A hot runner assembly is a complex combination of heated plastic parts molded into cavities in the hot-runner machine. The cavities are typically the hollow parts of the molded model to be created. Most hot runner machines use extrusion pressure to force hot plastic through the cavities and into the parts. The plastic material flows from the heated portions into the parts, which cools and hardens while it is being shaped. This hot runner mold technique is most commonly used on plastic models, such as plastic model cars or other hot vehicle models.
The basic theory behind how a hot runner mould works is that plastic heated by the extrusion process passes through a chamber that contains a liquid medium. This liquid medium is typically a highly viscous and hot form of gasoline, commonly referred to as Ethylene or tetra-acetic acid, which is injected into the hot runner mould. When the plastic reaches the end of the chamber, it is cooled to create a solid structure of plastic particles. The plastic then cools and hardens while being shaped, as the hot parts continue to flow into the plastic structure. Once the plastic mold is formed, it is generally fired at a high temperature in a high pressure environment, such as a bake-off, low pressure injection machine, or by a hot air gun. These types of plastic injection molds are popular for hot-jetting, hot-line manufacturing, or cold tube casting applications.
A cold runner mould uses a cold roller type of hot runner system. The cold roller type of hot mould system typically consists of a heated plate with continuous rollers that apply for a continuous stream of hot plastic to the cavities. The cavities contain a large volume of solid polymer powder. This polymer powder is heated by a tungsten heater to an appropriate temperature. The resin is injected into the cavitation; however, the speed and duration of heat exposure are variable.
In a cold runner mould, the continuous rollers apply a constant rate of hot plastic materials into a cavity. The cavitation slows considerably once the resin is cooled and the particles of polymer become denser and more resistant to aeration. In this type of system, there is no need for a hot filler material. Therefore, cold runner molds are used for a wide variety of hot plastic materials, such as plastics with high levels of resistance to ozone, most common polyethylene (PE), polypropylene (PP) and polystyrene (PS) sheets.
The advantages of the cold-runner mould are significant when compared with hot-mould methods. The primary benefit is increased production rates. This can be up to five times faster than hot-moulding operations. Because the cavity is internally heated, the plastic materials stay in an elastic state, which means they can be easily shaped and molded, resulting in a better quality product. The plastic materials also have a longer lifespan, because the internal heating process does not change the chemical makeup of the plastic materials.
Cold-runner systems produce material flows that are controlled, because the viscosity of the plastic materials remains low during the process. There is also less need for any kind of cooling unit or ventilation because the material flows are very low during the manufacturing process. This enables the manufacturers to provide greater design flexibility and control, as there is a lower need for expensive cooling equipment. Another benefit of the cold-runner mould is that the temperature of the nozzles can be controlled to a precise temperature. The nozzle designs are typically chosen to obtain the best possible material flow at any given time.
A hot-runner machine will have a cavity filled with molten polymer and a large nozzle. As the hot-runner moves through the mould, material flows from the lower chamber into the upper chamber, and then it makes its way out of the mould onto the casting plate. Once the material has safely made it's way to the casting plate, it must cool down so that it can be inserted into the mould. This cooling down process requires a source of heat in order to force the material down into the mould. The material will cool at a specific rate, which is dictated by the viscosity of the molten polymer, therefore there is a need to ensure that the viscosity is closely monitored and the cooling rate consistently achieved.
The length of time that a molten polymer spends in the mould depends on many factors. This includes the speed of the moulding cycle time, the ambient temperature, the surface tension of the polymer and the type of material being used. If these factors are closely observed, then the amount of time spent inside the mould will be shorter than if these factors were variable. In addition to reducing cycle times, this will also help to ensure that no heat enters the mould. This helps to keep the overall cost down of the tool, as reducing heat entry will reduce the temperature of any meltdowns that may occur, this will reduce the amount of heating required, and therefore reduce the risk of damage to the tools and production machinery.
