Injection molding is one of the most widely used manufacturing methods in the plastics industry. The method makes it possible to manufacture everything from simple packaging to advanced technical components with high dimensional accuracy and repeatability. In this article, we are addressing those of you who want to deepen your knowledge of injection molding technology, material selection, tool design and process optimization.
What is injection molding?
Injection molding is a cyclical manufacturing method in which a thermoplastic – or sometimes thermoset – is heated to a liquid state and injected under high pressure into a mold. There, it fills a cavity before cooling, solidifying, and forming into a finished part. The technology is particularly well suited to manufacturing products with complex shapes in large volumes, where each part must exhibit exactly the same properties.
An injection molding machine consists of three main parts. The injection unit is responsible for melting the plastic material through a combination of heat and mechanical energy from a screw. The melt is then fed into the mold, which is usually made of hardened steel or aluminum. Finally, the locking system holds the mold halves together during injection – a step that can be done hydraulically or with electric servo drives, depending on the machine type.
Material selection
The choice of material is crucial to both the function of the product and the efficiency of the injection molding process. Common thermoplastics include:
- PP (Polypropylene): Low density, chemical resistant, good processability.
- ABS (Acrylonitrile-butadiene-styrene): High impact resistance and good surface finish.
- PA (Polyamide): Very strong and durable, used in technical components.
- PC (Polycarbonate): Transparent, impact resistant, often used in optics and safety products.
Fillers (e.g. fiberglass, talc) and additives (UV stabilizers, antistatic agents) modify the properties of the plastic and adapt it to the requirements of the final product.
Injection molding step by step
- Plasticizing: Granules are fed into the screw and melted through heating zones.
- Injection: The molten plastic is pushed into the mold with high pressure.
- Holding pressure: A secondary pressure prevents shrinkage while the plastic solidifies.
- Cooling: The plastic cools in the mold until it is solid enough not to deform when opened.
- Eject: The finished blank is ejected from the mold via ejectors.
Specialized variants
For advanced applications, variants such as:
- 2K injection molding: Two materials in the same part, often to combine hard and soft zones.
- Gas-assisted injection molding: Creates hollow structures with reduced weight.
- Insert molding: Embedding metal parts into the plastic part during the process.
- Micro injection molding: For details on the millimeter or micrometer scale.
Quality control and measurement technology
For the process to deliver repeatable quality, careful monitoring of a large number of parameters is required. Melt temperature, tool temperature, injection pressure, holding pressure and cooling time are some of the factors that directly affect the dimensions, surface quality and mechanical properties of the part.
Process optimization is often done iteratively. By adjusting one parameter at a time and analyzing the results, a so-called process window can be defined – a span where production occurs with high stability and quality. Real-time data from sensors in machines and tools is often used to control the process with high precision.
At the same time, design work is crucial. Excessive material thickness, sharp corners or incorrectly placed injection points can cause shrinkage marks, air pockets or deformations. Designing for injection molding therefore requires a good understanding of both material behavior and the limitations of the mold tool.
Sustainability aspects
Injection molding is often considered a resource-efficient manufacturing method. The process is optimized to minimize material waste, low energy losses and fast cycle times. The tools have a long life, which allows initial investments to be spread over large series.
Environmental impact can be further reduced by choosing recyclable polymers or using bio-based materials. In addition, machine developments in recent years have led to increased energy efficiency, especially through the transition to electric injection molding machines.
Closing reflection
Injection molding is a powerful manufacturing technology that combines high volume capacity with precision. With the right material, well-designed tooling and optimized process control, parts with advanced geometry can be manufactured with high quality and low unit cost.
For those who develop products with demands for durability, function and cost-effectiveness, injection molding is a choice that combines technology and economics. Whether it is technical details for the automotive industry or plastic enclosures for electronic products – the possibilities with this manufacturing method are many, and the potential grows in step with technological developments.
