1. Acrylonitrile-butadiene-styrene plastics (ABS)
Acrylonitrile-butadiene-styrene plastics (ABS) is a general thermoplastic engineering plastics. Its specific properties are as follows:
The appearance of ABS engineering plastics is opaque and ivory color granules. Its products can be made into colorful and high gloss. The relative density of ABS is about 1.05, and the water absorption rate is low. ABS has good bonding with other materials and is easy to be printed, coated and coated. The limiting oxygen index of ABS is 18%~20%. It is a flammable polymer with yellow flame, black smoke and special cinnamon flavor.
ABS has excellent mechanical properties, excellent impact strength, can be used at very low temperature: ABS has excellent wear resistance, good dimensional stability, and oil resistance, and can be used for bearings under medium load and speed. The creep resistance of ABS is larger than that of PSF and PC, but smaller than that of PA and POM. The bending strength and compressive strength of ABS are poor in plastics. The mechanical properties of ABS are greatly affected by temperature.
The hot deformation temperature of ABS is 93 ~ 118 C, and the annealed products can be increased by about 10 C. ABS can still show some toughness at - 40 C and can be used in the temperature range of - 40 ~ 100 C.
ABS has good electrical insulation and is almost independent of temperature, humidity and frequency. It can be used in most environments.
ABS is not affected by water, inorganic salts, alkali and a variety of acids, but it is soluble in ketones, aldehydes and chlorinated hydrocarbons. Stress cracking occurs when it is eroded by glacial acetic acid and vegetable oil. ABS has poor weatherability and is easy to degrade under the action of ultraviolet light. After half a year outdoor, the impact strength decreases by half.
Application of ABS resin
ABS resins are widely used in automobiles, electronic appliances and building materials. The use of automotive field includes automotive dashboard, body outer panel, interior decorative panel, steering wheel, sound insulation board, door lock, bumper, ventilation pipe and many other components. In the electrical field, it is widely used in refrigerators, TV sets, washing machines, air conditioners, computers, photocopiers and other electronic appliances. Building materials, ABS pipe, ABS sanitary ware, ABS decorative board. In addition, ABS is also widely used in packaging, furniture, sports and entertainment products, machinery and instruments in industry.
Important indicators of car interior decoration include beauty, low odor, good mechanical properties, heat resistance, weather resistance and so on. Automotive materials can meet the use requirements of various interior components, materials with the following conditions: good fluidity; excellent impact resistance; easy processing and forming; easy coloring, spraying; low odor; good corrosion resistance; Matt effect. ABS is also used in 3D printing.
2. Polycarbonate (PC)/ABS Alloy
In order to overcome the above shortcomings of ABS, PC/ABS alloy is prepared by blending polycarbonate (PC) with ABS. The mass ratio of PC/ABS is usually 70/30. The influence of thickness, temperature and other factors on impact performance is considered comprehensively. The alloy combines the formability of ABS material with the mechanical properties, impact strength, temperature resistance and ultraviolet resistance of PC. It can be widely used in automobile interior parts, various machines, communication equipment, household appliances and lighting equipment.
Automobile interior and exterior trim: dashboard, decorative column, instrument front cover, grille, interior and exterior trim
Business equipment case and built-in components: notebook/desktop computer, photocopier, printer, plotter, display
Telecom, mobile phone case, accessories and smart card (SIM card):
Electrical Products: Electronic Products Shell, Meter Shell and Shell, Household Switches, Plugs and Sockets, Cable and Wire Pipes
Household appliances: washing machine, hair dryer, inside and outside parts of microwave oven
Processing conditions of PC/ABS injection moulds:
Drying must be done before processing. The humidity should be less than 0.04%. It is suggested that the drying conditions should be 90-110 C for 2-4 H.
The melting temperature is 230 - 300 C.
Temperature of die: 50 ~ 100 C.
Injection pressure depends on the plastic parts.
Injection speed should be as high as possible.
Chemical and physical properties: PC/ABS has the comprehensive characteristics of PC and ABS. For example, the machinability of ABS and the excellent mechanical properties and thermal stability of PC. The ratio of the two will affect the thermal stability of PC/ABS materials. PC/ABS blends also show excellent fluidity. The shrinkage rate is about 0.5%.
3. Problems and Solutions of Injection Molded PC/ABS Products
1. Silver filament is the most common problem of PC/ABS materials. Silver filament, also known as silver stripe, splash, flower and so on, is a silvery white filament stripe phenomenon on the surface of products along the flow direction.
The main reason is the interference of gases, which are mainly divided into three components: air: melt adhesive and air involved in the ejection stage; moisture: water contained in the material itself; pyrolysis gas: gas produced by high temperature hydrolysis/thermal decomposition.
Solution: First check whether the material is dry enough, after confirming that the material is dry enough, then adjust the injection molding process to improve the defect of silver wire. At the same time, the bad injection silver wire is also related to the exhaust of the die.
2. Flow mark problem is caused by material injection. The reason is that material fluidity is poor. Flow mark is different from craze and appearance is different.
Solution: It can be avoided by increasing the temperature of the material so as to improve the fluidity, properly improve the fluidity of the material in the mould and reduce the injection speed.
3. Shrinkage and indentation are caused by insufficient filling of materials in the die cavity.
Solutions: Appropriate increase of mould temperature and material temperature to improve material fluidity, prolong injection holding time, increase injection pressure, increase injection speed to improve mold filling, can also increase gate size, heat gate runner to reduce and eliminate shrinkage of products;
The indentation is caused by improper material temperature and improper product design. When the material temperature is too low, not only shrinkage will occur, but also indentation will occur. If the material temperature is too high and the die temperature is too high, the melt will shrink excessively during cooling, thus producing indentation.
Solution: Increase injection speed.
4. Warpage deformation of injection moulded parts is caused by unreasonable design of parts, improper location of gate and unreasonable processing conditions of injection moulding, resulting in internal stress, uneven or excessive shrinkage, excessive temperature of moulds or uneven temperature of moulds, resulting in difficult demoulding or uneven cooling of the mucous film of injection moulded parts. Warpage deformation will also occur.
Processing technology: lengthening injection molding cycle, reducing injection temperature, adjusting injection pressure and injection speed appropriately, reducing ejection speed, increasing ejection area and maintaining ejection force balance;
Product design: increase wall thickness, add reinforcement ribs and fillet reinforcement;
5. Spot problem is usually caused by melt rupture when melt expands into the cavity during high-speed injection.
In the aspect of moulding technology, we should increase the material temperature, nozzle temperature and slow down the injection speed to reduce the appearance of speckles.
Mould aspect: Increase mould temperature, add spillway, increase gate size and modify gate shape.
6. Solutions to the pitting problem: poor dispersion, adding dispersant or oil, raising temperature and adding back pressure. Also see if it's a mold problem. Use other materials.
Whether the baking temperature and time are enough. Appropriate adjustment of die temperature.
7. The peeling problem of plastic products is closely related to the breakdown of fluid under high shear stress. At low shear stress or rate, small disturbances caused by various factors are suppressed by melt, while at high shear stress or rate, disturbances in fluid are difficult to suppress and develop into unstable flow. When a critical shear force is reached, the fluid will rupture.
Material aspect: Because the bad peeling caused by mixtures needs to be eliminated in the first step, the compatibility between PC and ABS is partially compatible, so proper compatibilizer must be added in the modification process to improve its compatibility.
Mold aspect: The principle of die design should follow the direction of minimizing shear as far as possible. Generally speaking, products with dense grain surface are more prone to peeling (caused by friction and shearing of melt in the cavity and the inner wall of the cavity during high-speed filling); at the same time, too small gate size will lead to excessive shearing of melt through the gate, which will lead to peeling of the product surface;
Injection process: The main direction is to avoid excessive shear. When filling is difficult, high-speed and high-pressure can be used to improve the product. Therefore, in the actual injection process, we can also consider increasing the injection temperature/mold temperature and improving the material fluidity to reduce the flow resistance in the actual filling process, so as to avoid excessive shear caused by high-speed and high-pressure.
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