High Density Polyethylene Manufacture

High density polyethylene can be manufactured by different methods. Prior to its transformation, additives are added, this is called formulation.
Among the transformation methods are Extrusion, Injection, Blowing, Rotomoulding, Thermoforming, and Compression. By means of one or more of these techniques it is possible to transform the different types of HDPE.

Extrusion Moulding

Extrusion moulding consists of introducing the high density polyethylene in the form of pellets into a cylinder in which the screw is located. The material is pushed, compacted and finally melted in the cylinder and then extruded in the head, coming out in the form of tube, film, sheet, profile and others. The part exiting the extruder passes through a water bath which cools the part progressively.
Extrusion moulding is a continuous manufacturing process and the most widely used for the transformation of high density polyethylene, but it has the disadvantage of producing unfinished parts which require further treatment after extrusion moulding. Such is the case of lamination which requires thermoforming moulding.

Spindle and cylinder

The screw is the fundamental part of the extruder, its design varies according to the material, shape and purpose of the articles to be manufactured. For high density polyethylene, screws with long feeding and dosing zones and short compression zone are recommended; this guarantees the homogenisation of the material.
Cylinders must be manufactured to withstand high operating pressures and temperatures, as well as abrasion. Special devices for heating and cooling the cylinder zones are installed on the outside of the cylinder.

Temperature profiles

The temperature varies according to the grade of the material to be processed as well as the type of product to be obtained and sometimes even the machine itself.

Extrusion to manufacture pipes and profiles

The equipment necessary for the manufacture of pipes and profiles consists of an extruder, die, former, cooling vat, variable speed puller and coiler or cutter.

Given pipe

The most important thing in pipe extrusion is the die design and the distribution of the melt flow within the die. The flow is divided into three streams, which after being forced through the mandrel, the streams rejoin and form a single tubular stream.
The molten material has three joining lines that must weld together perfectly, without leaving any marks or flaws in the final product, because the flowing plastic is under pressure and in a semi-liquid state.
The outer diameter of the pipe is calibrated using the vacuum former, by means of which the extruded pipe is subjected to vacuum while cooling. The vacuum former is recommended as it provides a better finish.
The cooling bath consists of a separate bath with circulating water through which the extruded tube passes. The temperature of the cooling water is recommended to be 30 to 50 oC. The puller is the element that generates the force that keeps the plastic in motion within the extrusion line. The coil or the cutter is the final place depending on the product to be manufactured.

Extrusion for the manufacture of tubular film

For the manufacture of HDPE tubular film, a distance between die and rollers of 1.50 to 2.50 m maximum is recommended, depending on the width of the film; also, a blowing ratio between 3.5:1 to 6:1, the height of the cooling line greater than 0.40 but less than 0.80 metres and a die opening of 1.0016 to 1.524 mm.
Extrusion conditions affect the impact strength of the film by determining the orientation of the molecules; in general, the plastic has better mechanical qualities in the direction of orientation.
Polyethylene molecules have a very short relaxation time, i.e. after a force orients the molecules, they return to their previous position almost immediately. Only those polymer chains that are oriented just before the cooling line maintain the orientation provided.
In this sense, the characteristics of the bubble, such as cooling line height and blowing ratio, will affect the impact resistance.
The bubble's internal cooling system is a device that is continuously renewing the air that supports the bubble itself. This system improves cooling efficiency by removing the hot air that has already cooled the film and introducing fresh air. The rhythm of this exchange keeps the blowing ratio at an exact and constant magnitude.

Source:UNIVERSITY OF SAN CARLOS DE GUATEMALA FACULTY OF ENGINEERING SCHOOL OF CHEMICAL ENGINEERING ERNESTO ROCA GIRÓN