Design Guide

Certain component characteristics are best suited to PIM. Early identification of a match with these characteristics ensures technical and economic success. The considerations involve the materials, properties, component size and shape, tolerances, production costs, production quantity, and delivery schedule. Powder injection molding is best applied to the fabrication of complex, small components, especially those that would otherwise require multiple assembly or forming operations in alternative production routes. Further advantages occur when the material is expensive or difficult to process by alternative techniques. However, the shape complexity that can be produced is limited. Various shapes require secondary machining operations or complicated tooling that increase the manufacturing expense.

Tolerances are another concern. Precise tolerances prove difficult to achieve; production variations on a dimension are ±0.1% for special cases, and ±0.3% is more typical. Besides dimensional variability, successful component design must allow for noncritical placement of the parting line, ejector pin marks, and gate. It is also desirable to include a slight draft or taper of 0.5 to 2° to assist in ejection from the mold. Because precision tooling is expensive, the production quantity must justify the initial tooling cost. These factors need to be evaluated early to ensure feasibility. Greatest success is attained by early discussions of the manufacturing approach so that features can be incorporated that will aid PIM processing.

To emphasize the contemporary fabrication range, Table 7 summarizes the minimum, maximum, and typical features. Some explanation is in order. Components are characterized by the geometric aspects, including the largest dimension and the wall thickness. For PIM, the wall thickness is usually small, and it might be less than 10% of the largest dimension. Further, the variation in thickness over the part is usually small. The blades of scissors are an example where the length is much greater than the thickness, and the thickness is not highly variable. It is thickness that determines debinding time and is the main limitation of PIM.

Table 7 General criteria for powder injection molding

Attribute

Minimum

Maximum

Typical

Thickness, mm

0.2

25

10

Thickness variation

None

100x

2x

Longest dimension, mm

2

1,000

100

Tolerances, % (standard deviation)

0.03

2

0.3

Number of dimensional specifications

20

1,000

100

Mass, g

0.02

20,000

40

Material

Simple element

Composites

Alloys

Properties

Unimportant

Highest attainable

Handbook

Cost per part, $

0.20

400

2

Production quantity per year

200

20,000,000

150,000

Another gage of PIM candidates is through the information content, which is akin to the number of dimensions on the engineering drawing. A simple shape, like a flat washer for a bolt, has only a few features, while a microcomputer circuit has many millions of features; both would be poor applications for PIM. Common PIM successes involve several dimensions in the 20 to 40 g range-- a wristwatch case is one example that matches well with the technology. The typical mass is low, and similar attributes are evident in components for electronic packaging, computer disk drives, surgical tools, and firearms. On the other hand, metal, ceramic, and carbide bodies have been fabricated that are several kilograms in mass and up to 1 m in maximum length. These are somewhat lower-precision structures used in electric power turbines, steel mills, and furnace construction. The general production limit for precise components is 250 g, although several special processes have evolved to produce precise components as large as 1 kg.

Only in a few instances are mechanical or physical properties unimportant, such as in jewelry and wristwatch cases. More typically the goal is to attain near-handbook properties for the selected material. Likewise, only rarely are properties pushed to the limits for any material. In these cases secondary hot isostatic pressing or other treatments might be employed to ensure maximum properties. From a cost consideration, typical prices are moderate, but that assumes a high production volume to justify the cost of tool construction, and dimensional tolerances that are compatible with the inherent process variability. Thus, a general processing window exists that proves most feasible. The typical parts are small enough to be held in a hand, but can be as small as the tip of a pencil. This implies that very large objects are difficult or expensive to produce by PIM, as contrasted with other forming technologies.

Geometric Considerations. In a simple sense, if a shape can be formed from plastic, then PIM is viable. Tables 7 and 8 provide general quantitative and qualitative component design guidelines.

Table 8 Nominal powder injection molding component design guidelines

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