ABS PLASTIC INJECTION MOLDING

ABS is one of the most used plastics today. Its application extends across several industries. These include automotive, food, construction, consumer goods, aeronautics, and much more. ABS has no known toxicity. So in recent years, its application includes medical products. Their broad range of applications is thanks to their unique diverse chemistry. ABS is a strong stiff synthetic thermoplastic polymer. It has a repeating unit consisting of acrylonitrile, butadiene, and styrene. This chemistry gives it some desirable properties in application and use. The versatility of injection molding allows forming ABS into diverse products. You’ll find ABS in car engine casings, legos, electrical sockets, and much more. You can recognize ABS by its shiny appearance and rigid form. 

Why ABS?

ABS is a thermoplastic polymer. It has an amorphous structure and melts temperature between 190 and 270oC. But it is injection molded between 40 and 80oC. A terpolymer of acrylonitrile, butadiene, and styrene produced by emulsion polymerization. It is a low cost, high impact resistant, and lightweight. It has an average density of 1.07g/m3. It can withstand an impact of 200 -215J/m. ABS is a low-cost material. A kilogram of ABS pellets can get purchased for between $0.8 to $2.5. The pricing varies with grade and type of additives used.  

Chemical structure of ABS

Industry Experience and Data 

ABS has been around for several decades. There exists an archive of data on its chemistry and processing techniques. There is lots of experience in the industry of ABS processing. This means you won’t have to labor too hard to find a meld specialist for your ABS related needs. There are also many tools and machinery available dedicated to processing ABS. 

Regrind and Recycle

In the injection molding process, recycling is often necessary. In the cold runner process, for example, the runners need recycling. ABS recycles well. The runners and other scraps can go into regrind and get recycled. This is important in reducing waste. The amount of recycling in the feed depends on the standards followed. ABS has good heat resistance. This means it tolerates moderate to high recycled content. ABS is heat stable and can withstand a repeated heating cycle.

Aesthetic appeal

Looking at its physical properties. ABS has a glossy surface appearance. This is desirable in many products. It also has a high rigidity. ABS is also receptive to modification. Additives get added to ABS to change its properties as desired. This makes it possible to achieve ABS with a wide range of properties. 

ABS gives strong products with a glossy appearance

Abrasion resistance

ABS has good resistance to scratch. This means product injection molded with ABS keeps its gloss for a long time. This also makes them great for products like toys. During their usage of life, these toys get exposed to lots of scratches. This may make them lose their value. The abrasion resistance of ABS is a result of surface intermolecular bonds. 

Dimensional Stability

The shrinkage value of ABS is around 0.8%. This is on the lower end of shrinkage values for common plastics. Shrinkage values can be over 6% for some plastics. The low relative low shrinkage gives ABS good dimensional stability. Products formed from ABS will keep their shape and dimensions to a good extent. The shrinkage is also to a larger extent isotropic. There is a need to include this in the mold design. The extent to which it gets included depends on the precision demand. 

ABS can achieve very thin walls with thickness as low as 0.0045 inches. So when injection molding with ABS the limitation of thickness is not the material. As long as it is injection moldable, ABS works.  This thanks to the excellent mechanical properties of ABS.

Versatility

The chemical structure of ABS is also an interesting one. ABS is a terpolymer comprising acrylonitrile, butadiene, and styrene. Each of these compounds contributes to the inherent properties of ABS. For example, the glossiness gets attributed to styrene. The rigidity is due to the presence of acrylonitrile. The toughness is down to the butadiene compound. Each one contributes to different functional groups. The interaction between these groups also contributes to the properties of the polymer. ABS can undergo different modifications resulting in a range of properties. Processing temperature is also varied to achieve ABS products with different properties. At low processing temperature ABS attains good impact strength and toughness. When molded at high temperatures, it has better thermal resistance and good shine. So processing conditions depend on the target function and appearance. Other properties like photo degradability get improved with additives. Antioxidants, plasticizers, and stabilizers are common additives used in ABS. ABS is also often blended with other plastics like polycarbonate (PC). An example application of the ABS/PC blend is in electrical sockets and casings.

Chemical Resistance

ABS is resistant to a lot of common chemicals and solvents. This includes water, alcohol, gasoline, and acetone. Such resistance to a wide range of chemicals makes it safe in a lot of applications. For example, when used in engine parts it has a high chance of contacting oil and water. 

Good Adherence to Colors and Coatings

ABS adheres well to pigments. In its raw form, it has an opaque ivory appearance. Colors come out with excellent radiance and uniformity. ABS also works well with coatings. In particular, they are good with metal coatings. This is important for both aesthetics and function. For example where a metallic appearance is the aim but must not conduct. This good adherence also extends to adherence to glues. So it works well in cases where post-processing is necessary. For example where parts get molded separated then get glued. 

Post-processing

ABS has good stiffness yet is flexible enough to manipulate. This makes it easy to carry out a range of post-processing on it. ABS parts can get machined, drilled, or have parts sawed off. For example in the production of large parts from ABS. Separating the runners might need automated force and/precision. This is possible with ABS while maintaining a good finish. 

Injection Molding ABS: Things To Consider

ABS is not a difficult plastic to injection mold. It melts at a low temperature and has good dimensional stability. Below are some of the considerations specific to ABS. These are precautions taken when running standard injection molding of common plastics. 

Processing temperature

The temperature of injection molding affects the property of the final product. For more impact resistant ABS gets processed at the low end of the temperature range. For better gloss and thermal resistance, it gets processed at higher temperatures.  ABS begins to soften at around 40oC. It gets processed between 40 and 80oC. For injection molding, the plastic doesn’t need to completely melt. It only needs to get to its moldable soft form. The melting temperature of ABS is between 190 and 220oC. This means ABS will tolerate any temperature between 40 and 220oC. This gives a wide window for injection molding. While it is not a good idea to work too close to the melt temperature. It will not have as much impact as it would other more sensitive plastics. 

The barrel temperature gets adjusted along the length of the machine. At the end closer to the hopper it is about 195 to 205oC. At the center of the barrel, the temperature is around 210 – 225oC. Closer to the end of the barrel temperature reaches a peak of around 225 to 250oC. The temperature gets held at this level in the runners. The exact temperature setting depends on the type and grade of ABS used. The mold gets precooled so the temperature is much lower in the tens of celsius. 

Illustration showing the temperature variation across the barrel for ABS

Injection Molding Pressure and Gates

The injection molding pressure depends on two main factors. One is the wall thickness of the molded product. The other is the size of the gate used. Where the gate size is wide the injection pressure is around 70 to 110MPa. This works for products with thick walls. For smaller gates, the pressure is higher at 120 to 150MPa. This works best for products with thinner walls. At higher pressures, the processing temperature is higher. The choice of gate size and the pressure relates to the melt temperature. The mold gets held at a pressure between 60 to 70MPa. This reduces internal stress in the final product. The melt gets injected at a low to moderate speed. With the screw working at an rpm between 30 and 60. ABS hardens as it cools so there is no need for speedy mold filling. 

Product Prototyping

Often it is necessary to have a prototype for the part to be injection molded. There are different reasons for this. This could be to have a part as a guide from which the mold is then fabricated. Often the injection molder handles both prototyping and part production. ABS works in a variety of processes. This makes it a good choice for prototyping injection molded parts. ABS works for 3D printing. A popular prototyping technique. In particular fused deposition modeling. It serves as a practical option. The samples fed into the 3D printer are the same for injection molding. This reduces time and effort in ordering a different sample for prototyping. Using the same material also gives a better representation of the final product.

The fact that ABS works well in processing also makes it good for prototyping. The machining and drill So CNC machining would work well with ABS. Where the prototype parts get machined in separate stages, ABS adheres well to glue. This makes the joining of parts more effective.  

Sourcing ABS for Injection Moulding? Here are some brands 

ABS gets produced under many brand names. There are many grades of ABS. The different grades get targeted at specific applications. Thanks to the versatile chemistry of ABS it has many modification options. Additives get added to improve properties like flexibility, heat, and radiation resistance. Modifications could also have to do with the quality of coloring used. For example, Clariant introduced medical-grade ABS with good color quality. So based on these many possibilities of ABS, the following are some of the existing brands of ABS. It lays out its target use and manufacturer/ supplier.

Table showing different brands of ABS from different suppliers. 

Conclusion

ABS is a very temperature tolerant plastic. It can withstand a broad range of processing temperatures. Its properties can get tweaked as desired by processing at different temperatures. Injection molding of ABS is no more demanding than that of other plastics. ABS also offers good options for prototyping using methods like 3D printing and CNC. ABS is a good option for low-cost injection molded products. Its application should be at temperatures below ~50oC. 

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