IX Международная студенческая научная конференция Студенческий научный форум - 2017

McKelvey defined plastics processing as “operations carried out on polymeric materials or systems to increase their utility” [1, p. 23]. These types of operations produce flow, chemical change, and/or a permanent change in physical properties.

Plastics processing techniques can be grouped into three categories:

- forming operations

- bonding operations

- modifications.

Although forming operations always involve flow, thermoplastic processes, such as extrusion, thermoplastic injection molding, thermo-forming, and rotational molding, produce physical changes in the polymer whereas chemical change occurs in the casting of liquid monomers.

Reactive extrusion and thermoset injection molding induce both chemical and physical change in the plastics materials. Bonding operations join two or more materials by causing one or both joining surfaces to become molten or flow.

Modifications include surface activation, mixing, and polymer modifications. Surface activation improves adhesion or printability of plastics materials.

Mixing reduces the nonuniformity of the polymer composition. Polymer modifications, such as annealing molded parts and radiation of plastics parts, change the amount of orientation, crystallinity, and/or cross-linking in the plastic.

Injection molding is one of the most widely used processes for manufacturing

plastics parts. It is a major processing technique for converting thermoplastics and thermoset materials into all types of products for different end uses: from automotive to electronics, medical to sports and recreation, and building and construction to consumer products. Injection molding is a relatively new method of producing parts. The first injection molding machines were manufactured and made available in the

early 1930s, whereas other manufacturing methods that may be familiar date back

more that 100 years.

According to the Injection Molding Division of the Society of Plastics Engi-

neers, injection molding is defined as a method of producing parts with a heat-meltable plastics material. This is done by the use of an injection molding machine. The shape that is produced is controlled by a confined chamber called a mold. The injection molding machine has two basic parts, the injection unit, the clamping unit. The injection unit melts the plastic and conveys or moves the material to the confined chamber or mold. The purpose of the clamping unit is to hold the mold in a closed position during injection to resist the pressures of the conveying or injection and forming of the material into a specific shape, and then opens after cooling to eject the part from the mold. Besides injection molding machine this process requires an ancillary equipment such as material-feeding and conveying equipment, dryers, mold temperature controllers, chillers, robotics and conveyers [2, p. 461].

Rosato describes the three basic operations that exist in injection molding. The first is raising the temperature of the plastic to a point where it will flow under pressure. This is done both by heating and by grinding down the granular solid until it forms a melt at an elevated temperature and uniform viscosity, a measurement of the resistance to flow. In most injection molding machines available today, this is done in the barrel of the machine, which is equipped with a reciprocating screw provides the vigorous working of the material along with the heating of the material. This part of the process is referred to as the plasticating of the material.

The second operation is to allow the molten plastic material to cool and solidify in the mold, which the machine keeps closed. The liquid, molten plastic from the injection molding machine barrel is transferred through various flow channels into the cavities of a mold, where it is formed into the desired object. What makes this apparently simple operation so complex is the limitations of the hydraulic circuitry used in the actuation of the injection plunger and the complex flow paths

paths involved in filling the mold and the cooling action in the mold.

The third and last operation is the opening of the mold to eject the plastic after keeping the material confined under pressure as the heat, which is added to the material to liquefy it, is removed to solidify the plastic and freeze it permanently into the shaped desired for thermoplastics.

A variety of materials can be injection molded.

The purpose of this article is to break down the basic parts of the injection

molding process as if you were actually taking a walking tour down the entire process. This tour is divided into four phases. The first phase is the material feed phase. Here the focus is on material handling: how the material is dried and the preparation of the material to be injection molded. The second phase is the melt-conveying phase. Our discussion is concentrated on the important aspects of how material goes from a solid pellet to a molten polymer. The emphasis here is on the screw, the barrel, and the nozzle. The melt-directing phase entails how the melt gets to its final destination, the mold cavity. In this section the sprue, runners, gates, and gate lands are reviewed as to what they do and how they affect the molding process. The last stop is the melt-forming phase. Here we discuss how to design a tool or part for the injection molding process.

Material Feed Phase

When a plastic material begins its journey through the injection molding process, the first thing that is considered is how the material is delivered and stored until it is used. The next step is to determine how the material will flow to the individual machines for molding, and finally, what process is needed to prepare the material so that it can be molded. Other side processes, such as color and additive feeding, also need to be considered if these apply [3, p. 215].

In this section we focus on the following issues for material feed. The first is that of drying the material, a process used in preparing most thermoplastic materials for injection molding. We then explain why materials need to be dried and what needs to be considered. Then the hopper and the concept of bulk density are reviewed, how this relates to sizing storage space for materials, the elements of

material mass flow, and the time and conditions involved in drying the material.

Melt-conveying Phase

When the word conveying comes to mind, a number of different methods are

visualized. For example, a moving belt moving articles from one location to another is an example of conveying. Another example is the use of an auger, a screw like device that moves grain, powder, or even objects like rocks through a cylinder opened on both ends. The auger acts as the conveyor where the material is transferred within the flights of the auger in channels that have the same depth throughout the length of the auger [4]. The open-ended cylinder acts as a guide to the conveying of material by keeping the material moving linearly. The auger–cylinder example can be used to explain melt conveying in the injection molding process.

In injection molding, an open-ended cylinder, referred to as a barrel, acts as a guide for the pellets and moves the pellets and melt from the hopper to the mold where the part is made. The auger, referred to as a screw, conveys material down through the barrel from the barrel to the mold. However, what is different in the screw and barrel from the auger and cylinder example discussed earlier is that the channels of the screw do not have a constant depth. The screw at the hopper end of the barrel will be deep, and moving forward toward the mold end of the screw, the depth of the channel becomes shallow. As all this is taking place, the inside opening of the barrel stays at a constant diameter. So, in terms of conveying, material is fed at the deep channels and conveyed into shallower channels, which cause the material to compress and pack together. This compression process increases the friction of the material against the inside wall of the barrel, providing frictional heat. In addition to this, heaters are spaced on the outside diameter of the entire length of the barrel, providing additional heat. Therefore, the frictional heat of the material in the screw plus the heat applied on the outside of the barrel together provide enough heat to convert material in pellet form at the hopper end of the screw and barrel to material in a melt form midway down the length of the barrel to the end of the barrel and screw. This simplified example provides background on the meltconveying section.

Next, we go into more detail regarding this process by examining the barrel,

screw, external heating mechanisms, venting, and nozzle sections of the meltconveying phase.

The Barrel

The barrel is defined here as an open-ended cylinder that controls the linear direction of the melt-conveying process, from the hopper to the mold. This also provides a frictional surface for the plastic material, to assist in the melting of the plastic from pellet form to molten form and results in moving the material in a basically linear direction from the hopper to the mold.

One of the most important properties of the barrel is the material from which the barrel is made. The typical material is steel with a bimetallic liner. This liner is made from a steel alloy, typically a 4140 alloy. Most injection molding machine barrels are made to withstand burst strengths of approximately 22,000 lb/in². In special applications, barrels are made to withstand between 45,000 and 50,000 lb/in², especially for thin-wall injection molding [

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