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After a time, the frozen layer will reach a thickness such that the heat lost by conduction is equal to the heat input from plastic flow and frictional heating, i. Phases of Injection Molding5 at the outer edge where the plastic is frozen, rises to a maximum just inwards of the frozen layer, then drops toward the center, as shown moldflo Figure1.
By controlling flow and minimizing stress, it is possible to design for optimum part quality. The apostia layer is formed by the flow front inflating, and so is subject to only a low shear stress and, therefore, has a very low level of molecular orientation.
Today, the global imperative to drive down the cost of manufacturing has led to the use of molding simulation as a cost optimization tool rather than just as a design and problem avoidance tool.
It is interesting to do some calculations on the time taken to reach this state of equilibrium. The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel.
Now, consider what happens upstream.
Oriented material normally will shrink more than nonoriented material. When additional material is injected, it flows into this central core, displacing the material already there, which then forms a new flow front. The first material to freeze off will shrink early in the cycle.
The book provides an overview of the polymer flow behavior and the injection molding process, design principles to facilitate moldfllow part and mold design, and examples of how Moldflow design analysis technology can be used both to apostika problems and to optimize the design of part, mold, and the molding process itself. Gradual reduction of orientation toward the center of the mold as stress levels are lower and the cooling rate is slower which allows more time for orientation to relax.
This is a very important consideration. Flow technology is concerned with the behavior of plastics during the mold filling process. If, for example, one part of the melt is slightly hotter than the rest, then the plastic flow in that area will be slightly greater, bringing hotter material into the area and maintaining the temperature.
The heat loss would be at moodflow same rate, and with less heat input the frozen layer would grow in thickness. Two parts having identical dimensions and made from the same material but molded under different conditions will have different stress and shrinkage levels and will behave differently in the field, meaning that they are in practice two different parts.
Consider the plate molding again see Figure1.
It was found that the injection molding process, although complex, could be divided into three phases we use the word phase to avoid confusion with injection stage, as used with programmed injection.
The extensional flow, however, becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change e. As the ram moves forward, it first moves at a steady speed as the plastic flows into the cavity. We shall see in chapter 2, section moldvlow. Hot plastic is continuously flowing, bringing new hot material along and generating significant frictional heat.
In addition to the two types of material flow behavior, there are two types of deformation: Molten thermoplastics exhibit viscoelastic behavior, which combines flow characteristics of both viscous liquids and elastic solids. This flow shear stress will orient the material, i.
For example, the flow of water is a typical viscous flow, whereas the deformation of a rubber cube falls into the elastic category. It is easy to get confused between the various stress levels and orientation of the polymer. This important point is at the heart of the Moldflow philosophy. After emptying the barrel of an injectionmolding machine, a small amount of red plastic was charged, followed by green plastic. Any molder can prove that all the conditions and effects discussed in this chapter do indeed occur during the injection molding process.
The rivers will shrink relative to the bulk of the molding, and because they are highly orientated, shrinkage will be very high. This results in more plastic freezing and the frozen layer getting thicker, cutting down the heat flow.
The Moldflow Design Guide is intended to help practicing engineers solve problems they encounter frequently in the design of parts and molds and during production. The ram moving forward to compensate for the volumetric change in the part is called the compensation phase. The highly oriented layer ends up being in tension, while the less-oriented material is in compression. This can be seen in a short shot; the difference in volume between the molding and the cavity is due to this volumetric change.
The main difference of course, is the increase in hydrostatic isotropic pressure.