Ejection from moulds

C-E Shaping phase (cooling off to temperature at which components are ejected from mould) HP Hydraulic Pressure tc Cooling time... 

An injection moulding is in the form of a flat sheet 100 mm square and 4 nun thick. The melt temperature is 230°C, the mould temperature is 30°C and the plastic may be ejected from the mould at a centre-line temperature of %°C. If the runner design criterion is that it should be ejectable at the same instant as the moulding, eshmate the required runner diameter. The thermal diffusivity of the melt is 1 x 10 m /s. 

Nucleating agents in PET also allow faster demoulding times, that is, the injection moulded part can be ejected from the mould more quickly or at a higher temperature. 

When the material is sufficiently heated, it is transported by a moving plunger under high pressure (sometimes up to 1500 bar) through a narrow channel into the mould cavity (Figure 11.12a and 11.12b). In the cooled mould the product cools down under pressure. After complete solidification the mould is opened and the product is ejected from it (Figure 11.12c). 

Once the moulding is ejected from the mould, the average residual stress on the cross section falls to zero according to Eq. (6.5). There are many possible residual stress distributions across the thickness of injection mouldings, because of the varied pressure histories in the mould. If the cooling of one side of the mould is more effective than the other, parts of the moulding may bow. 

This calculation neglects the taper angle of 1° or 2°, needed on the rib to allow easy ejection from the mould. 

Generally, the lower halve contains a cavity, while the upper part has a projection. After preheating the material, the mould is closed. As the pressure is increased, the polymer material will deform and fills up the mould cavity. In case of thermosets, the pressure and the temperature has to be maintained until the material is fully cured. For thermoplasts, it suffices to cool the mould, thereby fixating the polymer s shape. When the mould is cooled down, the object is ejected from the mould and the process can be repeated. 

Early clarifying and nucleating agents sometimes resulted in difficult ejection from the mould, or gave rise to organoleptic problems. The suppliers have addressed these issues in recent years. 

When the desired length of the parison is reached, the mould is closed and the parison is inflated by internal air introduced via the die-head assembly. The mould walls are vented and a vacuum may be applied. The molten plastic is thus forced to conform to the shape of the mould cavity. The article is then cooled, solidified and ejected from the mould. 

The techniques of forming thermoplastics are obviously influenced by the fact that the molecules of thermoplastics do not cross-link on heating and therefore can be maintained in a softened state while being made to flow under pressure into a new shape. The properties of each thermoplastic, however, sometimes limit the nature of the possible processes in which it will be involved. Some, for example, will be flexible enough to be ejected from a mould having undercuts in it others will be too brittle to withstand such a treatment. It is therefore preferable to know about each plastics in terms of properties and applications in order to appreciate which forming techniques may be most suited to it. 

Injecting wax Wax pattern ejected into mould from mould... 

Ejects the moulded parts from the tool. The mechanism is activated through the clamping unit. The ejection force may be hydraulic, pneumatic or mechanical. 

When the mould opens the plate is separated by means of a delved action mechanism (e.g., chains or length bolts), so breaking the restricted gate. The mouldings are then ejected from one daylight and the sprue and nmner system are ejected from the other. 

The injection pressure and holding pressure selected must be as high as necessaiy to fill the cavity sufficiently fast, completely and efficiently, but, on the other hand, as low as necessaiy to produce low-stressed injection moulded components and avoid difficulties when the components are ejected from the mould. 

High mould temperatures cause the component to cool slowly, which is necessary, for example, with the majority of semi-crystalline thermoplastics, in order to obtain components that are to size and have constant dimensions. The crystallisation of these compounds must be completed in the mould, i.e., it must be over before the components are ejected from the mould. Otherwise, after-crystallisation occurs over the course of time, which in every case causes alterations in dimensions, and frequently also leads to warping of the component. 

The upper limit for the mould temperatures is naturally determined by the maximum temperature at which the components can be ejected from the mould, which is specific to each material, e.g., for amorphous thermoplastics it lies at least 10 °C below the freezing temperature. High mould temperatures lead to slower cooling, which means longer cooling times, and therefore longer cycle times. 

The convention extrusion blow moulding process may be continuous or intermittent. In the former method the extruder continuously supplies molten polymer through the annular die. In most cases the mould assembly moves relative to the die. When the mould has closed around the parison, a hot knife separates the latter from the extruder and the mould moves away for inflation, cooling and ejection of the moulding. Meanwhile the next parison will have been produced and this mould may move back to collect it or, in multi-mould systems, this would have been picked up by another mould. Alternatively in some machines the mould assembly is fixed and the required length of parison is cut off and transported to the mould by a robot arm. 

This type of mould, also often referred to as a three plate mould, is used when it is desired to have the runner system in a different plane from the parting line of the moulding. This would be the case in a multi-cavity mould where it was desirable to have a central feed to each cavity (see Fig. 4.38). In this type of mould there is automatic degating and the runner system and sprue are ejected separately from the moulding. 

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