Hot Runner

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Post on 10-Apr-2015

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Hot runner system represent the most extensive technological development from conventional runner system to a system ready to be installed.

Large moulding such as automotive dash boards , bumpers,computer housing etc. could be produced in practice without multiple melt supply.

Nozzles upto 8mm length and runners upto 1800mm are applied in such large moulds.

All materials can be moulded in hot runners,even reinforced plastics and structural foam.Certain limitation are applicable for thermally sensitive plastics and in particular flame resistant material.

Saving in material for runner cost for regrind Shorter cycles,time for demoulding runner system is omitted, cooling time is not determined by slow freezing,no runner break off.

Smaller machines are adequate, runner system does not create reactive forces,short size is reduced by volume for runners or larger shot size is available for filling larger number of cavities,shorter opening stroke than for three plate moulds.

mould making has recourse to mould standards.

Omission of demoulding the runner system permits simpler automation of processing. Heated runners allow the realization of long flow paths. Option to place the gate at an optimum location.

Diameter of runners can be kept large resulting in less pressure loss Longer holding pressure is made possible

Balancing of the runner system can be done by temperature control or mechanically Use of shut-off nozzles may avoid formation of knit lines

More rejects at least during start up More work in mould design Higher mould costs for installation More susceptible to trouble such as leak or failure of heating elements.

Hazard of thermal degradation of sensitive materials because of long flow path and high shear velocity No pressure control from gates

HOT RUNNER SYSTEMSThe sprueless moulding principles have been well known since the 50s, even though their use on a vast scale only dates back to the beginning of the 70s. Hot runner injection moulds are gaining wide acceptance as they give cheaper production as a result of shorter cycle times and scrap reduction. The advantages are that they have no sprues, no runners and that the machine plasticising capacity is used almost completely

HOT RUNNER SYSTEMSIn fact, the molten material remains under optimum viscosity conditions while it is being transferred from the feed bush through the runners up to the mould cavities. In the feed runners, the mass of molten material should be kept constantly hot, that is in a fluid state, ready for the new shot. On the conceptual standpoint, hot runners are not thermally influenced by moulding breaks and across their cross sections temperature is always constant

HOT RUNNER SYSTEMSHot runner cross sections may be very small, thereby ensuring constant filling stages over time. Runners into which resin flows must be designed in such a way to have special characteristics

HOT RUNNER SYSTEMS- the size must be such not to create high shear speeds which would result in pressure drops and overheating; - cross sections must not be too small to avoid material degradation and problems in colour change; - runners are to be balanced and sized in such a manner not to cause problems when filling the impressions.

HOT RUNNER SYSTEMS

HOT RUNNER SYSTEMSFurthermore, a number of practical and easy-to-service systems are used to get even temperature. Types most broadly used are: heater bands, cartridge bands, spiral bands, low or high voltage bands, heat tubes. Another critical design element is the gate, both for the nozzle construction and quality of moulded part.

HOT RUNNER SYSTEMSGates are divided into four types: - circular gate; - ring gate; - side (edge) gate; - valve gate; .

HOT RUNNER SYSTEMSCircular shape nozzles, used for non critical materials and applications, are made from high conductivity metals. Thus, they are heated up by simple contact with the distributor; in addition, at the border they are insulated by means of the same resin to inject which flows inside a hollow space located between the bush and mould. In order to maintain even thermal profile along the nozzle, the nozzles must not be too long, in particular L = 2*D. The nozzle can sometimes be equipped with a hot tip .

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HOT RUNNER SYSTEMS

HOT RUNNER SYSTEMS- Circular shape nozzles, used for non critical materials and applications, are made from high conductivity metals. Thus, they are heated up by simple contact with the distributor; in addition, at the border they are insulated by means of the same resin to inject which flows inside a hollow space located between the bush and mould. In order to maintain even thermal profile along the nozzle, the nozzles must not be too long, in particular L = 2*D. The nozzle can sometimes be equipped with a hot tip.

HOT RUNNER SYSTEMSRing type nozzles are similar to circular ones. Theonly difference is that ring types have a central tip which prevents the nozzle from cooling prematurely. The tip may be constructed in a variety of shapes and materials. High thermal conductivity materials can be used (Be-Cu alloys). Sometimes, tips are to be subject to nickel plating processes for major protection as certain polymers (e.g.: PA) react with these alloys; in some other cases, when abrasive resins are to be processed, hard metal is used

HOT RUNNER SYSTEMS

HOT RUNNER SYSTEMSSIDE (EDGE) GATEGates with side entrance are more critical that previous gate styles. They allow side gating of parts and are used for pieces which must be injected at the centre of a hole with multiple injections or for parts which do not accept capillary gate. The insulating material at the bottom of the nozzle is very near to the part surface in the injection area. The width of the gate is to be less than part thickness. Therefore hot resin must be brought very close to the part (0.5-1 mm), but this makes the local, mechanical resistance of the mould quite critical and the part itself is overheated.

HOT RUNNER SYSTEMS

HOT RUNNER SYSTEMSValve gate systems control gate opening and closing through a rod operated via a pneumatic circuit. In this case it is possible to operate with high injection diameters without affecting the part aesthetical appearance. In addition, the larger injection diameter permits very low filling times to be achieved without incurring in speed gradients in excess. This also permits cycle time improvements since lower temperature of both molten material and mould can be used. This also results in shorter cooling times

HOT RUNNER SYSTEMS

HOT RUNNER SYSTEMSValve gate systems control gate opening and closing through a rod operated via a pneumatic circuit. In this case it is possible to operate with high injection diameters without affecting the part aesthetical appearance. In addition, the larger injection diameter permits very low filling times to be achieved without incurring in speed gradients in excess. This also permits cycle time improvements since lower temperature of both molten material and mould can be used. This also results in shorter cooling times

HOT RUNNER SYSTEMSValve gate systems control gate opening and closing through a rod operated via a pneumatic circuit. In this case it is possible to operate with high injection diameters without affecting the part aesthetical appearance. In addition, the larger injection diameter permits very low filling times to be achieved without incurring in speed gradients in excess. This also permits cycle time improvements since lower temperature of both molten material and mould can be used. This also results in shorter cooling times

HOT RUNNER SYSTEMSValve gate systems control gate opening and closing through a rod operated via a pneumatic circuit. In this case it is possible to operate with high injection diameters without affecting the part aesthetical appearance. In addition, the larger injection diameter permits very low filling times to be achieved without incurring in speed gradients in excess. This also permits cycle time improvements since lower temperature of both molten material and mould can be used. This also results in shorter cooling times

The objective of a Hot Runner System is to distribute the material from the machine nozzle to each gate, while minimizing adverse effects to the materials properties.

Two hot runner system types: Complete Hot Runner System Manifold System

A complete hot runner system is supplied to the customer as a fully assembled unit, ready to be fastened to the cavity plate and installed into the machine press

A manifold system includes all components that make up the heated flow path except the hot runner plates. Drawings that outline the necessary plate machining to accommodate the manifold and related components are included with every shipped manifold system

The simplest hot runner mould for replacing the conventional sprue bushing including the reversed sprue busing with a hot sprue bushing or heated nozzle.It feeds directly into the cavity and has the the following advantages:

-Control of temperature -Substantial independence of material and cycle -Suitable for large moulds -Less pressure drop and clean gate separation.

The purpose of the nozzle is to provide a now path for the plastic melt from the machine's cylinder to the mould. In the simplest design the nozzle butts on to the sprue bush of the mould. There are two standard designs in common use and they differ only with respect to the form of seating that is made with-the sprue bush .One design incorporates a hemispherical end and the other design is flatended. The small length of reverse taper in the bore at the front end of the nozzle is such that the sprue is broken just. inside the nozzle. This helps to keep the no

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