For previous reading, please refer to Mold Design Guidelines (Two-Color Mold).
Runnerless molding molds are injection molds designed for thermoplastic materials. They utilize heating or insulation to keep material in runners constantly molten, achieving runnerless or minimal runnerless molding.
Runnerless molding molds offer many advantages, primarily:
High material utilization and full utilization of injection molding machine's plasticizing capacity. Low pressure loss in runners facilitates cavity filling and shrinkage compensation, preventing sink marks, shrinkage cavities, and deformation in molded parts. Shorter molding cycles and increased production efficiency. Automatic gate cutting enhances automation. Reduced injection pressure allows for smaller clamping tonnage.
Runnerless molding molds also have their disadvantages, primarily:
Molds with hot runners have increased closure height, potentially requiring larger injection molding machines. Heat from hot runner nozzle and hot runner plate affects temperature of front mold through thermal radiation and thermal conduction. Therefore, mold design should minimize heat transfer and enhance front mold cooling. Mold costs are relatively high.

After years of development, runnerless solidified molding molds now primarily adopt following two structural forms:
1. Molds using hot runner nozzles for direct or indirect feeding, referred to as hot runner nozzle molds. Their basic structure is shown in Figure 13.1.1.

 

2. Molds with hot runner plates and two-stage hot runner nozzles, referred to as hot runner molds. Their basic structure is shown in Figure 13.1.2.

 

13.1.1 Examples of Hot Runner Nozzle Mold Structures
(1) Hot runner nozzle mold structure with point gate feeding, as shown in Figure 13.1.3. This structure is only suitable for single-cavity molds and is limited by gate location.

 

(2) Hot runner nozzle mold structure with hot runner nozzle end face participating in molding, as shown in Figure 13.1.4. Suitable for single-cavity molds, with nozzle marks on the surface of plastic parts. End face of hot runner can be machined.

 

(3) Hot runner mold structure with some conventional runner forms, as shown in Figure 13.1.5. This type of mold can mold multiple plastic parts at the same time, but disadvantage is that it will produce some cold material in runner.

 

13.1.2 Examples of hot runner mold structures
(1) Hot runner mold structure with end of secondary hot runner participating in molding. As shown in Figure 13.1.2.
(2) Hot runner mold structure with secondary hot runner needle-point feeding. As shown in Figure 13.1.6
In addition, various different mold structures can be produced according to structure and feeding method of secondary hot runner, but their basic requirements are same.

 

(1) Injection volume: Appropriate hot runner should be selected according to size of plastic parts and different plastic materials. Suppliers will generally give maximum injection volume of each hot runner relative to different flowability plastic materials. Because different plastic materials have different flowability. In addition, attention should be paid to size of hot runner nozzle, as it affects not only injection volume but also other factors. If nozzle opening is too small, molding cycle will be prolonged; if nozzle opening is too large, it will be difficult to close, leading to drooling or stringing.
(2) Temperature Control: Temperature of hot runner nozzle and hot runner plate directly affects normal operation of mold, and their temperatures are generally controlled separately. Regardless of whether internal or external heating is used, temperature in hot runner nozzle and hot runner plate should be kept uniform to prevent localized overheating or undercooling. Furthermore, heater power should be sufficient to raise hot runner nozzle and hot runner plate from room temperature to required operating temperature within 0.5 to 1 hour; heating time for hot runner nozzle can be even shorter.
(3) Insulation: Hot runner nozzle and hot runner plate should have good insulation from mold panel, A-plate, and other parts. Insulation media can include asbestos boards, air, etc. Except for contact areas such as positioning, support, and sealing, thickness of insulating air gap for hot runner nozzles is typically around 3mm; thickness of insulating air gap for hot runner plates should be no less than 8mm. See Figures 13.2.1, 13.2.2, and 13.2.3.
Support between hot runner plate and mold panel/A plate uses insulating pads with insulating properties, made of materials with low thermal conductivity.
Panels of hot runner nozzle and hot runner mold should generally be padded with 6-10mm thick asbestos boards or bakelite boards for insulation. Thickness of insulating pads is generally 10mm.

 

In Figure 13.2.3, to ensure good insulation, following requirements should be met: D1 ≥ 3mm; D2 depends on dimensions of hot runner nozzle step; D3 ≥ 8mm, determined by thickness of central insulating pad; D4 ≥ 8mm.
(4) Heat Insulation Pads: Heat insulation pads between hot runner plate and other parts of mold not only provide heat insulation but also support hot runner plate. Number of support points should be minimized, and forces should be balanced to prevent deformation of hot runner plate. Therefore, heat insulation pads should minimize their contact area with other parts of mold. A common structure is shown in Figure 13.2.4. Structure shown in Figure 13.2.5 is a heat insulation pad specifically for center of mold; it also serves as a centering point.
Heat insulation pads are made of materials with low heat transfer efficiency, such as stainless steel and high-chromium steel. Specific structures of heat insulation pads provided by different suppliers may vary, but their basic assembly relationships are same, as shown in Figure 13.2.6.

 

(5) Positioning: To prevent rotation and overall displacement of hot runner plate, and to accommodate its thermal expansion, a combination of center positioning and slot positioning is typically used. Specific structure is shown in Figure 13.2.7.
Due to thermal expansion, centerline of elongated slot that serves as positioning element must pass through center of hot runner plate, as shown in Figure 13.2.8.

 

 

(6) Thermal Expansion: Because hot nozzle and hot runner plate expand due to heat, expansion amount should be estimated during mold design, design dimensions should be corrected to ensure that expanded hot nozzle and hot runner meet design requirements. In addition, a certain clearance should be reserved in mold, and there should be no structure that restricts expansion. As shown in Figures 13.2.9 and 13.2.10, hot runner nozzle mainly considers axial thermal expansion, while radial thermal expansion is compensated for by clearance of mating parts; hot runner plate mainly considers length and width directions, thickness direction is adjusted by clearance between heat insulation pad and mold plate.
Thermal expansion is calculated by following formula: D = D1 + Expansion Amount Expansion Amount = D1 x T x Z
D¾ dimension after thermal expansion, which should meet working requirements of mold; D1¾ design dimension in non-heated state; T = Hot runner nozzle (hot runner plate) temperature - Room temperature (℃); Z¾ Linear expansion coefficient. For general medium carbon steel, Z = 11.2 x 10⁻⁶; for H13 steel, Z = 13.2 x 10⁻⁶.

 

Hot runner nozzles and secondary hot runner nozzles used in hot runner molds and hot runner molds, although their structures are slightly different, have same function and selection method. For convenience, hot runner nozzles and secondary hot runner nozzles are collectively referred to as hot runner nozzles.
Due to complex structure and manufacturing of hot runner nozzles, different specifications of series products from professional suppliers are usually selected during mold design and manufacturing. Each supplier has different series standards, their hot runner nozzle structures and specification markings are different. Therefore, when selecting hot runner nozzles, it is essential to clearly identify supplier's specification markings and then determine appropriate specifications based on following three aspects:
(1) Injection Volume of Hot Runner Nozzles: Different specifications of hot runner nozzles have different maximum injection volumes. This necessitates that mold designers select appropriate specifications based on size of plastic part to be molded, required gate size, and type of plastic material, and take a certain safety factor. Safety factor is generally around 0.8.
(2) Permissible Gate Types for Plastic Parts: Whether hot runner tip is allowed to participate in molding, and shape of hot runner tip structure, all affect selection of its specifications. Gate type will affect selection of hot runner length; see following section on determining hot runner length for details.
(3) Distance between Gate and Axial Fixing Position of Hot Runner: Axial fixing position of hot runner refers to plane on mold that mounts and restricts axial movement of hot runner. Position of this plane directly affects length of hot runner.
To better understand influence of gate, distance between gate and axial fixing position of hot runner on length of hot runner, following are several common hot runner structures (mainly referring to tip shape), corresponding gate shapes, and methods for determining their lengths.
Structure 1: As shown in Figure 13.3.1, this type of hot runner allows its tip to participate in molding of plastic part. Tip can be machined to adapt to different plastic part shapes. Size of gate after machining should meet mold requirements. Figure 13.3.2 shows several machined forms.

 

Hot runner length L = L1 - Z; Z is amount of thermal expansion.
Thermal expansion Z = L x 13.2 x 10⁻⁶ x [Hot nozzle (hot runner plate) temperature - room temperature] (℃)
Structure 2: As shown in Figure 13.3.3, this is a commonly used structure. Point gate satisfies surface requirements of plastic part and prevents stringing at gate.

Calculation method for hot runner nozzle length "L" varies depending on gate structure, as shown in Figure 13.3.4.
GATE "A": L = L1 - Z; GATE "B": L = L1 - Z - 0.2 mm; GATE "C": L = L1 - Z - J - 0.2 mm where Z is thermal expansion.

 

Thermal expansion Z = L x 13.2 x 10⁻⁶ x [Hot runner nozzle (hot runner plate) temperature - room temperature] (℃)
Structure 3: As shown in Figure 13.3.5, this is used for plastic parts where gate quality requirements are not high, as there will be a small amount of residual plastic material at gate.
Hot nozzle length L = L1 - Z - J; Z is thermal expansion.
Thermal expansion Z = L x 13.2 x 10⁻⁶ x [Hot nozzle (hot runner plate) temperature - room temperature] (℃)

 

Structure 4: As shown in Figure 13.3.6, this is a needle valve structure. Needle valve is controlled by a separate mechanism. Needle valve generally passes through hot runner plate, so thermal expansion of through-hole positions on hot runner plate should be calculated appropriately. This type of structure is mainly used for high-flowability rubber materials to prevent drooling at gate.
Hot runner nozzle length L = L1 - Z - J; Z is amount of thermal expansion.
Thermal expansion Z = L x 13.2 x 10⁻⁶ x [Hot runner nozzle (hot runner plate) temperature - room temperature] (℃)

 

In previous sections, we mainly introduced types, structural forms, and requirements for selecting hot runner nozzles for commonly used runnerless solidified material molds. However, when designing molds, we should also pay attention to other accessories and their selection.
(1) Mold base
Mold base used for hot runner nozzle molds is same as that for general molds; only a reasonable plate thickness needs to be selected.
Because hot runner molds require addition of a hot runner plate, a support plate needs to be added between plate A and panel, and sufficient space needs to be reserved to ensure installation requirements of hot runner plate.
(2) Connecting Cables and Interfaces
To individually heat and control temperature of hot runner nozzle and hot runner plate, each component has two interfaces: one power input interface and one temperature output interface. In hot runner nozzle mold, connecting cables and their interfaces are relatively simple. When connecting cables are long enough, they can be directly connected to temperature control box, as shown in Figure 13.4.1; alternatively, form shown in Figure 13.4.2 can be used. In hot runner mold, there are many components requiring temperature control. Generally, power cable and temperature control cable are each integrated onto one interface, connected to temperature control box in the form shown in Figure 13.4.2; alternatively, each component can be connected to temperature control box separately, choosing either connection form in Figure 13.4.1 or 13.4.2 depending on actual situation.

 

When designing mold, need for connecting cables and their connectors should be determined based on interface forms of hot runner nozzle and hot runner plate provided by supplier, interface form of temperature control box used by manufacturer, distance between mold and temperature control box. If required, they generally need to be ordered separately.
(3) Temperature Control Box
Based on number of temperature control modules, temperature control boxes can be divided into single-group and multi-group temperature control boxes. Each temperature control module controls one heating element. Temperature control modules are classified by their maximum current, generally 15A and 30A. When using a temperature control box, power supply and temperature connection cable interfaces must match interface type of temperature control box. We usually use a single-group temperature control box. Temperature control box used in VT-PL is DSS-15-02/01 model from D-M-E Company of Canada, using single-phase DC power, with a maximum voltage of 240V and a maximum current of 15A. Please refer to temperature control box instruction manual for specific usage conditions.
In addition, other accessories include voltage converters, a mobile bracket (for mounting temperature control box), and (quick-change) fuses, which can be selected according to actual needs.