Blind risers: cold necks and hot risers - part 3 of 3.
To protect itself from this type of problem, the foundry should have a set of guidelines that specify the riser neck length and cross-sectional area to be used on low-production work. A set of the guidelines should also be available to patternmakers who construct risers and gating systems.
Guidelines should also be established for top risers using a Washburn, or breaker core, and for side blind risers necked down to reduce metal removal costs. For top blind risers on steel castings, the breaker core thickness should be about 1 0% of the riser diameter and the cross-sectional area of the neck should be about 25% of the cross-sectional area of the riser.
The length (or thickness) of the neck for side blind risers should be the minimum required for ease in riser removal. The maximum length of the neck should not exceed half the diameter of the riser. The cross-sectional area of the neck should not be less than 50% of the riser cross section. A lesser percentage can lead to premature neck freezing. While these guidelines are adequate for jobbing work, experimenting with smaller necks on high-production jobs may prove profitable. Smaller necks permit risers to e removed with less labor nd equipment costs. However, the foundry engineer should be cautious when using very small necks on top risers because metal segregation can occur in the casting at the neck area. The result can also lead to a localized high concentration of carbon and other alloying elements at the riser contact. Cold Metal in Blind Risers
Occasionally, blind risers fail to work because they are too far from the ingate. By the time they fill, the metal is too cold for the riser to function properly. The riser may be only partially filled or clearly show the shape of the firecracker core without the hole extending down into the riser. Difficult to predict and usually occurring only with side risers, cold metal may result from slow pouring or low metal temperature.
Side blind risers work best when they are "hot" risers-when an ingate enters the riser. This improves directional solidification by warming up the riser and neck cavities before and after the mold cavity is filled. It also reduces finishing costs by permitting the riser and ingate to be removed in one operation.
If it is not possible to route hot metal into the blind riser, there are other solutions such as opening the riser through the cope or enlarging its diameter. Though common, both are wasteful and may precipitate other problems. The best solution is to use an insertable, exothermic, insulating sleeve that can be rammed up in the mold or put in place after the mold is made. These sleeves, which are available f rom several foundry suppliers, usually contain a firecracker core and are reliable.
Most blind riser problems result from insufficient process control; others are a result of technological change that has forced the foundryman to use blind risers even if they are not preferred over open risers. A greater degree of process control is necessary with blind risers than open risers.
Process control must begin in the sampling stage by saw cutting risers to observe the shrink pattern. Molders must be taught to watch for broken or overbaked firecracker cores, omit washing firecracker cores and prevent metal from getting into core vents. Similarly, riser removal personnel must learn to look at blind risers before cutting and to contact engineering when blind risers do not appear to have worked properly. me
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|Title Annotation:||Cast Facts|
|Author:||Rowe, Charles A.|
|Date:||Apr 1, 1991|
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