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By the time the material in the river flows freezes, the bulk of the material will have already frozen off and shrinkage will have occurred. The result is high-stress tensile members throughout the molding, a common cause of warpage.

The pressurization phase—from the point of view of flow behavior—is very similar to the filling phase. The extensional flow, however, becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change e. The process of filling the mold can be distinctly analyzed with the ability to predict pressure, temperature, and stress.

Any molder can prove that all the conditions and effects discussed in this chapter do indeed occur during the injection molding process. The main difference of course, is the increase in hydrostatic isotropic pressure.

Tags Jay Shoemaker Ed. If the injection rate were raised, the frozen layer would be thinner Figure1. On the other hand, if the material were kept under stress and the plastic snap frozen, most of the orientation would be trapped in the frozen plastic Figure1.

At the same time, heat is being lost through the frozen layer to the cold mold surface. If, on the other hand, there is another area that is cooler, the flow will be less, so there will be less heat input, and the plastic will get colder until it eventually freezes off. This means shear stress during filling, shown on Moldflow plots, can be used as a design parameter.

Pearson – Logo Design Love A. Most of the stress in plastic parts occurs during the compensation phase. This flow pattern is often called fountain flow or bubble flow because the flow front is like a bubble being inflated with hot plastic from the center. The flow of this displaced material is a combination of forward flow and outward flow.


This flow shear stress will orient the material, i. After emptying the barrel of an injectionmolding machine, a small amount of red plastic was charged, followed by green plastic. We shall see in chapter 2, section 2.

Guide to Creating Iconic Brand 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 in Figure1. Compensating flow is unstable.

Hot plastic is continuously flowing, bringing new hot material along and generating significant frictional heat. As the ram moves forward, it first moves at a steady speed as the plastic flows into the cavity.

This is the filling phase. Flow technology is concerned with the behavior of plastics during the mold filling process.

The ram moving forward to compensate for the volumetric change in the part is called the compensation phase. It is easy to get confused between the various stress levels and orientation of the polymer.

It is critical for engineers in the plastics field to have access to flow analyses and their interpretations, including the simulation of cooling and warp effects, to aid in the successful design and manufacture of parts and molds. While this knowledge alone can somewhat improve quality, it is only with the use of Moldflow analysis during the initial design stage, with the mold designed for the optimum filling pattern, that these effects can be controlled and the full benefits obtained.

Consider the plate molding again see Figure1. A Resource for Plastics Engineers.

You would think that plastic flowing uniformly through the thin diaphragm would top up the thick rim. By controlling flow and minimizing stress, it is possible apoatila design for optimum part quality. This must not be confused with shear rate, which is the rate of plastic mokdflow over the next layer. This can be seen in a short shot; the difference in volume between the molding and the cavity is due to this volumetric change.

Moldflow Design Guide – . A Resource for Plastics Engineers

This allows more time for the level of orientation to relax, so the residual orientation drops rapidly toward the center. In practice, the plastic during the compensation phase flows in rivers that spread out like a delta, as illustrated in Figure1.


When the mold is filled, the ram will slow down, but it still moves quite some distance because plastics are very compressible materials. 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.

The layer of plastic just on the inside of the frozen layer is subject to maximum shear stress and freezes the instant flow stops, trapping almost all the orientation. When a viscous liquid flows, the energy that causes the deformation is dissipated and becomes viscous heat.

It was found that the injection molding process, although complex, could apostula divided into three phases we use the word phase to avoid confusion with injection stage, as used with programmed injection. On the other hand, when an elastic solid. However slight the temperature variation, natural instability will amplify it.

Initially, the frozen layer is very thin, so heat is lost very rapidly. As the total filling time is measured in seconds, the frozen layer reaches an equilibrium state early in the filling cycle. It is useful to think about how the thickness of this frozen layer will vary. Consider how this pattern will affect the residual stress level. When additional material is injected, it flows into this central core, displacing the material already there, which then forms a new flow front. Although fluids are usually assumed to be incompressible, molten plastics have to be considered to be more like a gas.

This was investigated using a centrally gated mold shaped like a dinner apkstila with a thick rim around the outside as shown in Figure1.

Moldflow Design Guide

The way the plastic flows into the mold is of paramount importance in determining the quality of the part. As the plastic flows it is subject to shear stress, also called flow shear stress. This phase lasts until the mold is just filled.