This blog is translated from German with DeepL.
I want to make a part out of plastic. But which plastic do I choose?
This question is very versatile, not always easy to answer right away, and is often treated too lightly.
Apart from the fact that there is usually not “one” right plastic, it often also depends on what is already in stock or what experience has already been gained.
The manufacturing process influences the material selection options
The first thing I need to know is what manufacturing process will be chosen for my part, because this greatly affects the range of materials available.
In general, this can be categorized as follows:
- Mechanical manufacturing
→ Assortment of semi-finished products.
→ Here, the thickness matters, as plates can be used for thicker parts and rolled-up foils for thinner parts. Accordingly, it is important to know the range of plate or film manufacturers.
- Injection molding or extrusion
→ Here, the choice of different granulate manufacturers is virtually endless.
- Generic processes (“3D printing”, STL, SLS, etc.).
→ Here it is important to know the materials of the various service providers. In the case of PLA (“3D printing”), the selection can usually be researched by oneself. With other processes such as SLS, STL, etc., many manufacturers use materials they have developed themselves or only have a small quantity of materials in operation. This means that I almost have to choose the manufacturer for the material selection.
- Various other processes, e.g. for the production of thermosets and elastomers.
In this article, we will primarily look at the case of injection molding, since this is where the largest quantities are produced and thus the choice of material for series production must be made with appropriate care.
Requirements for the design are requirements for the material
This sounds obvious, but in the heat of the moment it is often not taken into account, or only at a late stage. My choice of material therefore starts at the beginning of the project (or already in the bidding phase) with the list of requirements (specifications, or whatever document describes my design).
Certain cornerstones can be laid here. Some possible criteria: Operating temperatures, chemical resistance, environmental impact, transparency or specific color or soft surface. In addition, standards to be met such as fire safety, food grade, dishwasher safe, gamma sterilizability, USP VI, etc. Here it is also important to know whether a certificate is required.
I have to know and consider these requirements before I start the design. Of course, this applies not only to the choice of “which plastic” but in advance to the general choice (is plastic / thermoplastic suitable at all?).
So we have to help the customer to list or define all requirements. Not to be neglected is also the view of the end user. The feel and look of a material can be decisive for the success of the product. However, the user’s favorite color may not be feasible because, for example, the FDA/NSF certificate would expire if the material were dyed.
Also the further use such as labeling (printing, labeling, etc.) are often forgotten, but can be decisive for the choice of material.
Picture: Symbols on the take-away tableware we developed (see experience letter N° 10). Each symbol is also relevant to the choice of material.
Material considerations in part design
Often, decisions about the right plastic are not made until the part is almost finished being designed. This is not exactly the ideal strategy. It is much better if I draw up an initial parts list before the effective design, i.e. based on the first concept.
This forces me to already think about the number of parts, the manufacturing process, the assembly, the part design and the choice of materials.
So, if I create a first rough bill of materials before I start the design, I already know what to look for in the design.
A simple example: if my choice of material for a part is primarily based on the general stiffness of a part, this may conflict with a design idea of snapping or lead me to make the snapper on the other part or to geometrically adapt it according to the material properties.
Another factor that must always be taken into account is the planned choice of material with regard to injection-molded design. There are various principles, such as constant wall thicknesses, avoidance of material accumulation, ribs not too thick in relation to the base wall thickness, not too thin wall thicknesses, etc. Anyone who often designs injection molded parts knows that these principles are not always equally easy to follow. If I have roughly chosen my material and know its properties, I can also judge how delicate such effects are. For example, some materials are more prone to sink marks, while others are more tolerant. Some flow better, which makes the issue of thin walls less delicate. In addition, flanking requirements must also be considered here. If, for example, my fire protection requirement is UL 94 V0, I have to work with minimum wall thicknesses (even in areas as thin as these). If I already know my material exactly, the UL94 condition also results automatically (e.g. UL94@1.2mm means I must not be smaller than 1.2mm anywhere).
Conversely, of course, a detailed concept (example: we want to snap) can also lead to the fact that I have to adapt my original material list and choose a material that is better suited for snapping (or, if necessary, find a compromise between two requirements).
So we’re at the next material influencing factor:
The part design influences the material choice
Plastic clothespins with film hinge and spring function.
Conversely, if the general requirement does not constrain the material too much as described above, my design of the part can influence the material. Some examples:
- Bending elements such as snaps, flexible geometries, etc. require a plastic that is reasonably flexible and not too brittle.
- If something has to be as stiff as possible, we choose a stiffer plastic or one reinforced with glass fibers, for example.
- Sliding functions or friction situations require a plastic with good sliding properties (tip for not so experienced plastic users: take a few plastics in your hand and rub them. If one of them feels “slippery”, this is not a bad sign).
- Two plastics to be welded must be the same or similar enough.
- Two materials rubbing together at high speed should not be identical to avoid fusion during application.
Or very simple but often forgotten: if I need a specific color for my device, I should use no or few different materials for the visible parts if possible, because not all materials give the same color when dyed. In addition, this also helps with economy if only one material can be dyed with the specific color and therefore in larger quantities.
- When it comes to detail selection (not just “I’ll take a POM” but e.g. “I’ll take a Hostaform C27021”) I have to make sure that I take a good flowing type, e.g. for thin-walled parts or non-optimal flow paths. This is also just one example of countless possible requirements. Here, clarification with the raw material manufacturers is often mandatory. Often, an experienced injection molder or an in-house plastics specialist can also he
Availability and price should not be neglected
That the raw material price cannot be neglected is absolutely clear in most industries: if the Fiat Uno will do, we don’t have to use a Rolls-Royce. And the differences are massive. From 1.00-2.00 CHF/kg to several hundred CHF/kg you can find everything. Here the plastic pyramid helps for a rough overview. Depending on the size of the company, the purchasing department must also be involved as early as possible.
When it comes to the final detailed decision, availability often plays a role:
- Global: designing a part that is produced on another continent also requires clarification of whether the material is also available there (usually available on online portals such as materialdatacenter.com).
- In-house: if I can use a material already used for other parts, this allows a larger purchase quantity and easier storage.
- In small quantities or on short notice: if I want to do trials, e.g. to evaluate if this material is ideal for my specific requirement, I certainly don’t want to have to wait two months and/or buy 100kg. This is the latest time to talk to raw material manufacturers or their distributors.
Image source: Minihaa, Wikipedia: “High performance plastics”.
Coarse or fine selection?
If I want to choose the perfect material, the material selection process can consume quite a bit of time and effort, and I have to consult with some specialists or plow through a wide variety of tables.
But the material requirements are not always equally delicate. Accordingly, it is also necessary to weigh up how much effort is put into the material selection and how precisely a material selection must be defined.
To explain, here are a few example situations:
- If I have critical requirements (functional or regulatory) and, for example, an FEM or mold flow analysis, I have to define the precise material (e.g. Grivory GV-5H black 9915). This is the only way I can ensure that my part will then match my simulations as closely as possible.
- If I don’t have a critical requirement and don’t know the existing raw material range at the manufacturer, I do him a favor if I only define the polymer (e.g. ABS). Then he can take the ABS he has in stock in large quantities and give me a better price. Of course, it is also better, clearer and less prone to conflict if I define the exact material, but clarify this with him beforehand. But what if the manufacturer is not yet known? It is like with general tolerances: if I can live with the inaccuracy, I may leave it rough. But be careful: often supposedly simple plastic parts are underestimated and mistakes in the choice of material can be very expensive.
But what exactly do I choose now?
So far, we have talked a lot about the procedure but have not yet given any concrete tips. This is because the various granule manufacturers have an almost infinite range of materials (including fillers or reinforcing materials), each of which can massively change the properties. Also, the list of available materials is always in flux and, for example, more and more bio-based plastics are also coming onto the market, whose properties and processing are sometimes very different from the classic polymers. As already mentioned, a complete selection process is very time-consuming and even a selection of the lists would be as long as this article.