The Plasticating and Injection Unit

Key Questions What functions must be fulfilled by the plasticating and injection unit  

Which components make up this stractural unit Into which zones is the plasticating screw divided  

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The plasticating and injection unit. A plasticating and injection unit is shown in Fig. 3.45. The major tasks of the plasticating unit are to melt the polymer, to accumulate the melt in the screw chamber, to inject the melt into the cavity, and to maintain the holding pressure during cooling. 

A typical arrangement for a direct screw-transfer injection-molding machine for thermosets is shown in Figure 2.16. The machine has two sections mounted on a common base. One section constitutes the plasticizing and injection unit, which includes the feed hopper, the heated barrel that en closes the screw, the hydraulic cylinder which pushes the screw forward to inject the plasticized material into the mold, and a motor to rotate the screw. The other section clamps and holds the mold halves together under pressure during the injection of the hot plastic melt into the mold. 

The main functions of the plasticating and injection unit can be divided as follows (Figure 3-1). 

Figure 3-1 Main functions of the plasticating- and injection unit...

The individual functions of the plasticating and injection unit are described here as they apply to the molding of Compact Disks, produced from a crystal clear material, so that the laser can read the information. The CD is therefore made from an amorphous thermoplastic. Being opaque-apart from other material properties-disqualifies thermosets and elastomers from CD production. 

Let s pursue the subject of thermoplastics, which are supplied to the machine in granulate form. This granulate is loaded from the sack into the hopper by hand. Larger quantities may be fed automatically from a silo. The hopper is installed on top of the plasticating and injection unit. Figure 3-2 shows a cross section of a plasticating and injection unit. 

Above all, the plasticating and injection unit must convert this gramdate into a homogeneous melt, capable of filling the mold cavity completely. In other words, the ensuing melt must be pourable. For that purpose, the molding compound is heated, until it melts (Figure 3-3). This is achieved by friction... 

The plasticating and injection unit must therefore be heatable. With thermoplastics, this is usually achieved by clamping electric heaterbands around the cylinder. Heating can be set to a certain temperature profile. 

Another task of the plasticating and injection unit is illustrated by colored molded parts, such as salad bowls, coffee mugs, trash cans, or bottle crates. To manufacture such a colored part with an injection molding machine, the desired colorant must be admixed with the granulate. The reason for this is that most thermoplastics are either colorless or light colored and translucent in their natural states. In some cases, this admixing has already been performed by the manufacturer of the thermoplastic. The particles are then colored... 

Once the material has been uniformly prepared, it is important to ensure, that it can be conveyed into the mold itself This must occur very rapidly. It requires high pressure because of the material s viscosity. One of the plasticating and injection unit s main tasks therefore is the rapid injection of the melt into the mold. 

The second function of the plasticating and injection unit is to convey the plasticated material-i.e. the melt which has accumulated in the injection chamber-into the mold. This must occur as quickly as possible so that the material does not solidify in the cold mold during the injection operation. Material which solidifies and is then conveyed onward during the injection... 

The plasticating and injection unit must fulfill the fimctions of melting, homogenizing, storing and. ... 

The prepared, flowable material is transported from the injection chamber of the plasticating and injection unit into the mold. In other words, it is injection molded. ... 

The molding compound, which has been homogenized and melted in the plasticating and injection unit, is injected into the mold cavity in the injection operation. There the molding compound soUdifies to form the finished molded part. As soon as the molded part has soUdified, it is removed Ifom the mold by opening the mold at the parting line. 

The Plasticating and Injection Unit (Lesson 3) The Clamping Unit (Lesson 5)... 

Short dwell times of the melt in the plasticizing and injection unit, even at very small injection volumes,... 

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