Custom Shape

When the drawing stops being a stock geometry

Most magnets are asked to be a shape that already exists: a disc, a ring, an arc, a block. A custom outline is the case where the assembly will not take any of them — a magnet that has to follow a housing rib, clear a boss, sit in a pocket with one corner cut away, or carry a feature that no standard profile has.

That is a legitimate reason to ask, and it is also the point where the conversation changes. On a stock geometry the questions are dimensions and magnetization; on a custom outline the first question is which forming route can produce the outline at all, because the answer decides what the part can be made of.

The two routes open here are both moulded

This page lists injection-moulded ferrite and injection-moulded NdFeB. That is not a coincidence: a free outline is produced in a mould, and moulding is what makes an outline that a die could not press or a wheel could not cut economic at volume.

The two routes differ in what they give. Moulded ferrite is the cheaper of the pair and needs no coating. Moulded NdFeB gives more flux from the same outline and requires surface protection, because the material corrodes unprotected. Both are net-shape processes: features that would be machined on a pressed part are formed in the tool, which is where the cost sits and also where the savings are, since a formed feature is not a second operation.

What an outline has to be able to do before it can be made

A mould is not a drawing converter. Several things it cannot do are worth knowing before the outline is frozen:

• Draft. The part has to leave the tool. Vertical walls on a deep feature are a tooling decision, not a tolerance.
• Sharp internal corners. A corner that is sharp on paper is a point where the tool wears and the part concentrates stress.
• Thin walls. Moulding handles thin sections better than pressing, but a wall still has to fill completely and still has to survive handling.
• Undercuts and side actions. Possible in a mould, and each one is a tool cost that belongs in the quotation rather than in the sample.
• Inserted or overmoulded parts. A magnet moulded into a carrier is a different specification from a bare magnet, and the two are not quoted on the same outline.

Moulded usually means isotropic, and that is a trade

Free geometry is bought with magnetic strength. Moulded material is normally isotropic, so it has no preferred direction established during forming, and its residual flux sits below what a pressed or sintered grade of the same family reaches. The grade tables on the material pages set that ceiling; ferrite and NdFeB are tabulated separately because the gap between them is large.

What this means in practice is that a custom outline is a good answer for a position that needs a shape, a fit or an assembly feature, and a poor answer for a position that needs the maximum flux from a small volume. Where the flux requirement is the binding constraint, the usual resolution is to change the geometry back towards something a pressed or sintered route can hold, not to ask a moulded part to beat its grade.

The magnetic pattern still has to be specified

A free outline does not make the magnetization free. The pattern is still axial, diametrical, radial or multipole, it still needs a fixture, and it still needs a datum the poles are dimensioned from. On an uneven outline the datum is the part most often missing from the drawing, and it is the one the fixture is built from. The directions and patterns, and what each one asks of the drawing, apply to a moulded part unchanged.

What to send for a custom-geometry review

• The outline as a dimensioned drawing, with the features that carry function marked and the datum named.
• What the part has to do mechanically: locate, retain, seal, or carry another component.
• The flux or field the circuit needs, and the operating temperature.
• The magnetization direction and pattern, with N and S, and the pole count where there is one.
• The annual quantity, because a moulded outline is a tooling investment and the volume is what carries it.

The table further down this page lists the parameters a custom drawing has to settle before the part can be quoted.