Principles of Centrifugal Rubber Mold Casting : Chapter 9 (Part 2)

1. 9.6 A note on turbulence
2. 9.7 How gate design affects the way a cavity fills
3. 9.8 How to cut a gating system
4. 9.9 Cutting Review
5. 9.10 Gating and venting guide

9.6: A note on turbulence

Turbulence is a known cause of casting problems. Molten metal should flow through gates in a smooth, uninterrupted stream. When the metal encounters sharp angles, roughness, protrusions, or is forced to change directions suddenly in the gating system. its flow becomes turbulent and confused. Turbulence that will last all the way to the cavity can be created even as early in the course of the metal’s passage through the mold as the funnel or the basin if the metal is poured carelessly or the basin is rough or irregular. When turbulence develops in the metal at any point between the ladle and the cavity, the result will probably be dirt, porosity, and roughness in the castings.

Fig. 9.6: How a cavity fills.

Both the moldmaker and the caster must work to ensure that when a mold is cast, the metal will pass smoothly from the ladle, through the gates, and into the cavity. Gates should be designed to have as few sharp angles as possible. In Fig. 9.4(VI), the gate has been cut only as far as the ingate. The corner that the metal must turn in order to pass from the gate to the ingate is sharp and abrupt. Some turbulence will almost certainly develop in the flowing metal at this point. In Fig. 9.4(III), the gating has been designed so that the metal flows into the cavity in the direction of the machine’s rotation. The moldmaker has been careful to extend the gate beyond the ingate to form an appendix. This appendix eliminates the sharp, abrupt angle and allows the metal to flow more smoothly and without turbulence from the gate into the ingate. Any turbulence that may develop will develop in the appendix and be confined to it. The flowing metal will pass smoothly by it into the cavity. Roughness and waviness in the gate walls caused by unsteady cutting or a dull blade will cause turbulence. Rough or irregular gates also tend to burn out faster than smooth ones. The moldmaker must therefore be careful when he is cutting the gating system to make each incision with a single, sure. strong, steady motion. He must also be careful to change the blade in his knife immediately at the first sign of dullness.

Fig. 9.7: Auxiliary vent.

9.7: How gate design affects the way a cavity fills

Most moldmakers and casters assume that molten metal completely fills the gate and runner channels of a mold while it is flowing through them during casting. As a part of their continuing program of research into how casting alloys work in rubber molds, the Oster Group conducted a preliminary study to test this assumption and to determine exactly how metal flows in different types of gates and how it behaves as it enters and fills the mold cavity. Because the study was preliminary, factors such as alloy, temperature, and machine speed were not strictly controlled. But the study did serve to show that what actually happens inside a mold during the casting cycle is quite different from what most people in the industry think. A series of strictly controlled research projects based on this study is now being tested. Findings will be reported to the industry as each is completed.

To make the study, a transparent plexiglass cover was substituted for the top half of a mold set and the casting machine spin pressure plate. The mold was then spin cast. During the casting cycle, a high speed stroboscopic movie was made of the mold interior. The model used to make the cavities was a flat rectangle measuring 1” x 1½”. Three cavities were made. One was di- rect gated and the other two were side gated, one on the left and the other on the right. The mold had no runners; all gates ran directly from the basin. The machine’s rotation direction was clockwise.

The film that was made revealed the following important differences among the three cavities in how the metal flowed into them and how they filled:

Side gates: Neither of the side gates filled completely with molten metal during the spin cycle. In each, the metal flowed along one side of the gate channel, the side opposite the direction of rotation of the mold. In both side gated cavities, the metal entered from the ingate in the corner of the cavity and filled the cavity by flowing diagonally across to the opposite corner. The metal moved smoothly in the cavities, and little or no turbulence was evident. Both side gated cavities filled more quickly than the direct gated cavity. The cavity gated on the right side (same side as direction of spin) filled the fastest of the three, although the cavity gated on the left side (side opposite the direction of spin) was only a fraction of a second behind it.

Direct gate: The gate channel filled completely with metal. The metal en- tered the cavity as a sharp stream and hit its back wall with a great deal of splashing and turbulence. The cavity filled from the back to the front against the stream of entering metal. The direct gated cavity took longer to fill completely than did the two identical cavities that were side gated.

Suggested explanation: (Fig. 9.6) The side gates filled their cavities faster than the direct gate because the side gate channels did not fill up completely with molten metal as the direct gate channel did. Because metal flows only on one side of a side gate, a side gate is never more than partly full during the casting cycle. Thus, there is room in it for displaced air and gas to move out of the cavity as molten metal enters. In a direct gate, the metal fills the gate channel full and turns the cavity into an air trap by, in effect, plugging it so that air and gasses displaced from the cavity by entering metal cannot escape through the gate. Even generous venting does not seem to compensate for this loss of the gate’s venting function. This, then, is why side gates fill their cavities more quickly than direct gates: side gates are able to function as auxiliary vents, but direct gates cannot.

Conclusion: An efficient gate performs a two-fold function: it feeds metal to the cavity and acts as an auxiliary vent at the same time. Even though a direct gate provides a straight channel for the metal from the basin to the cavity, a direct gate is not an efficient gate for most castings because it cannot function as an auxiliary vent. In addition, direct gates seem to produce more turbulence in the metal, especially in the cavity where the metal first splashes against the back wall and then must flow against itself in order to fill.

Unless limited space in the mold de- sign makes direct gating from the basin necessary, side gating, back gating, or direct gating from a runner system should always be used because these gate types will perform more efficiently and produce cleaner, finer-grained castings than direct gating.

Auxiliary: The results of this study suggest a new technique for improving mold performance and for getting side and back gated cavities that will not fill properly to fill: the technique of cutting an ‘auxiliary vent’. (Fig. 9.7) The auxiliary vent consists of a tab-shaped channel cut outward from the side of the gate and then drilled to the backside of the mold and channeled like any other vent. It should always be cut on the same side of the gate as the direction of rotation, since the side of the gate opposite the direction of rotation is the side along which the metal flows in the spin cycle. The auxiliary vent enables the gate to perform more effectively as a vent. In tests, cavities that had previously been very difficult to fill, filled easily and produced clean. sharp castings after they had been given auxiliary vents.

9.8: How to cut a gating system

The moldmaker at work: Cutting a gating system is not as easy as it looks to the novice moldmaker who has never tried it. When he does try it for the first time, confusion about where the gate should enter the cavity, how deep the cuts should be, and fear of the knife will all create the hesitation and timidity that produce ragged, irregular cuts. And, as all novices discover, cured rubber is a lot tougher than it seems when they first see an experienced moldmaker working on it.

If you’re easily intimidated - don’t opt for moldmaking - try ballet dancing. But if you are self-confident (a true Dale Carnegie), then the best way to begin is to watch an experienced moldmaker carefully. The quick, deft motions that he makes as he transforms the mold into a maze of channels are the result of the skill that he has acquired through long practice. His skill is your guarantee that cutting molds is really easier than you will think it is after you’ve tried it for the first time for yourself.

When you observe an experienced moldmaker for the first time, the most important thing to watch is his technique. The most important part of his technique is the kind of motions he makes: smooth, clean, sure, steady. without hesitation. (For some, that’s only after the third morning cup of coffee!) Smoothness is everything in cutting molds. It is better to work slowly at first, stopping and taking a deep breath before each cut in order to produce smooth, clean channels, than it is to work fast if the result of speed is ragged, sloppy, irregular work. Only with practice does the skill come that enables a moldmaker to cut quickly without hesitation.

The second most important part of the moldmaker’s technique is the way he positions his work. He always places the mold directly in front of him. With his right hand he holds the knife, and with his left hand he grasps the mold. An experienced moldmaker moves his knife hand as little as possible. He uses his left hand (or, if he is left handed, his right hand) to rotate the mold clockwise or counterclockwise in order to bring the portion that he wants to cut to position beneath the waiting knife. When cutting the wheel for a runner system, he does not move the knife at all. He simply inserts the blade in the rubber, cutting edge toward himself, and then, while holding the blade perfectly rigid and stationary, rotates the mold half against the stationary blade with his left hand.

When cutting spokes and gate channels, experienced moldmakers always cut pulling the knife toward themselves. This is contrary to every rule of self preservation that has ever been formulated. But cutting toward oneself is the only way to ensure that each cut is smooth, clean, and straight. Consequently, moldmakers must stay awake and alert to what they are doing and must cut with an absolutely steady hand (as if their lives depended on it. They do!).

To cut gates and spokes, the moldmaker positions the mold set so that the portion to be cut is the part of the mold farthest from him and directly opposite him across the mold basin. He then inserts the blade at the point of the channel farthest from himself and cuts toward the mold basin. Each cut must be made with one smooth, strong, unhesitating motion.

The only secret behind an experienced moldmaker’s skill and assurance is: practice, practice, practice. The price of that skill: poor cuts, sloppy cuts, frustration, poor molds. But the end result is the satisfaction of making good molds that produce beautiful castings!

A note about preforms for runners: Preforms of various configurations are available from most casting supply houses. They eliminate some cutting by curing the runner system directly into the mold during vulcanization. They are made of brass and resemble a wagon wheel. To create a runner system for a particular mold, the moldmaker needs only to choose the appropriate preform and position it in the uncured mold set along with the models and basin preform prior to curing. Runner preforms do not include the gates to the cavities. These the moldmaker must still cut by hand.

Runner preforms offer some advantages. They save the moldmaker time when he is making many copies of the same mold. They produce perfectly smooth and even runners that help keep metal turbulence to a minimum. There is also some evidence that pre- formed runners do not burn out as quickly as runners cut by hand, so using preforms may help lengthen mold life.

The disadvantages are that preforms are difficult to position in relation to models, and they limit the ways in which the mold can be laid out. They also sometimes make it difficult to de- sign a mold to produce a variety of different castings. Finally, it is a fact that a truly skilled moldmaker can actually cut the runners and gates faster by hand than it would take him to hunt up the appropriate preform for a given mold and position it in the uncured mold set. What affects the amount of time required to cut a mold is not the runner system, but the number and type of gates that must be cut. Preforms do not cure the gates into the mold. Brass preforms, when used, should first be polished and rhodium plated before being used, because brass can react chemically with rubber.

Step by step cutting procedure:

1. Layout: Position the freshly cured and still warm mold half in front of you. Although experienced moldmakers can cut a cold mold as easily as they can a warm one, it is best for a novice to begin with a mold that is still hot, since the rubber will be softer and easier to cut. Begin by marking on the mold with a ballpoint pen the locations of the runners, gates, and vents that you want to cut. Always know where you are going before you start cutting! Use a compass to scribe two concentric circles around the basin approximately ½” apart and at least ½” in from the cavities toward the basin. (Fig. 9.8) These lines locate the wheel. From the basin radiating outward to the wheel, draw four to six equally spaced spokes. Fewer or more spokes may be necessary, depending on the number of cavities and their size. Each spoke channel should be about ½” wide.

At each cavity, mark the exact point at which the ingate will be cut. ¼” away from each ingate mark draw two lines to the runner to indicate where each gate will be cut. (These lines can be imaginary after you become experienced.) The size and configuration (back, side, or direct) of the gates will depend on the size and type of casting, the alloy used, and the amount of room available in the mold. The best gates are side or back gates that “flow” in a smooth curving arc from the runner or basin to the cavity in the same direction as the rotation of the machine. Abrupt corners in the gating design should be avoided if possible, since they generate turbulence that results in castings of inferior quality. Before starting to cut, go back and check to make sure the entire mold is correctly laid out.

1. Cutting the runners: If a preform has not been used, begin by cutting the runner system, wheel first, then spokes. To cut the wheel, insert the shipper’s knife with the blade toward you in one of the two scribed lines. The knife should be slanted outward from the runner channel’s center at a 45° angle. The depth of cut should be about ¼”. While holding the knife perfectly rigid and stationary with your right hand, use your left hand to rotate the mold in a counterclockwise direction against the knife’s edge. Continue to rotate the mold until you arrive at your starting point. Withdraw your knife, and insert it in the second scribed circle, again inclined at a 45° angle from the wheel channel’s center. The two opposite (or “complementary”) 45° angles will form a channel with a ‘v’ shaped cross section of 90°. Again, cut by rotating the mold in a counterclockwise direction while holding the shipper’s knife perfectly rigid. Both incisions must be made without any hesitation and the mold must be turned steadily at a constant rate of speed to ensure that the walls of the wheel channels are as smooth as possible.

When the incisions have been completed, use the tip of the scalpel to pry free a section of the rubber until you can grasp it with your fingers. Then pull the rubber out of the wheel channel with a smooth, moderately quick motion. (Fig. 9.9) It should lift out easily and cleanly and leave a ‘v’ shaped channel. Note that the ‘v’ channel may sometimes have a flat bottom, because the 14” depth of the two 45° cuts may not be deep enough for them to meet at the 90° angle in the channel’s bottom. (Fig. 9.10) The rubber in the channel bottom should tear smoothly. If it does not, make the incisions deeper so that they meet.

1. Sprue runners or spokes: Position each spoke to be cut directly opposite yourself across the basin and cut by pulling the knife from the runner toward yourself and the basin. Rotating the mold with your left hand, cut the right hand side of each spoke channel with the shipper’s knife. Proceed around the mold until all the spokes have been cut. Continuing to turn the mold, proceed around a second time cutting the left hand side of each spoke channel.

Fig. 9.8: Scribing the wheel for a runner system.

Fig. 9.9: Removing the wheel channel after cutting.

Cut all the way to the basin each time with a single, smooth slice. Each incision should be approximately 4” deep and at a 45° angle outward from the channel’s center to produce ‘v’ shaped channels similar in cross section to the wheel channel. After the spokes have all been incised, lift each out by using the tip of the scalpel to pry the rubber until you can grasp it with your fingers. If the mold contains two or more concentric ranks of cavities, additional wheels should be cut between each rank. Outer runner wheels are connected to inner runner wheels with more spokes. Spokes leading to an outer wheel should be placed so that they alternate with the spokes leading from the basin into the wheel. Spokes leading out of a wheel should never be positioned at the same points as the spokes leading into it. This staggering ensures even distribution of metal and controls the velocity of the molten metal as it is distributed throughout the gating system. Cut the outer spokes to the same size and cross section as the inner spokes.

Fig. 9.10: Cross sections of the runner channel. (a) Dimensions; (b) After cutting; © After removal.

1. Gates: (Fig. 9.11) Use the shipper’s knife to cut the gates. The gates should have the same 90° ‘v’ cross section as the runners. When cutting the gates it is more efficient to make both incisions for each gate at the same time. If runners are used, cut the gates to the outer rim of the main wheel channel. If runners are not used, cut the gates directly to the mold basin. Begin the gate channels with the appendix, and cut them to pass approximately ¼” from where they will enter the mold cavity. Cut toward the runner or basin. The ¼” between the gate and the mold cavity will accommodate the ‘ingate’ which is the actual opening into the mold cavity.

Fig. 9.11: Cutting the gate channel.

Fig. 9.12: Cutting the ingate.

1. Ingates: (Fig. 9.12-9.16) The ingates must be cut as smoothly and cleanly as possible because the way that the metal flows through them will have a decisive effect on the quality of the finished castings. The precise size and configuration of an ingate for any particular cavity will depend on the type of casting, the cavity’s size. and the pasty range of the alloy used.

The following dimensions are approximations for a typical squash mold producing medium size flat castings. (Fig. 9.13) The slot shaped opening in the cavity wall should be approximately ⅛” wide and 1/32” deep. The ingate channel should be approximately ¼” long from cavity to gate. Its width should taper from ⅛” wide where it enters the cavity to between ¼” and ½” wide at the point where it joins the gate. The ingate channel may be curved slightly, or straight.

Fig. 9.13: Ingate - Dimensions

Use the scalpel to cut the ingate. (Fig. 9.12) Make sure the blade is razor sharp. If it is not, change it. because even slight dullness will make it difficult to cut clean and smooth ingates. Position the cavity to be cut directly in front of you so that you are looking into the cavity straight at the point in its wall where the ingate will enter. Hold the scalpel with the blade horizontal, flat side parallel to the mold parting plane. Place the tip of the scalpel against the cavity wall at the location of the ingate, 1/32” below the mold parting plane. Holding the tip of the blade firmly against the rubber, but not so firmly that it breaks the surface, angle the scalpel so that the blade points downward into the rubber at a 45° angle. Maintaining the 45° angle of attack, make an incision on the width of the ingate by pushing the point of the scalpel directly down into the rubber and toward the gate. (Fig. 9.14, 9.15) This incision should be at least 3/16” deep. Turning the mold with your left hand, make the same incision in each of the cavities in the mold.

Fig. 9.14: Ingate-Angle of attack.

When all of the cavities have had this first incision made, prepare to make the next two incisions by positioning the first cavity to be cut so that the gate toward which you will be cutting is between you and the cavity. These two final cuts will be made from the cavity toward yourself and to the gate. They will form the walls of the ingate channel. Begin the second incision at the edge of the cavity. Slice downward into the rubber to the first incision, a depth of approximately 3/16” at a 45° angle inclined away from the center of the ingate channel. With the same motion cut backward to the gate to form one wall of a channel with a ‘v’ shaped cross section. Make the third incision at an opposite 45° angle to the same depth to form the other wall of the channel. The incisions should be made so that the width of the channel tapers from between ¼” to ½” wide at the gate channel to ⅛” wide at the edge of the cavity.

Fig. 9.15: Ingate - View from above for making the incision.

If the three incisions have been made correctly, the result will be a wedge of rubber that can be lifted cleanly from the ingate channel. From the gate channel, insert the tip of the scalpel beneath this wedge and lift it until it can be grasped with the fingers and pulled out of the mold. (Fig. 9.16) If it will not lift out, make the incisions a little deeper. If each of the three incisions was made smoothly and quickly. without hesitation, the resulting ingate will have smooth clean walls that taper to a narrow slot that opens into the cavity.

Rotating the mold with the left hand, proceed to the rest of the cavities.

Fig. 9.16: Peeling the rubber out of the ingate channel after cutting.

1. Topgates: If the mold requires topgating, the top gates can be cut using the same techniques that were used to cut the bottom gates.

The technique for locating the top gates so that they match and align precisely with the gates already cut in the bottom mole half is simple. After cutting the bottom gates, talc the bottom half of the mold set thoroughly. Gently miter the mold halves together, being careful not to disturb the talc, then slap the mitered mold once against the bench. When the mold is opened, the bot- tom gates will be outlined in talc on the top mold half. Mark the outline with a ball pen and then cut the top gates.

Another method of locating top gates is to run the mold set one or two passes in the CRMC machine. The heat of the molten metal will make an outline of the bottom gates in the rubber of the top mold half. If steps 1-6 have been followed carefully, your mold now has a complete gating system. But it is not yet ready to cast. It still needs vents. Detailed instructions for venting your mold are given in Chapter 10.

Fig. 9.17: Chart of gating and venting suggestions for various casting shapes.

Fig. 9.18: ‘Derringer’ mold. (1) Mold design was ‘squash’; (2) Mold rubber was a silicone compound; (3) Gating is from a double ‘spoke’ to a partial ‘wheel’ to a side gate; (4) Venting is both drilled and channeled on both sides of each mold half.

Fig. 9.19: ‘Filigree’ mold. (1) Mold design is ‘squash’; (2) Gating is top and bottom; (3) 3 gates are used directly from basin; (4) Note gate to jumpring; (5) Vents are standard and also drilled and channeled on mold backside.

Fig. 9.20: ‘Owl eyes’ mold. (1) Note quantity of ‘direct’ gates. Gating is top and bottom: (2) Venting is on top mold half only; (3) Note use of two types of locator nuts; (4) Mold design is ‘squash’.

Fig. 9.21: ‘Sails’ mold. (1) Mold design is ‘squash’; (2) Gating is direct and only on the bottom mold half. 4 gates per cavity; (3) Venting is both standard and drilled and channeled on mold back. Note how vents have been cut to small nipples on the sail mast.

Fig. 9.22: ‘Open trapezoid’ mold. (1) Mold design is ‘squash’; (2) Cavities are back gated, top and bottom. Note that some of the cavities required two gates. Gating uses a ‘wheel’ and ‘spokes’; (3) Venting is drilled and channeled on mold backside; (4) Flashing due to improper air pressure setting in CRMCM.

9.9: Cutting Review

9.10: Gating and venting guide

The many variables involved in the CRMC process such as alloy used, rubber type and supplier, mold type, machine speed, and direction of rotation all affect gating and venting design. Study the gating and venting designs of all the molds used to illustrate this book carefully to familiarize yourself with the many design possibilities. However, there can be no guarantee that the gating and venting suggested will always work. Trial and error based on experience is ultimately the only guide. But remember, the book and the Oster Group are available to help you when you have any casting problem.

PREVIOUS CHAPTER:

Chapter 9: Gating (Part 1)

NEXT CHAPTER:

Chapter 10: Vents