Principles of Centrifugal Rubber Mold Casting : Chapter 9 (Part 1)

What gates do and how they work in pewter moldmaking
The parts of the gating system
Gates and the solidification shrinkage feeder head
Gate Size for Pewter Molds
Types of Gates

9.1: What gates do and how they work in pewter moldmaking

The gating system of a mold is the series of channels through which the molten metal flows from the basin to the mold cavity. The gating system is either cut into the rubber after the mold has been cured or it is cured into the rubber with a preform during vulcanization.

Most moldmakers cut the gating system into the bottom half of the mold set, but there is no reason why a mold cannot be designed with its entire gating system in the top half. In certain kinds of molds, particularly modulated and sectional types, it may be advantageous to cut all of the gates in the top mold half. Top gating may also provide the best way of gating to the most inconspicuous part of a casting in order to minimize cleanup.

How gates work: The caster pours molten metal into the CRMC machine funnel. The metal drops down through the funnel into the basin of the spinning mold. Centrifugal force then pulls the metal from the basin into the channels of the gating system and through the gating system into the mold cavities.

Functions that gates perform: An adequate gating system is one that performs five functions in the casting process:

Delivers enough molten metal to each cavity to fill the cavity and produce a complete casting.
Fills the mold cavities quickly enough that the alloy does not have to be poured at an excessively high temperature to avoid premature freezing.
Channels the molten metal to the cavities in such a way that it reaches the cavities free of dirt and dross.
Allows the metal to flow smoothly and keeps turbulence to a minimum.
Provides a molten metal ‘solidification shrinkage feeder head’.

To design a gating system that performs these functions adequately for a particular mold, the moldmaker must consider carefully both the characteristics and peculiarities of the model to be cast, as well as the usual variables of the CRMC process: for example type and durometer of rubber; alloy used and temperature at which it is poured; speed, direction of rotation, and pressure of the CRMC machine. For the experienced moldmaker, designing the gating system for almost any mold is second nature. As for the novice, it is best first to understand how the gating system works, and then study and imitate drawings and photographs of gating systems that have been successful. Begin with the simplest and then work up to the most complex. After he has gained experience, intuition, and foresight, he can begin to experiment with original gating designs of his own.

9.2: The parts of the gating system

Funnel: The funnel guides the molten metal into the mold basin. How the molten metal is poured into the funnel affects significantly the way that the metal flows through the gates, and, ultimately, the quality of the finished castings. For a discussion of the different pouring techniques and the effect that each produces, see Chapter 2.3.

9-1 Fig. 9.1: Cross section of a mold. (a) Side view of a complete gating system cut into a mold; (b) Cross section of a cavity with ingate opening head on; © Looking from the basin across the runner system into the cavity.

Basin: The basin is the portion of the mold’s center into which molten metal is poured and from which it is distributed to the cavities. The basin may be formed during the curing process with a preform plug. Preforms to create concave, convex, and flat basins are available. The basin area of the mold may also be left flat, a configuration known as ‘blank’, or ‘unformed’. While individual moldmakers may express a preference for one configuration as opposed to the others, the concave basin is the design that is most frequently used.

Runners: The ‘runners’ consist of the system of channels that carry the molten metal from the basin to the gates. Not all molds are designed with runners. In some the cavities are gated directly from the basin. A system of runners consists of two kinds of channels: the ‘wheel’ and the ‘spokes’.

Spokes: The ‘spokes’ are the straight channels that radiate outward from the basin and carry molten metal from the basin to the wheel.

Wheel: The ‘wheel is a circular channel that runs concentrically with the circumference of the mold. It is fed with molten metal from the spokes and, in turn, feeds the gates to the cavities or another set of spokes.

The primary function of the runners is to form a rigid cast structure that links all of the castings in a mold together. This structure is called the ‘casting unit’. When the metal in the mold has solidified, all of the castings in it may be removed in a single motion by removing the casting unit. Compared with removing each casting individually, removing many castings as a single unit saves time during production and makes the castings much easier to handle. Casting units may be stacked so that the finished castings can be “broken off at any convenient later time. The gates and runners are then returned to the melting pot. Runners also perform a number of important secondary functions in the casting process. They help to streamline the flow of the metal through the gating system and, by reducing turbulence, they help to reduce dirt, dross, and gas entrapment in the castings. Thus, the runners help to clean the metal before it reaches the cavities. Runners also help to even out the distribution of molten metal in the gating system. For this reason, it is particularly important to include runners in the gating design for a mold that has many cavities arranged in two or more concentric ranks, since the more cavities a mold has, the more important it is to ensure even distribution of metal. (Fig. 9.2)

Gates: The gates are the channels that supply molten metal to the mold cavities. Their size and configuration control both the flow of the metal and the quality of the castings. How gates work is discussed in detail in sections 9.3 through 9.7, below.

In-gate: The ‘in-gate’ is the portion of the gating system through which the metal flows from the gates into the mold cavity. It is a narrow, tapered channel that leads to a thin, slot-shaped opening in the cavity wall. The metal forces its way through this opening like toothpaste squeezed from a tube. It is at the thin neck of metal that solidifies in this slot that the finished casting is bro- ken off the ‘casting unit’.

9-2 Fig. 9.2: ‘Pellet mold with 3 concentric ranks of cavities, each rank with its own runner system. (1) Mold design is squash. Size was 12”: (2) Mold was used to cast pure bismuth and required no venting: (3) Note concentric layout of pellet cavities and ‘wheels’ and ‘spokes used to distribute metal to cavities. Outer wheel has 4 ‘spokes”. Middle wheel has 8 ‘spokes’. Inner wheel has 16 ‘spokes’.

The ingate must be cut to enter the cavity at as inconspicuous a part of the casting as possible. It is always better to gate to a flat rather than a rounded surface because the breakoff will be cleaner and easier. A carefully cut and correctly tapered ingate will leave an insignificant mark on the finished casting that will require minimal cleanup after breakoff.

The ingate must be cut as smoothly and cleanly as possible, since the way the metal flows through it into the cavity has a decisive effect on the quality of the finished castings. A properly designed and smoothly cut ingate functions as a valve to smooth out and regulate the flow of the molten metal and reduce turbulence. It also helps to clean or ‘filter’ the metal by preventing dirt and dross from passing out of the gate channel into the cavity. The size and configuration of the ingate depends on the size of the cavity, the type of casting, and the alloy to be used.

Appendix: Many moldmakers cut a tapered extension of the gate channel past the cavity at right angles to the ingate. This ‘appendix’ to the gating system seems to act as a trap that collects dirt and dross and prevents them from reaching the mold cavity.

Vents: Although most moldmakers speak of a mold’s venting system as if it were a part of the gating system, the vents are not, strictly speaking, gates because they do not carry molten metal. The function of the venting system is to allow gasses and air to escape from the mold cavities as they are displaced by entering molten metal. The venting system is discussed completely in Chapter 10.

9.3: Gates and the solidification shrinkage feeder head

Besides delivering molten metal to the mold cavity, the gate performs a second important function: it maintains a ‘solidification shrinkage feeder head’ while the casting solidifies. The ‘solidification shrinkage feeder head’ is the quantity of molten metal that is poured into the mold over and above the amount needed just to fill the cavities, and that remains molten in the gates until after the metal in the cavities has solidified.

If a caster could see inside the cavities of a spinning mold, so that he would pour just enough metal into the mold to fill each cavity and no more, he would be surprised to find that when the mold had cooled and the metal solidified, the cavities would no longer be completely full. This would happen because a given amount of metal fills a greater volume; that is, it takes up more space when it is hot and molten than it does when it is cool and solid. This is why, when molten metal cools and solidifies in a mold cavity, it shrinks and pulls away from the cavity walls.

The ‘solidification shrinkage feeder head’ compensates for this natural shrinkage. When the caster casts the mold, he pours considerably more metal into the basin than is necessary just to fill the cavities. This extra metal remains molten in the gates and basin as the mold cools. As the metal in the cavities solidifies and starts to shrink, additional molten metal from the gates feeds into the cavities to fill the space created by natural shrinkage. Thus, the ‘solidification shrinkage feeder head’ compensates for natural shrinkage and ensures that the finished casting will be the same size as the cavity and reproduce the cavity’s details faithfully.

When the ‘solidification shrinkage feeder head’ does not feed the cavities continuously during solidification, the result is that castings have porosity. blowholes, and coarse crystal structure. The part of the casting nearest the in- gate which is the last part of the casting to solidify-will exhibit the most shrinkage and the most serious defects. There are two reasons why a ‘solidification shrinkage feeder head’ fails to function as it should. First, the caster may simply not pour enough metal into the mold. Second, the metal in the gates may solidify before the metal in the cavities has solidified. Metal solidified in the gates or the ingate forms a plug that seals the cavity off from the additional molten metal that is needed to compensate for shrinkage during solidification.

Preventing metal from solidifying prematurely in the gates and blocking the cavity off from the ‘solidification shrinkage feeder head’ depends on controlling two variables in the casting process so that each is adjusted to the other and to the size of the cavity, the type of casting, and the length of the pasty range of the alloy being used. These two variables are: gate size; and, temperature at which the alloy enters the mold. An alloy with a short pasty range may solidify prematurely in the gates if the gates are too small or if the caster uses a pouring technique that causes the alloy to dissipate too much heat in the funnel. Furthermore, the alloy must be poured at the correct pouring temperature: a minimum of 50°F above its liquidus. Even an alloy that has a long pasty range can solidify in the gates if it is poured at too cool a temperature. Thus, to ensure that the ‘solidification shrinkage feeder head’ is maintained during solidification, the moldmaker must be careful to design the gates so that they are large enough for the alloy that is to be used. The caster must be careful to run the alloy at the correct temperature, to pour enough metal into the mold, and to choose a pouring technique that is appropriate to the length of the alloy’s pasty range.

It is because the ‘solidification shrinkage feeder head’ must feed the entire cavity continuously during solidification if all shrinkage is to be compensated for. The ingate should always be cut to enter the cavity at the heaviest or thickest part of the casting. It is the heaviest part of a casting that shrinks most (simply because there is more metal there to shrink) and that freezes last. Because the smallest or lightest part of a casting will always freeze first, if the ingate enters the cavity at the lightest part-say, a jump ring-the heavy part of the casting will be effectively sealed off from the ‘solidification shrinkage feeder head’ by the early-freezing lighter part as soon as the casting begins to solidify. For this reason, it is always best to feed the cavity at the thickest, heaviest part of the casting.

9.4: Gate Size for Pewter Molds

The size of the gate channels varies from mold to mold. In general, larger gates produce castings with finer crystal structure, less shrinkage, and sharper, clearer details. This is because the larger gate holds more metal and provides a better ‘solidification shrinkage feeder head’. The trade-off is that the larger gate leaves a larger, more visible mark on the casting after break-off. Unless the gate can be positioned at an inconspicuous part of the casting or concealed with patterning, a large gate will necessitate more cleanup than a small gate.

The single most important factor to consider when planning the size of the gates for a particular mold is the pasty range of the alloy that the mold will be cast with. (See Chapter 4.5) Because an alloy with a short pasty range freezes more rapidly than an alloy with a long pasty range, the alloy with the short pasty range requires relatively larger gates if the ‘solidification shrinkage feeder head’ is to be maintained during solidification.

In designing gates for a production mold, then, the moldmaker must weigh the benefits to be gained from cutting gates larger than the minimum size necessary for the alloy to be used against the costs of the clean-up they will require. In general, for a production mold, the smallest gates that will produce castings of acceptable quality are the best gates for that mold. The moldmaker should always begin by cutting gates that are on the small side because, if the mold will not produce castings of acceptable quality after several trial passes, the gates can always be enlarged. However, gates that have been cut larger than is necessary cannot be reduced in size to leave a smaller mark on the castings.

In contrast with production molds, the gating in a model mold may be as large as the moldmaker judges will produce the best castings. Since the purpose of a model mold is to produce a limited number of high quality pieces, the amount of time and expense required for casting clean-up is not so important as it is for production molds.

Number of gates: The number of gates to any cavity should be as few as possible to keep clean-up to a minimum. Deficiencies in gating are better corrected by increasing the venting, by top gating, or by increasing the size of the gate (in that order) rather than by increasing the number of individual gates to a single problem cavity. The danger of feeding a single cavity with more than one gate is that the conflicting streams of metal entering the cavity may produce a ‘cold shut’ in the casting. A ‘cold shut’ is a structural defect produced by incomplete fusion of two streams of metal that are moving at different velocities and that have different temperatures as they impinge. A ‘cold shut’ shows up on the surface of the finished casting as a stream-like pattern of discoloration, porosity, and blow holes where the streams of metal from the gates met.

The “panic mold,” Fig. 9.3, is a classic example of too many gates. The moldmaker discovered that the mold cavities would not fill with only one gate to a cavity. So, he added another, and then another, and then still another, each time with the same results. By the time he had cut six gates, pure panic had set in. After mindlessly cutting seven gates-without ever thinking of cutting vents-the castings finally filled. The castings were badly flawed, with streaks of micro-porosity on the casting surface and several cold shuts in each. Additional operations to clean the marks left by the gates were also necessary. The method of gating and venting the mold that he should have used is shown by dotted lines on the illustration. The runner system should have been cut into the mold rather than cured in as a pre-form. This would have allowed the moldmaker to design the mold with an additional cavity.

9.5: Types of gates

There are several basic gate configurations. Each has its advantages, and each has its drawbacks. When designing a gating system for a particular mold, the moldmaker should not rely on habit, or worse, guesswork, but should choose the gate configuration that will meet the requirements of that mold best. Fig. 9.4a,b illustrate the various types as interpreted by two different moldmakers.

9-3 Fig. 9.3: ‘Panic’ mold.

Direct gate: A direct gate is a gate that runs in a straight line from the basin or runner directly to the front of the cavity. The single advantage of the direct gate design is that it takes up very little space in the mold. When production efficiency is the most important consideration, the direct gate is the best gate type because it allows the moldmaker to fit a larger number of cavities into a single mold than any other type of gate. The major disadvantage of gating directly from the mold basin to the cavity without cutting an intervening runner system is that this design seems to allow more dirt and dross to enter the mold cavity than other types of gating do. However, many moldmakers use direct gating and, with small castings, direct gates usually produce finished pieces of acceptable quality. When runners are not used, direct gates should, if possible, be led into the cavity at a tangent in the direction of mold rotation.

Back gate: A back gate is a gate that is channeled from the basin or runner past either side of the cavity and then led around to enter the cavity from the direction of the mold’s outside perimeter at the cavity’s back, the part of the cavity farthest from the basin. Of all gate configurations, the back gate produces the cleanest, highest quality castings, castings with the least amount of dirt, dross, and other inclusions. However, back gates take up more space in the mold set than other gate types and thus reduce the production efficiency of the mold by limiting the number of cavities the mold can contain.

Side gate: Any gate that enters a mold cavity at the side is a side gate. Side gates ates can have any number of configurations: curved; right angle; tangential. Like back gates, side gates produce cleaner castings than direct gates. But side gates also take up space in the mold and limit the number of cavities the mold can contain.

Runner gate: A runner gate is any gate direct, back, or side that is channeled from a wheel and spoke runner system. Direct runner gates produce cleaner castings than direct gates without runners.

9-4ab Figs. 9.4a,b: Typical squash molds illustrating 2 moldmakers’ interpretations of standard gate types. (9.4a) Gating is direct from the basin, and also through a partial ‘wheel’. Gating is on the bottom mold half only. Venting is a standard design. Both ‘pie’ and ‘ingate’ have been used. 1. Direct gate. II. Direct gate through a partial ‘wheel’. III. Direct side gate. IV. Direct tangential gate. V. Direct side gate with a heavy appendix made by drilling out the rubber using a burr’. VI. Side gate with appendix’. VII. Direct tangential gate through a partial ‘wheel’. VIII. Same as VII. IX. Direct back gate.

9-4b (9.4b) Gating uses a ‘wheel’ and ‘spoke’ runner system. Gating is top and bottom. Venting is a standard design. Both pie’ and ‘Ingate have been used. 1. Direct gate. II. Side gate with an appendix. II. Side gal IV. Tangential gate. V. Side gate with an appendix Vi Back gate slight appendix: VII. Side gate with heavy appendix’ made by drilling out the rubber using a “burr.”

Top gate: Gate channels are usually cut in the bottom half of a mold set. The top gate is a gate that is cut into the top half of a mold and that corresponds- usually to a standard gate cut in the bottom half. (Fig. 9.5) A top gate is often cut to enlarge a bottom gate in order to get a cavity to fill when it will not fill with the bottom gate alone. The larger gate holds more metal and, so, it does not freeze and seal the cavity shut as quickly. This gives the cavity more time to fill. Top gating is also used in molds that are designed in such a way that gating in the bottom half is not possible.

9-5 Fig. 9.5: Cross section of a cavity gated top and bottom

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