The decision to invest in a hot runner system is a major step in reducing molding costs. Some of the major benefits of adding any type of runnerless molding technology include: reduced or eliminated regrind, decreased cycle time and improved process capability. The selection of the right system for any given application will enhance the financial benefit by minimizing mold building costs and maintenance expenses over the life of the mold.

Purchasing the Hot Runner System
A key component in any hot runner system is the nozzle or drop. Nozzles fall into four main categories: torpedo, valve gate, edge gate and multi tip. Edge gate nozzles are used to gate on a part at 90 degrees to the parting line. There are two varieties of such nozzles.

The sealed edge gate is the traditional design, but is limited by its ability to process some resins (e.g., PC). Hot tip edge gates, on the other hand, are able to process a much wider range of resins, but one should pay attention to the design of such products. Some hot tip edge gate designs result in more costly mold construction because they require split cavity blocks to allow for the installation of the nozzle.

When pursuing a hot tip edge gate, ease of tip installation from the parting line should be a major deciding factor. Such nozzles do not require split cavity blocks and offer parting line accessible tips for ease of installation and removal. Also, dummy tips may be installed in place of a standard tip when a cavity needs to be shut down.

Hot runners may be purchased in the form of a bolt-on hot half that directly bolts to the cavity plate. These packages are delivered as pre-assembled and wired assemblies ready to attach to the cavity plate. A hot half greatly reduces the time needed by the moldmaker to complete the final mold assembly; however, all hot halves are not built the same way or offer the same degree of serviceability.

Most hot halves are designed consisting of two plates: the manifold frame/nozzle retainer plate—which houses the manifold and nozzle—and the top clamp plate. Traditionally, nozzles are installed from the back of the nozzle retainer plate and all heater and thermocouple wiring is routed within this plate.

Removal of nozzles for maintenance requires the complete disassembly of the hot half. A more logical and maintenance-friendly approach is to purchase a hot half with front mount nozzles. A hot half of this design has the nozzles installed from the front of the nozzle retainer plate and the wiring is contained in channels machined on the front face of the nozzle retainer plate for ease of accessibility.

The advantage is the easy removal of the nozzle by simply splitting the cavity plate from the hot half thereby exposing the entire nozzle and its wiring. The nozzle can be removed without disassembly of the hot half. In fact, the nozzle may be removed and/or exchanged quickly while the mold is still in the press if steps are taken in the design and build of the mold to latch the cavity plate to the “B” side of the mold.

Do It Yourself
Some mold builders will elect to buy the hot runner components, machine the hot half plates and install the system themselves to save money. In these cases, there are two options available.

The first is to purchase the standard hot runner components as individual items. The system will be delivered unwired and requires assembly. The installation entails installing the nozzles and manifold then wiring all components and testing the electrical aspects of the system. Although there may be a perceived cost advantage, there is a risk in doing this. Incorrect installation or failure to machine the plates correctly to meet the cold clearance requirements of the hot runner within the manifold frame plate can result in a catastrophic leakage of the hot runner. Also, the installation of the hot runner components will consume valuable mold building time not to mention the cost of labor.

A more attractive solution is to purchase a unitized or drop-in hot runner that is delivered in a fully assembled state and ready to—as the name implies—drop into the hot half plates machined by the mold builder. The nozzles are screwed into the manifold at the factory thereby eliminating any potential for leakage even if the cold clearance is not correct. The systems are shipped pre-wired and tested. In the case of valve gate systems, they are also pre-plumbed and tested. The advantage to the moldmaker is a greatly reduced installation time and the elimination of any risk.