Design Rules - MIM Expertenkreis

1. Filling of the cavity

BWall thickness influences the feedstock flow

      ■ Homogeneous filling in spite of
      ■ different flow path lengths
      ■ Homogeneous cooling
      ■ Homogeneous distribution
      ■ of filling pressure

Top:
inhomogeneous form filling,
recognizable from the timelines

Bottom:
homogeneous form filling due to
increased wall thickness towards
the corners

1. Formfüllung

2. Selection of the gate position

A gate position close to thick-walled areas:

      ■ prevents freezing of the feedstock
      ■ before form filling is completed
      ■ reduces voids and hourglassing
      ■ during cooling
      ■ even distribution of shrinkage
      ■ over the complete part

Due to the quick cooling of the feedstock,
jetting can lead to internal weld lines,
to inhomogeneities and also to pores
(especially in thick-walled parts).
Form filling should preferably happen
with fountain flow:

      ■ Melt should be directed towards
      ■ a wall or obstacle
      ■ The gate should be designed
      ■ with suitable radii
      ■ Use gravity to advantage
      ■ (form filling from bottom to top)

2. Auswahl des Einspritzpunktes

2. Einspritzpunkt

3. Draft angles for ejection

MIM green parts exhibit less strength
and less shrinkage than usual polymers.
Thus they need large size ejectors and
large draft angles.

Draft angle > 1°
Ejectors: 4-6 times larger compared
to plastic injection moulding

3. Entformschrägen

4. Adjusting wall thickness

Goal:
Uniform wall thickness
for the complete part

wall thickness:
max. 25 mm;
good: 1-10 mm;
better: 1-5 mm

4. Wandstärke

5. Adjusting the wall thicknes

Save money and take care to achieve
a consistent (thin) wall thickness.

This reduces the danger of:

      ■ Cavities and pores
      ■ Residual stress after injection moulding
      ■ Part distortion during sintering

Avoid large differences in wall thickness
as they lead to higher tolerances.
If unavoidable, use continuous transitions.

6. Keep in mind for sintering:

Green and brown parts are not very strong
and shrink noticeably during sintering.

      ■ Design with sufficient contact
      ■ area to support
      ■ Flat or contoured support
      ■ Avoid projections
      ■ Use a smooth support with low friction
      ■ If necessary, use feedstock supports
      ■ that shrink with the part (have to be
      ■ removed after sintering)
      ■ Adequate ribbing prevents sagging

6. Sintern