Understand ABS injection molding

- Jul 13, 2019-

Because of its great mechanical strength and good comprehensive performance, ABS plastic plays an important role in the electronics industry, mechanical industry, transportation, building materials, toy manufacturing and other industries. Especially for the slightly large box structure and stress components, decorative parts requiring electroplating are inseparable from this plastic.Based on many years of practical experience, the ABS plastic injection molding process is analyzed as follows.

(1) the drying of ABS plastic has great hygroscopic property and sensitivity to water. Sufficient drying and preheating before processing can not only eliminate the firework-like bubble belt and silver silk on the surface of products caused by water vapor, but also contribute to the plasticization of plastics and reduce the color spots and moire on the surface of products.The water content of ABS material should be controlled below 013%.The dry conditions before injection are: dry winter festival in 75 ~ 80℃ below, dry 2 ~ 3h, summer rain days in 80 ~ 90℃, dry 4 ~ 8h, such as parts to achieve a particularly good luster or complex parts themselves, dry time is longer, up to 8 ~ 16h.It is often ignored that mist spots on the surface of products are caused by trace moisture.* refit the hopper of the machine into a hot air hopper dryer, so as to prevent dry ABS from absorbing moisture again in the hopper, but this kind of hopper should strengthen humidity monitoring, to prevent material overheating when production is interrupted occasionally.

(2) the relationship between temperature and melt viscosity of ABS plastics is different from that of other amorphous plastics.In the melting process when the temperature rises, its melting actually reduces very little, but once reaches the plasticization temperature (suitable processing temperature range, such as 220 ~ 250℃), if continues blindly to raise temperature, will certainly cause the heat resistance is not too high ABS thermal degradation but causes the melt viscosity to increase, the injection molding is more difficult, the mechanical properties of the products also declined.Therefore, although the injection temperature of ABS is higher than that of polystyrene and other plastics, it cannot have a loose temperature range like the latter.Some temperature control bad injection molding machine, when the production of ABS parts to a certain number, often more or less found embedded in the products of yellow or brown coking particles, and it is difficult to use the new material to empty injection and other methods to remove it.

The reason is that ABS plastic contains butadiene. When a certain plastic particle sticks firmly to some surface that is not easy to be eroded in the spiral groove at a higher temperature and is subjected to high temperature for a long time, it will degrade and carbonize.Since partial high temperature operation may bring problems to ABS, it is necessary to limit the temperature of each section of the drum.Of course, different types and composition of the ABS applicable temperature is different.Such as plunger type machine, furnace temperature maintained at 180 ~ 230℃;Screw machine, furnace temperature maintained at 160 ~ 220℃.It is particularly worth mentioning that ABS is sensitive to the changes of various technological factors due to its high processing temperature.Therefore, the temperature control of barrel front and nozzle is very important.Practice has proved that any minor changes in these two parts will be reflected in the product.The greater the temperature change, will bring weld seams, poor gloss, flash, bonding die, discoloration and other defects.

(3) the viscosity of ABS melt under injection pressure is higher than that of polystyrene or modified polystyrene, so the injection pressure is higher.Of course, not all ABS products need to apply high pressure, small, simple structure, large thickness of products can be used with a lower injection pressure.In the process of injection, the surface quality and the degree of silver filamentous defects are often determined by the pressure in the instant cavity of gate closure.The pressure is too small, the plastic shrinkage is large, and the cavity surface disengagement opportunity is large, the product surface atomization.Too much pressure, plastic and cavity surface friction strong, easy to cause sticky mold.

(4) medium injection speed is better for ABS.When the injection speed is too fast, the plastic is easy to burn or decompose out of gasification, so as to appear on the parts of welding seams, poor luster and gate around the plastic red defects.But in the production of thin wall and complex parts, or to ensure that there is a high enough injection rate, otherwise it is difficult to fill.

(5) the molding temperature of ABS is relatively high, and the mold temperature is also relatively high.Generally adjust the mold temperature is 75 ~ 85℃, when the production of products with a large projection area, the fixed mold temperature requirements of 70 ~ 80℃, the moving mold temperature requirements of 50 ~ 60℃.Special heating of the die should be considered when injecting large, complex, thin-walled parts.In order to shorten the production cycle and maintain the relative stability of mold temperature, after the parts are taken out, cold water bath, hot water bath or other mechanical shaping methods can be used to compensate for the time of cold fixing in the mold cavity.

(6) quantity control when ABS plastics are injected by general injection molding machines, the injection amount per time only reaches 75% of the standard injection amount.In order to improve the quality and size stability of the parts, the uniform surface gloss and color, it is advisable to require the injection volume to be 50% of the calibrated injection volume.