Plasticator melting operation

The RP processing methods that process well over 50wt% of all plastics is injection molding. This process as well as a few others uses a plasticator to melt plastics. It is a very important component in a melting process with its usual barrel and screw (or screws). If factors such as the proper screw design and/or barrel heat profile are not used correctly fabricated products may not meet or maximize their performance and very important not provide for low cost parts. 

1 Feed Zone This is the part of the screw that picks up the plastic compound at the feed opening (throat) plus an addidonal portion downstream. Many screws, particularly for extruders, have an initial constant lead and depth section, all of which is considered the feed section. This section can be welded onto the barrel or a separate part bolted onto the upstream end of the barrel. The feed section is usually jacked for fluid heating and/or cooling. 

2 Tmnsition/Melting Zone It is the section, also called the compression zone, of a screw between the feed zone and metering zone in which the flight depth decreases in the direction of discharge. In this zone the plastics starts in both solid and molten state with target to have all molten upon leaving this zone. 

3 Metering Zone This section is a relatively shallow portion of the screw at the discharge end with a constant depth and lead usually having the melt moves 3 or 4 runs of the flight length. 

The primary purpose for using a screw is to take advantage of its mixing action. Theoretically speaking, the motion of the screw should keep any difference in melt temperature to a minimum. It should also permit materials and colors to be blended better with the result that a more uniform melt is delivered to the mold/die. 

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Yet another geometry is the cone and plate viscometer. This generally operates without a positive hydrostatic pressure and, although often used for plastics melts, is not suitable for rubbers because of excess slipping. 

As with other mold operation process mold release agents are usually required because the plastic melt can adhere to the surface of the mold cavity. Molds with very little or no draft usually require a release agent. Like other molding operations, a textured cavity can provide a textured product surface. Most texturing of cavities is by chemical etching so it is important to use the appropriate mold material to create a particular texture. An effective release is needed at the parting line to aid in demolding. 

A mold must be considered as an important part of IM. It is a controllable complex device providing different functions or capabilities to permit molding the desired product (Table 3). If not properly operated, handled, and maintained, it will not be an efficient operating device. Under pressure, hot melt moves rapidly through the mold. Basically, temperature-controlled water (with ethylene glycol if the water has to operate below its freezing point) circulates in the mold to remove heat from TPs with TSs electrical heaters that are usually used within the mold provide the additional heat required to solidify the plastic melt in the cavity. Air is released horn the cavity to eliminate melt burning or voids in the products. 

In the manufacture of products, there is always a challenge to utilize advanced techniques, such as imderstanding the different plastic melt flow behaviors, operational monitoring and control systems, testing and quahty control, and so on. However, these techniques are only helpful if the basic operations of fabricating are understood and characterized, to ensure the elimination or significant reduction of potential problems. 

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