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Undercuts should be avoided if possible through redesign of the part. Ideally, the mold tool should open in a direction parallel to the movement of the machine platen.


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In the figure above, hanging the form of the hole reduces initial costs and also maintenance cost during production.

For some complex parts the ideal situation will not exist and mechanical movement of one sort or another will be required.  A description of possible movements is given below:

  • Deflection – Dependent on the material and amount of undercut it may be possible to deflect the part out of the mold.
  • Inserts – The use of removable inserts that eject with the part is an option, certainly for prototype tooling. The disadvantages are the inserts must be removed from the ejected part and repositioned in the mould thus possibly extending the cycle time.
  • Cams – Cams or hydraulic/pneumatic cylinders move part of the mold out of the way to permit part ejection. These increase the complexity of the mold making it more expensive and also mean a controller is required to operate them during the molding cycle. Cycle times will also be affected.
  • Slides – By means of angled pins and rods mounted in the mold it may be possible to move the part of the mold forming the undercut in the direction of the angled pin during the opening sequence of the mold. This then allows ejection of the part.
  • Stepped parting line – By repositioning the parting line it may be possible to eliminate undercut features; although this may add to the complexity of the tool it is the most recommended solution.

The figure below shows an example of a sliding cam. The cam pins that operate the cams are mounted under a maximum angle of 20° - 25° in the injection side.  The angle is limited because of the enormous force that is exerted on these pins during mold opening and closing.

Cammed mold for part with undercut cams move in vertical direction when mold is opened.

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