Hot Runners

Published on November 2017 | Categories: Documents | Downloads: 45 | Comments: 0 | Views: 456
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INTRODUCTION

ADVANTAGES&DISADVANTAGES

The use of hot runner systems is increasing due to the fact that they provide a more efficient method of molding plastic parts. The cost savings can be substantial, and the part quality is very often much better than a cold runner system. With the increasing sophistication of today's plastic injection molds, the use of a hot runner system is becoming an industry standard. Many applications simply are not possible with the old standard sprue and cold runner system, and hot runners offer a great way to maintain uniform wall thickness and eliminate knit lines and flow lines. Hot runners offer the following advantages: − No loss of melt and thus less energy and work input. − Easier fully automatic operation. − Longer holding pressure, which leads to less shrinkage. − Shorter cycles; cooling time no longer determined by the slowly solidifying run‐ners; no nozzle retraction required. − Machines can be smaller because the shot volume, around the runners, is reduced, and the clamping forces are smaller because the runners do not generate reactive forces since the blocks and the manifold block are closed. − Superior quality because melt can be transferred into the cavity at the optimum sites. − Gates at the best position; thanks to uniform, precisely controlled cooling of the gate system, long flow paths are possible. − Pressure losses minimized, since the diameter of the runners is not restricted. − Artificial balancing of the gate system can be performed during running production by means of temperature control or a special mechanical system (e.g. adjustment of the gap in a ring‐ shaped die or use of plates in low channel). The disadvantages are: − Considerably higher costs involved. − Much more complex to work with. − More work involved with the start up of the production cycle. − Thermal isolation from the hot runner manifold block is problematic. − The risk of decomposition and production stoppages in the case of materials with low thermal resistance. − More susceptible to breakdowns, higher maintenance costs (leakage, failure of heating elements, and wear caused by filled materials). − Risk of thermal damage to sensitive materials because of long flow paths and dwell times, especially on long cycles. − More efficient temperature control required, because a non‐uniform temperature cycle will cause different melt temperatures and thus non‐uniform filling.

INSULATED RUNNERS Insulated runner moulds have oversized passages formed in the mould plate. The passages are of sufficient size that, under conditions of operation, the insulated effect of the plastic (frozen on the runner wall) combined with the heat applied with each shot maintains an open, molten flow path. The main advantages of the insulated systems over the heated systems is the low cost, the ability to have thorough color changes, low pressure loss due to the large diameter runners, and that it introduces less heat to the part‐forming cavities. The moulds are designed such that a parting line is located along the melts flow channel. This parting line can easily be opened while the mould is still in the moulding machine. Once the melt has frozen, this parting plane can be opened and the runner completely removed. This technique provides for very thorough color changes as well as for servicing the mould if the runner inadvertently freezes off. If this system is used, it is normally limited to low tolerance parts and commodity plastics such as polyethylenes, polypropylenes, and polystyrenes.

Internally heated manifold and nozzle. INTERNALLY HEATED RUNNERS

This system takes advantage of the insulating effect of the plastic melt to reduce heat transfer (loss) to the rest of the mould. The second consists of a cartridge‐heated manifold with interior flow passages. The manifold is designed with various insulating features to separate it from the rest of the mould, thus reducing heat transfer (loss). Internally heated systems eliminates most leaking problems, provides good isolation of the heater from the surrounding mould and provides good gate tip control. As heaters are internal, there is no need for a separate manifold block, which must be heated and insulated from the surrounding mould with an air space. Plastic around the perimeter of the flow channel freezes against the colder mould, solidifies, and provides a thermally insulating boundary of plastic. This reduces the challenge with externally heated hot runner systems of insulating the heat from the part‐forming cavity where you are trying to freeze the plastic. In addition, without the air gap surrounding the manifold in an externally heated system, leakage concerns are virtu‐ally eliminated. Therefore, these systems are not recommended for use with thermally sensitive materials. In addition, the pressure drop during mould filling will be the highest of any of the hot runner molds per amount of material in the runner. The flow cross section can be increased to reduce the pressure but will result in increased residence time for the molten material. The development of the frozen layer around the perimeter of the flow channel continues to create a challenge with regard to color changes.

Internally heated manifold and nozzle.

EXTERNALLY HEATED RUNNERS Externally heated systems have the ability to provide the lowest pressure drop com‐pared to other systems, except possibly for insulated hot runners. The flow channels are cylindrical in cross section and generally have a larger diameter than a cold runner system. The cylindrical flow channel is the most efficient shape for melt flow. As cooling of the runner and regrind are not issues in the case of hot runners, a larger diameter is permissible. Both the larger diameter and the fact that there is no growing frozen layer in the runner system, contribute to the relatively low pressure drop of these systems. This type of system may also provide the most homogeneous melt temperature of all hot runner systems. A melt injected at a temperature of 260 ºC can be surrounded by a flow channel that is heated at 260 ºC. Therefore the temperature gradient across the flow channel can be reduced, in contrast to an internally heated system, which is surrounded by a cold wall. Disadvantages of an externally heated runner system include the potential for leaking of molten plastic and the amount and location of the required heat from the heaters. Improper design or operation can result in plastic leaking between the nozzle and the manifold. This leaking plastic can engulf the manifold, entombing it and destroying heaters, wires, and thermocouples. In

addition, the external heat source is in direct conflict with the cooling of the mould. The hot nozzles often must be surrounded by an airspace, which helps insulate them from the cavity.

Externally heated manifold and nozzle. Advantages and disadvantages of hot‐runner systems.

MAINTENANCE OF HOT RUNNER SYSTEMS

Hot runner systems require a new level of expertise on the part of the mold maker. However, this does not mean that it is overly complicated or highly specialized. A well designed system is relatively free from maintenance, once it is properly installed. It can, and does happen, that mold makers become frustrated with hot runner systems and swear that they are too much trouble. This is usually due to the fact that the system is not properly installed or incorrectly operated in the molding process.

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