Resin Vacuum Cast Process and Its Points for Attention
WU Hai-jun
(Tianshui Changcheng Universal Electric Appliance Factory, Tianshui 741000, China)
Abstract: The process of resin-cast techndogy is introduced.The questions that are easier to meet in operation and its points for attention are analysed.
Key words: Dry-type transformer; Vacuum cast; Expoy resin; Technology
1 Overview
Epoxy materials in electrical products in China since the 1980s, more and more widely. As adhesive prepreg insulation material as the primary insulation material widely used in areas such as transformers, transformers, insulators, voltage level range 35kV, 10kV and below development from the past to the processing means is pouring. Each manufacturer to improve the quality of products pouring continuous casting equipment, technological innovation, by atmospheric pressure casting the semi vacuum casting vacuum casting and full vacuum insulator automatic pressure gel casting (referred to as the AFG process). Called full vacuum, that is, from the ingredients of the epoxy resin, mixing to the final casting completion, the whole process is carried out is less than the degree of vacuum of 0.1kPa.
Full vacuum casting advantage is that the whole process of the ingredients are in a vacuum condition can be effectively extracted to the air of the epoxy mix and to reduce the bubbles ingredients. Full vacuum casting process, as much as possible the air out of the epoxy resin within the casting mold, thus reducing the bubbles within the cast product, improve the strength and mechanical properties of the products electrical insulation, effectively reducing the amount of partial discharge of the inside of the product.
Vacuum casting technology has played a significant role to improve the quality of epoxy resin cast, increase the rate of qualified products, reduce production costs, improve the production environment, the protection of production workers healthy. So in recent years, the vacuum casting technology and equipment is widely used in the production of transformers and dry type transformers. I plant also from Switzerland in the mid-1990s, the introduction of epoxy resin cast three phase dry type transformer production equipment and manufacturing technology.
Casting process and Precautions
I plant the introduction of gating system, casting process flow diagram is shown in Figure 1.
Figure 1 casting process flow diagram
Due to the different vacuum casting equipment, casting process is also different. However, the overall process is much the same. The below I plant equipment, for example, talk about the production process and considerations.
2.1 batching process
This process is divided into the ingredients of the epoxy resin and the curing agent ingredient. I plant equipment using the Ciba-Geigy Corp. of Switzerland two-component epoxy resin material, composed of an epoxy resin CY225, and a curing agent HY227. Batching process is a certain amount of epoxy curing agent (both mucoid be heated in winter) inhalation dosing tank, the tank is low vacuum, the premix tank. Stirring heated to 60 ° C, its flowability of the best state, the ratio of 1:1.5 filler (usually microsilica), was added and stirred, degassed. Mixture was stirred to be uniform, no large amount of foam and the temperature reaches 60 ~ 65 ℃ can be fed into the degassing tank film degassing epoxy resin or curing agent. This process is about 2.5 to 3H, the degree of vacuum is 0.1 to 0.12kPa. Curing agent the ingredients with epoxy ingredients process is exactly the same.
This process is prone to the problem: low winter temperatures, the viscosity becomes very high in the ingredients, materials, and even did not flow. At this time, can the container be heated. Required heating must be uniform, slowly. Otherwise, the above material is not melted, metamorphic damage while at the bottom of a material because the temperature is too high, and even combustion, resulting in losses.
2.2 thin film degassing process
The membrane degassing process is a very important process. Degassed will determine the quality of pouring the amount of bubbles, the bubble is less, the higher the quality of the casting.
Film degassing is widely used as a degassing effect in the current casting equipment ideal method for degassing. Spiral lifting device materials to enhance on degassing umbrella, shed from the umbrella surface around the material to form a very thin layer of material the umbrella surface. The material gas in the umbrella surface will form bubbles escape into the vacuum tank, the vacuum pump extracting tank. Mixed with filler material, the viscosity will increase, fluidity, mixing the gas in the material is difficult to be filtered in a short time, but also a small amount of gas and water will be dissolved in the material, and therefore must a range of 60 ~ 65 ℃ long stirred degassed. Under normal circumstances, must be at least continuous degassing 24h, as much as possible so as to removal of the material in the water and gas.
Prone to problems in this process:
(1) in the ingredients after the degassing tank pump feeding, we must be very careful. If the resin pump into the curing agent tank, or the curing agent is pumped into the the resin degassing tank, the tank must be within 4 ~ 6h material discharge all. Otherwise, once cured, will result in off cylinders scrapped serious consequences.
In order to prevent this from happening, the device discharge valve at the bottom of the tank is equipped with emergency manual discharge valve for the tank body material is discharged when the unusual happens. Discharge port solenoid valve in serious condition, but also can be removed, in order to speed up the discharge of the mix.
(2) the degassing process should be carried out continuously to ensure that you can use at any time, to avoid downtime. Must re-degassing degassing completed materials downtime for any reason (such as weekends, holidays or maintenance equipment), the material back into use. At this time, the boot sequence is first heated degassing tank to reach> 45 ° C and then, to open the vacuum pump. When the the tank vacuum degree requirements, for several minutes, then open the mixer degassing. How much the degassing time can be material and downtime length adjusted appropriately. Otherwise, the material will be entering into a large number of air affect casting quality.
2.3 final mixing casting process
This process is a resin and a curing agent mixture, so that after degassing and vacuum casting process after the final uniform mixing. The casting process for manual operation, manual control volume and flow, so that the mixture smoothly into the mold. The device uses a natural pouring methods rely on the difference in height of the upper and lower tanks. In the casting must be good or bad based on the degree of complexity of the mold cavity of the casting process of appropriately controlling material flow Usually transformer windings horizontal casting can be controlled at 1.5 to 2kg/min or not clogged vent as the standard for Control.
Prone to problems in this process:
(1) the end of the mixing process takes too long. If the mixing time is too long, it will force the casting time. Or after 2 ~ 3h, the material viscosity will rapidly increase, affecting the quality of the stream feed speed and pouring. Serious cause product obsolescence or conveying pipes get clogged, damaged equipment. Should be appropriate to the temperature of 65 ~ 70 ℃, time 30 ~ 45min Stir casting.
(2) material pouring down spray. Must be checked before pouring the final mix tank air pressure slightly higher than pouring cans. Because the equipment used to rely on the weight of the atmospheric pressure difference and material, so that the material naturally flows into the mold the natural casting method for casting, if the upper and lower tank Pressure no pressure or the pressure is too small, will make the casting speed is reduced, the flow is slow, extended pouring time. When the tank pressure is higher on the tank, it will cause on the the tank material down spray.
(3) pouring rate is generally 1.5 ~ 2kg/min appropriate. As compared narrow cavity, preferably to not clog the gate discharge hole. The pouring rate must be uniform, not too fast or too slow. Too fast, the resin mixture was too late the full cavity overflow began at the gate, we poured a full illusion, heat curing, the material slowly fills the entire cavity caused at the gate Queliao; slow, than 2h, the material viscosity would over time rapid growth, the longer, the greater the viscosity, the more unfavorable casting. In this case, often within the cured product there will be a large number of honeycomb or the stomata or larger area Queliao of blotchy.
(4) After the pouring is completed, must be rapidly pouring member pushed into a curing oven for curing avoid stay in a low-temperature environment to prevent the decline in mold temperature, affecting the flow of the material within the mold. After casting is completed, should be strictly in accordance with the equipment operating procedures, the final mixing tank the conveying pipe valve cleaning. Regularly with cleaning fluid and a thorough cleaning equipment, clogging conveying pipe and valves, to clean or replace.
(5) after the completion of the mold assembly, first pouring tank uniform heating 6 ~ 8h, the mold temperature of 80 ~ 85 ℃, pulls out the vacuum, for casting. If the mold internal temperature of less than will appear after curing Queliao phenomenon.
The viscosity of the resin mixture and the temperature, time relationship shown in Figure 2.
Figure 2 The viscosity and temperature vs. time
> 60 ℃, the viscosity is small, and most favorable to the casting. If the temperature is lowered, the viscosity will rapidly increase, the flow difficulties. Sometimes, even if the mold surface temperature to meet the requirements, the internal copper foil and the insulating material, or copper and the temperature of the insulating material is not meet the requirement, and pouring the mixture after exposure to the inner winding, the temperature of the contact surface will decline rapidly, the material viscosity increase , clogging can not enter into the mold cavity, the back of the material, can also cause Queliao.
2.4 curing and form removal process
Pouring after the completion of the mold must be carried out in accordance with the time curve of the curing temperature of the material requirements in the curing oven heat curing. This process, as long as the uniform temperature of the curing oven, the upper and lower furnace temperature deviation <3 ° C will be able to properly cure. When the the local resin layer is very thick, should be after the end of the gel process, remove this part of the mold, so that a thick layer of resin material is exposed to direct heating, in the air, and to achieve the optimum curing effect. It should be noted, Although the surface of the material cured, but did not fully hardened, not by the extrusion pressure, otherwise it will be deformed damaged. After the curing, the mold is demolished.
Prone to the problems of this process:
(1) when the mold into the curing oven of about 1 ~ 2h, the need for each mold to be checked. The If found depth gate recessed inside the die, or Queliao pouring remaining material poured into the sprue, until filled. Half an hour later, check again, as there are shortage of material, may continue the previous step. Such measures can only be carried out before the resin mixture gel. Before this, the material is a liquid state, poured into the new material, the two can be miscible. After the start of the gel, the first gate at the surface of the resin mixture starts to solidify, the solid oxide film to form a layer of light, when the new material is not poured into the compatibility with the old material. Form a sandwich in the surface after curing, easy to peel, so entering the the curing oven about 4h, it should not be supplemented feed.
(2) casting mold is generally made of steel and aluminum into the distance insert member is fixed on the mold, before cooling Note should be removed these fasteners, in order to avoid the cooling, is inconsistent due to shrinkage, great internal stress, and even cracks in the inserts. Optimal disassembly time should be in the mold after curing, cooling before. Between the mold and the pouring member are maintained at the curing in a thermal expansion state, no relative deformation, so disassembly easier. The pouring member itself does not generate additional stress, can improve the mechanical properties and the fatigue strength of the product.
(3) during the cooling process, if the temperature change is large, in the pouring member will generate internal stress. This force was spread state, by the support of the inner winding, and not obvious. External factors, such as running current, voltage shock, mechanical shock, or encounter greater changes in ambient temperature, pouring surface cracks or cracks, it will cause a very serious potential quality risks. The resin layer is thicker, the larger the temperature change, the greater the internal stress generated. Especially at the uneven thickness of the resin layer, the most prone to cracking. Once we form removal, the Chai Wan mold, inadvertently a 80mm x 65mm x 000mm pad fell to the ground. Few minutes which appear more than 5mm of the hump, visible internal stress is a very serious dangers. Therefore, before casting workpiece temperature is not completely down, with the ground or other temperature difference must not be larger object contact, to avoid the hidden internal stress. Generally should be used to cool with the furnace slow cooling method.
3 devices should pay attention to the problem
3.1 Cleaning
Can be divided into regular cleaning and washing at any time. The cleaning operation at any time after each casting, emptying final mixed residual resin inside the tank and the pouring tube, close all valves, according to a final mixing tank feed method to the end of the curing agent mixture feed injection mixing tank. About 20kg (the extent to normal stirring, the amount is more, cleaning the better), vacuum stirring 20min, open the valve, the outflow process of curing agents, switch outlet valve several times, cleaning valve. Then, open casting valve operation with the former. The curing agent can be used as a pad after cleaning castable use, or back into the degassing tank degassing. As follow the rules of this material as the next casting material, because these curing agents will be most exposed to the air in the cleaning, the material will be mixed with large amounts of air in the mixing tank, the degassing effect poor, and ultimately in the casting compound containing a lot of air, to cast an adverse impact.
Regular cleaning was divided into long-term cleaning and short-term cleaning. Long-term cleaning in over a long period a thorough cleaning of the equipment, including pipelines, each inside the tank, and clean-up of the various valves. This cleaning can generally be carried out once a year. The specific operation will be approximately 500L solvent (organic solvent) available banana water, acetone, n-methyl pyrrolidone, etc., by the dosing tank Batch, degassing, the final mixing, pouring, washing process sequentially for each tank, the piping, valves. The time may be slightly longer than the normal operation to ensure a thorough cleaning.
The short-term cleaning on the end of the mixing tank and pouring cleaning of pipes and pouring valve. Even if each casting is completed, the final mixing tank and fill pipe, valve are carefully cleaned, still unavoidable in a small amount of the final mixing stick to the inside, over time, it will cause the valve switch is not working, pipes get clogged, damage to the tank wall sticky sediments agitator blade, affecting tank heating speed, and clogging of the sensor probes. The final mix tank to enhance efficiency, will affect the lifting mechanism. Serious, stick blender. So, in a month or a quarter of the equipment to conduct a clean. The main content is to remove the pouring valve, heating after soaking in a solvent, scrape the sticky sediments. Remove the pouring tube, and heated to 120 to 140 ° C, wherein the viscosity plot logistics out. If you have cured plug metal pipes, can be placed on the fire, baking, clog combustion discharged. Scrape the wall remnants Open final mixing tank heating, remove the vacuum sensor probe, filter cleaning solvent.
3.2 pipeline heating and insulation
Conveying pipeline and pouring pipeline, most of the heating device. When the ambient temperature is low, the material viscosity is increased, the flow is difficult. If the temperature is too low will cause the conveying pump idling, reducing the life of the pump. This situation often occurs in the northern winter.
There are two ways to resolve this situation: one is to raise the ambient temperature, such as the installation of heating device. Installation of heating systems is difficult at a cost of more production will cause some inconvenience. Another heating pipes and valves directly. Heating wire wound according to the requirements in the pipeline, according to the size of the selection of power to choose whether to add insulation layer. Power, you may not put the power of small must add insulation layer. Each of the pipes are respectively mounted control switches, and only when needed for heating.
3.3 regular inspection and calibration
Periodically check the electronic scale or flowmeter calibration. Directly affect the ratio of the degree of accuracy of the scale is accurate. The ratio of the respective components of the epoxy resin must be strictly controlled, otherwise it will affect the curing time of the reaction, the reaction efficiency and the mechanical, electrical properties of the cured product.
4 Conclusion
Epoxy resin vacuum casting system has been widely used in electrical components manufacturing industry, the epoxy resin as an insulating material in different products on the performance of different requirements. To assimilate the technology based on technology, equipment, local improvements, to improve performance or to make it more of a certain kind of adapt to production needs, it is very necessary.
