The use of a sprue bushing to directly gate into a single-cavity mold provides several advantages in the construction and operation of the mold over the cold runner type. The mold base size is only dependent on the part requirements and will use the smallest press needed.
When using a cold sprue bushing,

a sprue is created that must be removed, leaving a mark, which often must be removed by secondary operations if appearance or maximum height of the sprue remainder are part considerations. To prevent this, a hot sprue bushing is used, which often leaves an unacceptable mark, reduces the operating window and raises the level of internal part stress (warpage).

  Use of a valve gate system will not produce a sprue, will reduce unacceptable marks, and maintain a good operating window while providing a part with the least possible stress. Most valve gate systems require a larger mold—both in the mold base footprint and height—since they must use a hot runner manifold to offset the valve gate operating system from mold centerline and must fit a manifold and the operating mechanism in the mold. Better designed systems will accomplish this with no increase in mold base footprint, and often without an increase in mold height because the valve gate is on the centerline of the mold and does not use an external operating mechanism.                            

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