Injection Molding Equipments

In specifying an injection molding system it is necessary to decide on the type of injection machine, the size of the machine in terms of shot capacity, platen size, pressure, the number of mold cavities per platen, and the layout and size of runners and gates.

In specifying an injection molding system it is necessary to decide on the type of injection machine, the size of the machine in terms of shot capacity, platen size, pressure, the number of mold cavities per platen, and the layout and size of runners and gates.

Delivery Systems The three main types of injection molding machines were described in Section 9.3. They differ mainly in the way in which they heat, plasticize, and deliver the mix, using a simple ram, reciprocating screw, or separate screw and ram. Screw extruders provide better temperature control and homogeneity and are now generally preferred to the simpler systems.

Simple Ram Machines Whelan's summation [2] of the advantages and disadvantages of these cannot be improved upon and therefore it is quoted directly.
These cost less than other types and deliver mix efficiently to the mold as a result of the ram fitting very tightly in its cylinder. One disadvantage is that, depending on the ratio of the shot volume to the barrel volume there may be three, four or five shots in the barrel, all of which need to be pressed forward to deliver one shot. This causes an undesirable pressure drop and heat generation in the nozzle region of the barrel where it cannot be used to advantage.

Another disadvantage is that the mix is heated slowly by the thermal convection or conduction from the barrel of the machine. It cannot become hotter than the barrel until it is forced through the nozzle into the mold. For products having thin section, these machines may be adequate.

One modification is the insertion of an unheated torpedo in front of the nozzle to conduct heat away from the nozzle region. The separation necessary between torpedo and chamber walls depends on the viscosity of the mix.

Reciprocating Screw Machines Most of the early screw machines were of the reciprocating type but it is now generally accepted that the use of the pre-plasticizing screw as the injection ram

9 Molding of Rubber is only possible for low shot volumes (< 500 ccs) or very soft compounds. The advantage of screw machines in feeding material efficiently plasticized and thermally homogeneous, can only be realized if there is constant intake of feed compound into the feed port. In other words, it is important to ensure that the feedstock is not severed when the screw, acting as a ram, moves forward to inject the shot into the mold.

This can be achieved by having a positive drive arranged so that the feed strip hangs freely over the barrel opening. Another method is to machine a short tangential tunnel undercut at the front of the base of the feed throat where the material feeds into the barrel bore. This acts as a reservoir, or buffer, and smoothes out variations in the feed flow. It eliminates choking or balling up of the material in the feed throat.

Screw Machines with Separate Injection Chambers and Rams

These are the preferred basic design for rubber injection molding machines because they combine the advantages of both screw and ram machine. In the standard ªVº configuration the plasticated compound is fed through a check valve into an accumulation chamber. One disadvantage of this is that the first rubber fed through the check valve is actually the last rubber to be injected. This can lead to adhesion and build-up of rubber on the face of the injection piston, which can cure, break off, and cause rejects and molding problems.

Two modifications have been developed to circumvent this. In the ªfirst-in-first-outº system, the screw and ram, although separate, are in line. Initially, the injection ram is in the forward position and the injection chamber is empty. As compound enters through the ram, it is forced backwards by the incoming material until a limit switch controlling shot volume is activated. Injection then takes place through a special ball-type torpedo, which completes the plastication and thermal homogenization. As the material does not reach the final injection temperature until it reaches the nozzle, temperatures earlier in the system can be relatively low (~ 70 °C). Such inline systems have gained in popularity in recent years.

Shuttle Press and Multi-Station Rotary Press
One disadvantage of the single-station machines is that the injection system is idle during the curing and demolding stages. Shuttle presses are designed for applications where the time to inject and cure is approximately the same as the time required to unload, clean, and service the mold.
It is especially useful for the production of items containing inserts of tall moldings and of moldings with a long stripping time. The press is equipped with two sliding bottom platens, which are individually heated. After the press has opened, both bottom heating platens and molds shuttle, positioning the mold that was inside the press into the unloading station while simultaneously positioning the other mold inside the press.
The main advantage to this format is that unloading takes place on one mold while injection and curing takes place on another mold. Therefore, the time needed for stripping and loading inserts remains entirely outside the cycle time.
A logical extension of two-station shuttle presses is a multi-station rotary press. Several equipment manufacturers produce such systems, which have one injection unit that feeds a number of molds, carried on a rotating carousel. The economic, and practical, number of mold stations depends on capital costs of machine and molds, required production rate, and curing and stripping times. Such machines may have automatic ejection of parts and runner system, cleaning and spraying of the molds, and automatic loading of metal inserts.

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