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Let us consider the methods of  incorporating cavity and core. We have now seen that in general the core is incorporated in the moving half and the cavity in the fixed half. However, there are various methods by which the cavity and core can be incorporated in their respective halves of the mold. These represent two basic alternatives: (a) the  integer methods where the cavity and core can be machined from the steel plates which become part of the structual build-up of the mold, or (b) the cavity and core can be machined from samll blocks of steel, termed inserts, and subsequently, bolstered. The choice between these alternatives constitutes an important decision on the part of the mold designer. The final result, nevertheless, will be the same whichever method of manufacturer is chosen. In either design the plate or assembly which contains the core is termed the core plate and the plate or assembly which contains the cavity is termed the cavity plate.

When the cavity or core is machined from a large plate or block of steel, or is cast in one piece, and used without bolstering as one of the mold plates, it is termed and integer cavity plate or integer core plate. This design is preferred for singke-impression molds because of the strength, samller size and lower cost characteristics. It is not used as much for multi-impression molds as there are other factors such as alignment which must be taken into consideration. Typical mold designs which incorporate an integer cavity and core.

Of many manufacturing process available for preparing molds only two are nomally used in this case. There are a direct machining operation on a rough steel forging blank using the conventional machine tools, or the ‘precision’ investment casting technique in which a master pattern is made of the cavity and core. The pattern us then used to prepare a casting of the cavity or core by special process. A 4-1% nickel-chrome-molybdenum steel ( BS 970-835 M30) is normally specified for integer mold plates which are to be made by the direct maching method. The precision investment casting method usually utilizes a high-chrome steel. For more plastic tools fabrication and parts molding plastic visit our website: http://www.plastics-china.com, thanks!

Basic Terminology
Impression: The injection mold is an assembly of parts containing within it an “impression”  into which plastic material is injected and cooled. It is the impression which gives the molding its form. The impression may, therefore, be defined as that part of the mold which imparts shape to the molding.

The impression is formed by two mold members:
1, The cavity, which is the female portion of the mold, gives the molding its external form.
2, The core, which is the male portion of the mold, forms the internal shape of the molding.

Cavity and core plates: This is illustrated for a simple container. The basic mold in this case consists of two plates. Into one plate is sunk the cavity which shapes the outside form of the molding and is therefore known as the cavity plate. Similarly, the core which projects from the core plate forms the inside shape of the molding. When the mold is closed, the two plates come together forming a space between the cavity and core which is the impression.

Sprue bush: During the injection molding process plastic material is delivered to the nozzle of the machine as a melt; it is then transferred to the impression through a passage. In the simplest case this passage is tapered hole within a bush as shown. The material in the passage is termed the sprue, and the bush is called a sprue bush.

Runner and gate system: The material may be directly injected into the impression through the sprue bush or for molds containing serveral impressions it may pass from the sprue bush hole through a runner and gate system before entering the impression.

Register ring: If the material is to pass without hindrance into the mold the nozzle and sprue must be correctly aligned. To ensure that this is so the mold must be central to the machine and this can be achieved by including a register ring.

Guide pillars and bushes: To mould an even-walled article it is necessary to ensure that the cavity and core are kept in alignment. This is done by incorporating guide pillars on one mold plate which then enter corresponding guide bushes in the other mold plate as the mold closes.

Fixed half and moving half: that the various mold parts fll naturally into two sections or halves. Hence, that half attached to the stationary platen of the machine is termed the fixed half. The other half of the mold attached to the moving platen of the machine is known simply as the moving half. Now it has to be decided in which of the two halves the cavity or core is to be situated. Generally the core is situated in the moving half and the overriding reason why this is so, is as follows:

The molding, as it cools will shrink on to the core and remain with it as the mold opens. This will occur irrespective of whether the core is in the fixed half or the moving half. However, this shrinkage on to the core means that some form of ejector system is almost certainly necessary. Motivation for this ejector system is easily provided if the core is in the moving half. Moreover, in the case of our single-impression basic mold, where a direct sprue feed to the underside of the molding is desired the cavity must be in the fixed half and the core in the moving half.