Dimensional errors have diminished greatly with CNC control. The most common today involves plate flatness and parallelism. This is particularly common with larger systems and plates. An out of tolerance plate can cause nozzle seal damage or slight leakage that is often difficult to pinpoint.

Mechanical failure of the hot runner product is not common, but it does occur. Hot runner systems see intense cyclical pressure and the down sizing of systems to fit more into a mold frame invites fatigue failure. The most common failure is the gate seal. The gate seal is subject to dimensional errors, side loading due to thermal expansion, pressure fatigue and material anomalies. The second most common is a stress crack in the manifold. This can be the result of mounting plates that allow some flexing and is more common in larger systems or bridged systems. One of the most difficult plastic leakage situations to trouble shoot is when an internal crack leads to a threaded hole in the manifold.

Mechanical / Structural failure is still a cause of many hot runner problems. Hot runner systems are heated pressure vessels designed to maintain the temperature of the plastic melt uniformly. The heated pressure vessel must be mounted in, and isolated from, the mold frame. The mold frame is in effect a heat exchanger designed to remove heat.

There are two major engineering challenges: The first challenge occurs when the rigid mounting requirements of the heated pressure vessel encounters heat expansion. The hot runner system must be designed so that at full operating temperature, thermal expansion does not distort plate flatness of the mold frame. This is a common occurrence. Thermal expansion issues related to system mounting are often seen as a bow in the center of the tool. This is the common cause for parting line vent problems in the center of the tool. It can cause other tooling problems related to flash and shut off issues. In a worst case situation, it can crack plates or nozzle tips and manifolds over repeated cycles.

The second challenge concerns management of heat loss at the mounting locations. This heat loss is compensated for by building the pressure vessel with sufficient wattage to overcome loss for mounting. The challenge is to keep uniform temperature balance of the pressure vessel. A naturally balanced or computer balanced manifold can be seen as way out of balance if there are hot and cold spots in the pressure vessel.

Engineers have two tried and proven ways to help maintain uniform heat in the pressure vessel; first, bendable heaters, these enable the engineer to put extra heat where he needs it, and secondly strategic location of cooling lines in the mold frame mounting plates.

In conclusion: When trouble shooting a hot runner equipped mold:

• First look for a bow in the plates. This immediately identifies a thermal expansion issue that can be easily fixed.
  • Watch for broken mounting screws which are often a common sign of a thermal expansion problem.
  • Vents that are closed in the center of the tool are another sign of thermal expansion problems.
• Make sure cooling in the mounting plates is distributed in a way that maintains uniform temperatures. Note that linier expansion problems damage nozzle seals on the outer most areas of the tool.
• Watch for tell tale signs of plate structural weakness including small areas between the top clamp plate and the manifold retainer plate that look polished. Understand how your hot runner supplier has engineered the thermal expansion of your system and make sure that the system is functioning as designed. (Is the system being operated in accordance with the temperature parameters the system was designed for?)

  Finally, part weight imbalance issues, (especially those where part weight tends to vary from shot-to-shot), in perfectly balanced manifolds systems are generally resolved with better heat management between the pressure vessel and mounting plates.

  
  

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