This is an excerpt from Chapter 2 of The Engineering Thermoplastics Guide. Download the full guide below for the full text, including technical diagrams and application notes.
Injection molding is a melt-processing approach where thermoplastic material is heated and subjected to pressure, filling the interior of a mold. Afterward, the thermoplastics cool and regain rigidity, completing the molding cycle. After demolding the cycle can start from new. An injection-molding machine consists of an extruder for plasticizing the resin, a ram system for high-pressure material introduction into the mold, and a cooling device to facilitate part solidification. Key variables in the injection molding process include cylinder temperature, mold temperature, injection rate, holding pressure, back pressure, and the speed of the screw's rotation.
One of the key challenges of injection molding is that, due to differences in densities between the solid polymer and the melt, cooling results in dimensional shrinkage. In the case of some semi-crystalline polymers, this contraction can reach 20% of the original volume.
The inherent shrinkage can be compensated for by injecting additional material in a step known as the holding phase. However, a residual contraction between 0.5% and 2.5% is unavoidable with injection molding, and should always be kept into account when designing the molds.
Additive manufacturing of thermoplastics, also known as 3D printing, offers an alternative approach to creating complex parts rapidly, layer by layer, without the need for traditional tool and mold fabrication and with maximum design freedom. This technology has revolutionized the production of prototypes and products, resulting in substantial cost and time savings since its invention in 1986.
Due to their ability to melt and solidify into any desired shape, thermoplastics are the ideal materials for additive manufacturing and have been the first materials used for this technology. Among the different 3D printing processes utilized today for thermoplastics, most can be classified into two categories.
1. Extrusion 3D printing (E3DP). In E3DP, the material is heated to reduce its viscosity and extruded through the printer’s nozzle. For small-scale printers, the preferred approach is fused filament fabrication or FFF. Small-scale FFF printers extrude thermoplastic polymer filaments through a heated nozzle onto a heated bed, allowing for fast and cost-effective component production. However, these printers have limitations, including filament diameter constraints and smaller print areas. To address these challenges, large-scale extrusion printers for Big Area Additive Manufacturing or BAAM have been developed. These technologies implement more efficient feed systems and allow for faster production rates.
2. Powder Bed Fusion (PBF). In PBF, the powdered material is locally melted with a high-energy laser beam. Subsequent deposition and fusion of powder layers result in the gradual formation of the finished product. This process is suitable for large-scale industrial printing and can be used with engineering and advanced thermoplastics with higher melting temperatures.
For FFF, commodity plastics such as PETG and HDPE are a common choice. However, also engineering plastics such as polyamides and polycarbonates can be used with most professional extrusion 3D printers.
The technology for 3D printing with advanced thermoplastics is still in its infancy. Materials such as PEEK and PEI are challenging to extrude but can be used with PBF technology. New thermoplastic blends, composed of a mixture of advanced and standard thermoplastics, are also emerging as a suitable choice for additive manufacturing. 
The choice of processing method for thermoplastics depends on the specific requirements of the desired application. Injection molding stands out as an excellent choice for mass production of standard thermoplastics due to its automation capabilities, cost efficiency, and superior surface finish. However, challenges such as part warpage and shrinkage should be carefully considered.
Extrusion processes are ideal for producing parts with constant cross-sections, making them suitable for products like pipes and window profiles. While versatile and capable of high output, they have limitations when it comes to incorporating reinforcements and can result in anisotropic properties.
Compression molding is particularly well-suited for engineering and advanced thermoplastics with high melting temperatures, offering advantages like minimal material waste and the ability to maintain product performance. This method is especially valuable for producing high-performance engineering components.
Additionally, additive manufacturing, or 3D printing, has revolutionized rapid prototyping and product development. It is a valuable option for complex, customized parts, with extrusion 3D printing (E3DP) suitable for small-scale applications and Powder Bed Fusion (PBF) better suited for large-scale industrial printing, especially with engineering and advanced thermoplastics like PEEK and PEI.
Ultimately, the selection of the most suitable processing method should be based on factors such as material properties, production volume, design complexity, and cost considerations, ensuring that the chosen method aligns with the specific needs of each application.
You have just been reading an excerpt from Chapter 2 of The Engineering Thermoplastics Guide. Download the full guide below for the full text, including technical diagrams and application notes.