Principles of Centrifugal Rubber Mold Casting : Chapter 10

1. What vents do and how they work in a pewter casting mold
2. Vent configurations
3. Types of vents
4. Where to vent a cavity
5. Channeling vents
6. How changing rubber or pewter metal alloy affects venting
7. Using venting to improve flawed or inferior pewter castings
8. How to cut vents and vent channels for molten pewter

10.1: What vents do and how they work in a pewter casting mold

A mold’s venting system is the system of channels that corresponds to and complements its gating system. The gates are the entrances to the cavities, and the vents are the exits. However, vent channels are much narrower and shallower than gate channels.

Vents are necessary because, as molten metal flows into a mold cavity, it displaces that is, pushes aside and drives out the air in the cavity. The expanding gasses that the heat of the molten metal produces must also have a way to escape. Otherwise, the cavity will not fill completely.

The venting system provides the necessary escape paths or “airways” through which air and gasses can pass out of the cavity as the metal enters. The vents open out of the cavities into channels that run to the mold’s perimeter. The talc that the caster applies to the mold each time it is cast supplements the venting system by maintaining an infinitesimal amount of space between the two mold halves through which some air also escapes.

10.2: Vent configurations

All of the different types of vents are cut in either of two basic configurations or shapes. The shape of a vent controls the speed at which air can pass through it. Hence, the choice of a vent configuration determines, to some extent, the speed at which the cavity will fill. But the choice of one configuration over the other should depend more on the size of the cavity and the volume of air that must be exhausted.

The two vent shapes are the ‘pie wedge’ and the ‘ingate’:

Pie wedge: (Fig. 10.6) The ‘pie wedge’ is the smaller of the two vent configurations, and the simpler and more commonly used. It consists of three cuts made with the scalpel to form the outline of an isosceles triangle in the rubber adjacent to the cavity. The smallest angle, or ‘apex’, of this triangle just barely touches the cavity so that only the slightest slit in the cavity wall results. This ensures that the vent opening will leave no mark on the finished casting. The “pie wedge” of rubber that is lifted out leaves a shallow, triangular hollow that is the vent.

Ingate vent: (Fig. 10.2) The ‘ingate vent’ is similar in form to the ingate that feeds the cavity. (See Chapters 9.2. 9.8) It is larger than the pie wedge and is used when the volume of air and gas that must be exhausted from the cavity is too great for the smaller pie wedge. Because of its tapered shape, a correctly cut ingate vent will leave a negligible mark on the finished casting. Any metal that may flow into it during casting will break off cleanly after solidification.

10.3: Types of vents

There are seven different types of vents. The term ‘type’, as opposed to ‘shape’, designates the point in the cavity at which the vent enters, or the way in which the vent is channeled to the mold perimeter. Each of the seven vent types may be cut in either of the two basic shapes: pie wedge, or ingate. The seven vent types are:

Fig. 10.1: The Gate Vent

Gate vent: (Fig. 10.1) The gate vent enters the cavity on one or the other or both sides of the ingate at a 45° angle. It is used most often with direct gates.

Back vent: The back vent opens from the cavity at the point in the cavity nearest the mold’s perimeter.

Drilled vent: (Fig. 10.2) A drilled vent is a vent that is channeled from the cavity for a short distance along the mold’s parting plane to a hole drilled through the rubber to the outside of the mold half. From there, it is channeled along the mold’s back to the perimeter.

Fig. 10.2: The drilled vent, seen from above (a) and in cross section (b). Vent configuration shown is ingate.

Drilled cavity vent: (Fig. 10.3) A drilled cavity vent is a drilled vent that is drilled from any part of the cavity directly to the mold back and there channeled to the mold perimeter. Drilled vents are used when the gates intervene and block the vent channel’s usual path along the mold’s parting plane to the perimeter, or when space inside the mold is so limited that it is easier to cut the vent channels on the back.

Fig. 10.3: The Drilled Cavity Vent

Parts of cavities that may require drilled vents include blind holes, and any other part that is deep and narrow enough to become an air trap that will not fill.

Reserve vent: A reserve vent is an oversized vent cut in the ingate configuration. It is used wherever a smaller vent would be inadequate. Cavities for large or oversized castings may require reserve vents. A reserve vent may or may not be drilled.

Reservoir vents: (Fig. 10.4) Reservoir vents are a series of small pie wedge vents. They are cut where a single vent would be inadequate: for example. along irregular, deeply scalloped, filigreed, or pronged edges. Reservoir vents are usually drilled and channeled on the mold’s back.

Auxiliary vent: (Fig. 9.7) An auxiliary vent is a thin, tab-shaped channel cut from the side of a side or back gate on the same side as the direction of mold rotation and drilled to the back of the mold. The auxiliary vent promotes filling by improving the gate’s capacity to function as a vent.

10.4: Where to vent a cavity

There are no hard and fast rules for determining how many vents a cavity should have and where they should open into the cavity. As a general rule, the location of the most important vent can be found by drawing an imaginary line from the center of the basin through the center of the cavity. The vent should be cut into the cavity wall at the point where the line intersects it nearest the basin. If the cavity is direct gated, then two gate vents should be cut, one on each side of the ingate.

When determining the location of a vent, it is important to keep in mind that, because of centrifugal force, most cavities fill beginning in the part closest to the mold perimeter and, from there, inward toward the mold basin. As a general rule, then, since the vents must remain clear of metal and continue to allow air to pass until the cavity is full, the main vent or vents for a cavity should be cut into the part of the cavity that will fill last. This will almost always be the part directly opposite the part that fills first.

In addition to the main vent or vents, vents should probably be cut or drilled:

1. From any narrow appendage of the cavity that might become an air trap and fail to fill: a blind hole; an ear, leg. or tail of an animal casting; a jump ring: any small protuberance. (See Fig. 9.21, the ‘sail mold’)

2. From any part of the cavity that will not fill after several warm-up spins have brought the mold up to casting temperature.

3. From any part of the cavity in which flaws such as voids, porosity, or blowholes occur consistently.

All of the molds used to illustrate this book ran successfully in production. Their gating and venting designs provide excellent examples of the various options to consider when planning where and how to vent.

Fig. 10.4: Reservoir vents.

10.5: Channeling vents

All vents should be channeled to the perimeter of the mold, either along the parting plane, or, in the case of a drilled vent, along the mold’s back. Vent channels should always be cut as shallow as possible to prevent metal from flowing into them, solidifying, and clogging them. For the same reason, they should never run in a straight line, but should be cut to run to the mold perimeter in a zig-zag or a curve. Zig-zags and sharp angles break up the channel’s course and prevent metal from flowing through it. Straight vent channels are far more likely to clog.

When vents are drilled, the channels on the back of the mold can be cut to run either directly to the mold perimeter, or through a wheel and spoke system similar to the runners of a gating system.

A note on unvented molds and unchanneled vents: Many moldmakers do not include vent channels in their venting designs. Indeed, many molds are made that cast successfully in production, yet have no vents at all! An excellent example is the ‘Ingot’ mold, Figure 13.9. This mold originally had no vents, but cast successfully with a high lead alloy. However, when the attempt was made to cast it with a high tin alloy, it would not fill until a system of vents had been designed and cut into the cavities.

It is probable that many casters who get unvented cavities to fill manage to do so only by running their alloy at temperatures much higher than would otherwise be necessary. Nice high temperatures make metal more fluid; only extremely fluid metal will flow through a gate that is being forced to function as both feeder and sole vent.

The price a casting operation pays for not taking the trouble to cut vents is very high. Working metals at extreme temperatures results in rapid metal depreciation, excessive dross, and premature mold burnout. In addition, the gates must be cut much larger than would otherwise be necessary to enable them to perform their dual function as feeder and vent. Large gates are more difficult to break off, and clean-up is usually much more extensive and expensive. Finally, when gates are forced to function both as feeders and vents, much more dirt and dross seem to enter the cavity than happens when vents are cut as a separate part of the mold design.

On the whole, then, in spite of the successes that many seat-of-the-pants casters and moldmakers have had with unvented molds, it is best to include a separate venting system in the design of every mold, and, in most cases, probably best to channel every vent to the mold perimeter. The few extra moments spent cutting vents will be more than repaid by superior quality castings, increased mold life, lower fuel costs, slower alloy depreciation, and a lower rate of dross formation, all the result of preparing molds so that they will fill using alloys run at the lowest possible temperatures.

10.6: How changing rubber or pewter metal alloy affects venting

Changing the alloy used to cast a particular mold may change the number and locations of vents needed to get the mold cavities to fill. If a mold that has previously cast successfully fails to fill after a change in alloys, enlarging or redesigning the venting system may solve the problem.

Changing the rubber compound used to make a particular mold, or even changing rubber suppliers, may also change drastically the kind of venting the mold requires.

When either the rubber used to make a particular mold, or the alloy used to cast it, is changed, the new mold should be trial cast several times on the CRMC machine before the same venting design that was used in a previous copy of that mold is cut in a new copy.

10.7: Using venting to improve flawed or inferior pewter castings

Most casting problems can usually be more effectively corrected by increasing the venting to the problem parts of a cavity than by adding or enlarging the gates. It is always better to try to correct problems by first modifying the venting than by modifying the gating or raising the pouring temperature of the pewter metal.

Frequently, when flaws such as porosity, voids, and blowholes appear regularly in one location on all of the castings from a mold, the cause is inadequate venting. The easiest way to test whether this is the case is to reverse the CRMC machine spin direction and trial cast the mold several passes. If the problem then appears regularly in a different part of the castings, the venting is at fault. Cutting or drilling a vent to the part of the cavity where the flaws are located should eliminate them.

Because cavities in a mold fill beginning in the part closest to the out- side of the mold and, from there, inward toward the mold center, most of the common casting problems such as mis-runs, voids, and porosity usually appear in the part of the casting that lies nearest to the mold center. Any experienced moldmaker “worth his salt” can determine immediately which way a defective casting was positioned in its mold by observing where the problem is located on the casting. For example, a casting that has not filled completely will usually be most defective in the part of the casting that faced the center of the mold. After this part of the cavity has been opened with a vent, it should fill completely.

10.8: How to cut vents and vent channels for molten pewter

Cutting vents is not nearly so difficult as cutting gates. Because pewter metal does not flow through the vents, cuts do not have to be particularly smooth or regular. The only part of a vent that is really critical is the opening into the cavity. This must be as small as possible to minimize the mark left on the casting and to prevent metal from flowing into the channel and clogging it.

The most important things to consider before cutting a venting system are: what parts of the cavity should be vented (Chapter 10.4); and, how the vents can be positioned so that any marks they might leave on the finished castings will not be noticeable.

The scalpel is the best tool to use for cutting the vents themselves because it has a thin, pointed blade, and because it is easy to manipulate when making small, delicate cuts. The channels from the vents to the mold perimeter can be cut using either the scalpel or the shipper’s knife. (Fig. 10.5)

There are three basic principles to remember when cutting vents:

1. The vent should just touch the cavity wall so that the opening between the cavity and the vent is no larger than a barely visible slit or pin hole. This prevents metal from flowing into the vent, and ensures that the mark left by the vent on the casting will not be noticeable. If some metal does flow into the vent, a microscopic opening makes it easier to break it off cleanly after solidification.

Fig. 10.5: The vent channel.

1. The channels from the vents to the mold perimeter should be as shallow as possible.

2. Vent channels should be cut with sharp angles so that they run in a zig-zag course. Or, they can be sharply curved. Cutting a channel that runs an irregular course helps to prevent metal from entering the vent and clogging it.

Cutting procedures:

Pie wedge: (Fig. 10.6) To cut the basic pie wedge vent, make three shallow incisions approximately ½” to ¼” long to form the outline of an isosceles triangle adjacent to the cavity. The short side should be approximately 16” long. The apex of the triangle (its smallest or “sharpest” angle) should just barely touch the edge of the cavity.

Each cut should be angled inward so that the wedge of rubber that is removed is shaped like an inverted three-sided pyramid, with two faces longer than the third face. After this wedge is removed, a shallow vent will remain.

Cut the vent channel from the vent inward a bit toward the basin, then make a sharp angle and cut outward to the mold perimeter, or, if the gates intervene and block the vent channel on the parting plane, cut a short channel, drill from the end of the channel through the mold to the back. Finally cut a vent channel on the mold back from the opening of the hole to the mold perimeter.

Ingate vent: (Fig. 10.2) To cut an ingate vent, follow precisely the same procedures as those used to cut an ingate for a gating system. (Chapter 9.5) All dimensions are the same. The only difference is that the first incision (the one made with the blade held horizontally) is made at the edge of the cavity wall precisely at the level of the parting plane and not below it so that the ingate vent tapers down from the level of the parting plane at the cavity edge. This creates the thinnest possible opening between vent and cavity, and produces a vent that will leave only the slightest mark on the finished casting. The in- gate vent is channeled in the same way that the pie wedge vent is channeled.

Fig. 10.6: The pie wedge vent, seen from above (a) and in cross section (b).

Drilled vent: To drill rubber, use a sewing needle mounted in the flexible shaft drill rather than a standard tool drill. Drills tend to make ragged holes when they are used to cut rubber. Any roughness or irregularity in a drilled vent will trap solidified metal that will quickly clog the vent. Metal solidified in a rough drilled vent is almost impossible to remove. When a needle is used to drill the vent, the opening that results is smooth and clean. Any metal that forces its way into the vent during casting will slide out easily after solidification. A clean opening is especially important in the case of a drilled cavity vent.

To make a drilled vent, simply drill from the part of the cavity to be vented, or from the end of the vent channel, down through the rubber to the outside of the mold half. Cut a shallow vent channel on the mold back from the vent opening to the mold perimeter.

The opening of drilled vents on the back of the mold half can be located easily by standing the mold half on its side on the bench and bending it as much as possible. Bending will cause the holes to stretch open enough to become visible so that they can be marked.

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