Need to produce small and light parts has made thin-wall injection molding the most in-demand performance of injection molding machines. "Thin wall" is usually defined by lightweight electronic parts with a wall thickness of less than 1mm. For large automotive parts, "thin wall" can be 2mm. In short, thin-walled products require changes in processing technology: higher pressure and speed, shorter cooling time, changes in product ejection and gate arrangement. Following are requirements of thin-wall injection molding for injection machines and molds.

Standard injection machines can be used to produce a variety of thin-walled products. Performance of current new injection machine greatly exceeds that of 10 years ago. Advances in materials, gate technology and design have further broadened performance of standard injection machines for filling thin-walled parts. However, due to continuous reduction of wall thickness, a more special injection machine with high speed and high pressure performance is needed. For example, for an electronic part with a thickness of less than 1mm, it is normal for filling time to be less than 0.5 seconds and injection pressure to exceed 210MPa.
Hydraulic injection machine for thin-wall injection is designed with a pressure accumulator, which can frequently drive injection and mold clamping. All-electric injection machines and electric/hydraulic injection machines with high-speed and high-pressure performance are also available. In order to withstand high pressure of new injection machine, minimum clamping force must be 5 to 7 tons/inch (projected area). In addition, when wall thickness decreases and injection pressure increases, large mold plate helps reduce bending. Ratio of tie rod of injection machine for thin-walled products to thickness of mold plate is 2:1 or lower. In the production of thin-walled products, infinite loop control of injection speed, pressure and other processing parameters helps to control filling and holding pressure under high pressure and high speed.
As for injection volume, large diameter barrel is often too large. Recommended injection volume is 40% to 70% of barrel capacity. Total molding cycle of thin-walled products is greatly shortened, and it is possible to reduce minimum injection volume to 20% to 30% of barrel capacity. Users must be very careful when injection molding, because for materials, a small injection volume means that material stays in barrel for a longer time, which will lead to a decline in product performance.

 

Speed is one of key factors for success of thin-wall injection molding. Rapid mold filling and high pressure can inject molten thermoplastic material into mold cavity at high speed, thereby preventing gate from freezing. If a standard part is filled within two seconds, mold thickness is reduced by 25%, and it is possible to reduce filling time by 50%, which is exactly 1 second.
One of advantages of thin-wall injection molding is that when thickness is reduced, less material needs to be cooled. As thickness decreases, molding cycle can be shortened by half. Reasonable setting of melt conveying device makes hot runner and runner not hinder shortening of molding cycle. Use of hot runners and sprue bushings helps to shorten molding cycle to a minimum.
In addition, mold material should also be considered. P20 steel is widely used in molding of traditional products, but due to higher pressure of thin-wall injection molding, mold must be made very strong. H-13 and other hard steels add an extra safety factor to thin-walled molds. However, cost of a solid mold may be 30%-40% higher than that of a standard mold. But increased cost is usually offset by improved production performance.

 

Application range of plastic parts is becoming more and more extensive. Plastic parts are used from simple tableware, kitchenware to complex mechanical parts, electronic product shells, etc. However, for plastic parts with high precision requirements such as electronic product shells, precision requirements must be paid attention to from beginning of mold manufacturing during injection molding.
What are factors that determine accuracy of the plastic injection molding process? How can we improve precision of thin-walled injection molded parts?
1. Closing mold pressure: In principle, it should be greater than total pressure of plastic injection into mold, based on no burrs.
2. Pressure and speed have partly same relationship in mold. Purpose of acting in mold is to make raw materials enter mold evenly, thoroughly and appropriately to fill corners. Too low will cause short shots, shrinkage, too high to have hairs, over-saturation, sticking, scorching, easy damage to molds and high internal stress.
3. Speed of material determines condition of raw material in mold runner and finished product. Fast speed has burrs, oversaturation, and scorching. If it is slow, short shots, shrinkage, poor bonding and easy breakage will occur.
4. Temperature of raw materials is different, and temperature is different, too low to melt glue and cause uneven color, increase internal stress of finished product. If temperature is too low and pressure is too high, it may cause screw to break, too high, product has burrs, and temperature difference due to cooling causes shrinkage. Raw materials will decompose, turn yellow, change color, and break easily. Cooling time becomes longer, and air is not easily discharged.