Plunger moulding

PLUNGER STEEL/SILICONE RUBBER M/C (MOULDED SILICONE RUBBER END SEAL)... 

The pole has a eharaeteristie dimension of 8 mm from the rear of the bobbin to the reeess faee and has a toleranee assigned to it of 0.02 mm. From the pole reeess faee, the tube base toleranee is the last eomponent to make up the toleranee staek. The brass tube has been given a dimensional toleranee on its base of 0.2 0.025 mm. Note, the dimensional toleranee on the plunger is 28 0.05 mm, but the analysis will eoneentrate on the silieone rubber seal length of 6 mm beeause this is moulded onto the plunger and again is a mould related dimension. 

In the low-pressure systems a shot of material is injected into the mould which, if it did not expand, would give a short shot. However, the expanding gas causes the polymer to fill the mould cavity. One important form of the low-pressure process is the Union Carbide process in which the polymer is fed to and melted in an extruder. It is blended with nitrogen which is fed directly into the extruder. The extruder then feeds the polymer melt into an accumulator which holds it under pressure (14-35 MPa) to prevent premature expansion until a predetermined shot builds up. When this has been obtained a valve opens and the accumulator plunger rams the melt into the mould. At this point the mould is only partially filled but the pressurised gas within the melt allows it to expand. 

The earliest injection moulding machines were of the plunger type as illustrated in Fig. 4.30 and there are still many of these machines in use today. A predetermined quantity of moulding material drops from the feed hopper into the barrel. The plunger then conveys the material along the barrel where it is heated by conduction from the external heaters. The material is thus plasticised under pressure so that it may be forced through the nozzle into the mould cavity. In order to split up the mass of material in the barrel and improve the heat transfer, a torpedo is fitted in the barrel as shown. 

However, nowadays this type of machine is seldom used because it is considerably more complicated and more expensive than necessary. One area of application where it is still in use is for large mouldings because a large volume of plastic can be plasticised prior to injection using the primary cylinder plunger. 

Example 5.2 In a plunger-type injection moulding machine the torpedo has a length of 40 mm, a diameter of 23 mm and is supported by three spiders. If, during moulding of polythene at ITO C, the plunger moves forward at a speed of 13 mm/s, estimate the pressure drop along the torpedo and the shear force on the spiders. The barrel diameter is 25 mm. 

The solid metal cylinder or plunger which is moved by liquid pressure, air pressure or a combination of both, in such machinery as moulding presses, bale splitters, internal mixer floating rams and bottom doors, etc. 

Machines differ in their mode of heating, plasticisation of the mix and delivery of the mix to the mould cavity. This may be by simple ram (plunger or piston) or by screw. 

The final clamping action forces rubber through the sprue holes leaving a thin (0.13-0.25 mm) transfer pad. The mould is designed with load springs to break open the plates and to help support the top plate and plunger when bottom injection is used. 

To make a cupel, the cavity is nearly filled with hone-ash, which is first compressed slightly with the hands, and afterwards by means of the plunger, D, which is formed of the same metal as the mould, and has the lower end turned convex, so as exactly to correspond to the concave surface of the cupel, When the plunger Is introduced, it Is struck several times with a mallet till the hone-ash is well consolidated. The plunger is then withdrawn, and hy inserting at the lower and smaller end of the mould a solid wooden cylinder, which exactly fits the aperture, the cupel is F1 2i0-readily forced out, and is then placed to dry on the ledge which surrounds the furnace chimney—Figs. 230 and... 

Fig. 12. Schematic diagram of the experimental set-up for plastisol moulding into thin flat moulds 1 — loading of raw materials 2 — damper 3 — pump unit 4 — plunger 5 — discharge tube 6 — inlet channel (round runner) 7 — metallic bottom half mould 8 — top half mould (of lucite for low-temperature moulding and of steel for moulding into hot mould) 9 — pressure gauge 10 — stub tube for control 11 — drain cock 12 — conical flat tube instead of round point runner bolts for controlling the width of forming cavity within the limits of 0.15-1.5 mm...

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). 

Because of the much better temperature homogeneity of the injected material, the required injection pressure is two to three times lower than in a plunger machine, under comparable conditions regions where the polymer is hardly molten cause a considerable increase in viscosity. Plunger machines are, therefore, only used for mass production of very small articles. A variant is the combination of screw and plunger, each in their own cylinder, both of which are discharged into a chamber just in front of the injection point the screw forms and transports the melt, which is the injected into the mould by the plunger. 

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. 

As a mould for high pressure we generally use a cylinder and plunger for rather short stars, but for longer stars a split mould, which is described in 1. 3. 

The composition is loaded into a thin paper tube of about 10 mm in diameter, which is inserted into a copper plate mould with a funnel (Fig.63). The plunger is mm in diameter, which is a little smaller... 

In practice we take the ratio H/D less than 0.8, where H is the length of the pressed composition and D the diameter. To avoid such an inclined distribution of the density another mould like in Fig.65 may be used. The mould has two plungers,A and C, and a spacer D. On pressing D is removed and the composition is pressed from both sides, A and C, but this type of mould is not so popular, because it is somewhat inconvenient for... 

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