hotrunner system of plastic mold
The ideal injection molding system delivers molded parts of uniform density free from runners, flash and gate stubs from every cavity, leaving no runners, sprues, etc., for reprocessing (thus resulting in large material savings Hot runner and insulated runner systems have been devised to achieve this goal; they are also called hot manifold molds. In changing over from the earlier style runners in three-plate molds, the plate that delivers the fluid plastics is properly called a manifold. In effect, hot runners are the extension of the heated machine nozzle in the mold.
Alternatively, the cold runner type of mold used for thermosets is properly called a cold manifold mold; in this case, the manifold temperature must be kept below the curing temperature of the material.
Hot runner molds require more time to design, and require greater expense to manufacture than conventional molds. The production cost of molded products is very greatly reduced; other gains are increased production, no regrinding of scrap and the material is not contaminated. The gate mark (vestige) is limited to a small dot.
In consideration of the high cost of runnerless molds, it is desirable to consider the alternatives offered by the several systems, by various zone and entry locations to the manifold and by the type of tip. The design selected must facilitate use of the basic runner and gate system components for subsequent mold cavities that can run in this same base and system at a later date. Specialists are available with stock components that are very versatile and economical. Standard or custom made manifolds can be had with various port or entry locations.
One novel procedure employs a cast hot manifold and achieves the effects of direct gating in multi-cavity molds with integral heaters for the sprue plus hot edge or valves gating.
Other forms of hot manifold molding make use of a hot runner plate with cartridge heaters and with electrically heated exit nozzles gating directly into the part. An alternate is the insulated runner mold that makes use of very large area runners in the third plate.
In this case the “A’，plate and the back-up plate are latched together. Material flowing into the runners hardens as it contacts the runner surfaces but continues to flow or “tunnels” through the center of the runner. IN this case, the outer hardened core of material insulates the central fluid plastics as it moves into the cavity. Such molds use probes or hot tips at the cavity gate point to prevent freeze-up in the gate and to permit slower cycling speeds. Insulated runner molds are simpler than other hot manifold types and the lowest in cost.
Another definition for a hot runner mold is a mold utilizing electrical heating elements in hot tips at the cavity gate points, in addition to heaters in the manifold. An insulated runner mold is a mold utilizing electrical heating elements in hot tips at the cavity gate points in conjunction with a colder manifold section. These basic hot manifold systems without refinement introduced a variety of problems: material drooling or freezing in the gates, balancing flow, packing, thermal control, contaminents clogging nozzles and freeze-up in the runner system. All hot runner systems should have strainers to stop foreign material that could plug the nozzles. This problem is minimized when virgin material only is used.
A little discussed question that merits consideration as a basic issue is: how is the heat distributed in the material as it enters the cavity? There are two basic concepts of this heat transfer process that are to be considered: (1) When the internal heat from the probe is transferred into an annulus type runner, (2) When the heat transfer source is external and the runner configuration is that of a small cylindrical rod. The internally heated probe system is most widely accepted and is dominant in contemporary runnerless molds. The simple cylindrical system is much more costly and its advantages are not as obvious. Annulus type runner systems require careful balancing and sophisticated heat controls. Cylindrical runner configurations do not require close thermal control at every gate.
Heat pipes* are expected to improve the heating and cooling of the plastics by flowing the plastics over a tube. A heat pipe consists of a closed envelope usually a metal tube, containing a capillary-wick structure and a small amount of vaporizable liquid. It functions on the same evaporation-condensation principle as is used in closed-cycled heating and cooling systems. Heat energy from the source is transferred by conduction from end to end through the container wall where the fluid vaporizes. Vapor flows through the core to the condenser where the vapor condenses and returns through the wick by means of capillary action. It is expected that current research on this plan will improve the thermal efficiency of molds.
Many other considerations will dictate the mold design and the higher cost methods merit a careful analysis. One important consideration is to achieve the minimum thermal gradients in the melt as it enters the cavity. Different layers of temperature cause a laminar condition that can introduce warpage after molding.
Essential to good mold design for hot manifold operation is a balanced runner system and calculations that will give effective cooling of the cavity clusters. Thermal insulation for the hot manifold is achieved by the inclusion of air gaps and minimum contact areas between hot and cold portions of the mold. Doubling the air gap spacing increases the thermal insulation a factor of eight. The mold components must not be contacted in any way by the hot runner plates except for the minimal support areas essential for separation of the sections. Transite asbestos, glass-bonded mica are often used to minimize thermal transfer from the hot to cold components. Glass-bonded mica has an advantage since it can be ground to absolute flatness and parallelism the same as steel and it has total dimensional stability up to 700° F. Allowance must be made for differential expansion between the hot manifold element and the colder mold components. This is commonly accomplished by an arrangement of sliding mold sections between the manifold plate and cluster sprue plates. Flat ground surfaces provide the essential sliding surfaces or a piston-cylinder arrangement may also be used. Some type of probe or hot tip bushing is used with annulus gates to maintain temperature at the exit from the manifold. Manifold and tips are heated to maintain barrel temperature in the compound.
The minimal manifold area follows the cavity layout pattern and the manifold is cut to the minimum contour that will contain runners and heating elements. Fundamental manifold design considerations include heaters that parallel the internal runner passages. Thermocouples to maintain desirable temperatures are essential.
Since the hot tips are indexed positively by the cavities which are at lower temperature than the manifold, they must be free to move in compensation for their differential expansion. A positive seal must be maintained between the bushing and the manifold. the hot runner is free to expand in the directions from the fixed end by sliding in the bores of the spruce bushing or nozzle and in the cavity bore. The slidable ends are positively sealed by specially designed metal seals.
In some cases the distribution runner clusters may require external heating from a controlled source.
Hot tips, heated probes, hot sprue bushings and heater casts are important to improved hot manifold molding. By heating at the sprue tip in a heat controlled gate, the melt time is extended and material will not freeze in the sprue. Sprues and probes are heated by being enclosed in a cast beryllium copper body or by the use of an internal heating element; in some cases a thermocouple is included also.
How to clean hotrunner system: