【Overview】Mix epoxy resin, curing agent, fillers and other auxiliary materials evenly in specified proportions, then inject the mixture into molds under given conditions for curing and shaping to produce products identical to mold cavities. Common processes include vacuum casting (VC), atmospheric pressure gel process (APG), and new fully sealed epoxy resin technologies. Epoxy cast insulation parts for high-voltage switches require flawless appearance, stable dimensions and qualified mechanical, electrical and thermal properties. Vacuum casting is widely adopted to eliminate internal and surface air voids and bubbles, relieve internal stress and prevent cracking. Qualified casting materials, professional vacuum casting equipment and strict control over material pretreatment, mixing, casting and curing procedures are essential.
Pretreatment of raw materials: Raw materials are heated at a set temperature for a certain duration and treated under vacuum to remove adsorbed moisture, gas and low-molecular volatiles, realizing degassing and dehydration.
Mixing: The purpose is to evenly blend epoxy resin, fillers, curing agent and other components to facilitate chemical reactions. Mixing is divided into primary and secondary processes. Blending resin with fillers is primary mixing, and adding curing agent afterwards is secondary mixing. Primary mixing fully wets fillers with resin. The reaction between epoxy resin and anhydride curing agent is exothermic. Fillers with good thermal conductivity dissipate heat effectively and avoid heat accumulation, ensuring uniform internal stress and preventing shrinkage marks. Sufficient mixing time is required for even distribution of curing agent. Temperature and vacuum degree are critical parameters. Excessively high temperature sharply raises mixture viscosity and hinders degassing and casting. Vacuum removes moisture and air without vaporizing curing agent, hence vacuum level shall be properly controlled.
Casting: Place the assembled and preheated mould into the vacuum casting tank, or preheat it inside the tank. The mould temperature is slightly higher than that of the mixed material. Evacuate the tank pressure below 1330Pa and keep constant temperature. Pour the well-blended material into the mould. Maintain vacuum for a period after pouring to eliminate internal bubbles. Then release vacuum, take out the mould and send it to the curing oven. Control pouring speed and mould position properly to facilitate air exhaust and full mould filling.
Curing: Select proper temperature and curing duration. The performance of epoxy castings improves with rising crosslink density, yet mechanical strength declines once exceeding a critical value. Optimal curing temperature is determined based on this turning point. Curing is conducted in two stages. The first stage achieves initial shaping, and the secondary post-curing is carried out at a slightly higher temperature to complete full curing and acquire optimal performance. Pressurized gelation can be adopted to reduce shrinkage marks.
Demoulding: This process mainly controls demoulding temperature and cooling speed to reduce internal stress of castings, with temperature generally kept below 80℃.
Basic Characteristics The pot life of epoxy casting compound is 1 to 2 days at room temperature of 25°C, and 6 to 8 hours at 40~45°C. It features high reactivity at 140~160°C and gels rapidly within a short time. The mould temperature is 80~100°C higher than that of the casting material. The gelation reaction spreads gradually from the mould wall to the material center. Pressure is maintained throughout the curing process. The material is pressed into the cavity to compensate volume shrinkage gaps during gelation. The cured product gains compact internal structure with reduced internal stress, achieving improved insulation performance, mechanical strength, dimensional accuracy and product uniformity.
Process Flow:(1) Primary mixing: Weigh epoxy resin, curing agent, fillers, colorants and additives according to the formulated ratio, then put them into the primary mixing equipment for blending and vacuum degassing. Fillers shall be dried prior to stirring. Thin-film degassing is adopted to ensure thorough air removal.
# Automatic Pressure Gel Process Install the pressure cover on blended epoxy casting compound, then fill with filtered dry compressed air. Set air pressure between 0.2~0.5 MPa according to product specifications. Connect the device to the injection mechanism of hydraulic forming machine, and control switch and flow rate via manual ball valve. After commissioning the mould on the machine, start the automatic pressure gel forming process. Three key factors must be strictly controlled: 1. **Mould temperature** Determined by product size, shape complexity and curing characteristics of epoxy casting material, normally kept at 140~160℃. 2. **Pressurized injection duration** Full filling of the mould can be judged by normal overflow at the top. Improper injection time will impair product quality. 3. **Gelation time inside mould** It refers to the period from full mould filling to demoulding. Constant pressure shall be maintained in the cavity during this stage. Gelation time ranges from several minutes to dozens of minutes, varying with mould size. ## Post-curing Process Demoulded products shall be placed in an oven for secondary curing to enhance curing degree.
# Characteristics of APG Casting Technology 1. Constant pressure is continuously applied to epoxy casting compound to compensate shrinkage during gelation, enabling rapid gelation at high temperature. The finished products feature flawless surface, low internal stress, compact texture, high dimensional accuracy, excellent mechanical and electrical performance and high yield rate. 2. Short gelation and curing time greatly improves mold utilization and shortens production cycle. Only flash removal is required after demoulding before post-curing. The process delivers high productivity, low production cost and reduced labor intensity. Conducted in a fully sealed system, APG technology causes little environmental pollution and saves energy and raw materials. It also boosts the utilization rate of post-curing ovens. With advantages of high quality, efficiency and energy conservation, APG technology has been widely applied and rapidly developed in China's electrical industry.