{"id":8421,"date":"2026-11-04T09:28:50","date_gmt":"2026-11-04T09:28:50","guid":{"rendered":"https:\/\/www.lyafs.com\/?p=8421"},"modified":"2026-11-04T09:28:50","modified_gmt":"2026-11-04T09:28:50","slug":"sls-vs-slm","status":"publish","type":"post","link":"https:\/\/www.lyafs.com\/ja\/blog\/sls-vs-slm\/","title":{"rendered":"SLS vs SLM: Which Laser Process Fits the Part"},"content":{"rendered":"\n

SLS sinters. SLM melts. That one word difference decides which materials you can run, whether the part needs support structures, and what the surface looks like when it comes out of the chamber.<\/p>\n\n\n\n

Both processes use a laser and a powder bed, which is why they get confused. They are not interchangeable and they do not compete for the same job. This page separates them by what they physically do to the powder, then by surface and tolerance, then by cost, then by the failure modes each one produces.<\/p>\n\n\n\n

\"LaserCore-5300
LaserCore-5300 \u2014 an SLS platform. Coated sand and PSB wax, no support structures, 700 \u00d7 700 \u00d7 500 mm.<\/figcaption><\/figure>\n\n\n\n

The one-sentence difference<\/h2>\n\n\n\n

SLS heats powder until the particles fuse at their contact points, which leaves the part slightly porous and needs no support because the surrounding powder holds it up. SLM melts powder completely into a liquid pool that solidifies into near-full-density metal, which needs support because nothing else holds the overhangs.<\/p>\n\n\n\n

Everything downstream follows from that.<\/p>\n\n\n\n

What SLS actually does<\/h2>\n\n\n\n

Selective Laser Sintering bonds polymer, wax or coated-sand particles without fully melting them. The laser supplies enough energy to fuse the surface of each grain to its neighbours; the grain core stays solid. The result is a part that is structurally sound at the scale of a casting mould and dimensionally stable enough for pattern work.<\/p>\n\n\n\n

Two consequences matter in production. First, no support structures. The powder bed is the support, so a mould or a pattern can be nested anywhere in the build with nothing to cut off afterwards. Second, the surface is granular rather than smooth, because the part is made of partly fused grains rather than a melted pool.<\/p>\n\n\n\n

In a foundry, SLS runs coated sand for moulds and cores, and PSB wax for investment casting patterns. The same machine does both \u2014 the change is powder, not hardware. That flexibility is the reason the mid-range SLS band is often the first platform a mixed shop buys.<\/p>\n\n\n\n

\"Engine
An engine block pattern in PSB wax. No supports to remove, and the whole build nests inside the powder bed.<\/figcaption><\/figure>\n\n\n\n

What SLM actually does<\/h2>\n\n\n\n

Selective Laser Melting takes the metal powder past its melting point entirely. Each layer is a set of overlapping melt tracks that fuse into the layer below, and the finished part is 99 percent dense or better \u2014 close enough to wrought for most structural applications after heat treatment.<\/p>\n\n\n\n

The support requirement is the operational cost. An overhang needs an anchor, or the melt pool sags. Those anchors are printed in the same metal, cut off by hand or by EDM afterwards, and the cut surfaces usually need finishing. On a part with a lot of overhang, support removal can take longer than the build.<\/p>\n\n\n\n

SLM also builds residual stress into the part as a matter of course. The melt pool cools and contracts against the layer below, and the stress accumulates until the part is stress-relieved. Skip that step and the part will move when the supports are cut.<\/p>\n\n\n\n

\"AFS-M120
AFS-M120 \u2014 an SLM platform. Metal only, support structures on every overhang, 120 \u00d7 120 \u00d7 150 mm.<\/figcaption><\/figure>\n\n\n\n

Side by side<\/h2>\n\n\n\n
\n\n\n
<\/th>SLS<\/th>SLM<\/th><\/tr>\n<\/thead>\n
What the laser does<\/td>Fuses particle surfaces<\/td>Melts the powder completely<\/td><\/tr>\n
Materials<\/td>Coated sand, PSB wax, polymers<\/td>Stainless steel, tool steel, titanium, aluminium, superalloys<\/td><\/tr>\n
Density<\/td>Porous at grain scale<\/td>99%+ after processing<\/td><\/tr>\n
Support structures<\/td>None \u2014 the powder bed supports the part<\/td>Required on every overhang<\/td><\/tr>\n
Post-processing<\/td>Depowder, cure, inspect<\/td>Stress relief, support removal, machining, HIP where specified<\/td><\/tr>\n
Build rate<\/td>Higher per layer, no support overhead<\/td>Lower \u2014 support and stress relief dominate calendar time<\/td><\/tr>\n
Best fit<\/td>Casting moulds, cores, wax patterns<\/td>Finished metal parts, thin walls, lattices<\/td><\/tr>\n<\/tbody>\n<\/table><\/figure>\n\n\n\n

Where each one fails<\/h2>\n\n\n\n

Every process has a characteristic failure. Knowing what yours looks like saves a week of arguing with a supplier.<\/p>\n\n\n\n

SLS failure modes<\/h3>\n\n\n\n

Dimensional drift at the build edges.<\/strong> The powder bed near the chamber wall sits at a different temperature from the centre, and parts built at the edge come out slightly larger or slightly warped. Nest the critical dimensions toward the middle of the bed.<\/p>\n\n\n\n

Nesting that ignores thermal mass.<\/strong> Packing a bed tightly raises the local temperature and can fuse neighbouring parts together at their closest points. A high-density bed needs a different laser energy profile than a sparse one.<\/p>\n\n\n\n

Surface finish taken as a tolerance.<\/strong> The granular surface is not uniform, so a tolerance quoted on a nominal dimension can fail on the surface texture alone. Specify the tolerance and the surface separately.<\/p>\n\n\n\n

SLM failure modes<\/h3>\n\n\n\n

Support removal damage.<\/strong> The most common cause of a scrapped SLM part is not a bad build \u2014 it is a chisel in the wrong place. Thin walls and fine lattices are the usual casualties.<\/p>\n\n\n\n

Residual stress distortion.<\/strong> A part built without adequate stress relief will bow when the supports are cut. The distortion appears after the build, which is why it gets missed in inspection.<\/p>\n\n\n\n

Porosity at the build interface.<\/strong> Where a part sits directly on the build plate, the first few layers see different thermal conditions and are the usual location for lack-of-fusion porosity. Machining allowance on the base solves it.<\/p>\n\n\n\n

Surface finish and tolerance in practice<\/h2>\n\n\n\n

Suppliers quote Ra figures as though they were specifications. They are averages, and the number that matters depends on which face the tolerance sits on.<\/p>\n\n\n\n

What “as-built” really means<\/h3>\n\n\n\n

An SLS mould comes out at roughly Ra 6 to 15 \u00b5m. That is consistent across the part because the surface is defined by grain size, not by orientation. A casting mould at that roughness needs no downstream work \u2014 the sand surface is reproduced in the casting, which is often an advantage for as-cast texture.<\/p>\n\n\n\n

An SLM part comes out rougher on angled faces, because the stair-step of layer thickness shows up wherever the surface is not vertical. Horizontal top faces are the smoothest; a 45-degree face is the worst. The same part can read Ra 8 \u00b5m on one face and Ra 20 \u00b5m on another, which is why a single Ra number on a metal drawing is not a specification \u2014 it is a wish.<\/p>\n\n\n\n

Where machining enters<\/h3>\n\n\n\n

This is where the two processes diverge in cost, not in capability. An SLS mould is used as printed. An SLM part is usually machined on every surface that carries a tolerance or a sealing face, which means fixtures, datums, a machine tool and a second operation.<\/p>\n\n\n\n

The practical rule: if the drawing has a tolerance tighter than \u00b10.2 mm on a metal part, budget for machining before you compare processes. If the drawing is a mould or pattern with a casting tolerance of \u00b10.5 mm, SLS as-built will hold it.<\/p>\n\n\n\n

Cost per part: the comparison that does not get published<\/h2>\n\n\n\n

Machine price comparisons are easy to find and mostly useless. What decides a project is cost per acceptable part, and that number includes four things the brochure leaves out.<\/p>\n\n\n\n