Plastic isn't just plastic

For many people, plastic is just plastic: a cheap, versatile material that can be used to make all sorts of things. But anyone who has ever overseen an injection molding project knows that this view is only a small part of the story. Behind every injection-molded product lies a careful choice of material, and that choice largely determines whether or not a product fulfills its function.

Why are there so many types of plastic?

The answer lies in the application. Every product that incorporates plastic has its own specific requirements. Consider:

• Thermal properties - does the product need to withstand heat, or extreme cold?
• Chemical resistance - will the material come into contact with motor oil, solvents, cleaning agents, or sunscreen?
• Flame retardancy - is the product located in an electrical cabinet where a short circuit could occur, and must it not contribute to the spread of fire?
• Mechanical strength and stiffness - must the product be able to withstand impact, pressure, or vibration?

A standard, widely used plastic such as polypropylene (PP) or polyethylene (PE) is available in large volumes and is relatively inexpensive. For many applications, that’s exactly what’s needed. But as soon as the requirements become more stringent, these bulk plastics often no longer meet the criteria. That’s when you turn to more technical materials such as POM, polycarbonate, or ABS - and even within those categories, there are countless variants, each with slightly different performance characteristics.

Here’s a good example: the materials used in airplane interiors are virtually non-flammable and, in the event of a fire, emit extremely little smoke and toxic substances. That makes sense, you don’t want passengers to be in danger before the plane can land. But those kinds of high-quality materials are also many times more expensive than an average plastic, and therefore completely illogical for a simple, everyday product. These kinds of trade-offs, functionality versus cost, ultimately determine which material is suitable for a given application.

Specific to injection molding: it’s more than just the right properties

In injection molding, there’s another factor to consider in addition to the functional material requirements: processability. Not every material that seems technically suitable is also easy to process in an injection molding process. Here are a few points to consider:

Shrinkage and warping. Some materials have low mold shrinkage and are therefore ideally suited for products that must be highly dimensionally stable. Other materials shrink more and are thus less suitable when precise dimensions are crucial

Moisture sensitivity. Certain plastics must be thoroughly dried before they can be processed. If you skip that step, you run the risk of reduced product quality.

Wall thickness. Not every material can be processed into thin-walled parts. For products that must have a wall thickness of less than 0.8 to 1 millimeter, some materials are simply not viable.

Flow characteristics. Material that must flow around a hole or obstacle in the mold often leaves a visible flow line or seam. This is not only an aesthetic concern, but the material is also slightly less mechanically strong at that point.

Sensitivity to shear stress. When material is forced through a narrow opening under excessive pressure, some plastics may react negatively, resulting in a loss of quality in the final product.

A striking example of this is PET. Everyone is familiar with PET from soda bottles - a fully developed product, design, and material that is virtually identical worldwide. But as soon as you want to use PET in a completely different product, such as a protective component for a bicycle, it suddenly turns out to be a much more challenging material. Pre-drying is critical in this case; it flows less easily, causing molding pressures to rise significantly, and the mold must be specifically designed with adequate cooling. So PET can work very well outside the context of the bottle, but you have to give serious consideration to both processing and product design.

Knowledge and experience, translated into a clear assessment

With so many variables - functional requirements, processability, and costs - choosing the right material can quickly become a complex issue. In practice, it always starts with the application itself: what must the product be capable of, what will it come into contact with, and under what conditions must it function? These requirements are translated into specific material properties, which quickly narrows a long list of potential materials down to a small, manageable shortlist.

It is precisely this process - translating an application into the right material choice, with an eye for both functionality and processability in the injection molding process - where the expertise of an experienced injection molding partner makes all the difference. Not every material is suitable for every product, but there is a suitable solution for almost every challenge, provided you ask the right questions early enough.