Venting ... a lost art.

With venting still far too often neglected today, this article provides recommendations and guidance for proper practice.

The need to vent cores and molds has been recognized for centuries as a means of avoiding the adverse effects of gases entrapped in or evolved from cores, molds, coatings and binders during the pouring and solidification of castings. Without adequate venting, gases become entrapped and result in gas-related defects such as pinholes, blowholes, scabbing or other related defects. Proper venting techniques can result in fewer rejects, reduced casting finishing times and increased production economies - all for a more profitable casting operation.

Venting is more necessary in cores than in molds because a major portion of a core is in direct contact with molten metal and the core area where gases may escape is usually limited due to increasingly complex core geometries. While the venting of molds to allow the gas generated at the mold-metal interface to escape is not of great concern, mold vents are essential to relieving the buildup of core gas in the mold.

Venting of molds and cores can be as important to maintaining casting quality and reducing casting defects as having a good gating and risering system. Perhaps the least understood aspect of gating involves the use of vents to relieve the pressure of gases generated by the contact of hot metal with the mold or cores during the filling of the mold. This is particularly significant in high density molding operations where the mold is of significant density to virtually negate its ability to permit gas to pass through the mold.

Gas Evolution

As the mold is filled with metal, the heat preceding the metal and the heat penetrating the surface of the mold and core drives gases off from the mold and core. In general, these gases are generated at low temperatures and, because of their substantial volume, fill into the mold cavity because it offers the least immediate resistance to their flow. The gas pressures within the mold cavity are therefore increased and must be offset by the entering metal. This now makes it more difficult to fill the mold cavity, particularly during the last stages of filling, when the gas pressures may be quite substantial. Because of the low permeability of the high density compacted sand, the gases will, for the most part, be forced back into the core.

Table 1. Compaction Influence on Permeability/Gas Flow

Mold Hardness Impact on Permeability

Mold            Permeability
Hardness            No.

30                  550
40                  417
50                  341
60                  243
70                  212
80                  163
85                  138
90                  112
96                   88

Venting of the cores allows some of these gases to be released through the vents; however, the mold cavity also appears to generate gases. Most of the gases emerging from the core vents are released after the casting has been poured, and are of little or no concern to the filling of the mold cavity. Only after these gases have developed substantial pressures will they enter the metal after the mold cavity is filled. On the other hand, the mold itself may be vented to permit the ready escape of gas. It should be noted, however, that when these vents are filled with metal, they are no longer effective as mold vents.

Usually, these vents are placed at the upper sections of the mold cavity where they also serve to indicate when the mold has been filled with metal functioning as "flow-off." If a substantial amount of gas is evolved during mold filling (such as when a core is surrounded with metal), precaution must be taken against erosion of the core or mold at the vent by the escaping gases. Also, care must be taken against distortion of the mold cavity due to the pressures involved.

The generation of gases within the mold can develop backpressures that affect the rate of mold filling even to the point of negating the designed choke in the gating system. This fact, coupled with the decreased flow rate experienced during the latter stages of the mold filling, require that the sprue be maintained full at all times to affect the greatest possible pressure on the metal during the final stages of pouring. This is particularly significant in vertically parted molding because the volume flow rate of the metal being poured varies widely with changes in the hydraulic head, and the hydraulic head is usually quite small near the end of the pour. Incomplete filling of the mold, surging of metal flow that makes it appear that the mold cavity is filled, and entrapped gas pockets in the mold cavity may result if these precautions are not heeded.

Permeability

The screen distribution of the sand and its degree of compaction of the mold or core can significantly influence its permeability and the ease of gas flow through the sand (Table 1). Although green sand can have a permeability value as high as 110, the gas evolved during casting may be unable to pass through the sand mass rapidly enough to avoid blowholes or other related defects. A properly formed vent channel would then be required. Gas cannot pass through a sand mass as speedily and effectively as through a properly formed vent channel, regardless of the permeability of the sand. Additionally, the application of a core/mold coating greatly reduces permeability. For this reason, cores that are coated may only be able to vent through their prints - making properly designed vent passages essential. Poor vent connections and partial or complete blockage of vents increases the risk of producing castings without defects. Core prints should be a good fit into the mold, otherwise metal can penetrate into - and partly or completely block the vent channel.

Pressure buildup in the mold (backpressure) changes with grain fineness, amount of compaction and the amount and decomposition of binders and additives. These factors change the permeability of the mold, which can vary widely, thus causing the pouring time to also vary due to this backpressure. This variation in pouring time is the reason for casting problems with surface finish and other related quality problems. Venting can reduce or even eliminate this pouring time variation and subsequent defects. Figure 1 illustrates the issues that must be considered when developing a venting system in both horizontally and vertically parted molds. Also listed are the defects that will occur as a result of improper venting.

How to Vent

Vents and vent locators for manual venting must be mounted directly on the pattern whenever possible. On molds that require manual venting, special instructions should accompany the process sheets as to the size, number and location of the vents. Figure 2 shows some of the locations and types of vents to be used on both horizontally and vertically parted molds.

Rod-formed vents are round, cylindrical holes or passageways that extend through the core. There are several methods for making channels or open passages in cores using rods or wire. Textile tubing is an all-purpose venting material that is suitable for use with cores/molds for baked or nobake binder systems. The textile tubing is placed in the core or mold during its production where it remains to provide a flexible internal passage for gases to escape.

Vents can also be formed in cores by:

* drilling the core freehand or holding the core in a fixture and drilling ([ILLUSTRATION FOR FIGURE 3 OMITTED]);

* producing the core with a hollow interior. This is often the norm for shell cores;

* having fixed pins in the corebox;

* using a vent imprint board to squeeze-in the vent system while the half-core is still in its "green" or uncured state. Vents can be formed in molds by:

* using inverted "V" or "U" strips on the pattern plate to provide vents on the parting face;

* having fixed vent rods on the pattern;

* using a vent imprint board to squeeze a vent system into the parting face.

Fig. 1. This list indicates key considerations when venting horizontally and vertically parted molds, as well as defects that can occur from the lack of venting and improper venting practices. (1995 5-M Committee Cast Facts)

* Type of Vent

Horizontally Parted Molds - stem vent, parting line vent, core vent.

Vertically Parted Molds - parting line vent off casting, stack vent off casting, stack vent off core.

* Size of Vent

The size or diameter of the vent(s) is regulated by: weight of the casting in the mold; weight of the core in the mold; and pouring time required. A 0.5 in. diameter round stock and tapered. vent through the cope should be used for castings [greater than] 25 lb.

* Defects From Lack of Venting

Gas pockets on cope side (horizontally parted); gas at 12:00 (vertically parted); misruns and laps (slow pouring); pinhole porosity; runouts, penetration and poor surface finish; swells and other dimensional problems.

* Possible Defects from Venting

Sand in casting; vents breaking into casting, causing scrap; possibility of runouts with parting line vents.