Nozzles, Runners, and Gates


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The rubber is injected into the mold through the nozzle and, obviously, with multi-station systems, its shape and outside dimensions have to be such that it mates positively with each mold. As the inside nozzle diameter is increased, the pressure drop, injection time, and temperature rise, decrease. Nozzle diameters are chosen to give a temperature rise of ~ 25 °C and an injection time of 5 to 10 s. This balance is adjusted, if necessary, to avoid scorch during mold filling.
Nozzle diameters may range from 3 mm for a shot volume of 500 cm3, to 10 mm for a 4,000 cm3 shot volume. Nozzles usually have a reverse taper of 4 to 5 degrees and as short a land as possible. The nozzle bushing on the mold also has reverse taper and an inlet bore the same or slightly smaller than the nozzle.

These are the channels provided to convey the rubber from the injection point, the sprue, to the mold cavities; in cross section, they are usually trapezoidal, half-round, or round. They should be as short and direct as possible, and streamlined to reduce pressure loss. The thickness of the molding at the gating point governs the cross-section size of the runner. For multi-cavity molds, the lengths of runner channel between the sprue and each cavity should be the same so that all cavities are filled at the same rate and have the same effective pressure. If the product has to meet close tolerances, this requirement becomes essential. Where the flow branches there should always be a blind wall at 90°, this gives balanced flow as the leading edge of the flowing rubber hits the wall and divides equally.

If the runner system is extensive, venting the runners is advisable, to help reduce the volume of air and gases which have to be removed via the cavity vents and flashlines.

A reduction in scrap can be achieved by using a ªcoldº runner system. This depends on having a temperature-controlled platen containing the runners, which are constantly full of rubber at a temperature below that at which it will cure. From here, it is injected into the rest of the mold, from which the cold channel platen is effectively insulated. Obviously sufficient heat has to be generated in the hot channel run to raise the rubber temperature to the required level for curing. Gates
These are restrictive passageways from the runner to cavity which are sized to allow easy flow of the rubber into the cavity and at the same time raise its temperature to the final desired level. Generally they are full-round, fanned, with a bore one-half of that of the feed runner and as short as possible. The final sizing of gates is usually done by carrying out a practical molding test.
The gate position is often more important than the size or type of gate. It should be positioned so that it feeds the thickest section of the molding, preferably in the non-stressed area of the molding. If weld lines are likely to be a problem, the gate position is critical.
Obviously, the major design consideration with a mold is the shape and size of the required molding. However, within this restriction, there are a number of points that have to be considered in determining the optimum mold design for a particular product. Turk lists general mold design requirements, which are summarized here.
Molds should be rigid enough to avoid deflection under pressure, which can cause flashing.
• Guide dowels and bushes used in lining up the mold halves should mate easily and lock rigidly. They should also be sized to allow for thermal expansion.
• The working faces of the mold, particularly on the split line or closure faces, should be kept as clear as possible of screw fixing holes and joints.
• The use of good quality toughened steels is advisable, particularly for the areas that come into contact with rubber, to reduce wear.
• All surfaces coming into contact with the rubber should be polished and the cavities and core pins should be plated. This ensures good surface finish on the molded product.
• Shrinkage allowances must be made in the cavities and on the core pins. These depend on the type of rubber compound and such processing variables as mold surface temperatures, injection pressures, and cure times. Allowances should be made in the mold
Injection Molding of Rubber manufacture so that the final sizing of the cavities and core pins can, if necessary, be carried out after initial testing and proving.

• In the design of the molds, the transfer of the mold into and out of the machine has to be considered. Suitable tappings, holes, etc. should be provided for the attachment of lifting devices.

• Shallow grooves from the cavities to atmosphere on the split line of the mold and on the opposite side to the gate or feed points should be provided to allow escape of air and volatiles.

• Trim or tear off grooves should be provided around the cavities to give clean moldings and minimize extra after-trimming operations. The rubber is allowed to flow out through very narrow lands between the cavities and the trim grooves, the thickness of the rubber across the land being so thin that on removal of the moldings it is possible to tear off the excess rubber anchored to the moldings and formed by the groove cavities. To minimize flash, it is essential to achieve a good bite between the split line faces of the mold when clamping force is applied.

This can be achieved by providing land faces around the feed runner, gate, and cavity areas.

Automatic Ejection

To take full advantage of the short cure cycles obtained with injection molding, an automatic ejection system is required [2,8].

Thin sections can be stripped from the mold by a compressed air jet. Simply shaped and flat parts can be removed by a rotating brush. Delicate parts can be removed by a robot pulling device, which places each part on a turntable, rather than ejecting it into a tote box. Many rubber moldings, however, do not lend themselves to automatic ejection, but assistance in removal can be provided by mechanical or hydraulic mechanisms, which retain the molding in the most suitable part of the mold for easy stripping.

Ejection is better suited to thick moldings with good hot tear strength and those with bonded metal inserts. When ejector pins are used to push directly on the rubber, a mushroom or miter type of pin is recommended in preference to a straight pin, which may be inhibited in its action by rubber stretching.

The ideal mold design would produce either no flash or would make flash removal simple, as pointed out above. However, some parts, especially multi-cavity molds, will need deflashing. There are a number of techniques for deflashing. The most efficient involves chilling to ±150 °C and then tumbling or shot blasting. The latter will handle complex parts, even items with inaccessible internal flash.