Hot runner is a method of heating to ensure that plastics in runner and gate remain molten. Hot runner system generally consists of several parts such as hot nozzles, manifolds, temperature control boxes and accessories. Hot nozzles generally include two types: open hot nozzles and needle valve hot nozzles. Since form of hot nozzle directly determines selection of hot runner system and manufacture of mold, hot runner system is often divided into an open hot runner system and a needle valve hot runner system accordingly.
Spear nozzle (gun nozzle)
There is a spear head extending into gate neck in gate part. It is a heating probe whose temperature changes with operation cycle: it is a heating element made of special alloy. Front end of injection molding is energized, resin melts, injection molding is completed, power is turned off, and gate is frozen. Characteristics are short molding cycle and can mold precision products.

 

Valve gate hot runner nozzle
Advantages: can avoid drooling, large gate opening size, up to Ø12.5mm, so flow resistance is small, wear is small, and gate mark is small.
Elastic needle valve type nozzle
Valve core is opened and closed by spring force and plastic melt pressure respectively.
Advantages: 1. It can avoid drooling 2. It can reduce wire drawing 3. Reduce possibility of freezing at spout 4. It leaves almost no gate traces.

 

Hydraulic driven valve gate
It can accurately control filling time, close gate quickly and early under high temperature and high pressure, reduce internal stress of plastic part, and increase dimensional stability.

 

Hydraulic driven valve gate
Pre-pressing operation can shorten filling time and increase process ratio. For example, volume compression ratio of HDPE is 8.5% at a temperature of 204.4℃ and a pressure of 105MPa. Assuming that volume of plastic in barrel is 5 times volume of mold cavity, filling time can be shortened by 8.5% x 5 = 42.5% due to expansion when needle valve is opened. Process ratio (L/t) is increased from 240 to 310. Molding cycle is shortened by 1/3 or more.
You should learn to hold needle valve closing time correctly. If gate is frozen and needle valve is closed, not only will needle valve lose its function, but resin in gate will be squeezed toward plastic part and wrinkles will be formed around gate, or resin that has not been squeezed flat will become a protrusion.

 

Mold Masters Hydraulically Actuated Valve Nozzles

 

Air pressure driven valve gate
Air pressure driven is safer, will not cause pollution and other safety issues due to leakage.

 

Mold Masters Air Actuated Valve Nozzles

 

Hot runner mold with cold runner gate
Hot runner is followed by a cold runner, which is connected to product. As shown in figure, a cold main runner is connected to hot runner to reduce conflict between hot and cold.

 

Fan gate, ring gate, latent gate and other parts connected to hot runner do not need to be heated. After molding, they are frozen and ejected together with product. Cold runner part shown in the figure below uses a latent gate.

 

Hot runner plate
Function of hot runner plate is to send plastic melt to each hot runner nozzle through branch channel at a constant temperature.
Requirements:
1 Small pressure loss:
2 No dead corner for material stopping:
3 Balanced layout is adopted.
Classification:
Externally heated hot runner plate
Internally heated hot runner plate
Externally heated hot runner plate
1. Polished runner Ø5~12mm.
2. Rod-type and tube-type coil heaters can heat hot runner plate to 200~300℃ in 0.5~1 hour.
3. Air gap is used for insulation around nozzle head, and air or plastic insulation layer is used for insulation of nozzle head.
4) Shape: strip, X-shaped or H-shaped, etc., with balanced gate.

 

Other forms of hot runner plates

 

Thermal expansion of hot runner plate
Causes great thermal stress: both ends cannot be fixed.
Nozzle position movement: leave space for movement.

 

Internally heated hot runner plate
Feed nozzle is often equipped with an internal heating probe
Manifold is heated by an internal heating tube, and surface temperature of manifold plate is low, only about 40℃.
Advantages:
Heat loss is reduced by 75%, energy saving, and reduced risk of leakage.
Disadvantages:
Temperature in cross section of runner is uneven, which can easily bring cold material into cavity and reduce quality of product.
Thermosensitive resin will also decompose
Flow along annular gap, and flow resistance is much greater.

 

Internally heated hot runner mold assembly form

 

Heat pipe heating hot runner
Heat pipe technology comes from aerospace technology. Principle is shown in figure below. Liquid working medium is heated by electric heater in heating area and evaporates, and heat is diffused to cold end in the form of steam. Steam condenses and releases heat. Its thermal conductivity can reach more than 1,000 times that of copper.

 

Heat pipe is used in the case of mold main runner sleeve. Heat pipe is made into a jacket around main runner. Heating ring on main runner sleeve releases heat and transfers heat to feed nozzle head through heat pipe. Temperature difference can be controlled below 1.5~2℃. Heat pipes can also be used to heat runner plate.

 

Multi-nozzle example

 

Double nozzle for double layer mold