When A-MIM is not a fit

Too large

The edges are compared by size, so the part may be rotated. A part of 160 × 60 × 40 mm is above the guideline, one of 80 × 140 × 60 mm is below it.

Why it becomes difficult above this

• The green part is larger than the finished part. With 15–20 % linear shrinkage, it has to be printed 18–25 % larger. A final dimension of 150 mm becomes roughly 175–190 mm in the printer.
• Deviations grow with length. Shrinkage varies in percent, not in millimeters. Over 150 mm, the same variation has one and a half times the effect it has over 100 mm.
• Large part, soft phase. In the furnace, the part temporarily softens and has to shrink along with its support. The longer it is, the further it slides across the support and the more friction holds back individual regions – this causes warpage.

A manufacturer of binder jetting systems specifies an ideal part size of at most 150 × 100 × 50 mm for its large systems – although the build volume extends to 800 × 500 × 400 mm. The build volume is therefore not the limit; sintering is. An edge length of 200 mm is therefore usually already too much.

Too small

A cube with a 10 mm edge length has 1 cm³ and weighs just under 8 g in stainless steel.

Why it rarely makes sense below this

• Details are not formed. Bores below 0.75 mm are not reproduced in the sintered part, and slots below 0.5 mm cannot be depowdered. With a very small part, however, the details are precisely the reason for manufacturing it additively.
• The manual work does not shrink along with the part. Each green part is removed from the powder individually, cleaned and placed on the sintering support. For a 5 mm part this takes almost as much time as for a 50 mm part, but is spread over much less value.
• Small parts in large quantities are the core domain of MIM. An injection mold with several cavities produces them in seconds.

Too heavy, too massive

There is no fixed weight limit – wall thickness, geometry and support during sintering are what count. A large but thin-walled housing can work; a small solid block cannot.

Why massive parts cause problems

• The binder has to escape. During debinding, the binder migrates from the interior to the outside. The manufacturer's guideline recommends a wall thickness of at most 25 mm, and not in more than two directions at once. A solid cube with a 25 mm edge (15.6 cm³, just over 120 g) is therefore already at the limit, even though it stays far below 60 cm³.
• Massive regions shrink differently than thin ones. A block next to a thin wall densifies unevenly – the part warps or cracks at the transition.
• Weight presses on the support. The heavier the soft part in the furnace, the greater the friction while it shrinks along and the stronger the warpage.

Too filigree

• Below 0.75 mm the green part is too weak – the wall does not survive unpacking.
• Between 0.75 and 2 mm the wall is formed but remains at risk of breaking during depowdering; this costs scrap and manual work.
• Slender walls and pins: Height to thickness at most 8 : 1, for walls below 1 mm at most 4 : 1. A rib 1.5 mm thick should therefore not be taller than 12 mm.

The reason is always the same: before sintering, only the binder holds the powder together. In this state the part is as fragile as pressed sugar.

Unsuitable geometry

During printing, the part is supported in the powder bed – almost any shape is possible there. It becomes critical in the furnace: there it softens and carries its own weight.

Tight tolerances almost everywhere

The part shrinks by about 15–20 % linear during sintering, and not equally in all spatial directions. The shrinkage is compensated, but every variation affects the entire dimension:

Fits, sealing surfaces and threads are therefore post-processed. If this has to be done almost everywhere, the blank contributes hardly anything – then printing and sintering are added to the milling.

Too few parts

Why

• The process design is incurred once per part. Position in the build space, sintering support and shrinkage compensation are defined for each new part and often readjusted on first samples. First sample parts with a coordinated inspection program start at FIT at about 5,000 euros.
• With ten parts, each part bears a tenth of it, with a thousand only a thousandth.
• Laser powder bed fusion needs no sintering design. The metal is melted in the printer; the part does not shrink by 15–20 %. For single parts and small quantities of up to about 50 pieces, it is therefore usually more economical.

What counts is the part family: 30 variants of 20 pieces each are 600 parts on a common basic shape – this can be of interest for A-MIM if the variants differ only outside the sintering-relevant base body.

Too many identical parts

Why

• In MIM, the shape is created in the injection mold. The tooling is a high one-time effort, after which a shot takes seconds. The more identical parts, the less the tooling matters.
• A-MIM saves exactly this tooling – an advantage that hardly counts anymore at very large quantities of the same version.
• With a simple shape, also earlier: MIM suppliers state that cost-effective series production is possible in some cases from as few as 5,000 pieces per year. If the part is easy to demold and does not change, MIM should therefore also be compared below 50,000 pieces.

Simple shape

A-MIM plays to its strengths with channels, undercuts, consolidated functions and variants. A shaft, a flange or a plate with bores gains no advantage from it – but they additionally incur printing, debinding, sintering and rework of the functional surfaces.