Mould multi-cavity

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. 

A mould with a cooling device to allow cooling of the thermoplastic, which will give part solidification and allow its release from the mould. The mould can be mono- or multi-cavity. 

In injection moulding processors often use multi-cavity moulds to make several products in one cycle. The runners required to supply material to each cavity contain a considerable amount of material which, if the product itself is small, may be of the same mass as the parts themselves. This is evidently quite uneconomic. Hot rurmer systems solve this problem by keeping the material in the runners from solidifying between moulding cycles. 

Transfer molding is used for the manufacture of small components and is particularly useful with multi-cavity tools and where small inserts are to be moulded in. Materials used are polyester and epoxide dough moulding compounds, although a new liquid resin injection technique is reported. 

Provide a positive guide to the multi-cavity mould design. 

Direct gating to a cold runner in a multi-cavity mould. In this design, the CR is only partially replaced by an HR system. A simple open HR nozzle eliminates the sprue (Figure 1.3e), thus shortening the reqnired mould opening path and making it easier to separate the sprue protrusion from the moulding on the transporter belt. This method is used when the production volume or product shape does not jnstify the use of HR nozzles for each cavity. 

Gating via a distribution system (indirect) to a cold runner in a multi-cavity mould. 

The drawback to a spring-powered drive is indirect control. The gate is opened under pressnre from the melt, which causes an explosive mould filling, and is then closed as a result of a fall in the melt pressure in the channel. If the fall in pressure in the manifold charmels in a multi-cavity mould is not uniform, there may not be simultaneous gate closure. Spring-driven nozzles are rarely used, but their low price makes them an interesting alternative for simpler technical applications. 

Edge nozzles are used both as central nozzles with one or more injection points, and as manifold nozzles, thus making it possible to design multi-cavity moulds with a reduced number of nozzles. Edge nozzles have enabled the economic problem underlying moulding of small products to be resolved, in much the same way as multi-tip nozzles did. 

Low voltage heating is particnlarly recommended for the injection of very small mouldings in multi-cavity moulds - for two reasons ... 

One disadvantage of this type of heating is the thick, rigid leads (16-25 mm cross-section), requiring extra space in the mould for their location (they make it difficult to place cavities next to one another). Nozzles heated by a 24 V current with a maximum power of 25 A have been developed for applications with a small nozzle pitch or in large multi-cavity moulds, and this has enabled the lead cross-section to be reduced to 4 mm. ... 

For manifold design or selection, the following conditions should be met provision of identical cavity filling conditions in multi-cavity moulds minimisation of pressure losses in manifold channels ... 

Natural balance may only be used for identical products (in multi-cavity moulds), or with symmetrical cavity filling via multi-point gating (in a single-cavity mould). The channel cross-sections are equal or successively diminishing in more branching systems. To achieve balance, it is important to maintain the same size for all gates. 

Another example of the use of sequential injection is a multi-cavity mould for products of varying size, e.g., a box and a cover. The HR system in this case would require rheological balancing, or an alternative would be sequential filling, i.e., successive filling of cavities by nozzles opening one after another. 

Figure 9.7 Multi-cavity mould with in-line layout of nozzles with flat mounting a - method of nozzle installation b - single and double nozzles Reproduced with permission from Gunther Heiflkanaltecknik GmbH)...

Figure 9.18 shows a fragment of a multi-cavity mould for a feeder made of PR The long nozzle (1) is located in a bushing (2) with a double screw cooling channel leading right up... 

This class of material is utilized by the rubber compounder when he has a problem concerned with improvement of material flow in such products as intricate mouldings manufactured by compression or more often by injection moulding. To be economic, injection moulding needs compounds which will fill the mould rapidly and which will have consistent flow characteristics to ensure that all cavities of multi-cavity moulds are filled to make good products. 

Unlike applying the elastomeric encapsulants previously described, transfer moulding requires the use of a press capable of pressures up to 3 or 4 MPa and temperatures of about 175X, in combination with precision multi-cavity steel moulds which may hold several hundred discrete devices. For these reasons, transfer moulding is only justified for high-volume production (see Chapter 5). 

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