Explainer: Tolerances in Resin vs Metal AM

Why Tolerances Matter in Additive Manufacturing

Tolerances define how much a printed part can vary from its digital model. In traditional machining, tolerances are often tighter because the process removes material in small, controlled amounts. In additive manufacturing, the part is built by adding material layer by layer, which changes the relationship between the digital file and the finished object.

Engineers who design for additive processes need to understand how different technologies handle dimensional accuracy. A part that fits perfectly in a resin process may not fit in a metal process without adjustment. This difference affects sourcing decisions, post-processing budgets, and the final performance of the component.

When selecting between resin and metal 3D printing, tolerance requirements should be one of the first factors considered. The choice determines whether the part can ship as-is or needs additional work. It also affects the number of revisions needed during the design phase.

How Resin AM Achieves Tolerances

Resin 3D printing, often called stereolithography or similar photopolymerization processes, builds parts by curing liquid resin with light. The resolution of the light source and the thickness of each layer determine the smallest features the machine can produce.

Tolerances in resin AM are often tighter because the process has less thermal distortion and no large-scale shrinkage from solidification. The resin cures in place, and the dimensional changes are usually small and predictable. For small parts with fine features, this makes resin a strong choice when precision matters.

However, resin parts are sensitive to surface finish and support material. Supports must be removed carefully, and the build plate can leave marks that affect flatness. Engineers designing for resin need to account for these effects in the model. A part that looks flat in the CAD file may have a slight bow from the build plate after printing.

The material itself can also affect tolerances. Different resins have different shrinkage rates during curing. Some are more dimensional than others. When sourcing, it helps to ask the service bureau about typical tolerance performance for the specific resin used.

How Metal AM Achieves Tolerances

Metal 3D printing uses powder-based processes such as laser powder bed fusion or electron beam powder bed fusion. These processes melt small droplets of metal powder to form layers. The result is a part with a porous microstructure if not post-processed, and it is usually dense only after heat treatment or other methods.

Tolerances in metal AM are generally looser than in resin AM. The build process involves heating and cooling cycles, which cause shrinkage. The powder bed is also subject to flow and compaction effects that can change the final dimensions. The part is often printed with a margin of safety to account for these variations.

Post-processing plays a large role in metal part tolerances. Machining, grinding, and heat treatment are common steps. Each of these can change the dimensions of the part. A part that is slightly oversized after printing may be brought into tolerance by removing material. This adds cost and time, but it can make metal parts suitable for applications that require precision.

Metal parts also have support structures that must be removed. The supports leave marks and can affect surface finish. In some cases, the support removal process can shift dimensions slightly. Engineers need to design with these effects in mind, especially for parts with critical fits.

Comparing Tolerance Performance

The table below shows a general comparison of tolerance performance between resin and metal AM processes. These are typical ranges, and actual results vary by machine, material, and post-processing.

Factor	Resin AM	Metal AM
Typical Tolerance	Tighter, often within a few thousandths of an inch	Looser, often within a few hundredths of an inch
Shrinkage	Low, predictable	Higher, varies by material and process
Support Effects	Can affect surface finish and flatness	Can affect surface finish and dimensions
Post-Processing	Minimal, usually just support removal	Common, includes machining and heat treatment
Best For	Small, detailed parts with tight fits	Strong, complex parts where post-processing is acceptable

This comparison shows that resin AM is often better for parts where dimensional accuracy is critical and the part size is small. Metal AM is better for parts that need high strength or complex internal features, even if the tolerance window is wider.

The choice also depends on the application. A small bracket that must fit into a housing may work well in resin if the tolerance is tight. A large structural component that needs to withstand high loads may be better in metal, even if it requires post-processing to reach the final dimensions.

Designing for Additive Tolerances

Designing for additive manufacturing means thinking about how the part will be built and how the final dimensions will be achieved. The digital model is not the final part. It is a starting point.

For resin parts, designers should consider the build orientation. The side that touches the build plate may have a different surface finish than the top. If a part has a critical flat surface, it may be better to orient it so that surface is not on the build plate. This can improve dimensional accuracy and reduce the need for post-processing.

For metal parts, designers should account for shrinkage. Many metal AM processes have a known shrinkage rate that can be compensated for in the CAD model. If the part is printed with a larger dimension, it will shrink to the desired size during cooling. However, the shrinkage is not always uniform. Different parts of the model may shrink by different amounts. This can affect the final fit.

Support structures also affect the design. In both resin and metal AM, supports are needed to hold the part during printing. They add cost and time to remove. They can also leave marks on the part. Designers can reduce support volume by changing the geometry, but this may affect strength or surface finish.

When working with a service bureau, it helps to provide clear tolerance requirements. Specify which dimensions are critical and which are flexible. This allows the bureau to choose the right process and post-processing steps. It also reduces the risk of receiving a part that does not fit.

Sourcing Decisions Based on Tolerances

Sourcing decisions in additive manufacturing are often driven by tolerance requirements. If a part needs a tight fit and is small, resin AM may be the better choice. If a part needs high strength and can tolerate post-processing, metal AM may be the better choice.

Engineers should also consider the cost of post-processing. Resin parts often require minimal post-processing, which can reduce cost. Metal parts often require more post-processing, which can increase cost. However, the cost of post-processing may be offset by the ability to meet tighter tolerances or higher strength requirements.

Another factor is the number of parts needed. If the part is a one-off or low volume, the cost of metal AM post-processing may be high. If the part is high volume, the cost may be spread out. This affects the total cost of ownership.

When sourcing, it helps to ask the service bureau about their typical tolerance performance for the specific material and process. This information can help make a more informed decision. It can also help identify potential issues before the part is printed.

A Worked Example

Consider a small bracket that must fit into a housing with a tolerance of a few thousandths of an inch. The bracket has fine features and a complex shape. The engineer considers resin AM as a first choice because the process supports tighter tolerances and the part is small.

The engineer designs the bracket in CAD and specifies the build orientation. The side that touches the build plate is a non-critical surface. The critical fit surface is oriented away from the build plate to improve dimensional accuracy.

The engineer sends the model to a service bureau and asks about the typical tolerance performance for the resin used. The bureau confirms that the process can meet the tolerance requirement with minimal post-processing. The part is printed, supports are removed, and the part is ready for use.

In this example, the tolerance requirement drove the process selection. Resin AM was the better choice because it could meet the tight tolerance without extensive post-processing. The design was adjusted to reduce support effects and improve dimensional accuracy. The result was a part that fit as intended.

If the same bracket had been printed in metal AM, the engineer would have needed to account for shrinkage and post-processing. The part might have been printed with a larger dimension and then machined to the final size. This would have added cost and time. The result might still have met the tolerance, but the process would have been less efficient.