PM Design Factors
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The high precision forming capability of PM generates components with near net shape , intricate features and close dimensional precision pieces finished without the need of machine work. By producing parts with a homogeneous structure the PM process to enables manufacturers to make products that are more consistent and predictable in their behaviour across a wide range of applications Attention must be given to the following design factors in the light of limited lateral flow and also of the necessity of ejecting the green part in the direction of pressing: |
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- Length-to-Width Ratio. The applied pressure and, therefore, the density decreases over the length of the compact. Double-ended compaction assists in equalising pressure distribution but still leaves a lower density region at the middle section of the part. Ratios of length to width in excess of 3:1 are not recommended.
- Re-entrant Grooves, Reverse Tapers and Lateral Holes cannot be moulded into the compact because of the impossibility of ejection and must, therefore be subsequently machined, although elaborate, sometimes flexible die assemblies have been designed and patented to overcome this limitation.
- Bevels require feather-edged tools, which are fragile and easily fractured; so, if design permits, the bevelled edge of the component should end in a small flat.
- Abrupt changes in section should be avoided since they introduce stress raisers which may lead to crack formation as a result of the stresses induced by the elastic expansion - spring back - that takes place as the compact is ejected from the die.
- To a first approximation the size of part that can be made is a direct function of the capacity of the press available, but the complexity of the part and number of punch motions required also influence the equation.
- These same factors are relevant also to production rates: the simpler the part the easier it is to press at high speed. With such parts rates as high as 1 part per second have been achieved using mechanical presses.
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Hydraulic Press Hydraulic presses enable greater pressures to be used - up to 5,000 tonnes - but speeds are necessarily much lower, 10 parts per minute being a fairly representative high speed for parts of comparatively simple geometry .
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Designers should take into account design rules, and the experience of the supplier, and discuss with them the producibility of parts with boundary shapes and the dimensional tolerances compatible with a given shape-material.
Shape designs depend on the forming method that has been chosen, as with other processes, this may have some restrictions in terms of dimensions, weight, and profile freedom.
Some examples of these are outlined below:
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Guideline for Shapes Neither the diameter of the holes nor their distance from the edge should be less than 1.5mm.
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When the upper punch is withdrawn there is no longer any balance of forces acting on the compact: both elastic spring back of the lower punch that forms the hub and internal stress in the compact (still in the die) try to bend the piece: give it an adequate thickness.
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Avoid specifying narrow and deep splines, requiring the construction of dies with reduced and therefore weak sections. |
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Long and narrow teeth make flow of the powder mix difficult during filling of the die cavity and the die becomes fragile. |
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A fillet radius favours filling of the die cavity and increases the robustness of the part. |
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Rounded corners allow better filling and increase die life. |
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Thin walls having a thickness of less than 0.8mm limit the flow of the powder and should be avoided. |
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Completely conical parts may cause the upper punch (diameter "d") to jam in the die during pressing. |
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Heavy radiused corners would require very narrow, feather edges, punches. A small flat will improve both functioning and the life of the die. |
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Specification of chamfers with angles over than 45° should be avoided. Instead a small flat area should be designed eliminating the feather edge on the punch which will give it longer life. When the chamfers serve to contain the pressing burr on "h" depth of 0.2mm is sufficient. The flat area may be dispensed with if the parts have low density and the angle is |
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The depth of the bosses produced with the upper punch must not exceed: 1) For the shape A: p<0.3H 2) For the shape B: p<0.2H |
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Compact sections indicated by diameters "d" are produced by means of projections or recesses in the upper punch. When compaction is complete, during the withdrawal of the upper punch the friction between the punch and the compact on the walls of diameter "d" tends to make those sections crumble: tapers are required. |
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Helical gears with large helix angles create excessive pressure on the teeth of the die both in the pressing and in the ejection phases. |
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Shapes To obtain sufficient uniformity of compact density the s/L ration should not be lower than 1:4. From the point of view of the behaviour in service, in certain cases a 1:6 ration may be allowable. Other less favourable ratios should be considered case by case. In structural parts subject to high stresses the H/D ratio should not be greater than 5, for the same reason. |
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Teeth with modulus smaller than 0.3 may not guarantee sufficient mechanical strength; additionally the flow of powder during filling is difficult. |
