What should be paid attention to in heat-resistant ABS injection molding?

Time:2020-03-21 09:05:55 / Popularity: / Source:

Heat-resistant ABS plastic is one of five major synthetic plastics. Chinese name of ABS is polymer. It was first discovered in outbreak of World War II. With good impact resistance, heat resistance, low temperature resistance, chemical resistance and electrical properties, it also has characteristics of easy processing, stable product size, good surface gloss, easy to paint and color. It can also perform secondary processing such as surface spray metal plating, electroplating, welding, hot pressing and bonding. It is widely used in industrial fields such as machinery, automobiles, electronic appliances, instrumentation, textiles, and construction, and is a very versatile thermoplastic engineering plastic. According to its performance, ABS can be divided into fire-resistant grade, flame-retardant grade, impact resistance grade, conductive grade, transparent grade, electroplating grade, high temperature resistant grade, and other varieties. Processing can be divided into extrusion, injection, vacuum, blow molding , and other varieties.
heat-resistant ABS injection molding 

Heat-resistant ABS applications

The largest application areas of ABS resins are automobiles, electronics and building materials. Use in automotive field includes many parts such as automobile dashboards, body exterior panels, interior trim panels, steering wheels, sound insulation panels, door locks, bumpers, ventilation ducts, etc. In terms of electrical appliances, it is widely used in electronic appliances such as refrigerators, televisions, washing machines, air conditioners, computers, and copiers. In terms of building materials, ABS pipes, ABS sanitary ware, and ABS decorative boards are widely used in building materials industry. In addition, ABS is also widely used in packaging, furniture, sports and entertainment products, machinery and instrumentation industries.

Heat-resistant ABS processing

ABS, like PS, is a thermoplastic with excellent processing properties and can be processed by general processing methods. ABS has better melt fluidity than PVC and PC, but worse than PE, PA, and PS, similar to POM and HIPS. Flow characteristics of ABS are non-Newtonian fluids; its melt viscosity is related to processing temperature and shear rate, but is more sensitive to shear rate. ABS has good thermal stability and is not prone to degradation. ABS has a high water absorption rate and should be dried before processing. Drying conditions of general products are temperature 81 ~ 87 ℃, time 2 ~ 4h. For special requirements products (such as electroplating), drying conditions are 72 ~ 86 ℃, time 17 ~ 18h. ABS products are prone to internal stress during processing. Magnitude of internal stress can be tested by immersion in glacial acetic acid. If stress is too large and product is absolutely forbidden from stress cracking, annealing treatment should be performed. Specific conditions are to place in a hot air circulation drying box at 72 ~ 82 ℃ for 2 ~ 4h, and then cool to room temperature.

Heat-resistant ABS performance

ABS has good impact strength and surface hardness, good dimensional stability, certain chemical resistance, and good electrical insulation in a certain temperature range. It is opaque and generally has a light ivory color. It can be made into any other color products with high gloss by coloring. Appearance of plating grade can be decorated with electroplating and vacuum coating. General-purpose ABS is impervious to water and burns slowly. It softens during combustion. Flame is yellow, black smoke, finally burnt, has a special odor, but has no melting drips. It can be processed by injection molding, extrusion, and vacuum molding methods. ABS material are hygroscopic and requires drying before processing. Recommended drying conditions are at least 2 hours at 82 ~ 91C. Material temperature should be guaranteed to be less than 0.1%. Melting temperature: 211 ~ 281C; recommended temperature: 248C. Mold temperature: 26 ~ 72C. (Mold temperature will affect finish of plastic part, lower temperature will result in a lower finish). Injection pressure: 501 ~ 1000bar. Injection speed: medium to high speed.

Precautions for heat-resistant ABS in injection molding

Reducing internal stress of product is key to heat-resistant ABS injection molding. Here we share processing points of heat-resistant ABS resin molding from five aspects: mold design, material drying, molding temperature, holding pressure and time, and mold temperature.
Heat resistance of ordinary ABS is not good enough. Now, a series of heat-resistant ABS resins have been developed by adding heat-resistant agents such as fluorene-methylstyrene copolymer and N-phenylmaleimide copolymer. Heat distortion temperature (HDT) is 90-120℃.
Because molecular structure of heat-resistant ABS contains a highly rigid benzene ring and N-phenyl group, while improving heat resistance, it also increases steric hindrance effect of molecular chain, reduces relaxation rate of molecular chain, and easily makes product have  large internal stress, causing defects such as stress cracking, stress whitening, and product brittleness. Therefore, reducing internal stress of product is key to heat-resistant ABS injection molding.
heat-resistant ABS injection molding 

Product mold design

Product wall thickness requirements are uniform, and wall thickness difference of ABS products should be controlled within 25% to prevent local stress concentration caused by excessive wall thickness differences. Weaker pillars need to increase R angle or additional ribs to prevent pillars from breaking.
For buckle design, root of buckle needs to be chamfered. Size of R corner depends on wall thickness of product. Ratio between R angle and wall thickness should be above 0.3. With this ratio increasing, internal stress gradually decreases, but problem of shrinkage of product surface must also be considered. Design of mold cooling water channel should ensure uniformity of cooling, avoid internal stress caused by uneven cooling and shrinkage.

Material drying

Generally, heat-resistant ABS resin absorbs moisture in air during storage and transportation. Water absorption rate varies with air humidity, generally between 0.2 and 0.4%. Therefore, material must be fully dried to reduce moisture content of material to below 0.05%, preferably below 0.02%, otherwise surface defects such as water splashes and silver silk may occur. Heat-resistant ABS resin has a higher drying temperature than ordinary ABS, usually 80-95℃, and a drying time of 3-4 hours.

Molding temperature

Molding temperature is a parameter that requires special attention when processing heat-resistant ABS. Its setting is based on ensuring that heat-resistant ABS is fully plasticized. Temperature range above middle value of molding temperature recommended by supplier should be used as much as possible. Increasing molding temperature will significantly reduce viscosity of heat-resistant ABS, increase fluidity of resin, and make flow distance longer to ensure that material has sufficient filling capacity.

Holding pressure and time

For heat-resistant ABS, whether holding pressure and time are set properly will directly affect internal stress of part. Increasing holding pressure will make molecular gap smaller, range of chain segment reduced, melt volume reduced, density increased, force between molecules increased, which can improve shrinkage and internal quality of product, but it will cause internal stress to increase. Therefore, on the premise of ensuring appearance quality of product, a lower holding pressure should be selected as much as possible.
Holding time is set according to time until gate solidifies as cooling completes, and screw no longer exerts pressure on molded product during advancement. If holding time is too long, it is easy to overfill material, molecular gap becomes smaller, and internal stress becomes larger; if holding time is too short, product is prone to shrink and size is unstable. Holding time setting should be based on the shortest time when weight of product no longer changes as optimal holding time.

Mold temperature

When molding heat-resistant ABS, use a mold temperature machine to control mold temperature. Recommended mold temperature is 60-80℃. Higher mold temperatures tend to produce good flow, higher weld line strength, and less internal product stress, but molding cycle will be extended appropriately.
If mold temperature is lower than recommended temperature, internal stress of product will be too high, which will reduce performance of product, and may cause defects such as product brittleness, screw hole cracking, or paint cracking.

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