When using semi-crystalline engineering thermoplastics for injection molding, the correct choice of hot runner determines the function of the mold and the quality of the part. In this case, the temperature control must be more stringent than that of amorphous plastics. The type and installation of the hot runner system determines the performance of the components. This article describes the key issues that should be considered when selecting the right hot runner system for POM (polyoxymethylene), PA (nylon), PBT, and PET (polyester).
What can go wrong with using the wrong hot runner?
Improper hot runner systems can cause high pressure losses and can only be used at very high temperatures, which can lead to polymer degradation and the consequences described in Part 5 of this series, "Wrong Melting Temperatures". Due to localized overheating, streaks, stains, and surface defects may also occur. The degradation of plastic can lead to blistering and other adverse consequences.
What are the key factors that should be considered?
All of the plastics mentioned above have a recommended range of melting and curing temperatures, which requires effective thermal isolation of the hot runner from the runner and nozzle.
In order to use a naturally balanced runner, it is better to design the nozzle. This is the only way to ensure a uniform pressure loss and the same residence time of the melt in all cavities.
For small injection volumes, indirect gates are more suitable than direct gates, especially for glass fiber reinforced plastic materials. material through spray #x5634; The amount of heat generated in the polymer is gradually increased, so that the heat generated in the polymer is more controllable. The gates used for hot runner nozzles can be large, while parts machined using traditional methods have smaller gates. In any case, the cold material cavity should be located facing the hot runner, which is the only way to prevent cold material from entering the molded part through the nozzle.
The inlet, runner and each nozzle of the hot runner should be controlled separately so that the mixture of heat sensitivity of each component is balanced separately. Conditioning equipment should be used to guarantee that the temperature is kept constant during adaptation to the energy supply (e.g., PID).
Hot runner systems should use the same method for mechanical support as ejection systems. The mould is weak in the vicinity of the runner and must be compensated as much as possible. The use of a separate heating circuit close to the hot runner nozzle allows the mold surface temperature to reach the correct set point.
Hot runner and nozzle selection guidelines
Runners with fully criss-cross and system-integrated thermal conductors are the best choice. A central heating system with only one ring crossing can produce excessive pressure losses and should be avoided where possible.
For highly heat-sensitive materials such as polyoxymethylene and flame retardant grades, the bypass in the flow channel should be designed as perfectly as possible.
The nozzle should be an open external heating system with a complete criss-cross. The melt should not be divided into several strands in the gate area. The distribution of the feed connection should be adapted to the conditions of the embedding, so that an average temperature distribution can be obtained. If abrasion-resistant materials must be used to manufacture the nozzle heads, interchangeable nozzle heads can be used, which is an advantage. In addition, a small torpedo head nozzle head can be used if required.
Closed nozzles should generally not be used when processing acetal. If other plastics prescribe the use of needle valve nozzles, the nozzle/needle combination should be such that the pressure loss is as low as possible. If the above recommendations are followed, there are many different types of hot runner systems available in the market that can achieve good results.
