The production of refuse sacks - what is polyethylene?

Polyethylene - the most widely produced plastic around the world is a material that we encounter on a daily basis.
It can be used to make soft, pliant, as well as tough, hard and strong products, such as:

• Pipes
• Toys
• Cling film
• Bottles
• Fuel tanks
• Rubbish bags

 

What characteristics does it have?

• Good insulation properties
• Withstands aggressive substances
• Nearly unbreakable
• Withstands extreme climatic changes
• Surprisingly lightweight
• Can be easily made into a great variety of products
• Does not contaminate ground water

POLYETHYLENE IS: strong, safe and versatile.

 

How polyethylene is created

From naphtha to polyethylene:
Naphtha comes from oil, and is another term for petroleum spirit. When naphtha is heated to a high temperature (a process called cracking) a gas is released, called ethylene.

Ethylene is turned into polyethylene:
In the next step, this gas (ethylene) is passed through another reactor which is heated and pressurised. This reactor can be a large round container or a very long system of pipes. A white powder falls out of the end of the reactor and is then melted down and extruded. The extruded material is cooled under water and then cut with a rotary blade. The final product is a polyethylene granulate, which can then be used by manufacturers.

 

Polyethylene Granulate

 

 

Plastic recycling I

Here, previously used plastic is recycled into "new" plastic raw materials, so-called re-granulate. The chemical composition of the material remains unchanged.

 

 

Plastic recycling II

The separated plastic packaging material is checked for contaminants and shredded. The shredded material is washed, and any foreign material is removed while the plastic is still in the water. This works on the principle of the sink-or-swim separation where the different densities of the plastics can be taken advantage of. Polyethylene for example is less dense than water and floats. In this way, separating different types of plastic is easily carried out. The washed and graded plastic is dried and then granulated in an extruder.

The extruder works like a sausage machine. Inside the extruder, the plastic is melted down and then pressed through a perforated plate. The plastic extruded through this plate is rather like spaghetti and is cut up with a rotary knife. This is how granulate is produced.

Extruder to create the granulate

 

1^st Step: The production of plastic sheet

In a heated cylinder, a screw turns, which moves the plastic forwards and melts it. A tool is fitted in front of the screw and it is this tool which gives the molten plastic leaving the machine its desired shape, for example the plastic sheet. In the extruder the plastic is blown out like a balloon using a blower.

This way, a tube of plastic sheet is created and then cooled, laid flat and then wound onto a roll.

How a blow-extrusion plant works

 

2^nd Step: Bag production / blow-extrusion of plastic sheeting

After the "endless" tube of plastic sheeting is wound up onto the roll, it is taken to the bag finishing machine.

Bag finishing machine

 

3^rd Step: Inline / Offline bag finishing

A combined welding and perforating machine welds the bottom of the bag closed and adds the line of perforation holes to help tear off the bag. Prior to this, the tube of plastic sheeting is folded, in order to create the desired type of fold, e.g. side fold, star-sealed bottom, etc. The number of bags is determined electronically and the pre-defined number of bags is wound onto individual rolls.

Bag finishing machine

Bag finishing machine

 

4^th Step: The finished product

The freshly made bags on the roll:

Finished rolls leave the

conveyor belt and are packed in boxes

 

• An introduction to polyethylene