Although component design is complex, following a few fundamental principles will help minimize problems during injection molding and in part performance. In some instances it may not be possible to incorporate or follow all the suggested advice but the guidelines should increase your understanding of designing with thermoplastics.
Design recommendations
Wall thickness
Ribs and profiled structures
Gussets or support ribs
Bosses
Holes
Radii & Corners
Tolerances
Coring
Undercuts
Draft angle
Holes are easily produced in molded parts by core pins. Through holes are easier to produce than blind holes because the core pin can be supported at both ends.
Blind holes
Core pins supported by just one side of the mold tool create blind holes. The length of the pins, and therefore the depth of the holes, are limited by the ability of the core pin to withstand any deflection imposed on it by the melt during the injection phase. See information on bosses & cores.
As a general rule the depth of a blind hole should not exceed 3 times the diameter. For diameters less than 5 mm this ratio should be reduced to 2.
Through holes
With through holes the cores can be longer as the opposite side of the mold cavity can support them. An alternative is to use a split core fixed in both halves of the mold that interlock when the mold is closed. For through holes the length of a given size core can be twice that of a blind hole. In cases where even longer cores are required, careful tool design is necessary to ensure balanced pressure distribution on the core during filling to limit deflection.
Holes with an axis that runs perpendicular to the mold opening direction require the use of retractable pins or split tools. In some designs placing steps or extreme taper in the wall can avoid this. See section on draft. Core pins should be draw polished and include draft to help with ejection.
The mold design should direct the melt flow along the length of slots or depressions to locate weld lines in thicker or less critical sections. If weld lines are not permissible due to strength or appearance requirements, holes may be partially cored to facilitate drilling as a post molding operation.
The distance between two holes or one hole and the parts edge should be at least 2 times the part thickness or 2 times the hole diameter whichever is the largest.
For blind holes the thickness of the bottom should be greater than 20% of the hole diameter in order to eliminate surface defects on the opposite surface. A better design is to ensure the wall thickness remains uniform and there are no sharp corners where stress concentrations can occur.
Further strength can be achieved with gusset ribs or by attaching the boss to a sidewall.
Bosses adjacent to external walls should be positioned a minimum of 3 mm (.12 in) from the outside of the boss to avoid creating a material mass that could result in sink marks and extended cycle times.
A minimum distance of twice the nominal wall thickness should be used for determining the spacing between bosses. If placed too close together thin areas that are hard to cool will be created. These will in turn affect quality and productivity.
Bosses often serve as mounting or fastening points and therefore, for good design, a compromise may have to be reached to achieve good appearance and adequate strength. Thick sections need to be avoided to minimize aesthetic problems such as sink marks. If the boss is to be used to accommodate self tapping screws or inserts the wall section must be controlled to avoid excessive build up of hoop stresses in the boss.
General recommendations include the following:
Nominal boss wall thickness less than 75% nominal wall thickness, note above 50% there is an increased risk of sink marks. Greater wall sections for increased strength will increase molded-in stresses and result in sink marks.
A minimum radius of 25% the nominal wall thickness or 0.4 mm at the base of the boss is recommended to reduce stresses.
Increasing the length of the core pin so that it penetrates the nominal wall section can reduce the risk of sink marks. The core pin should be radiused (min 0.25 mm) to reduce material turbulence during filling and to help keep stresses to a minimum. This option does increase the risk of other surface defects on the opposite surface.
A minimum draft of 0.5 degrees is required on the outside dimension of the boss to ensure release from the mold on ejection.
A minimum draft of 0.25 degrees is required on the internal dimension for ejection and or proper engagement with a fastener.
发表于 2006-5-31 12:09:17
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发表于 2006-5-31 12:31:13
