Once a designer has created a model to be 3D printed we need to determine how strong the model needs to be. Generally this is determined by how the model will be used. Perhaps the model is statue that sits on a table. In this case strength is not critical. Perhaps the model is a tool, like a wrench, which would require a strong and rigid print. In either event we need to determine the correct number of perimeters (aka walls), the correct amount of infill, and the most appropriate infill geometry. This post may refer to a piece of software called a “Slicer”. A Slicer is a program that is used to change the CAD model file into the layer by layer commands that are used by the 3D printer to actually reproduce the original model.

3D prints are largely made up of two types of structures, perimeters and infill. Perimeters are the walls that make up the model. The Slicer has the ability to determine the thickness of the walls. The thickness is a multiple of the minimum perimeter width, which is typically around 0.40 millimeters. A wall that is one perimeter thick is very weak and will crush easily. A wall that is two perimeters thick is stronger, and so on. If strength is not an issue, one or two perimeters are usually used. But if we need a strong object we’ll need 3 or more perimeters. The exact number of perimeters will vary, but the point is that we first need to know the strength requirements in order to be able to choose a starting point that is close to what we need. So why not always just print with 3, 4 or more walls? The answer is time and material. More walls will use more material and take longer to print, which will mean higher costs and lower throughput for your customer.

Infill is the area between walls. Like the walls in a house the area can either be hollow (0% infill) or filled (something more than 0% infill). The more infill the stronger the part. Imagine an egg. The walls are the shell of the egg and in our example the inside of the egg is hollow. We can make the walls stronger by adding perimeters, but if we could also fill in the hollow space the eqq would be stronger still. We could just use 100% infill for maximum strength, but of course the “time and material” problem comes into play. There’s also a point of diminishing returns where adding more infill doesn’t add a corresponding amount of strength. The “default” amount of infill used is typically 20%, with 40% or 50% being the point at which you start to see diminishing returns. But there can be a large difference in strength between 20% and say 40%.

The combination of perimeters and infill can have a significant impact on the strength of a part, but it’s a tradeoff between time and materials. If time is of the essence you’ll want to use less material which may mandate using a stronger plastic. While that’s certainly an option, ever stronger plastics are usually more expensive and harder to print, so you’ll likely want to start with something easy to print like PLA or PETG, make a few test models with varying amounts of walls and infill and determine if any meet your needs.

A discussion of the pros and cons of different infill types is fairly complex. There are several good articles that cover the topic, but suffice it to say that just percentage alone does not determine infill strength. The infill pattern used is also important. Some lend strength in the XY axis, some in the Z axis, and some are generally good in all three axis. Here are some references:

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