Design rules for the manufacture of parts by injection molding of metals

Metal Injection Moulding (MIM) is currently a very competitive technology for the manufacture of small steel parts of high precision and complexity. It coexists with other technologies such as casting, investment casting or additive manufacturing. From a cost and functional requirements point of view, the selection of the most efficient technology to be used in a project is key. In this article we will explain the technical recommendations to get the most out of this technology.

MIM manufacturing process

The MIM – Metal Injection Moulding process is a technology suitable for manufacturing complex geometry, small to medium sized parts in a wide range of materials (stainless steels, low alloy steels, soft magnetic, tool steel and ceramics). 

It combines the technique and versatility of plastic injection molding with the sintering technique to produce high-density parts with very tight geometric tolerances. 

We have an article in which we explain in detail which are the steps of the manufacturing process with metal model.

Advantages and disadvantages of Metal Injection Moulding

The advantages offered by Metal Injection Moulding technology over other steel part forming technologies are:

• Improved dimensional tolerances
• Elimination of machining operations
• Reduced wall thickness for easier design
• Improved surface finishes

In the following article you can read more about the advantages and disadvantages of MIM.

MIM or Investment Casting?

Within the various metal parts manufacturing technologies, we can point out that the MIM covers parts of medium-high complexity and a medium-high batch size (minimum batches of 5,000 parts per year). This can be clearly seen in the graph. Investment casting, on the other hand, covers the range of complexity similar to MIM but for smaller batches. 

The current market trend is pushing investment casting technology towards larger and more complex parts, while larger batch parts are increasingly being manufactured in countries with lower labor costs.

What is the optimum MIM part like? Dimensional tolerances in Ecrimesa and recommended design rules

The optimal MIM part could be defined:

• Length less than 100 mm
• Ratio length/width must be less than 5
• Weight can vary between 0.5 and 50 grams
• Wall thickness: between a range of 0.5 and 15 mm

Currently, dimensional tolerances are governed by an ISO 2768 standard, although the Ecrimesa Group can offer for some dimensions a better range than the standard. It is necessary, in any case, to analyze this situation for each part.

Tolerance table

DIMENSION (mm)	ECRIMESA	EPMA (ISO 2768)	INVESTMENT CASTING (D2 P690:2010)
<1.5	±0.03
<3	±0.04	±0.05	±0.12
3-6	±0.05	±0.06	±0.12
6-9	±0.06	±0.075	±0.14
9-12	±0.07	±0.075	±0.17
12-15	±0.08	±0.075	±0.17
15-20	±0.10	±0.15	±0.17
20-25	±0.12	±0.15	±0.20
25-30	±0.15	±0.15	±0.20
30-45	±0.18	±0.25	±0.30
45-60	±0.24	±0.25	±0.30
>60	±0.5%	±0.4%

• Length: Ecrimesa can offer pieces up to 150 mm.
• Ratio length/width must be less than 5
• Weight can vary between 0.5 and 50 gr, although Ecrimesa can manufacture parts up to 200 gr, but in this case the machining savings must be high to compensate the high prices of the raw material.
• Minimum wall thickness we work with: 0.4 mm 
• Minimum internal radii: RO.2 
• Process surface roughness: Ra< 1.6 
• Flatness: 0.4 % of the maximum flat surface length

• Alignment and concentricity: shall be minimum 0.1 mm and 0.4 % of the maximum dimension.

• Perpendicularity: a minimum of 0.1 mm and 0.5 % can be guaranteed.

It is also of vital importance to ensure a reliable process, as well as a reasonable production cost and therefore a competitive price, to take into account a series of design rules when designing the parts: 

1. Uniformity of wall thicknesses or at least gradual transitions to eliminate deformations, shrinkage, cracks …etc.

2. Eliminate excess material to improve tolerances and reduce raw material costs.

3. Design flat surfaces to facilitate the debanding and sintering of the parts, since these are supported on metal plates and thus avoid deformations during sintering or avoid the manufacture of complex supports that make the process more expensive.

4. Reduce stress zones by using bores or radii in the inside and outside corners; radii above 0.2 mm are always helpful to improve injection and eliminate defects. Elimination of sharp edges is also very important.

5. An area must be defined for the injection point; it must be taken into account that a mark may be left in this area; if this positive or negative mark is admissible, it will mean a saving in the cost of the part by reducing removal operations.

6. Define a non-critical zone for the demolding line.
7. Define zones for injection ejectors.
8. Design the parts with an angle in the direction of the ejection of the parts from the mold, so as to aid in the ejection of the parts.
9. Provide some lateral planes in the external threads to be able to place the demolding line on them and that it does not impede the operation of the thread. Inner threads can be made, but due to the high cost of manufacturing the mold it is not recommended.
10. Size the boreholes properly: their diameter should be smaller than the thickness of the surrounding walls and their depth should not be more than five times the thickness of the material walls.
11. Details such as numbers, letters, logos can be made on the molds.

Ecrimesa Group’s work methodology: consulting, prototypes for validation

However, from Ecrimesa Group we collaborate in the design of your part to reduce the cost of the same, since we have very experienced technicians in the development of MIM parts, after more than 2,200 projects carried out for sectors as diverse as automotive, tools, defense, medicine, aeronautics…etc. 

Our project managers work on the drawings or 3D and anticipate those tolerances that will be obtained directly by process, those tolerances that should be increased, and those that should be machined if necessary. Before construction, the mold is sent to the customer for approval, as well as the part configuration with demolding lines, ejectors, injection point…etc. for validation.

If you want to learn more about our metal fabrication technologies or start your MIM project, please contact us: