Runnerless condensate mold is an injection mold for thermoplastic rubber materials. It uses heating or heat insulation to keep rubber material in runner in a molten state at all times, thereby achieving purpose of no runner condensate or less runner condensate.
1. Advantages of runnerless condensate mold are many, and main ones are:
(1) No runner condensate or less runner condensate, effective utilization rate of rubber material is high, and plasticizing capacity of injection molding machine can be fully utilized.
(2) Pressure loss of molten rubber in runner is small, which is easy to fill cavity and compensate for shrinkage, can avoid occurrence of dents, shrinkage holes and deformation of rubber parts.
(3) Molding cycle is shortened and production efficiency is improved.
(4) Gate can be automatically cut off, which improves degree of automation.
(5) It can reduce injection pressure and reduce clamping tonnage.
2. Runnerless condensate mold also has its corresponding disadvantages, which are mainly manifested as follows:
(1) Closing height of mold equipped with a hot runner plate is increased, and it may be necessary to use a larger injection molding machine.
(2) Heat in hot nozzle and hot runner plate affects temperature of front mold through heat radiation and heat conduction. When designing mold, heat transfer should be minimized and cooling of front mold should be strengthened.
(3) Mold cost is relatively high
Basic form of runnerless condensate mold
After years of development, runnerless condensate mold now basically adopts following two main structural forms:

1. Mold that uses a hot nozzle for direct or indirect feeding is referred to as a hot nozzle mold. Its basic structure is shown in Figure 12-1.

 

2. A mold with a hot runner plate and a two-stage hot nozzle is referred to as a hot runner mold. Its basic structure is shown in Figure 12-2.

(1) Hot nozzle mold structure with point gate feeding, as shown in Figure 12-3. This structure is only applicable to single-cavity molds and is limited by gate position.

 

(2) Hot nozzle mold structure with hot nozzle end face involved in molding, as shown in Figure 12-4. Applicable to single-cavity molds, with nozzle marks on the surface of plastic part. Hot nozzle end face can be processed.

 

(3) A hot runner mold structure with a few conventional runner forms, as shown in Figure 12-5. This type of mold can mold multiple plastic parts at the same time, but disadvantage is that some runner cold material will be generated.

 

(1) A hot runner mold structure in which end of secondary hot nozzle participates in molding. As shown in Figure 12-2.
(2) A hot runner mold structure with a secondary hot nozzle pin-point feeding. As shown in Figure 12-6.
In addition, a variety of different mold structures can be produced according to structure and feeding method of secondary hot nozzle, but their basic requirements are same.

 

1. Injection volume
Appropriate hot nozzle should be selected according to size of plastic part and different rubber materials. Supplier generally gives maximum injection volume of each hot nozzle relative to different fluidity rubber materials. Because rubber materials are different, their fluidity is different. In addition, attention should be paid to size of hot nozzle nozzle, which not only affects injection volume, but also has other effects. If nozzle mouth is too small, molding cycle will be extended; if nozzle mouth is too large, nozzle mouth is not easy to close, and it is easy to drool or draw.
2. Temperature control
Temperature of hot nozzle and hot runner plate is directly related to whether mold can operate normally. Generally, temperature is controlled separately. Regardless of whether internal heating or external heating is used, temperature in hot nozzle and hot runner plate should be kept uniform to prevent local overcooling and overheating. In addition, power of heater should be able to make hot nozzle and hot runner plate rise from room temperature to required working temperature within 0.5~1h, and heating time of hot nozzle can be shorter.
3. Insulation
Hot nozzle and hot runner plate should be well insulated from other parts such as mold panel and A plate. Insulation medium can be asbestos board, air, etc. Except for parts that need to be in contact such as positioning, support, and sealing, thickness of insulation air gap of hot nozzle is usually about 3mm; thickness of insulation air gap of hot runner plate should be no less than 8mm. As shown in Figures 12-7, 12-8, and 12-9.
Support between hot runner plate and mold panel and A plate uses an insulation pad with insulation properties. Insulation pad is made of a material with a low heat transfer rate.
Panels of hot nozzle and hot runner mold should generally be padded with 6~10mm asbestos board or bakelite board for insulation. Thickness of insulation pad is generally 10mm.

 

In Figure 12-9, in order to ensure a good thermal insulation effect, following requirements should be met: D1 ≥ 3mm; D2 is determined by size of hot nozzle step: D3 ≥ 8mm, and by thickness of central thermal insulation pad: D4 ≥ 8mm.
4. Thermal insulation pad
Thermal insulation pad between hot runner plate and other parts of mold not only plays a role in thermal insulation, but also supports hot runner plate. Supporting points should be as few as possible, and force should be balanced to prevent hot runner plate from deforming. To this end, thermal insulation pad should minimize contact area with other parts of mold. Commonly used structure is shown in Figure 12-10. Structure shown in Figure 12-11 is a thermal insulation pad dedicated to the center of mold, which also has function of center positioning.
Thermal insulation pad is made of materials with low heat transfer efficiency, such as stainless steel, high chromium steel, etc. Specific structure of thermal insulation pad provided by different suppliers may be different, but their basic assembly relationship is same, as shown in Figure 12-12.

 

5. Positioning
In order to prevent hot runner plate from rotating and shifting as a whole and to meet thermal expansion of hot runner plate, hot runner plate is usually positioned by combining center positioning and slot positioning. Specific structure is shown in Figure 12-13.

 

Affected by thermal expansion, center line of long slot that plays a positioning role must pass through center of hot runner plate. As shown in Figure 12-14.

 

6. Thermal expansion
Due to expansion of hot nozzle and hot runner plate due to heat, expansion amount should be estimated during mold design, design dimensions should be corrected so that hot nozzle and hot runner meet design requirements after expansion. In addition, a certain gap should be reserved in the mold, and there should be no structure that restricts expansion. As shown in Figures 12-15 and 12-16, hot nozzle mainly considers axial thermal expansion amount, and radial thermal expansion amount is corrected by gap of matching parts; hot runner plate mainly considers length and width directions, thickness direction is adjusted by gap between insulation pad and mold plate.
Thermal expansion amount is calculated as follows:
D=D1+expansion amount Expansion amount=D1·T·Z
D-size after thermal expansion, this size should meet working requirements of mold:
D1-design size in non-heated state;
7-hot nozzle (hot runner plate) temperature-room temperature (℃);
Z-linear expansion coefficient. Generally, medium carbon steel Z=11.2x10^-6; H13 steel Z=13.2X10^-6;

 

Hot nozzles and secondary hot nozzles used in hot nozzle molds and hot runner molds have slightly different structural forms, but their functions and selection methods are same. For convenience of following description, hot nozzles and secondary hot nozzles are collectively referred to as hot nozzles.
Due to complex structure and manufacturing of hot nozzles, series products of different specifications provided by professional suppliers are usually selected when designing and making molds. Each supplier has different series standards, their hot nozzle structures and specification markings are different. Therefore, when selecting a hot nozzle, it is necessary to clarify supplier's specification markings, then determine appropriate specifications based on following three aspects.
1. Hot nozzle injection volume
Hot nozzles of different specifications have different maximum injection volumes, which requires mold designers to select appropriate specifications based on size of plastic parts to be molded, required gate size, type of rubber, and take a certain insurance coefficient. Insurance coefficient is generally around 0.8.
2. Allowable gate forms for plastic parts
Whether plastic part allows top of hot nozzle to participate in molding, top structure of hot nozzle, etc. will affect its specification selection. Gate form will affect length selection of hot nozzle. For details, see following determination of hot nozzle length.
3. Distance between gate and axial fixed position of hot nozzle
Axial fixed position of hot nozzle refers to plane installed on mold to limit axial movement of hot nozzle. Position of this plane directly affects length of hot nozzle.
In order to better understand influence of gate and distance between gate and axial fixed position of hot nozzle on length of hot nozzle, following are several common hot nozzle structures (mainly referring to top shape), corresponding gate shape and method of determining its length.
Structure 1: As shown in Figure 12-17, hot nozzle of this structure allows its top to participate in molding of plastic part, and top can be processed to adapt to different shapes of plastic part. Size of gate after processing should meet requirements of mold. Figure 12-18 shows several forms that can be processed.

 

 

Length of hot nozzle L = L1-Z; Z is thermal expansion.
Thermal expansion Z = Lx13.2x10^-6x[hot nozzle (hot runner plate) temperature-room temperature] (℃)
Structure 2: As shown in Figure 12-19, this is a more commonly used structural form. Point gate can not only meet surface requirements of plastic part, but also prevent generation of wire drawing at glue inlet.

 

Calculation method of hot nozzle length "L" is different due to different gate structures. Gate structure is shown in Figure 12-20.
GATE "A": L = L1-Z;
GATE "B": L = L1-Z-0.2mm
GATE "C": L = L1-Z-J-0.2mm
Z is thermal expansion.
Thermal expansion Z = Lx13.2x10^-6x [hot nozzle (hot runner plate) temperature - room temperature] (°C)

 

Structure 3: As shown in Figure 12-21, it is used for rubber parts that do not require high quality at gate position, because there will be a small amount of residual rubber at gate.
Hot nozzle length L = L1-Z-J; Z is thermal expansion
Thermal expansion Z = Lx13.2x10^-6x [hot nozzle (hot runner plate) temperature - room temperature] (°C)

 

Structure 4: As shown in Figure 12-22, this is a needle valve structure. Needle valve is controlled by another mechanism. Needle valve generally passes through hot runner plate, so thermal expansion should be reasonably calculated at position of through hole on hot runner plate. This type of structure is mainly used for rubber with high fluidity to prevent drooling at gate.
Hot nozzle length L = L1-Z-J; Z is thermal expansion.
Thermal expansion Z = Lx13.2x10^-6x[hot nozzle (hot runner plate) temperature - room temperature] (°C)

 

In previous sections, types, structural forms and requirements for selecting hot nozzles of commonly used runnerless condensate molds were mainly introduced. However, other accessories and their selection should also be noted when designing molds.
1. Mold base
Mold base used for hot nozzle molds is the same as that of general molds. It is only necessary to select a reasonable plate thickness.
Because hot runner molds need to be equipped with hot runner plates, support plates need to be added between plate A and panel, sufficient space needs to be reserved to ensure installation requirements of hot runner plate.
2. Connection wires and their interfaces
In order to heat and control temperature of hot nozzles and hot runner plates separately, there are two interfaces on each component: one power input interface; one temperature output interface. In hot nozzle molds, connection wires and their interfaces are relatively simple. When connection wires are long enough, they can be directly connected to temperature control box, as shown in Figure 12-23; or they can be in the form shown in Figure 12-24. In hot runner mold, there are many components that need to control temperature. Generally, power cord and temperature control line are assembled on one interface and connected to temperature control box in the form of Figure 12-24; each component can also be connected to temperature control box separately, connection form of 12-23 or 12-24 can be selected according to actual situation.

 

When designing mold, it is necessary to decide whether connection line and its connector are needed based on the interface form of hot nozzle and hot runner plate provided by supplier, interface form of temperature control box used by factory, distance between mold and temperature control box. If necessary, it is generally necessary to order separately.
3. Temperature control box
According to number of temperature control modules, temperature control box can be divided into a single-group temperature control box and a multi-group temperature control box. Each temperature control module controls a heating component. Temperature control module is classified according to maximum current, generally 15A and 30A. When using a temperature control box, interface of the power supply and temperature connection line is required to match interface form of temperature control box. Usually we use a single-group temperature control box. Temperature control box used by VT-PL is DSS-15-02/01 model from Canada's D-M-E Company. It uses single-phase DC power, with a maximum voltage of 240V and a maximum current of 15A. For specific usage conditions, please refer to temperature control box instructions.
In addition, other accessories include voltage converters, mobile racks (for loading temperature control boxes), fuses (that can be quickly replaced), etc., which can be selected according to actual conditions.