# Designing a Part That Won't Break

> Layer direction, inside corners, wall thickness and ribs: the design choices that decide whether a printed part survives, and what they cost you.

Source: https://local3dprinting.com/blog/designing-for-strength
Published: 2026-08-30 (updated 2026-10-15)

If you're sending us a model, a handful of design choices matter more than anything we change at the printer. None of them are hard and most cost nothing. Before you start, it helps to know which way the force comes from, what happens if the part fails, and whether anything screws into it.

## Layers are the grain

A printed part is built from stacked layers and behaves like wood. Strong along the grain, weaker across it. Pull along the layers and you have full strength. Pull across them and you're relying on the bond between layers, which is roughly half as strong.

This is the biggest lever available and it's free. Tell us which way the load runs and we'll rotate the part on the plate so the layers work with you.

## Round your inside corners

Sharp internal corners concentrate stress and start cracks. A 2-3 mm fillet where two walls meet is often the largest single improvement you can make, and it changes nothing about how the part fits. Find the square corners in your design and soften them.

## Thick walls beat dense insides

People reach for "more solid" when they want stronger. In practice the outer shell carries most of the load. A couple of extra wall passes cost pennies and add real stiffness. Filling the middle costs a lot and adds much less. Aim for around 3 mm of wall on a working part.

## Ribs, not bulk

A solid block isn't the strong answer. It's slow, expensive, prone to warping, and no better in the direction that counts. Use ribs and gussets instead: thin reinforcing walls running along the load path. Look at the underside of any injection-molded plastic part and you'll see exactly this.

## Plan for fasteners

Fine printed threads strip. If something bolts into your part, design a pocket for a heat-set threaded insert or leave room for a bolt and a captive nut. Both need to be in the model before we print. Tell us which fastener you're using and we'll size the hole.

## When printing is the wrong process

If a part needs to be thin, stiff and carrying real load all at once, a bent metal bracket may beat a printed one for less money. Better to hear that up front than after it fails on the first install.

## Before you send it

Send the model with a note on how it mounts, which way the load pulls, and what it's attached to. We'll flag the weak spots before we print.