So-called runnerless aggregate mold means that in injection molding, melt in runner always maintains a hot flowing state. When mold is opened, only cured product needs to be taken out, and no runner aggregates are generated. Compared with traditional injection molds, this is an advanced injection mold technology and a hot spot in the development of plastic injection molding technology. Its biggest feature is that it can increase utilization rate of materials, reduce production costs, and ensure quality of parts.
Runnerless aggregate injection mold of thermoplastics refers to use of heat insulation or heating in mold to keep plastic melt in flow channel from nozzle of injection molding machine to gate of mold cavity always in a molten state, and can be continuously injected into mold cavity .
Thermosetting plastics use a hot runner injection mold, that is, melt in runner is kept at a set temperature by controlling temperature.

 

Runnerless aggregate molds are also called hot runner molds. Hot runners are not a new technology. They have been used in thermoplastic injection molds for more than 30 years. As early as 1940, E.R. Knowles applied for a patent for hot runner technology in United States.
It is estimated that hot runner technology is applied to more than 1/4 injection molds in Europe, and more than 1/6 in United States. In foreign countries, hot runner system components have been serialized and commercialized. Forecasts indicate that application proportion of hot runner technology will increase year by year. In recent years, hot runner technology has continued to develop and improve.
In China, hot runner technology has only gradually been applied after 1980s, it is still in the development and application stage. In injection molds, application ratio is only about 2% to 3%. But development prospects are very good, and potential market demand is very large.
Development of hot runner mold technology has following trends:
1) Develop and research various new nozzles, hot runner plates and related technologies to meet requirements of different plastics and products. Such as anti-leakage, abrasion resistance, high temperature resistance and thermal balance.
2) Micro thermal nozzle and heating element and temperature control technology.
3) Three-dimensional CAD and simulation technology of hot runner system.

(1). According to plastic performance and runner heat source method:

 

(2). Basic structure of hot runner system:

 

(3). Analysis and comparison of cold and hot runners:

 

Application examples of one mold with eight cavities
(A) Traditional cold runner.
(B) Hot runner nozzle replaces main runner and eliminates main runner condensate. Reduce runner waste by about 40% and shorten molding cycle by about 10%.
(C) Two hot nozzles are added to hot runner plate to reduce volume of main runner. Compared with figure (a), reduce flow channel aggregate by 60% to 70%.
(D) Each cavity uses a hot nozzle to remove cold runner. Cycle is short and thin-walled parts can be formed, and mold cost is high.

1) Three-plate mold is not used, and point gate can also be used for feeding; mold structure is simplified, requirements for mold opening stroke of injection machine are reduced.
2) Save raw materials; avoid process and cost of recycling, crushing and reusing runner aggregates.
3) Melt in runner is always in a molten state, and flow resistance is small, which is conducive to transfer of mold filling and holding pressure, improves surface quality and mechanical properties of product. It can realize molding of multi-point gate, multi-cavity mold and large-scale, thin-walled, long-process system.
4) There is no time to cool down and take out runner aggregates, shorten forming cycle; easy to automate production.
5) Pressure loss in runner is small, which reduces required filling pressure and reduces clamping force of injection machine. There is no condensate in pouring system, which reduces injection volume and can give full play to capabilities of injection machine.
6) Needle valve gates can be used to control gate closing time and ensure quality of product molding.

1) Mold structure is complex, manufacturing cost is high, and maintenance is difficult; hot runner system is prone to failure and high operating cost. Not suitable for small batch production.
2) Initial production preparation time is long, and mold debugging requirements are high.
3) It is not suitable for molding of plastics with poor heat sensitivity and fluidity, plastic parts with long molding cycles.
4) Runner plate is prone to thermal expansion, is more sensitive to melt leakage and heating element failure.
5) Temperature control requirements are strict, requiring precise temperature control components and systems.

1) Melting temperature range is wide, viscosity change is small, and thermal stability is good. (Not easy to decompose at high temperature, good fluidity at low temperature)
2) Melt viscosity is sensitive to pressure. No pressure does not flow, lower pressure can flow.
3) Plastics have low specific heat capacity, are easy to melt and solidify.
4) Heat distortion temperature of plastic is high, and product can be quickly demolded from mold.
In theory, almost all thermoplastics can be injection molded without runners. At present, the most widely used materials are: PE, PP, PS and ABS and other materials.

There is no auxiliary heating device in runner, but poor thermal conductivity of plastic is used to design runner section size to be very large (often more than 30 mm), so that plastic melt close to surface of runner will quickly condense due to lower mold temperature and form a frozen layer, while melt in the center of runner keeps melting and flowing. In order to keep runner unobstructed, speed of plastic melt flowing through runner should be as fast as possible, so that melt in runner is continuously replaced, and there is not enough time to completely freeze.
Main features of adiabatic runner are low cost; easy to replace materials in production; runner diameter is large, and pressure loss is small; when runner condensate is frozen, it is easy to remove parting surface by opening. But because of its large size, plastic heating time is prolonged. Temperature control is not ideal, and it is not suitable for processing heat-sensitive plastics. Less applications.
It is usually used for products with low processing accuracy and short molding cycle, molding of small general-purpose plastic products such as PE, PP, and PS.

Also known as adiabatic main runner, it is a single cavity adiabatic runner with the simplest structure. It is only suitable for products whose molding cycle is less than 20 s.
So-called pit nozzle is a sprue cup set between injection machine nozzle and cavity gate. Inner volume of cup is about 1/3 to 1/4 of volume of product. A frozen layer is formed around cup wall to insulate heat, air gap between runner cup and mold plate also plays a role in heat insulation.

 

Structure of pit nozzle
(a) 1-Nozzle of injection machine; 2- Storage well; 3-point gate; 4- Main runner cup; Picture (b) Gate size; Picture (c) 1- Spring; 2- Positioning ring; 3 -Storage well; 4-Nozzle

1) Main runner gate
Multi-cavity adiabatic runner has a circular cross-section, and diameter is often Φ16～32mm. The longer molding cycle, the larger diameter should be.

 

Air gap between runner plate and movable mold plate is to reduce contact area. Figure (a): Beginning of gate protrudes into runner, so that part of sprue is in thermal insulation of runner's insulating skin. Figure (b) A heating ring is added around direct gate bushing. There is an air gap between gate bushing and movable mold plate for heat insulation, there is a heating ring between gate bushing and runner plate. If molding cycle is long, a heating rod can be inserted in the center of gate to heat.

 

Multi-cavity adiabatic runner injection mould with sprue gate
1- sprue bushing; 2- fixed mold plate; 3- runner; 4- solidified thermal insulation layer; 5- runner plate; 6-direct gate bushing; 7- movable template; 8-core; 9 -Heating ring; 10-Cooling water pipe.
2) Point gate
Parts formed by point gate have no gate aggregate, but gate is easy to freeze, which is only suitable for products with a short molding cycle. A heating probe is set at guiding part at beginning of gate, which can heat gate and can form products with a long period. Probe body is usually made of beryllium copper alloy with good thermal conductivity.

 

Point gate multi-cavity adiabatic runner injection mould
1- Runner closing lock plate; 2- fixed mold fixing plate; 3- heat insulation layer; 4- runner; 5- main runner bushing; 6-parting surface closing lock plate; 7- runner plate; 8-type Core fixing plate; 9- stripping template; 10- core; 11- movable mold backing plate; 12- guide sleeve; 13- guide post

Hot runner is to set a heater inside or around runner, so that plastic melt in runner is always in a molten state.
Adiabatic runner needs to remove solidified substance in runner before each use, while hot runner only needs to heat solidified plastic in runner to melting temperature, and shoot it to empty, then it can be reproduced. Its application range is wider than that of adiabatic runners, and it is also suitable for forming larger products with multiple point gates.
Hot runner system consists of two basic units: manifold and drop. Runner plate is installed in fixed mold part to transfer melt from nozzle of injection machine to cavity plate, then melt is directly transferred to cavity by hot nozzle, or indirectly feeds material to multiple cavities through a cold runner. Nozzle usually passes through cavity plate and forms a 90° angle with runner plate.
Hot runner mold has heating, temperature measurement, heat insulation and cooling devices at the same time. Hot runner plate is heated and insulated, nozzle is same. Runner plate and each nozzle have independent heating elements and temperature control systems. Hot runner molds have high requirements for temperature control accuracy, and it is a difficult problem to prevent thermal balance imbalance.

In hot runner mold, gates are respectively connected to runners that maintain molten state and products that need to be solidified, temperature difference between the two is more than 100℃. It is required that melt passes smoothly during injection, and gate is quickly closed when mold is opened to avoid melt leakage. At present, commonly used gate sealing methods are:
1) Open gate closed by heat balance
Thermal balance of gate opening and closing is achieved by adjusting temperature of outer heating ring or inner heating probe of sprue sleeve. Structure and temperature adjustment method are simple and cost is low. Disadvantage is that it is easy to block or wire drawing at gate, which requires high temperature settings.
2) Side gate closed by heat balance
Gate is cut through opening of mold, gate structure and temperature adjustment method are simple, and there is no wire drawing. Disadvantage is that gate is easy to block, scope of application is limited by shape of product.
3) Gates closed by circulating heating and thermal insulation
It is necessary to install gate heating and thermal insulation devices that are compatible with molding cycle. Structure and temperature adjustment are relatively simple, gate is closed and reliable, but a higher-precision temperature control system is required.
4) Gate closed by spring action valve stem
Valve stem is opened by resin pressure and gate is closed by action of spring. Structure is simple, gate is closed reliably, spring is required to have good heat resistance and valve stem to slide flexibly.
5) Mechanical valve gate
Pneumatic and hydraulic systems are used to force valve stem to move, so that gate can be closed and opened. Structure is reliable, forming conditions are wide, cycle is short, and gate resistance is small. However, structure is complicated and manufacturing cost is high.

1) Extended nozzle
It is a special nozzle that lengthens nozzle of ordinary injection machine so that it can directly contact gate part of mold. It is heated by an electric heating coil, has a temperature measurement and control system. Nozzle temperature is required to be 15-20℃ higher than that of barrel. Nozzle mouth is actually gate of cavity, and point gate with diameter of 0.8～1.2mm is commonly used.
Because high temperature nozzle directly (or indirectly) forms plastic part, mold must be insulated to prevent high temperature of nozzle from affecting curing of plastic part. Air gaps and plastic skins are commonly used for thermal insulation. After injection and holding pressure, nozzle should be separated from mold, contact area between nozzle and mold should be minimized.
Extended nozzle has a simple structure and is often used in single-cavity molds. Commonly used are spherical, conical and other forms.

 

(a) Spherical nozzle (b) Conical nozzle

 

(C) Forming nozzle (d) Insulated nozzle
Extension nozzle structure
Figure (a) Nozzle extends into sprue sleeve, nozzle is positioned by shoulder and bears force. There is a bushing to increase air gap between nozzle and sprue sleeve.
Figure (b) End face of nozzle is a part of cavity, with an intermediate bushing, an air gap slot, and cooling water.
Figure (c) Nozzle must be positioned on injection seat to withstand pressure. Front end of nozzle is matched with hole, thermal expansion and flash must be considered.
Picture (d) shows heat-insulating nozzle, bowl-shaped plastic heat-insulating skin, thickness of center is 0.4～0.5mm, and outside is 1.2～1.5mm. Pressure-bearing shoulder is embedded with a PTFE gasket to ensure strong rigidity of bottom of sprue cup.
2) Multi-cavity hot runner injection mold
There are many structural forms and a wide range of applications. It is characterized by a runner plate heated by a heater. Main flow channel is connected to the top, there are divided flow channels and multiple nozzles.

 

Multi-cavity hot runner mold structure with main runner type gate
1- Main runner bushing; 2- Hot runner plate; 3- Fixed mold fixing plate; 4- Spacer block; 5- Sliding pressure ring; 6-Nozzle sleeve; 10-heater; 11-side plate; 12-main runner type sprue cup; 13-fixed mold cavity plate; 14-movable mold cavity plate.
3) Structural design of hot runner plate
Good heating and insulation facilities are required to ensure effective heater installation and temperature control. According to number and location of gates, there are many forms.
Hot runner plate design:
• Diameter of circular runner is generally 5～15mm.
• End hole of shunt is sealed with a fine-tooth plug.
• Air gap or asbestos board for heat insulation. Commonly used air gap is 3～8mm.
• Sufficient strength and rigidity of hot runner plate.
•Made of medium carbon steel or carbon alloy steel.

 

Structure of hot runner plate
1- heater hole; 2- branch runner; 3- feed nozzle mounting hole
4) Heating method of hot runner plate
• Internal heating
Internal heating is to heat a large diameter flow channel, and a rod heater is installed on axis of flow channel. Outer wall of runner is cold, outer plastic freezes to insulate heater and mold. Power consumption can be reduced by about 50%, and there is no problem of thermal expansion of runner plate. Leakage can be better eliminated, end of gate can be controlled with a heated probe.
Internal heating may cause material to stay and cause decomposition. Therefore, it is not suitable for heat-sensitive plastics. In addition, mold filling pressure in runner is high.

 

Internal heating runner and nozzle
1- cooling water hole; 2- heating nozzle; 3- melt channel; 4- internal heater
• External heating
Externally heated runner plate is suspended in mold, often with heating rods or curved heating tubes arranged on the outside of runner. An air gap is used for heat insulation of runner plate, and a heat insulation sheet is also used. Heat loss is an issue that must be considered. Thermal expansion of runner plate needs to be compensated to prevent leakage. Hot nozzle is installed on runner plate. External heating can minimize pressure loss of mold, runner is generally circular and large in diameter. Externally heated runner plate and nozzle are suitable for heat-sensitive and high-viscosity plastics. Runner has no cold skin and runner has a large flow rate. External heating runner is more expensive than internal heating.

 

Nozzle is key element of hot runner mold. To maintain molten state of plastic in nozzle, heat insulation must be as perfect as possible, and some nozzles need to be heated internally or externally. Cavity needs to be cooled. Temperature difference between the two is usually 100-200℃, so nozzle design should first meet thermal balance requirements. It is necessary to avoid too much cold material in nozzle to solidify and block, but also to avoid overheating of plastic, casting or drawing, or even thermal decomposition. Secondly, thermal expansion caused by temperature difference should be considered. Again, pay attention to leakage of melt, which will cause flash and affect normal operation.

 

Commonly used hot runner nozzle structure:
External heating
Internal heating
Spring needle valve type

Structure of various hot runner nozzles

 

①Flat nozzle
Straight gate form

 

Flat nozzle form with split gate
• Point gate form

 

Several single-gate flat nozzle forms
②Point gate nozzle

 

Single-gate point nozzle form and sub-gate point nozzle form
③Valve nozzle

 

Cylinder, cylinder type
④Special nozzle

 

One-core multi-head type and multi-core multi-head type
2) Heating method of nozzle
①External heating nozzle
Heat source comes from heating ring around nozzle. Melt flow resistance in nozzle is small, and length is not limited. Due to structural constraints, temperature at gate at the front end of nozzle is relatively low. Due to temperature difference, heat balance is not easy to control. Heat utilization rate of externally heated nozzle is low, and there should be an adiabatic air gap of 3～5mm around heating ring.

 

Contact nozzle multi-cavity hot runner injection mold
1- fixed mold floor; 2- spacer; 3- stop pin; 4- plug; 5- heater; 6- hot runner plate; 7- side support plate; 8- direct contact nozzle; 9- heating Circle; 10-fixed model cavity plate; 11-moving mold plate
②Inner heating nozzle
Heat comes from heating rod in the center of shunt shuttle. Heating rod power can be adjusted by voltage. Gap of melt channel around shunt shuttle is generally 3～5mm. Gap is small, flow resistance is large, and heat dissipation is fast; gap is large, radial temperature difference of melt is large. If nozzle is longer, an electric heating coil is needed to assist external heating.
Temperature of internally heated nozzle can be effectively controlled because high-temperature cone-shaped tip extends into gate.

 

Internally heated hot runner nozzle
1- fixed mold plate; 2- nozzle; 3- tapered tip; 4- splitter edge; 5- heating rod; 6-insulation layer; 7- cooling water hole.
3) Needle valve nozzle
A needle-shaped valve core that can be opened and closed is placed in nozzle, so that gate becomes a valve. Injection pressure holding phase is turned on; cooling phase is turned off. Diameter of gate can be enlarged to avoid blockage of foreign matter, also prevent casting and drawing of melt of the gate. Suitable for various viscosities, especially low-viscosity plastics.
Opening and closing of valve core can be driven by melt pressure or hydraulic pressure.

 

Spring needle valve hot runner nozzle
1- fixed mold bottom plate; 2- hot runner plate; 3- pressure ring; 4- compression spring; 5- piston rod; 6-positioning ring; 7- main runner bushing; 8- heating ring; 9- needle valve core; 10- heat insulation layer; 11- heating ring; 12- nozzle body; 13- nozzle head; 14- fixed mold cavity plate; 15- stripping mold plate; 16- core
Molding characteristics of valve nozzles:
• There are no gate marks on the surface of product, and surface of gate is smooth.
• A larger diameter gate can be used to speed up filling speed of cavity. Reduce injection pressure and reduce product deformation.
•Prevent phenomenon of wire drawing or casting in the gate when mold is opened.
•When screw of injection machine retreats, it can prevent melt in cavity from flowing back.
• It can cooperate with sequence control to reduce weld line of product.

1) Requirements for thermal balance of hot runner system
Hot runner system must meet thermal balance requirements, and its heat loss should be compensated by heating. Hot runner system in an ideal state should be isothermal. Requirement for control of hot runner system is to keep deviation of required temperature to a minimum. For this, following conditions should be met:
• Accurate design of heating element power;
• Heating element is installed correctly in system structure;
• Reasonably determine heating location and temperature measurement point;
• Adequate heat insulation measures and effects.
From user's point of view, conditions that should be met are:
• Good durability;
• Easy to replace;
• Good damage resistance, corrosion resistance, and not easy to leak;
• Line connection is safe and reliable.
2) Type of heater
Commonly used heaters for hot runner molds are:
•Nozzle heating commonly used coils and band coil heaters;
• Rod and tube heaters are commonly used for runner plate heating.
3) Temperature control of hot runner system
•Accurate temperature control is key factor to realize automatic operation of hot runner system. Common method is to control contactor with a temperature control meter.
• Control principle is to control opening and closing of heating element by judging temperature of mold. When mold temperature is lower than set value, contactor is closed, all voltage is applied to heating element, and its temperature rises rapidly; when temperature reaches set value, contactor is disconnected.
•Thermocouple is installed near flow channel. Hysteresis of thermocouple temperature measurement makes its temperature control accuracy lower. Output control device of pulse width hot runner temperature control system selects high-power bidirectional thyristor output, which has stable operation, reliable performance and long service life of heating elements.

4) Application examples of no runner aggregate mold

 

Hot runner injection mould for runnerless aggregate injection moulding of thermosetting plastics.

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

 

1-Moving mold fixing plate; 2-Pushing board; 3-Pushing rod fixing plate; 4-Pushing rod; 5-Insulation board; 6-heating rod; 7-moving mold backing plate; Insert; 10-core; 11- fixed template; 12- water hole; 13- hot runner plate; 14- positioning ring; 15- heat insulation plate; 16- fixed mold fixed plate; 17- side plate; 18- Lock template; 19-heat insulation board; 20-nozzle.

For a thermosetting plastic injection mold with one mold and one cavity, a nozzle whose temperature is controlled by a temperature adjustment medium can be specially designed and manufactured to replace nozzle of original injection machine and extend it into mold.
Extended nozzle is directly connected with gate, leaving scars on plastic part after molding. There is an air gap around nozzle to insulate heat, nozzle leaves mold after injection and pressure retention. Nozzle temperature is strictly controlled, and material will be solidified if it is too cold or too hot. Spacer nozzle can easily remove solidified material when injection fails.
 
(A) Extension nozzle (b) Sleeve nozzle

1) There must be good heat insulation measures between hot runner plate and mold plate to prevent temperature of runner plate from rising and malfunction.
2) Mold temperature must be accurately controlled and allowed to fluctuate within 5℃. Temperature of runner plate and each nozzle should be controlled separately.
3) Hot runner should adopt a circular cross section to facilitate heat preservation and filling flow of melt, general diameter is 6 to 8 mm. When there are fiber fillers, the larger value should be used. Runner should not have dead ends, grooves and other stagnant areas. Surface roughness of runner should be consistent with that of cavity, and it is best to be chrome-plated to ensure wear resistance.
4) Nozzle aperture is generally not less than 4mm, and has an inverted cone of 0.5°～1°, which is convenient for gate demoulding.
5) Parting surfaces should be provided on hot runner plate, a hook-type opening and closing lock plate should be provided 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.