A quick guide to dimensional accuracy for 3D printing

Dimensional accuracy refers to how well a printed object matches the size and specifications of the original file. For accurate parts, choose the printing process and materials that are most appropriate for the design.

What impacts dimensional accuracy?

3D printing is an imperfect process. Decisions that impact the final dimensional accuracy can be made while designing the object, choosing a printing method and selecting the materials that the object will be made from. Once the design is finalized, the 3D printing service can take steps to make sure that the final product is as accurate as possible.

Machine accuracy

• Machine accuracy: Some 3D printers are simply more accurate than others. Many manufacturers provide standard dimensional accuracies for well-designed parts made on a well-maintained machine.

Materials

• Materials: Choosing the correct material for your object will greatly increase the accuracy to which it can be printed. Standard SLA resin has a high dimensional accuracy compared to flexible SLA resin.

Object size

• Object size: In general, small objects can be printed with higher accuracy than large ones. Bigger objects have more room for manufacturing errors.

Warping and shrinkage

• Warping and shrinkage: The processes involved in 3D printing have a chance to warp and shrink the materials. Large expanses of material, flat surfaces, and unsupported structures can all warp and should be avoided in your final design.

Support structures

• Support structures: The presence of support structures may be necessary to achieve a high level of accuracy. However, the removal of these support structures will affect the surface finish of the product.

Post processing

• Post processing: Decisions made during the final stages of production can impact or preserve the final accuracy. Proper cooling procedures are necessary to keep objects in the desired shape.

Dimensional accuracies of common printing processes

Most 3D printing processes are chosen to fulfill a specific purpose. Prototypes are made with FDM printing and complex objects are made with selective laser sintering. Dimensional accuracy should be taken into consideration when selecting a process. Even if the printing method has already been chosen, an understanding of the standard accuracy and the common reasons for failure will help with the design process.

FDM

Fused deposition modeling (FDM) is one of the most common 3D printing processes and is often used to produce quick prototypes or functional parts. Industrial FDM printing has a dimensional tolerance of ± 0.15% and a lower limit of ± 0.2 mm.

FDM 3D printing creates objects by extruding thermoplastics through a heated nozzle. Objects are printed one layer at a time, and these layers cool at different speeds depending on their size and structure. This can cause warping and slight variations in the accuracy of the final object. Materials that need higher print temperatures, like ABS, have a greater risk of warping.

SLA

Stereolithography (SLA) printing produces smooth, visually accurate parts made from cured resin. Industrial SLA printing has a dimensional tolerance of ± 0.15% and a lower limit of ± 0.01 mm.

The resin used for SLA printing takes time to harden completely. When large, unsupported spans of material are printed, they are likely to warp underneath their own weight or the weight of the surrounding layers. The material might also warp when the object is peeled away from the printing bed during the final stage of the printing process.

SLS

Selective laser sintering (SLS) is a particularly accurate process that is often used to produce complex geometries. SLS printing has a dimensional tolerance of ± 0.3% and a lower limit of ± 0.3 mm.

SLS printing uses lasers to sinter layers of powder together. Although this process is more accurate than FDM, there is still a chance that the layers will not cool at the same rates. This can result in warping, especially in larger SLS parts. The solution is often to leave the object in the powder bed until it is fully cooled.

Material jetting

Material jetting is the most accurate 3D printing process. Material jetting has a dimensional tolerance of ± 0.1% and a lower limit of ± 0.05 mm.

Heat is not used in the material jetting process, so warping and shrinking problems are unlikely to occur. However, thin walls and extremely detailed features might not be printed correctly. Objects made from material jetting are also not as durable as those made with FDM; they might warp when exposed to environmental conditions like heat or humidity.

Metal 3D printing

Metal 3D printing works very similarly to other 3D printing processes. Metal printing has a dimensional tolerance of ± 0.1 mm.

Although there are several different metal 3D printing services, the most common ones use technologies similar to selective laser sintering. Metal powders are heated and fused together into layers; because these layers might cool at different temperatures, warping is a consistent issue. Metal printing requires support structures, and a heat-based stress relief treatment is often used to prevent warping after production is complete.

Quick tricks for higher dimensional accuracy

In general, material jetting has the highest dimensional accuracy. SLA has high accuracy for parts smaller than 1000 cubic centimeters, and SLS has high accuracy for parts greater than 1000 cubic centimeters. FDM is a relatively accurate and cost-effective solution for most parts, especially when the lower limits are respected.

Once the correct process has been chosen, try these tips to increase the accuracy of the final product:

Avoid large, flat surfaces support structures

• When creating designs, always avoid large, flat surfaces. Use support structures to hold up areas that might be susceptible to sagging.

Multiple small parts

• When large objects are needed, consider printing multiple small parts that can be connected later on.

Standard materials

• Unless a specific feature is required, choose standard materials that are less susceptible to warping.

Lower limits

• Pay attention to the lower limits of the chosen process. The digital design should not have any features or details smaller than these limits.

Calibrated and working heated printing beds

• If operating a printer directly, make sure that the device is calibrated and in working order before production starts. Consider using heated printing beds to keep objects at a uniform temperature until printing is complete.

Check the 3D Hubs Knowledge Base for information about specific printing processes and design considerations to improve the accuracy of printed objects. Concerns about a specific design can often be brought up with the 3D printing services before production begins.

3D Hubs Design for Manufacturing analysis will automatically check any design for viability with a chosen printing process. Upload a file to see if it meets the requirements for size, details and wall thickness. If no flags are listed, the object can be created with the dimensional accuracy normal for that printing process.

This article was published in collaboration with 3D Hubs.