Monday, September 19, 2011

MOLDED HOLES


  • ·         Molded holes with optimum geometry and size usually cost less than the machined holes if large quantities of parts are to be made.
  • ·         Molded holes in the direction of the mold closure (perpendicular to the parting line) are less expensive than holes perpendicular to or at an angle to the mold closure.
  • ·          For holes perpendicular to or at an angle to the mold closure, a post molding drilling should be considered because it requires special retracting mechanisms for automatic molding.
  • ·         Molded holes may be blind, through, stepped, recessed step or intersecting. The holes can be round, square, elliptical, etc. the round holes being always the cheapest because simple dowel pins can be used as mandrels in the mold to form the holes.
  • ·         When the length to diameter ration of the pins forming the holes, is excessive, the pins may be distorted or broken off due to the severe bending force to which it might often be subjected to. The ratio is known as “slenderness ratio”.
  • ·         Using two hole-pins support in compression molding involves the risk of flash entrapment because the resin is introduced prior to mold closing. This possibility is minimized during injection molding or transfer molding because resin is introduced after the mold is closed. The entrapped flash can cause the pin to bend or break in the next shot. Two pins butted together in the center can minimize the problem by revealing any flash on either pin when the mold opens. The two pins should have different diameter to minimize the possibility of misalignment.
  • ·         While molding the holes by injecting the material into the mold, which flows around the pin to form the hole, the material joins itself somewhere behind the pin to form a weld line forming an area of potential weakness. If this proves harmful (detrimental) in the part operation, the way of injection and gate location should be changed such that it will minimize the weak areas. The weak areas may be in the side wall, edge or the area between the holes.
  • ·         To eliminate the weld line weakness and mark the hole location, short pin is used to mold a shallow hole which can then be drilled further to obtain a complete hole.
  • ·         The holes should be placed at minimum 2d apart or from the edge or a side wall to minimize the weld line weaknesses.
  • ·         Holes to be tapped or to be used with self tapping screws should be countersunk.
  • ·         Feather edge should be avoided because it can lead to cracking at the edges of the part or mold.

No comments:

Post a Comment