Hot runner injection mold for non-runner aggregate injection molding of thermosetting plastics.

During injection molding of hot runner, plastic in the runner should always be kept in a molten state as in barrel of injection machine. For this reason, a low-temperature zone must be independently set at flow channel of mold, and temperature is roughly in the range of 105-110℃. Hot runner plate adopts hot water or hot oil circulation for heat preservation, heat is taken away or supplemented by temperature measurement and temperature adjustment system. Mold cavity is a high-temperature zone, and temperature is about 145-180℃. After material is injected into cavity, it is cross-linked and solidified under heat and pressure to form an infusible and insoluble substance with a network structure. Heat insulation between low temperature area and high temperature area is the key to temperature control, and asbestos cement board or epoxy glass fiber board is usually used for heat insulation between them. At the same time, it is necessary to insulate fixed plate of fixed mold and fixed plate of movable mold. Air-gap insulation is also a commonly used means of insulation. There are air gaps around hot runner plate and nozzle. Nozzle is at interface between high and low temperature, so it should be made of alloy steel with poor thermal conductivity, or high-strength plastic such as PI can be used to inlay nozzle mouth, and upper end of nozzle needs to be kept at a low temperature by a temperature-regulating medium.
Hot runner injection molding requires materials to maintain good fluidity in runner, be sensitive to pressure, and solidify quickly after entering high-temperature cavity.
Hot runner injection molding can save 15% to 35% of raw materials, and can produce multiple pieces in one mold, so it is a promising molding process. But it has strict requirements on temperature control, high technical difficulty and high mold cost.

 

1-moving mold fixed plate; 2-push plate; 3-push rod fixed plate; 4-push rod; 5-insulation plate; 6-heating rod; Insert; 10-core; 11-fixed formwork; 12-water hole; 13-hot runner plate; 14-locating ring; 15-insulation plate; Locking template; 19-insulation board; 20-nozzle.

For a one-cavity thermosetting plastic injection mold, nozzle can be specially designed and manufactured with a temperature-controlled medium to replace original injection machine nozzle and extend into mold.
Extended nozzle is directly connected to gate, leaving scars on plastic part after molding. There is an air gap around nozzle for heat insulation, nozzle leaves mold after injection and pressure maintenance. Temperature of nozzle is strictly controlled, and material will solidify if it is too cold or too hot. Spacer nozzles allow easy removal of cured material in the event of injection failure.

 

(a) Extended nozzle (b) Sleeve nozzle

1) There must be good heat insulation measures between hot runner plate and template to prevent temperature of runner plate from rising and causing failure.
2) Mold temperature must be accurately controlled, and fluctuations within 5℃ are allowed. Temperature of runner plate and each nozzle should be controlled separately.
3) hot runner should adopt a circular cross-section to facilitate melt insulation and filling flow, with a general diameter of 6-8mm. A larger value should be taken when there is fiber filler. There should be no stagnation areas such as dead ends and grooves in the runner. Surface roughness of runner should be consistent with cavity, preferably chrome-plated to ensure wear resistance.
4) Hole diameter of nozzle is generally not less than 4mm, and it has an inverted cone of 0.5°~1°, which is convenient for gate demoulding.
5) Parting surface should be set separately on hot runner plate, it should be equipped with a hook-type opening and closing lock plate to prepare for need to take out solidified material from runner.
6) Volume of runner should be smaller than the total volume of plastic injected at one time to prevent plastic melt from staying in runner for too long and solidifying.