The Injection Machine

Melting (plasticating) the plastic is accomplished in a plasticator (screw in barrel as described in Chapter 3). This melt is forced into a clamped mold cavity. The liquid, molten plastic from the injection cylinder of the injection machine is transferred through various flow channels into the cavities of a mold where it is finally shaped into the desired object by the confines of the mold cavity. What makes this apparently simple operation complex is the limitations of the hydraulic or electrical circuitry used in the actuation of the injection plunger and the complicated flow paths involved in the filling of the mold (Chapter 17). Finally opening the mold to eject the plastic after keeping the material confined under pressure as the heat in the melt is removed to solidify the plastic into the shape desired. 

Raw polymer is fed into the injection machine in pellet or granule form. These pellets are fed into a hopper connected to the end of a cylindrical barrel. Injection molding will accomplish its cycle in separate zones within the apparatus [3], A... 

Unfortunately factors that will affect scorch occur at all stages of the process and are influenced by compound design, mixing, rubber storage conditions, the injection machine, mould design and operating conditions. 

Rubber stock must not be placed upon wooden pallets or on the floor, no matter how clean it is thought to be. Such practice will cause mouldings to contain pieces of everything to be found in a press shop and will certainly lead to unseen and unsuspected damage to the injection machine. 

Some compounds contain fillers such as silica and silicates that have a significantly abrasive effect on the screw and barrel. Special steels may be appropriate to minimise wear, and additionally, the amount of wear should be measured and recorded on a regular basis (at least annually). Changes in machine performance can then be avoided by planned replacement of eroded machine parts. If wear goes unchecked then the performance of the injection machine will change and affect product quality. The first sign of this could be an onset of moulding rejects. 

Plan for the material to be prepared to the correct strip profile for the injection machine. 

When the cavity temperature is stable at the desired value, start to dry cycle the tool, mimicking the expected cycle. After several such operations, and whilst the tool is open, feed the rubber strip into the injection machine pre-plasticiser. This rubber will be used initially to purge the system of the previous rubber used (see Section 6.3 on purging). 

Injection machine. Part volume should be more than 20% of the maximum injection capacity of the injection machine. 

Your mold is done. Make sure the knob iasert and guide pins are in positon. Clamp the mold, together, prepare the injection machine and inject the mold. 

The amount of wear and expected life of the injection machine components are of great importance to the molder. Not only is the equipment rendered unserviceable but worn equipment can cause loss in plasticizing capacity and loss in injection pressure resulting in the inability of the machine to fill the shot or pack the shot properly. This produces unsatisfactory moldings. 

To facilitate colour changes, designs with an openable distributor are used, which makes it easy to clean the channels after removal of the mould from the injection machine. 

In processing practice, the resistance of the plastic to the action of heat governs the admissible time for which the plastic may remain in the injection machine cylinder and the EIR of the mould. 

The task of an HR system is to feed the melt to the mould cavities in the state in which it leaves the injection machine cylinder, although it is not able to adjust the homogeneity of the melt. If an HR system is not correctly designed, selected or built, it may cause disruption to production and flaws in the items moulded. 

Function. Central nozzles and manifold nozzles differ only in that the former are designed to interact directly with the injection machine nozzle. Both a central and a manifold nozzle may be in the form of a hot sprue bushing leaving a small sprue and intended mainly for gating to a cold runner. 

Hydraulic drives. A special pump or else the injection moulding machine s own hydraulic system is used to power a hydraulic drive. The drive system comprises a pressure regulator (the pressures used are 2-8 MPa, depending on the manufacturer) and hydraulic manifolds. The injection machine control system is normally used to control the drive the injection signal is used to open the nozzle and the end of holding phase signal to close the nozzle. Special time relays may also be used to provide control in a different seqnence. 

The channel must be polished. In the centre of the manifold is a sprue bushing (1), on the opposite side is a support pad (3) taking the pressure from the injection machine cylinder and a pin (4) aligning the manifold. A second pin (5) located in the end of the manifold sets its angular position. The manifold is pressed against the HR nozzles located in the mould by pressure pads (6) on the opposite side of the manifold. The transfer from the... 

For a manifold with a single nozzle (Figure 4.61b), it is possible to shift the one injection point from the mould axis if the injection machine does not have the potential for coaxial movement of the injection cylinder. 

A melt filter (Figure 4.83d) is used with small gates which are easily blocked as a result of mechanical impurities in the melt. A filter must be used when recycled material is being injected. The filter may be located in an open sprue bushing (see also Figure 4.60) or in the injection machine nozzle. A drawback to the filter is the substantial pressure loss, as much as 30%, depending on the type of filter. For this reason, filters with a filter area of at least 125 mm are recommended. 

To simplify cleaning, and also colour changes, a split manifold design has been developed, where the manifold may be rapidly dismantled when the mould is removed from the injection machine. The flow channels may be located in one or both halves of the manifold, and they may intersect as a result of the use of heaters set in one end of the channel. The ends of the heaters must be supported to prevent their being skewed by the flow of melt. To clean the manifold, the heating should be switched on, the manifold opened up and the heaters and torpedoes pulled out of the frozen sprue. 

The set is equipped with all the essential power (plug-in sockets), cooling and compressed air connections, and also with installation pillars and bushings, as well as threaded holes for attachment to the cavity plate. The set is thus ready to be installed on the mould, and it is also possible to remove the set from the mould on the injection machine with subsequent ease of replacement of a damaged tip or nozzle heater. The set is tested prior to delivery. 

The design of the enclosed HR set (see Figure 4.97) shown in a position separate from the rest of the mould is based on a similar principle this position is therefore prior to installation with the mould or after opening on the injection machine for servicing of the nozzles. 

Satisfactory operation of the system in practice is attained when a heated torpedo is introduced into the gate opening. The frozen runner is removed on the injection machine after separation of the mould in the channel plane through the use of clamping strips (3), which can be moved to the required mould parting line. 

Since the critical point for many plastics is the time spent by the melt under a thermal load, attention should also be given to the combined time spent in the HR and the injection machine cylinder (see Chapter 3.1). When small HR moulds are used, one major problem is selection of an injection machine small enough not to be operating below the lower limit of the machine s nominal injection capacity (0.25 without an unnecessary increase in the multiplicity of the mould. 

However, angled injection moulding machines, and especially large ones, make up only a tiny fraction of the injection machines in use. 

The manifolds most frequently used have a very long heated sprue bushing located on the mould axis and operating on the principle illustrated in Figure 9.10. As can be seen, the melt flowing out of the sprue bushing may, when the mould is taken away from the injection machine nozzle, gets into the ejector plate space. 

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