Mould ejection

PRESS OPEN MOULDING EJECTED FORKED STRIPPING TRAY IN... 

Problems in automation of mould ejection of elastomers, and particularly thermoplastic polyurethane (TPU) which adhere strongly to the monld, may have an impact on reproducibility of the cycle time, and thus also on the reproducibility of the thermal parameters of the process. 

As may be seen from the examples given, in many large moulds with deeply shaped cavities it is essential to nse various types of very long nozzle. Elongated central nozzles are used in cases where the moulding ejection system is located in the fixed mould half. 

An example of the effect on production rates is provided by injection moulding. The longer it takes after injection for solidification of the polymer to occur, the longer will be the overall cycle. (Provided the moulding is not distorted on ejection it will only be necessary to form a rigid skin to the moulding.)... 

Figure 8.12. Typical process for the compression moulding of thermosetting plastics, (a) Load, (b) Mould, (c) Eject, stripper in. (d) (After Groves )...

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. 

Example 5.11 A polyethylene injection moulding is in the form of a flat sheet 100 mm square and 3 mm thick. If the melt temperature is 230°C, the mould temperature is 30°C and the plastic may be ejected at a centre-line temperature of 90°C, estimate... 

During the blow moulding of polypropylene bottles, the parison is extruded at a temperature of 230°C and the mould temperature is 50°C. If the wall thickness of the bottle is I mm and the bottles can be ejected at a temperature of I20°C estimate the cooling time in the mould. 

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. 

The explosive compositions can be pressed directly into a container or mould, and ejected as pellets as shown in Figure 7.3. 

Plastic closures can be made by injection moulding, where the mould has two parts - a core which has the inside features of the closure and a cavity which has the features of the outside of the closure. Molten plastic is injected into the mould, which is water-cooled. The plastic solidifies, and the mould opens and the completed closure is ejected. If the closure is a two-piece design (i.e. fitted with a loose or flowed in-liner), then this is added later. Another technology for moulding plastic closures for the beverage market is compression moulding. A hot pellet of PP is positioned in a mould, similar to the cavity of an injection mould. As the mould closes, the pellet is squashed and defoimed into the shape... 

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). 

Ejection After a cooling period the blow mould opens and the core rod is rotated to the ejection position. The finished article is stripped off the core rod and leak-tested prior to packing. The preform and blow mould can have many cavities, typically three to sixteen depending on the article size and the required output. There are three sets of core rods, which allow concurrent preform injection, blow moulding and ejection. 

After a cooling period the mould is opened and the final article is ejected. To speed production several identical moulds may be fed in cycle by the same extruder unit. The process is not unlike that used for producing glass bottles, in that the molten material is forced into a mould under air pressure. 

Injection moulding A plastic processing technique to produce solid parts with a high degree of precision. The material is injected into a mould by a plunger, and a press keeps the mould closed while the material cools. At the end of the process, the mould is released and the part ejected. 

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