EVA is a thermoplastic elastomer with soft texture, good toughness and low temperature resistance. However, it has low melt viscosity, extremely strong fluidity, easy mold sticking, easy thermal oxidation and yellowing, and is sensitive to temperature, shear, mold and process. The following is a complete set of precautions that can be directly used in the workshop.
Material Drying and Pre-treatmentEVA has low moisture absorption, but it will still absorb trace moisture after long-term storage or humid environment, resulting in bubbles, silk lines and surface pits on products. Hot air drying is recommended before processing at 50 to 60 degrees Celsius for 1 to 2 hours. Temperature should not be too high, otherwise the material will soften and stick. Recycled materials should be clean without impurities and oil. The addition ratio is generally not more than 20%, and must be mixed evenly with virgin material to avoid affecting product gloss and mechanical properties. Raw materials should be stored sealed in a dry and ventilated environment to prevent dust pollution.
Injection Molding Machine and Screw Application RequirementsEVA has good fluidity and low viscosity. It is suitable to use ordinary three-stage screw with length-diameter ratio of 1:20 to 1:22 to avoid excessive shear causing material decomposition and yellowing. Screw speed should not be too high to prevent excessive shear heat. The nozzle adopts straight-through open structure with appropriately increased diameter to avoid local overheating caused by small nozzle. The barrel and screw must be thoroughly cleaned before processing EVA to remove residual easily decomposed materials such as PVC and POM to prevent pollution of EVA and causing discoloration and peculiar smell. Clean barrel with PE or PP material before shutdown to avoid EVA residual high-temperature aging.
Temperature Control Key PointsEVA is extremely sensitive to temperature. Low temperature will cause poor plasticization, material shortage and rough surface. High temperature will easily cause decomposition, yellowing, stickiness and strong odor. Barrel temperature is recommended to be controlled in sections: first stage 150 to 160 degrees Celsius, second stage 160 to 170 degrees Celsius, third stage 155 to 165 degrees Celsius, nozzle temperature 155 to 165 degrees Celsius. Adjust specific temperature according to EVA grade, melt index and product thickness. The higher the melt index and the thinner the product, the lower the temperature appropriately. Mold temperature is controlled at 20 to 40 degrees Celsius. High mold temperature is easy to stick and slow cooling. Low mold temperature leads to poor surface gloss and obvious welding marks.
Injection and Holding Process ControlEVA has good fluidity, so injection speed should not be too fast, otherwise it will cause air entrapment, jetting marks, scorching and gas trapping. Adopt medium and low speed steady injection: slow mold sealing at front end, uniform filling at middle section, slow down to holding at end. Injection pressure is based on just filling cavity without obvious material shortage. Excessive pressure will produce flash, high internal stress and product deformation. Insufficient pressure will cause depression and shrinkage. Holding pressure and holding time should be short. EVA has small shrinkage rate. Long holding time will easily cause product stress concentration and ejection deformation. Stop holding after gate solidification.
Mold Design and Venting RequirementsEVA fills fast, so mold venting must be good, otherwise it is very easy to cause bubbles, gas trapping scorching and weak welding marks. Set vent grooves at flow end, welding mark position and cavity dead angle with depth of 0.015 to 0.025 mm to ensure smooth gas exhaust. Runners and gates avoid sharp corners and narrow structures. Gate size is appropriately increased to reduce local shear overheating. Cavity surface needs polishing treatment, polishing direction is consistent with demolding direction to reduce mold sticking risk. Demolding draft is appropriately increased. Ejection mechanism should be evenly distributed. Ejector pin or ejector block area should not be too small to avoid product deformation, whitening and fracture during ejection.
Cooling and Demolding ControlEVA has high elasticity and low thermal conductivity, so cooling must be sufficient and uniform, otherwise products are easy to warp and deform. Cooling water channels are as close to the cavity as possible and evenly arranged to ensure consistent cooling speed of all parts of the product. Try not to use oily release agent during demolding, which is easy to remain and affect appearance, printing and bonding. If necessary, select dry release agent with a small amount of light spraying. Handle products gently after ejection to avoid stacking extrusion deformation. Products with high size requirements can use shaping fixture for auxiliary cooling and shaping.
Common Problem Prevention in ProductionBubbles and pores on products are mostly caused by damp raw material, poor venting or high temperature. Strengthen drying, optimize venting and reduce material temperature. Product yellowing, stickiness and peculiar smell are usually caused by high temperature, excessive shear or long residence time. Reduce temperature, slow down screw speed and shorten material residence time in barrel. The main reasons for product mold sticking are high mold temperature, rough cavity and small demolding draft. Reduce mold temperature, polish mold and increase demolding draft. Product size instability and deformation are mostly caused by insufficient cooling, long holding or unbalanced ejection. Extend cooling time, shorten holding and optimize ejection structure.
Production Safety and MaintenanceEVA decomposition at high temperature will produce slight odor, so the workshop should be well ventilated. Regularly clean mold parting surface, cavity and vent grooves to prevent carbon deposits, cold materials and oil from affecting product appearance. Control barrel temperature during production to avoid long-time empty injection at high temperature causing material aging. Record process parameters during shift handover to keep temperature, pressure and speed stable and ensure product quality consistency.
