Plastic Extrusion Process

The extrusion of polymers is quite different from extrusion of metals like aluminium. The “billet” material here is not in the form of a bar stock. Rather it comes in a powder or pellet form. The substance stated above is added to a machine. This machine utilizes a screw-type action for helping in mixing and pushing the plastic material via the die opening. The polymer is first fed onto a rotating screw, heated up and mixed. As soon as the melt reaches the die, it is allowed to pass through a screen pack. This screen pack consists of a series of wire meshes perfectly supported by a stiff plate (also known as a breaker plate) comprising small axial holes. An elementary polymer extrusion process line is shown here.

Plastic extrusion process is actually polymer extrusion process only. It involves forming of a continuous piece of plastic as plastic material is forced via a shaping orifice with optional heat. The opening via which the resins are forced actually gives the product its basic shape and form. This process results in consistent thickness and gauge control.

A basic process for manufacturing extruded Plastic Profiles goes like this. Plastic material in the form of pellet is fed an Extruder. The plastic is then conveyed forward, continuously by use of a rotating screw. It lies inside a heated barrel that gets softened by friction and heat. Then softened plastic is consequently forced out via a die into a cool water directly. Here the product solidifies. The extruded material is then conveyed onwards into take-off rollers. This rollers do the act of pulling the softened plastic from the die.

The die is a sort of metal plate that is placed at the end of an extruder. A section is cut out from its interior. Actually the cross-section of the product gets determined by this cutout and speed of the take-off rollers.

There are various types of extrusion process available depending on the various types of materials and user specification. With a Coextrusion process it's now possible to produce several different plastic types in a same production line.

Extrusion is generally a process that is fully-automatic. It has continuous dimension checks and automatic machine adjustment, this makes it possible for the manufacturers to make simple tubes, rods to even complex window profiles. Extrusion process also enables manufacture of large products like pipes with diameter up to 150cm.

Dual Durometer Extrusion
A most common type of variation on the straight extrusion is the Dual Durometer Extrusion. Here, typically a "side machine" which is about one fourth or even less as compared to the size of main machine runs in tandem with the primary machine, feeding a separate material for example flexible vinyl with rigid PVC to the die. Here the streams gets merged into a single extrusion constituted of double bonded profiles. They often display two hardnesses or dual durometers. The following image describes a typical dual durometer extrusion.

Advantages of Dual Durometer Extrusion
As two materials with different physical properties are joined, there is a combination of structural integrity as well as flexible function into one part. To illustrate the point as a rubber gasket is attached to a metal part for providing a closure seal, the dual durometer extrusion process integrates both functions into a single piece. Thus cosiderably reducing assembly cost and at the same time enhancing performance and function.

Design considerations in plastic extrusion process
Here's some of the design consideration in plastic extrusion process.

Geometry of the part
In a plastic extrusion process material melts down in a gradual manner as it travels through the barrel, and subsequently out of the die. Since it is typically in the form of a liquid as it exits the die, the wall thicknesses must remain uniform, along with the symmetrical shape. Otherwise, there is every chance that a greater pressure on one of the side can force the profile sideways, thus creating a "bow", in place of a straight part. A perfectly balanced shape permits maximum running speed, at a low cost. Hollowness in the profile can create knitlines, in the place where there is separation and rejoining of the material.

 

Material used
Most of the thermoplastics can be extruded, the long list includes materials like HDPE, LDPE, ABS, acetates, polystyrene, polypropylene, butyrates, acetals, nylons, polyphenylene sulfides, polycarbonates, thermoplastic polyesters and rubbers among others. Materials like nylon, are difficult to extrude as they become very fluid as they are melted. Others like acetal or butyrate comes with an objectionable odour thus resulting in very few takers for them.

Tolerances
As plastic extrusions are not contained fully by metal tooling, tolerances as a rule must typically be looser than the other types of molding processes. Specialized tooling can hold tighter tolerances, however the following are the "normal tolerances":

Basic Die Design
A tooling system for the basic die design comprises:

Flat Plate Die
Plate that has the shape of the part cut through the die. However for guiding the material there is no transition. The flat plate die design is considered a low cost alternative for low volume production.

Semi-streamlined Die
Here at the back of the die the corners and edges have radii for helping in the transition of material into the die.

Fully Streamlined Die
In this case the die forcefully channels the material uniformly throughout the die. This design however is most expensive and well suited for high volume jobs.

Secondary Operations
Secondary operation for plastic extrusion process includes cutting, drilling, mitering of corners (especially for gaskets or frames), belling (to increase the diameter at the end), taping and cutting according to length. This process can be performed during extrusion (on-line), or after the process of extrusion (off-line). Off-line cutting generally is more complex and costly, but are known to hold tighter tolerances.

Parameters in Plastic Extrusions
There are certain basic standards which need to be met by plastic extrusions whenever the finished profiles are going to be used for industrial application. Efforts are to be made to remove defects like tearing and necking. Some of these parameters are:

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