Hot Runner Systems in Injection Molding
Hot Runner Systems
Hot runners are cassifed according to the ways they are heated: insulated-runner sys tems ( It is not described in this artice) and genuine hot-runner systems.
The later can be further sub-cassified according to the types of heating: internal heating and external heating.
Heating is basicaly perormed electricaly by cartridge heaters, heating rods, band heaters, heating pipes and coils， etc. To ensure uniform fow and distribution of the melt, usualy a relatively eaborate control system comprising several heating circuits and an appropriate number of sensors is needed. The operating voltage is usualy 220 V to 240 V, but smal nozzles fequently have a low volage of 5 V, and also 15 V and 24 V operating voltage.
Runner systems in conventional molds have the same temperature leve as the rest of the mold because they are in the same mold block. If, however, the runner system is located in a special manifold that is heated to the temperature of the melt, al the advantages listed below accrue. Runner manifolds heated to mel temperature have the task of distributing the mel as far as the gates without damage. They are used for al injection molded thermoplastics as wel as for crosslnking plastics, such as elastomers and thermosets.
In the case of thermoplastics, these manifolds are usualy refered to as the hot-runner system, the hot manifold, or simply as hot runners. For crosslinking plastics, they are known as cold runners.
A. Hot-Runner Systems
Hot-runner systems have more or less become established for highly-automated production of molded plastic parts that are produced in large numbers. The decision to use them is almost always based on economics, i. e. production size. Quality considerations, which played a major role in the past, are very rare now because thermoplastics employed to day are almost al so stable that they can be processed without dificulty with hot-runner sys tems that have been adapted accordingly.
Hot-runner systems are available as standard units and it is hardly worthwhile having them made. The relevant suppliers ofer not only proven parts but also complete systems tailored to specifc needs. The choice of individual parts is large.
B. Economi Advantages and Disadvantages of Hot-Runner Systems
1 Economic Advantages
Savings in materials and costs for regrind.
Shorter cycles; coolng time no longer determined by the slowly solidifying runners; no nozzle retraction required.
Machines can be smaler because the shot volume - around the runners一is reduced, and the camping forces are smaler because the runners do not generate reactive forces since the blocks and the manifold block are cosed.
2. Economic Disadvantages
Much more complicated and considerably more expensive.
More work involved in running the mold for the first time.
More susceptible to breakdowns, higher maintenance costs (leakage, failure of heating elements, and wear caused by filed materials) .
3. Technological Advantages
Process can be automated (demolding) because runners do not need to be demolded. Gates at the best position; thanks to uniform, precisely controled coolng 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; balancing can be perormed during running production by means of temperature control or special mechanical system (e. g. adjustment of the gap in a ring-shaped die or use of plates in fow channel Natural balancing is beter). Selective infuencing of mold filing; needle valve nozzles and selective actuation of them pave the way for new technology (cascade gate system: avoidance of fow lines, in-mold dec- oration) .
Shorter opening strke needed compared with competing, conventional threeplaten molds. Longer holding pressure, which leads to less shrinkage.
4. Technological Disadvantages
Risk of thermal damage to sensitive materials because of long fow paths and dwel times, especialy on long cyces.
Elaborate temperature control required because non-uniform temperature control would cause diferent melt temperatures and thus non-uniform filling.
C. Design of a Hot-Runner System and is Components
Hot-runner molds are ambitious systems in a technological sense that involve high tech nical and fnancial outay for meeting their main function of conveying melt to the gate with out damage to the material Such a design is demonstrated.
D. Externaly /Internaly Heated Systems
The major advantages and disadvantages of the two types are immediatey apparent from reaserches.
E. Externaly Heated System
1 Advantage
Large fow channels cause low fow rate and uniform temperature distribution.
2. Disadvantage
The temperatures required for external heating have to be very much higher. For PA 66， for example， the mold temperature is approximately 100°C and the manifold temperature is at least 270 °C ; this means there is a temperature diference of approximately 170°C from the mold block, which means:
Special measures required for fxing the hot - runner nozzles to the gates because of the considerable thermal expansions.