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, Rotomolding, Thermoforming, and Compression. By means of one or more of these techniques it is possible to transform the different types of high density polyethylene.
Extrusion Molding
Extrusion molding 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 extrusion head, coming out in the form of pipe, film, sheet, profile and others. The part exiting the extruder passes through a water bath that cools the part progressively.
Extrusion molding is a continuous manufacturing process and the most used for the transformation of high density polyethylene, but it has the disadvantage of manufacturing unfinished pieces which need a subsequent treatment after extrusion molding. Such is the case of lamination which requires thermoforming molding.
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 homogenization 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
Most important 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 failures in the final product, this is possible because the flowing plastic is under pressure and in a semi-liquid state.
The outside 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 an independent bath with circulating water, through which the extruded tube passes. It is recommended that the cooling water temperature be 30 to 50 oC. The puller is the element where the force that keeps the plastic in motion inside the extrusion line is generated. The coil or the cutter is the final place depending on the product to be manufactured.
Extrusion for tubular film manufacturing
For the manufacture of HDPE tubular film, a distance between the die and the rollers of 1.50 to 2.50 m. maximum is recommended, depending on the film width; 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 meters and a die opening of 1.0016 to 1.524 mm.
The extrusion conditions affect the impact resistance of the film, as they determine 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, that is, after a force orients the molecules, these acquire 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 continuously renews 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 maintains 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