Hot runner design must strictly adhere to drawings provided by hot runner supplier.

① Low pressure loss within runner, good melt flowability, uniform density, and significantly reduced internal stress in product, thereby significantly minimizing product deformation, significantly improving surface quality and mechanical properties.
② Runner waste is completely or largely eliminated, resulting in high material utilization.
③ Because added heat generated by waste is eliminated, mold cooling cycle is limited to cooling time of plastic part, shortening molding cycle and improving production efficiency.
④ Hot runners all have automatic gate cutoffs, which can increase degree of automation.

Hot runners can be divided into two main categories: needle valve type and open type. Open type can be further divided into two types: large and small hot runners; as shown in Figure 2-1.

 

Figure 2-1 Types of hot nozzles
① Features of needle valve hot runners: 1. They allow for multiple gates, with valves controlled to achieve sequential flow and balanced material flow. 2. They also minimize gate traces, reducing heat loss from mold and maintaining uniform temperature distribution at hot runner. 3. They use a needle to cut off sealant, leaving no residual sprue. They can be placed directly on non-exterior surfaces of the product without post-processing.
② Features of open hot runners: 1. They do not require valves to control flow, resulting in a simpler structure and lower price than needle valve types. 2. They utilize cooling shrinkage of melt to cut off sealant. There is a certain amount of sprue residue, so it cannot be directly installed at part's glue point. It must be transferred through cold runner system. 3. Wiredrawing will occur at hot gate.

① Because contact area between hot nozzle and mold is high, and valve needle of a needle-valve hot nozzle is in constant motion, contact area is susceptible to wear. Therefore, a nozzle sleeve is required at contact area between hot nozzle and mold. Sleeve is usually made of SKD61 heat-treated material, as shown in Figure 2-2.

 

Figure 2-2 Types of hot nozzles
Typically, nozzle sleeve of a needle-valve hot runner is designed as a conical shape, while nozzle sleeve of an open hot runner is designed as a rectangular shape. This is because needle-valve hot runners generally come into direct contact with product, so the smaller clamping line, the better. A rotary nozzle sleeve can meet this requirement. Open hot runners generally come into contact with a cold runner section, so clamping line requirement is less stringent. Machining process for a rotary nozzle sleeve is complex, while machining process for a rectangular nozzle sleeve is simpler. If requirements are met, choose a rectangular nozzle sleeve.
② Because hot runners involve a relatively fast forming process, individually controlled cooling water must be designed near gate to prevent overheating at gate, which could cause stringing during demolding. This also shortens cooling time and reduces costs. A common cooling method involves running water through nozzle sleeve, as shown in Figure 2-3.

 

③ Each hot runner has heating and temperature-sensing components. Their outlets must face upper side of mold for easy installation, as shown in Figure 2-7 of "Mold Design for Multi-Point Needle Valve Hot Runners."
④ Because hot runner temperature control connector is installed on upper side of mold's fixed mold section, water inlet and outlet of fixed mold section should avoid facing upper side. If this is necessary for structural reasons, drainage grooves should be provided in mold plate, as shown in Figure 2-4. This prevents water from leaking into hot runner system, burning hot nozzle during installation and removal of mold from molding machine.
⑤ Hot runner specifications: Size is generally provided by hot runner company. Length is a fixed series value and is determined according to assembly drawing created by mold designer. Thickness of hot nozzle head can be determined based on available space. If space is limited, a hot nozzle with a smaller tip can be selected.

 

Figure 2-5 Schematic diagram of single-point needle valve hot runner structure
② Schematic diagram of assembly structure of a single-point hot-dip nozzle on mold base is shown in Figure 2-6a). When ordering a mold base, all dimensions and tolerances shown in diagram must be clearly indicated on mold base order drawing for control purposes. Exceeding tolerances will affect operation of nozzle. Furthermore, dimensions of nozzle sleeve and fitting tolerances between nozzle and nozzle sleeve must be carefully controlled, as shown in Figure 2-6b).

 

Figure 2-6 Single-point needle valve hot runner
Note: Design considerations for a single-point open hot runner are similar except for nozzle shape.

① Schematic diagram of multi-point needle valve hot runner structure is shown in Figure 2-7:

 

Figure 2-7 Schematic diagram of multi-point needle valve hot runner structure
② Schematic diagram of assembly structure of a multi-point hot-dip nozzle on mold base is shown in Figure 2-8. When ordering a mold base, all dimensions and tolerances shown in diagram must be clearly indicated on mold base order drawing for control purposes. Exceeding tolerances will affect operation of nozzle.
③ To facilitate nozzle installation and guidance, multi-point hot runner systems require nozzle guide pins. Length of nozzle guide pins must protrude at least 20mm beyond nozzle mouth to protect nozzle, as shown in Figure 2-8.
④ Due to large temperature difference between hot runner plate and hot nozzle, water vapor is easily generated at nozzle mounting hole on hot runner plate. To prevent water vapor from forming droplets and flowing into temperature sensor or heater cable mounting slots, potentially burning them, a drainage groove is provided on ground side of hot runner plate at the lowest point corresponding to hot runner mounting location, as shown in Figures 2-7 and 2-8.

 

Figure 2-8 Schematic diagram of multi-point needle valve hot runner assembly
Note: Design considerations for multi-point open-type hot nozzles are similar, except for nozzle shape.

Single-nozzle hot runners typically have a built-in hose (see Figure 2-5). Free end has a PT1/8" thread and requires a PT1/8" adapter: model JEFS11 (PT1/8" female adapter at both ends). Adapter is then connected to a water quick connector, which is then connected to solenoid valve that controls opening and closing of needle valve via a central 8" hose. Solenoid valve is then connected to high-pressure gas, thus controlling needle valve. Multi-nozzle hot runners do not have their own inlet and outlet hoses. Each nozzle requires an inlet and outlet channel in mold, as shown in Figure 2-7. However, principle is same as for single-nozzle hot runners, except that gas circuit does not require an adapter. To meet needs of molding plants, it is now stipulated that all parts of gas duct that control opening and closing of needle valve using water quick connectors must be purchased from Misumi, model M-PC 8-01. (Quick connector for solenoid valve is not required.)

 

Figure 2-10 Connection of needle valve hot nozzle gas line

① Temperature control box: This controls temperature of hot runner system and heated mold. System's nozzles and manifold heaters are controlled via thermocouples to maintain a molten plastic state. Temperature is optimized based on specific properties of plastic, as shown in Figure 2-11.

 

Figure 2-11 Temperature control box
②) Temperature control wiring: Cable connecting temperature control box to mold has two terminals: a male and a female connector. End connecting to mold is female, while mold itself has a male connector (e.g., if you order a 24-pin terminal block (male connector)). End connecting to temperature control box is male, while temperature control box itself has a female connector, as shown in Figures 2-12 and 2-13.
③ Wiring method for heating mold: When wiring connectors, start with heater wires on upper left side of side with the most reverse connection slots. Arrange them from top to bottom. All odd-numbered rows of connectors can only be connected to heater wires, and all even-numbered rows of connectors can only be connected to temperature sensing wires. Temperature sensing wires are arranged one by one according to their corresponding heaters from top to bottom. Connect temperature controller + terminal to the left of even-numbered rows on the side with the most reverse connection slots, and temperature controller - terminal to the right side with the fewest reverse connection slots. (See Figure 2-14.)

 

Note: Odd-numbered connectors connect to heater wires. Even-numbered connectors connect to temperature sensor wires.
Left-hand side (side with more reverse-connection slots) has "+" temperature control connector.
Right-hand side (side with fewer reverse-connection slots) has "-" temperature control connector.
As shown in figure, red is positive terminal, and black is negative terminal.
④ Temperature control wiring method for hot nozzle is shown in Figure 2-15.

 

Note: Odd-numbered connectors (1, 3, 5, 7, 9, 11) connect to heating wires.
Even-numbered connectors (2, 4, 6, 8, 10, 12) connect to temperature sensing wires.
Left temperature control connector (side with more reverse connection slots) is + terminal.
Right temperature control connector (side with more reverse connection slots) is - terminal.
As shown in figure, red is positive terminal and black is negative terminal. Wiring diagram is as shown.
A 24-pin terminal block can only connect to a maximum of four hot nozzles. If a mold set has more than four hot nozzles, use two or more terminal blocks.
Figure 2-15 Temperature control wiring of hot nozzle on the 24-pin terminal block (male plug)
⑤ Securing junction box to mold: A certain amount of space should be left between junction box and mold plate to facilitate wire routing. This space can be achieved using either a support plate or a support column. Support plate method is shown in Figure 2-16a, and support column method is shown in Figure 2-16b.

 

Figure 2-16 Fixing junction box to mold
Support plate method is generally used in small molds when available space for junction box on A and B plates is limited, as shown in Figure 2-17. Note that support plate should be kept clear of air spaces where there are parting surfaces. Support column method is generally used in large and medium-sized molds when there is sufficient space for junction box on A and B plates, as shown in Figure 2-17. This method facilitates processing and saves material.

 

Figure 2-17 Support plate form
1-8 Hot runner procurement process and precautions
① Purchase process:

 

② Purchase Notes: When purchasing hot runners, you should confirm hot runner supplier's delivery status of hot runner system accessories. If supplier does not provide them, 3D person should draw up a drawing or purchase them. These mainly include: locating rings, cylinder heads, cylinder seals, hot nozzle sleeves, etc.