Introduction of the plastic extruder

Plastic extruder is used to produce the plastic products from the thermoplastic materials,by the extrusion technology.

The structure of the plastic extruder is as follow:

The production procedure is as follow:

1. Preheating of the screw and barrel.

2,The thermoplastic materials will loaded in the hopper througn vacuum or screw loading device. The hopper is top mounted to the barrel. The materilas will be loaded by gravity in to the barrel.

3. Then start the plastic extruder. The rotating screw normally turning at up to 120 rpm. It forces the plastic beads forward in the barrel . During this procedure,there is extra heat .Extra heat is contributed by the intense pressure and friction taking place inside the barrel. In fact, if an extrusion line is running a certain material fast enough, the heaters can be shut off and the melt temperature maintained by pressure and friction alone inside the barrel. In most extruders, cooling fans are present to keep the temperature below a set value if too much heat is generated. If forced air cooling proves insufficient then cast-in heater jackets are employed, and they generally use a closed loop of distilled water in heat exchange with tower or city water.

4.At the front of the barrel, the molten plastic leaves the screw and go through a screen-changer to remove any contaminants in the melt. The screens  are a thick metal puck with many holes drilled through it. The pressure at this point can exceed 34 Mpa. The screen changer assembly also serves to create back pressure in the barrel. Back pressure is required for uniform melting and proper mixing of the polymer, and how much pressure is generated can be 'tweaked' by the number of screens, their wire weave size, and other parameters. This breaker plate and screen pack combination also does the function of converting "rotational memory" of the molten plastic into "longitudinal memory".

5.After passing through the screen changer,the molten plastic enters the die. The die is what gives the final product its profile and must be designed so that the molten plastic evenly flows from a cylindrical profile, to the product's profile shape. Uneven flow at this stage would produce a product with unwanted stresses at certain points in the profile. These stresses can cause warping upon cooling. Almost any shape imaginable can be created so long as it is a continuous profile.

6.Now he product must be cooled . Plastics are very good thermal insulators and are so they are difficult to cool quickly. Compared with steel, plastic conducts' heat away 2000 times more slowly. In a tube or pipe extrusion line, a sealed water bath is acted upon by  vacuum to keep the newly formed and still molten tube or pipe from collapsing. For products such as plastic sheeting, the cooling is achieved by pulling through a set of cooling rolls.

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