the die head, it is pulled upwards (or rarely downwards) and inflated with air to form a “bubble”. The blown film needs to be solidified, and this happens with the help of air or water coolers. The solidified product is then cut to the required length or processed further as needed. This method called blown film extrusion. Still with here? Then let’s proceed to the next stage.
Bag formation
Interestingly, the plastic film that exits the extruder is not flat— it is actually tubular. Next time you go shopping, check how the platic shopping bags look. Most of them are seamless along the sides because the bags are cut from a continuous plastic tube making them more durable.
When plastic film is set and rolled, it can be sent through a flexoprinting press. They can be either integrated into the line or a standing-alone units depending of the production processes, possibilities and available space. Flexoprinting allows manufacturers to personilize brands and offer diverse products and services for the consumers.
Clean film or printed film is fed into a packaging machine that cuts and seals the film at the required intervals. The bottom of each section is welded together with heated sealing bars creating a strong seam and required shape of the bag’s base (star seal, flat seal etc.). The sealing module with sealing bars allows the machine to reach a high production rates (over 350 bags/min). The sealed bags are guided to the folders allowing all possible folding combinations to be made in order to obtain the required roll width.
Finishing
Finally, the finished rolls can be automatically labelled with an adhesive paper band and conveyed to the automatic boxing line (forming of cardboard boxes, filling, closing of the boxes and palletising). Each batch of bags is a subject to various tests and measurement like bag thickness. This is required to ensure that the item is manufactured properly and suits to the purpose, and also to avoid wastful material use if bag matrial is too thick.