Hot runner mold
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Hot-runner mold - injection mold in which the runners are kept hot and insulated from the chilled cavities. Plastic freezeoff occurs at gate of cavity; runners are in a separate plate so they are not, as is the case usually, ejected with the piece.
Hot runner molds are two plate molds with a heated runner system inside one half of the mold.
A hot runner system is divided into two parts: the manifold and the drops. The manifold has channels that convey the plastic on a single plane, parallel to the parting line, to a point above the cavity. The drops, situated perpendicular to the manifold, convey the plastic from the manifold to the part.
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Types of Hot Runner Molds
There are many variations of hot runner systems. Generally, hot runner systems are designated by how the plastic is heated. There are internally and externally heated drops and manifolds.
Externally heated hot runners
Externally heated hot runner channels have the lowest pressure drop of any runner system (because there is no heater obstructing flow and all the plastic is molten), and they are better for color changes none of the plastic in the runner system freezes. There are no places for material to hang up and degrade, so externally heated systems are good for thermally sensitive materials.
Internally heated hot runners
Internally heated runner systems require higher molding pressures, and color changes are very difficult. There are many places for material to hang up and degrade, so thermally sensitive materials should not be used. Internally heated drops offer better gate tip control. Internally heated systems also better separate runner heat from the mold because an insulating frozen layer is formed against the steel wall on the inside of the flow channels.
insulated hot runners
A special type of hot runner system is an insulated runner. An insulated runner is not heated; the runner channels are extremely thick and stay molten during constant cycling. This system is very inexpensive, and offers the flexible gating advantages of other hot runners and the elimination of gates without the added cost of the manifold and drops of a heated hot runner system. Color changes are very easy. Unfortunately, these runner systems offer no control, and only commodity plastics like PP and PE can be used. If the mold stops cycling for some reason, the runner system will freeze and the mold has to be split to remove it. Insulated runners are usually used to make low tolerance parts like cups and frisbees.
Disadvantages
Hot-runner mold is much more expensive than a cold runner, it requires costly maintenance, and requires more skill to operate. Color changes with hot runner molds can be difficult, since it is virtually impossible to remove all of the plastic from an internal runner system.
Advantages
They can completely eliminate runner scrap, so there are no runners to sort from the parts, and no runners to throw away or regrind and remix into the original material. Hot runners are popular in high production parts, especially with a lot of cavities.
Advantages Hot Runner System Over a Cold Runner System include:
- no runners to disconnect from the molded parts
- no runners to remove or regrind, thus no need for process/ robotics to remove them
- having no runners reduces the possibility of contamination
- lower injection pressures
- lower clamping pressure
- consistent heat at processing temperature within the cavity
- cooling time is actually shorter (as there is no need for thicker, longer-cycle runners)
- shot size is reduced by runner weight
- cleaner molding process (no regrinding necessary)
- nozzle freeze and sprue sticking issues eliminated
Process: Injection molding cycle Reaction injection molding, Reinforced reaction injection molding, Liquid injection molding, Gas assist injection molding, Coinjection molding, 2-shot injection molding, Fusible core injection molding, Rapid injection molding, Micro injection molding
Equipment: Injection molding machine(Vertical, Horizontal), Injection molding press, Injection mold (Hot runner, Cold runner), Injection unit (Injection ram, Injection screw), Clamping unit (Toggle , Hydraulic, Hydromechanical), Plunger injection molding, Injection molding machine manufacturers, Types of injection molding machines
Terms: Injection pressure, Injection rate, Clamping pressure
See also: Plastic thermoforming • Blow molding • Plastic welding • Plastic extrusion • Rotational molding
