Screen Pack
A very fine mesh sizing in micrometers is used at the end of barrel and screw or start of die. The main purpose of screen pack is to minimized or eliminate the chances of any contamination to pass and go to the final products. These contamination can be results in poor product quality. These contaminations include metallic parts, any other high temperature plastic/resin mixed or any wood and other hard high melting temperature. Screen pack actually is a sandwich like layers of different micron pore size mesh. Screen pack itself has not much strength that can bear the high pressure of plastic melt. So to avoid deformation of screen pack a breaker plate is used.
Breaker Plate
Breaker plate is used for different purposes one like to support the screen pack. Breaker plate is a circular plat having wide gaps that allow the molten plastic to pass through the breaker plate. Besides supporting the screen pack breaker plate also changes the direction of melt flow from circular motion of the melt due to rotation of screw to linear flow of melt to die.
Sources of contamination in Plastic
Screen pack is installed to plastic machines to remove any foreign or not melted materials. What are the sources of these particles? From where these get contaminated in the raw materials? These are some question we will find the answers to these. Sources can be divided into portions.
Raw Material Contaminated
Contaminating while mixing or compounding
Contaminating while processing
Contamination from poor handling of material
Raw Material
Raw material may be contaminated. The chances of contamination in new/virgin raw material are minimum and rare cases. While if raw material is recycled material then it always have content of contamination percentage may be different. Master batch is used for different purposes master batch may also contaminate the material. Types of contaminations in recycled material.
Different types of plastic
Aluminum pieces
Metallic particles
Wood particles
Fabric or fiber
Size difference of granules of same material
Mixing Section
Mixing section may also contribute to the contamination in the material. Any foreign material particles may get mixed. There is some time material contaminated if mixer is not completely cleaned.
Poor purging of Machine
When processing material change then we need to remove all the material through the machine this process of cleaning the plastic machine is called purging. Mostly Polystyrene (PS) and Polyethylene (PE) are used to perform this process. Sometime material remains in the machine not removed completely. This remained material causes the contamination. If material is of high melting point and processing temperature then it can be removed easily by screen pack and breaker plate.
Handling Of Plastic Raw Material
Handling of plastic raw material needs much care form contaminations and loss of material. If two different types of plastic get mixed then it is very difficult to separate them.
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