It can produce 1,500 bottle caps in one minute, which is so efficient that you can't believe it! It benefits from efficient and leading injection molding technology and multi-cavity hot runner precision molds. Let's take a look at the injection molding process and its molds.
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The concept of non-runner aggregate injection mold
The so-called non-runner condensate mold means that in injection molding, the molten material in the runner is always kept in a hot flow state. When opening the mold, it is only necessary to take out the cured product without generating runner aggregate. Compared with the traditional injection mold, this is an advanced injection mold technology, and it is a hot direction in the development of plastic injection molding process. Its biggest feature is that it can increase the utilization rate of materials, reduce production costs, and ensure the quality of parts.
The non-runner condensate injection mold of thermoplastics refers to the method of heat insulation or heating in the mold to keep the plastic melt in the flow channel from the nozzle of the injection molding machine to the gate of the mold cavity always in a molten state, and can Continuous injection into the mold cavity.
Thermosetting plastics use a warm runner injection mold, that is, the melt in the runner is kept at a set temperature by temperature control.
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02
Development of technology of non-runner aggregate mold
The non-runner condensate mold is also called the hot runner mold. The hot runner is not a new technology. It has 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 the 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 the United States. In foreign countries, hot runner system components have been serialized and commercialized. It is predicted that the application proportion of hot runner technology will increase year by year. In recent years, hot runner technology is still developing and improving.
In China, hot runner technology has been gradually applied since the 1980s, and is still in the stage of development and application. In injection molds, its application ratio is only about 2% to 3%. But the development prospects are very good, and the potential demand in the market is very large.
The development of hot runner mold technology has the following trends:
1) Develop and research various new nozzles, hot runner plates and related technologies to meet the requirements of different plastics and products. Such as leakage prevention, wear resistance, high temperature resistance and thermal balance, etc.
2) Miniature thermal nozzles and heating elements and temperature control technology.
3) Three-dimensional CAD of hot runner system and its simulation technology.
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Types of non-runner aggregate molds
(1) According to the properties of the plastic and the heat source of the runner:
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(2) The basic structure of the hot runner system:
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(3) Analysis and comparison of cold and hot runners:
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Application example of one mold and eight cavities
(a) Traditional cold runner.
(b) The hot runner nozzle replaces the main channel, and the condensate in the main channel is omitted. Reduce runner waste by about 40%, and shorten molding cycle by about 10%.
(c) Two hot nozzles are added to the hot runner plate to reduce the volume of the main flow channel. Compared with Figure (a), the flow channel aggregate is reduced by 60% to 70%.
(d) Hot nozzles are used for each cavity, eliminating cold runners. The cycle time is short, and thin-walled parts can be formed. High mold cost
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Features of non-runner condensate injection mold
1. Benefits of using non-runner aggregate injection mold
1) Instead of a three-plate mold, point gate feeding can also be used; the mold structure is simplified and the requirements for the mold opening stroke of the injection machine are reduced.
2) Saving raw materials; avoiding the process and cost of recycling, crushing and reusing the flow channel aggregate.
3) The melt in the runner is always in a molten state, and the flow resistance is small, which is conducive to the transfer of filling and holding pressure, and improves the surface quality and mechanical properties of the product. It can realize multi-point gate, multi-cavity mold and large-scale, thin-walled and long-flow molding.
4) There is no time for cooling and taking out the runner condensate, shortening the forming cycle; easy to automate production.
5) The pressure loss in the runner is small, which reduces the required mold filling pressure and reduces the clamping force of the injection machine. There is no coagulation in the gating system, which reduces the injection volume and fully utilizes the capacity of the injection machine.
6) A needle valve gate can be used to control the closing time of the gate to ensure the molding quality of the product.
2. Limitations of using non-runner aggregate injection molds
1) The mold structure is complex, the manufacturing cost is high, and maintenance is difficult; the hot runner system is prone to failure and the operating cost is high. Not suitable for small batch production.
2) The initial production preparation time is long, and the requirements for mold debugging are high.
3) It is not suitable for plastics with heat sensitivity and poor fluidity and plastic parts with long molding cycle.
4) The runner plate is prone to thermal expansion and is sensitive to melt leakage and failure of heating elements.
5) Strict temperature control requirements require precise temperature control components and systems.
3. Plastic materials suitable for non-runner aggregate injection molds
1) The melting temperature range is wide, the viscosity change is small, and the thermal stability is good. (High temperature is not easy to decompose, low temperature fluidity is good)
2) Melt viscosity is sensitive to pressure. No pressure, no flow, low pressure can flow.
3) Plastic has a low specific heat capacity and is easy to melt and solidify.
4) The thermal deformation temperature of the plastic is high, and the product can be released from the mold quickly.
In theory, almost all thermoplastics can be injection molded without runners. At present, the most widely used materials are: PE, PP, PS and ABS.
05
Thermoplastic Concrete Injection Mold without Runner
1. Insulated runner
There is no auxiliary heating device in the runner, but the poor thermal conductivity of plastic is used to design the cross-sectional size of the runner to be very large (often more than 30mm), so that the plastic melt close to the surface wall of the runner will die due to the lower mold temperature. Rapidly condenses to form a frozen layer, while the melt in the center of the flow channel remains molten and flowing. In order to keep the flow channel unblocked in this system, the speed of the plastic melt flowing through the flow channel should be as fast as possible, so that the molten material in the flow channel is continuously replaced, and there is not enough time to completely freeze.
The main features of the adiabatic runner are low cost; easy replacement of materials during production; large diameter of the runner and small pressure loss; when the condensate of the runner freezes, it is easy to remove it by opening the parting surface. But because of its large size, the plastic heating time is prolonged. The temperature control is not ideal, and it is not suitable for processing heat-sensitive plastics. There are fewer applications.
It is usually used for products with low processing precision and short molding cycle, and the molding of small general-purpose plastic products such as PE, PP, and PS.
(1) Pit nozzle
Also known as adiabatic sprue, it is a single-cavity adiabatic runner with the simplest structure. Only applicable to products whose molding cycle is less than 20 s.
The so-called well-pit nozzle is a main flow cup set between the injection machine nozzle and the cavity gate. The volume inside the cup is about 1/3 to 1/4 of the volume of the workpiece. A frozen layer is formed around the cup wall for heat insulation, and the air gap between the runner cup and the template also plays a role of heat insulation.
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The structure of pit nozzle
Figure (a) 1- injection machine nozzle; 2- storage well; 3- point gate; 4- main channel cup; Figure (b) gate size; Figure (c) 1- spring; 2- positioning ring; 3 - storage well; 4 - nozzles
(2) Multi-cavity adiabatic runner
1) Sprue gate
The multi-cavity adiabatic flow channel is a circular cross-section, and the diameter is usually Φ16-32mm. The longer the molding cycle, the larger the diameter should be.
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The air gap between the runner plate and the movable template is to reduce the contact area. Figure (a) The beginning of the gate protrudes into the runner, so that part of the straight gate is in the insulation of the heat insulation skin of the runner. Figure (b) A heating ring is added around the direct gate bushing, there is an air gap between the gate bushing and the movable template for heat insulation, and there is a heating ring between the gate bushing and the runner plate. If the molding cycle is long, a heating rod can be inserted in the center of the gate for heating.
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Sprue gate multi-cavity adiabatic runner injection mold
1-main runner bushing; 2-fixed mold plate; 3-runner; 4-cured insulation layer; 5-runner plate; 6-direct sprue bushing; 7-moving template; 8-core; 9 - heating ring; 10 - cooling water pipe.
2) Point gate
The parts formed by the point gate have no gate aggregate, but the gate is easy to freeze, so it is only suitable for products with a short molding cycle. A heating probe is installed at the leading part of the gate, which can heat the gate and form products with a long cycle time. The probe body is usually made of beryllium copper alloy with good thermal conductivity.
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Point gate multi-cavity adiabatic runner injection mold
1-runner closing lock plate; 2-fixed mold plate; 3-insulation layer; 4-runner; 5-main channel bushing; 6-parting surface closing lock plate; 7-runner plate; 8-type Core fixed plate; 9-remove template; 10-core; 11-moving mold backing plate; 12-guide sleeve; 13-guide column
2. Hot runner injection mold
The hot runner is to install a heater inside or around the runner, so that the plastic melt in the runner is always in a molten state.
The insulated runner needs to remove the solidified material in the runner before each use, while the hot runner only needs to heat the solidified plastic in the runner to the melting temperature, and then empty it. It can be re-produced. Its scope of application is wider than that of adiabatic runners, and it is also suitable for forming larger products with multiple point gates.
The hot runner system consists of two basic units: the manifold and the drop. The runner plate is installed in the fixed part of the mold, and the molten material from the nozzle of the injection machine is transferred to the cavity plate, and then the molten material is directly transferred to the cavity by the hot nozzle, or indirectly supplied to multiple cavities through a cold runner. material. The nozzles typically pass through the cavity plate at a 90° angle to the runner plate.
The hot runner mold has heating, temperature measurement, heat insulation and cooling devices at the same time. The hot runner plate is heated and insulated, and the same is true for the nozzle. The manifold and each nozzle have independent heating elements and temperature control systems. Hot runner molds have high requirements for temperature control accuracy, and preventing heat balance imbalance is a difficult problem.
(1) Closing of hot runner gate
In the hot runner mold, the gate is respectively connected to the runner in the molten state and the product to be solidified, and the temperature difference between the two is more than 100°C. It is required that the melt passes through smoothly during injection, and the gate is quickly closed when the mold is opened to avoid leakage of the melt. Currently commonly used gate closure methods are:
1) Open gate closed by heat balance
The thermal balance of gate opening and closing is achieved by adjusting the temperature of the outer heating ring or inner heating probe of the gate sleeve. The structure and temperature adjustment method are simple and the cost is low. The disadvantage is that the gate is easy to be blocked or drawn, and the temperature setting requirements are high.
2) Side gates closed by thermal balance
The gate is cut off by opening the mold, the gate structure and temperature adjustment method are simple, and there is no wire drawing. The disadvantage is that the gate is easy to block, and the scope of application is limited by the shape of the product.
3) The gate is closed by circulating heating and heat insulation
It is necessary to set up gate heating and thermal insulation devices suitable for the molding cycle. The structure and temperature adjustment are relatively simple, and the gate is closed and reliable, but a high-precision temperature control system is required.
4) Gate closed by spring action valve stem
The valve stem is opened by the resin pressure, and the gate is closed by the action of the spring. The structure is relatively simple, and the gate is closed reliably. The heat resistance of the spring is required to be good, and the valve stem can slide flexibly.
5) Mechanical valve gate
Use the pneumatic and hydraulic system to force the valve stem to move, so as to realize the closing and opening of the gate. The structure is reliable in action, the forming conditions are wide, the cycle is short, and the gate resistance is small. But the structure is complex and the manufacturing cost is high.
(2) The structure of the hot runner
1) Extended nozzle
It is a special nozzle that lengthens the nozzle of the ordinary injection machine so that it can directly contact the gate of the mold. It is heated by an electric heating coil and has a temperature measurement and control system. The temperature of the nozzle is required to be 15-20°C higher than that of the barrel. The nozzle mouth is actually the gate of the cavity, and a point gate with a diameter of 0.8-1.2mm is commonly used.
Since the high-temperature nozzle directly (or indirectly) forms the plastic part, the mold must be insulated to prevent the high temperature of the nozzle from affecting the curing of the plastic part. Air gap and plastic skin insulation are commonly used. After injection and pressure holding, the nozzle should be separated from the mold to minimize the contact area between the nozzle and the mold.
The extended nozzle has a simple structure and is often used in single-cavity molds. Commonly used are spherical, conical and other forms.
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(a) spherical nozzle (b) conical nozzle
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(c) Forming nozzle (d) Adiabatic nozzle
The structure of the extension nozzle
Figure (a) The nozzle extends into the sprue sleeve, the nozzle is positioned by the shoulder and bears the force, and there is an increase in the distance between the nozzle and the sprue sleeve.
Air gap bushing.
Figure (b) The end face of the nozzle is a part of the cavity, with an intermediate bush, an air gap slot, and the introduction of cooling water.
Figure (c) The nozzle must be positioned against the injection seat to withstand the pressure. The front end of the nozzle matches the hole, and thermal expansion must be considered
Bulge and flash.
Figure (d) is a heat-insulating nozzle, with a bowl-shaped plastic heat-insulating skin, with a central thickness of 0.4-0.5mm and an outer side of 1.2-1.5mm. The pressure shoulder is embedded with a PTFE gasket. Ensure the strong rigidity at the bottom of the sprue cup.
2) Multi-cavity hot runner injection mold
It has many structural forms and is widely used. It is characterized by a runner plate heated by a heater. It is connected to the main flow channel, and is provided with a flow channel and a plurality of nozzles.
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Sprue type gate multi-cavity hot runner mold structure
1-main flow channel bushing; 2-hot runner plate; 3-fixed mold plate; 4-pad; 5-sliding pressure ring; 6-nozzle sleeve; 7-screw; 8-plug; 9-rotation stop ; 10-heater; 11-side plate; 12-main channel type sprue cup; 13-fixed model cavity plate; 14-moving model cavity plate.
3) Structural design of the hot runner plate
Good heating and insulation facilities are required to ensure effective heater installation and temperature control. According to the number and location of gates, there are many forms.
Hot runner plate design:
• The diameter of the circular runner is generally 5-15 mm.
• The end hole of the shunt channel is sealed with a fine-toothed plug.
• Air gap or asbestos board for thermal insulation. Commonly used air gap 3 ~ 8 mm.
•The hot runner plate has sufficient strength and rigidity.
• Made of medium carbon steel or carbon alloy steel.
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The structure of the hot runner plate
1-heater hole; 2-runner; 3-feed nozzle installation hole
4) The heating method of the hot runner plate
• Internal heating
Internal heating is to heat a large-diameter flow channel, and a rod heater is installed on the axis of the flow channel. The outer wall of the runner is cold, and the peripheral plastic freezes to act as a thermal insulation, so that the heater is well isolated from the mold. It can reduce power consumption by about 50%, and there is no problem of thermal expansion of the runner plate. Leakage is better eliminated and the end of the gate can be controlled with a heated probe.
Internal heating may trap material, causing decomposition. Therefore, it is not suitable for heat-sensitive plastics. In addition, the mold filling pressure in the runner is high.
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Internal heating runner and nozzle
1-cooling water hole; 2-heating nozzle; 3-melt channel; 4-internal heater
• External heating
The externally heated runner plate is suspended in the mold, and is usually arranged on the outside of the runner with a heating rod or a curved heating tube. Air gaps are used for heat insulation of the runner plate, and heat insulation sheets are also used. Heat loss must be considered. The thermal expansion of the runner plate needs to be compensated to prevent leakage. Hot nozzles are installed on the runner plate. External heating can minimize the pressure loss of the mold, and the flow channel is generally a circular large diameter. The externally heated runner plate and nozzle are suitable for heat-sensitive and high-viscosity plastics, the runner has no cold skin, and the flow of the runner is relatively large. Externally heated runners are more expensive than internally heated.
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(3) Hot runner nozzle
The nozzle is a key element of the hot runner mold. To maintain the molten state of the plastic in the nozzle, it must be as perfect as possible insulated, and some nozzles also need internal or external heating. The cavity needs to be cooled. The temperature difference between the two is usually 100-200°C, so the nozzle design should first meet the heat balance requirements. It is necessary to avoid solidification and clogging caused by too much cooling material in the nozzle, and to avoid casting or drawing or even thermal decomposition of the plastic due to overheating. Secondly, thermal expansion caused by temperature difference should be considered. Again, pay attention to the leakage of the melt, which will cause flashes and affect the normal work.
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Commonly used hot runner nozzle structures:
external heating
Internal heating
Spring Needle Valve
1) Structural forms of various hot runner nozzles
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①Flat nozzle
Straight gate form
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Split gate flat nozzle form
• Point gate form
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Several forms of single gate flat nozzle
②Point gate nozzle
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Single gate point nozzle form and split gate point nozzle form
③Valve nozzle
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cylinder, oil cylinder
④Special nozzle
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One-core multi-head type and multi-core multi-head type
2) The heating method of the nozzle
①External heating nozzle
The heat source comes from the heating ring around the nozzle. The melt flow resistance in the nozzle is small and the length is not limited. Due to structural constraints, the temperature at the gate at the front of the nozzle is relatively low. Due to the temperature difference, the heat balance is not easy to control. The heat utilization rate of the externally heated nozzle is low, and there must be a heat insulating air gap of 3 to 5mm around the heating ring.
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Contact nozzle multi-cavity hot runner injection mold
1-fixed mold floor; 2-cushion block; 3-rotating 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 template
②Inner heating nozzle
The heat comes from a heating rod in the center of the shuttle. The heating rod power can be adjusted by voltage. The melt channel gap around the shunt shuttle is generally 3-5mm. The gap is small, the flow resistance is large, and the heat dissipation is fast; the gap is large, and the radial temperature difference of the melt is large. If the nozzle is longer, an electric heating coil is required to assist external heating.
The temperature of the internally heated nozzle can be effectively controlled because the high-temperature conical tip extends into the gate.
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Internal heating hot runner nozzle
1-Fixed template; 2-Nozzle; 3-Taper tip; 4-Splitting edge; 5-Heating rod; 6-Insulation layer; 7-Cooling water hole.
3) Needle valve nozzle
A needle-shaped spool that can be opened and closed is placed in the nozzle to make the gate a valve. The injection packing phase is on; the cooling phase is off. The diameter of the gate can be increased, which avoids the clogging of foreign matter, and prevents the casting and wire drawing of the gate melt. Suitable for various viscosities, especially low-viscosity plastics.
The opening and closing of the spool can be driven by melt pressure or hydraulic pressure.
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Spring Needle Valve Hot Runner Nozzle
1-fixed mold bottom plate; 2-hot runner plate; 3-pressure ring; 4-compression spring; 5-piston rod; 10-heat insulation layer; 11-heating ring; 12-nozzle body; 13-nozzle head; 14-fixed mold cavity plate; 15-release plate; 16-core
Forming characteristics of valve nozzles:
•No sprue marks are left on the surface of the product, and the surface of the sprue is smooth.
•Ability to use larger diameter gates to speed up cavity filling. Reduce injection pressure and reduce product deformation.
•Prevent the phenomenon of wire drawing or casting at the gate when opening the mold.
•When the screw of the injection machine moves back, it can prevent the molten material in the mold cavity from flowing backward.
• It can cooperate with the sequence control to reduce the product weld marks.
(4) Heat balance and temperature control of the hot runner system
1) Requirements for heat balance of hot runner system
The hot runner system must meet the requirements of heat balance, and its heat loss should be compensated by heating. Ideally, the hot runner system should be in an isothermal state. The requirement for the control of the hot runner system is to keep the deviation of the desired temperature to a minimum. For this, the following conditions should be met:
• Accurate design of heating element power;
• Heating elements are installed correctly in the system structure;
• Reasonably determine the heating position and temperature measurement point;
• Adequate thermal insulation measures and effects.
From the user's point of view, the conditions that should be met are:
• Good durability;
•Easy to replace;
• Good damage resistance, corrosion resistance, not easy to leak;
•The line connection is safe and reliable.
2) Type of heater
Commonly used heaters for hot runner molds are:
• Commonly used coil and strip coil heaters for nozzle heating;
•Rubber and tube heaters are commonly used for runner plate heating.
3) Temperature control of the hot runner system
•Accurate temperature control is the key factor to realize the automatic operation of the hot runner system. A common method is to use a temperature control meter to control the contactor.
• Its control principle is to control the opening and closing of the heating element by judging the temperature of the mold. When the mold temperature is lower than the set value, the contactor is closed, all the voltage is applied to the heating element, and its temperature rises rapidly; when the temperature reaches the set value, the contactor is disconnected.
• Thermocouples are installed near the flow path. The hysteresis of thermocouple temperature measurement makes the temperature control accuracy lower. The output control device of the pulse width modulation hot runner temperature control system adopts high-power bidirectional thyristor output, which has stable operation, reliable performance, and long service life of the heating element.
(5) Application examples of non-runner aggregate molds
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06
Thermosetting Plastic No Runner Congeal Injection Mold
Warm runner injection mold for non-runner aggregate injection molding of thermosetting plastics.
1. Forming principle
During the injection molding of the warm runner, the plastic in the runner should always be kept in a molten state as in the barrel of the injection machine. For this reason, a low-temperature zone must be independently set at the flow channel of the mold, and the temperature is roughly in the range of 105-110°C. The warm runner plate adopts hot water or hot oil circulation for heat preservation, and the heat is taken away or supplemented by the temperature measurement and temperature adjustment system. The mold cavity is a high-temperature zone, and the temperature is about 145-180°C. After the material is injected into the cavity, it is cross-linked and solidified under heat and pressure to form an infusible and insoluble substance with a network structure. The heat insulation between the low temperature area and the 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 the fixed plate of the fixed mold and the fixed plate of the movable mold. Air-gap insulation is also a commonly used means of insulation. There are air gaps around the warm runner plate and the nozzle. The nozzle is at the 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 the nozzle mouth, and the upper end of the nozzle needs to be kept at a low temperature by a temperature-regulating medium.
Warm runner injection molding requires materials to maintain good fluidity in the runner, be sensitive to pressure, and solidify quickly after entering the high-temperature cavity.
Warm 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.
2. The structure of the warm runner injection mold
1) Structure of multi-cavity warm runner injection mold
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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-warm runner plate; 14-positioning ring; 15-insulation plate; Locking template; 19-insulation board; 20-nozzle.
2) Single cavity temperature main channel injection mold
For a one-cavity thermosetting plastic injection mold, the nozzle can be specially designed and manufactured with a temperature-controlled medium to replace the original injection machine nozzle and extend into the mold.
The extended nozzle is directly connected to the gate, leaving scars on the plastic part after molding. There is an air gap around the nozzle for heat insulation, and the nozzle leaves the mold after injection and pressure maintenance. The temperature of the nozzle is strictly controlled, and the 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.
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(a) Extended nozzle (b) Sleeve nozzle
3. Key points of mold design
1) There must be good heat insulation measures between the warm runner plate and the template to prevent the temperature of the runner plate from rising and causing failure.
2) The mold temperature must be accurately controlled, and fluctuations within 5°C are allowed. The temperature of the runner plate and each nozzle should be controlled separately.
3) The warm runner should adopt a circular cross-section to facilitate melt insulation and filling flow, with a general diameter of 6-8 mm. 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. The surface roughness of the runner should be consistent with the cavity, preferably chrome-plated to ensure wear resistance.
4) The hole diameter of the nozzle is generally not less than 4mm, and it has an inverted cone of 0.5°~1°, which is convenient for gate demoulding.
5) The parting surface should be set separately on the warm runner plate, and it should be equipped with a hook-type opening and closing lock plate to prepare for the need to take out the solidified material from the runner.
6) The volume of the runner should be smaller than the total volume of plastic injected at one time to prevent the plastic melt from staying in the runner for too long and solidifying.
