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Present-day technologies such as insert, coinjection and multicomponent molding would not be possible without hot runner systems. Large parts such as automobile bumpers can be produced only with hot runner systems due to the restrictions on the flow path/wall thickness ratio and the presence of knit lines. Hot runner technology is presently playing an increasingly important role in injection molding production.

Although the hot runner system is incorporated within the injection mold, its tasks and functions are completely different from those of the mold itself. The system constitutes a self-contained unit with specific, highly precise requirements placed on its installation, connection and operation. For this reason, state-of-the-art hot runner systems increasingly are being supplied in ready-assembled form as injection halves. This prevents installation errors, simplifies connection of the system and reduces the amount of time required for installation in the mold.

Advantages of Unitized Systems
On unitized systems, the nozzles and manifold block constitute a single drop-in unit. The melt channel runs from the manifold block directly to the nozzle without any potential misalignment or dead spots. Any potential for plastic leakage between the nozzles and the manifold block is eliminated since the nozzles are inserted within the manifold by a thread connection. This eliminates the tendency for plastic leaks between the nozzles and manifold, which can occur during heat expansion of conventional bushing-type system designs. Production is then interrupted in order to effect repair.

Unitized systems are centered in the injection mold with minimal contacts made of materials that are poor heat conductors and do not require any clamping or pre-tensioning of the mold plates. The minimal contact in unitized systems provides highly accurate and stable temperature profiles, with energy consumption substantially below that for conventional systems (see Figure 1).

Figure 1: Unitized hot runner system (also shown integrated). Nozzles are threaded into the manifold and wiring is placed in covered channels. The system becomes a self-contained drop-in assembly.

The design of unitized systems makes it possible to install prewired hydraulic connections directly on the system, independent of the mold to be used. The hydraulic mechanism driving the valve gate also can be mounted directly on the system, making the injection mold more flexible and enabling it to be used on machines without additional control valves. The electrical and hydraulic connections are configured to customer specifications (see Figure 2).

Figure 2: Twelve-cavity unitized system with electrical and hydraulic connections, including control manifold and valves.

All the systems undergo electrical continuity, temperature and pneumatic or hydraulic tests prior to delivery. The customer thus obtains a checked, ready-to-install system that can be easily assembled in the mold and brought directly into operation.

When routine maintenance needs to be performed on the system or the mold, the unitized system can be removed from the mold again by this same simple procedure. It can then be tested or repaired independently of the mold.

Conventional Hot Halves Designs
Hot halves were developed from hot runner systems; the nozzles are not permanently attached to the manifold block, but float on the nozzle flanges. These systems require retainer plates to keep the system together (see Figure 3).

Figure 3: Structure of a hot half.

Given that the average temperature differential between the hot runner system and the mold is approximately 200'C for most plastics and that the system is in contact with the mold plates, it is possible for increased temperature and energy losses to occur. Dead spots also may result at the transition between the manifold block and the nozzles.

Hot halves typically must be completely dismantled when the hot runner needs servicing. Since the nozzles are not connected to the manifold, the electrical and hydraulic connections must be uncoupled-either entirely or in part-and clamped together again once repairs are completed. This involves a considerable amount of extra work compared with unitized systems, with a renewed risk of assembly and installation errors.

Integrated Hot Runner Systems
The unitized system offers considerable improvements in quality and addresses the cost of repair favorably. Since the integrated system is based upon using the unitized prewired design for installation in the mold, the chance for pinched or severed heater and thermocouple wires is eliminated during assembly. All wiring from the heater to the junction box is encased within a conduit specially designed for that purpose. This is always a benefit whenever the mold/hot runner system is disassembled. Also, since the unitized manifold system is fully functional before being installed in the mold, full testing of electrical and thermal functions are performed at the factory before shipment.

An integrated hot runner system further provides the injection molder the added benefits of the following:

• Routine maintenance, which can be performed without removing the hot runner system from the molding machine. While conventional injection halves also allow some maintenance to be performed when in the molding machine, it is limited to the very front of the nozzle area due to the retainer plate. Servicing beyond this point requires the hot runner manifold system to be removed from the machine in order to expose the manifold system.
• Simple hot runner system/mold interchangeability. Inserts can easily be interchanged without mold removal.
• Valve gate operational plumbing, which can be serviced without disassembly.

• Gate tip exchange
• End cap exchange
• Nozzle heater exchange
• Nozzle thermocouple exchange
• Manifold thermocouple exchange
• Complete hot runner inspection

	
Figure 4: Integrated hot runner valve gate system. Valve gate connections and plumbing are also attached to the manifold and become part of the self-contained assembly.		Figure 5: The mold strap in the position shown will capture both the core and cavity plates as the moving platen is retracted.

	
Figure 6: Remove the socket head screw used to fix the cavity plate to the top plate.		Figure 7: Integrated hot runner system is fullly exposed inside the press.

	
Figure 8: If necessary, only the top plate and the integrated hot runner system must be removed from the press.		Figure 9: Once outside the press, the integrated hot runner system is easily detached from the top plate.

Figure 10: Once outside the press, the integrated hot runner system can be completely refurbished. When complete, the integrated hot runner system only must be re-installed for further production.

Since the hot runner system remains firmly attached to the stationary plate during this work, the mold can be repositioned again immediately after the service work is complete and can be ready for production again in just a few minutes. Integrated systems can thus save several hours-or in large systems perhaps days-during routine hot runner system maintenance.

Unitized systems provide a considerable contribution to reducing the number of errors and saving time for mold manufacturers during the manufacturing and sampling of new molds (see Chart 1). In addition, integrated systems permit additional savings for injection molders by reducing downtime during injection molding production.