Principles of Centrifugal Rubber Mold Casting : Chapter 11

1. The parting line problem, causes and cures
2. Methods of controlling parting lines
3. How to make a flat-back or ‘dummy’ mold for pewter casting
4. How to make a base mold to correct parting lines
5. Chapter 12: Insert Molds

11.1: The parting line problem, causes and cures

The ‘parting plane’ is the plane along which the top and bottom halves of a mold set come together. After being cast, a line will be visible on the casting that is caused by imperfect alignment of the two mold halves along this “plane of demarcation.” On inexpensive castings, these parting lines may not be objectionable, but they should be controlled on more expensive castings or whenever they may detract from the appearance of the finished pieces.

There are two ways of dealing with parting lines: “correction” and “control.” (Fig. 11.1) “Controlling” the parting line refers to dealing with the problem before the mold is made by designing the mold or model so that the parting line is disguised or is positioned in an inconspicuous part of the casting. “Correction,” on the other hand, involves dealing with the problem after the mold has been made by modifying the finished mold itself.

Sometimes parting lines can be controlled by positioning the models so that the parting line falls in an inconspicuous part of each casting. It may not always be possible to do this, however, because of such factors as the modulation of the models, the manner in which they must lie in the mold for gating, or their basic design. When this is the case, it may be necessary for the moldmaker to correct the parting lines by modifying the production molds or the finished castings.

Fig. 11.1: Parting lines.

11.2: Methods of controlling parting lines

Design: The simplest method of concealing a parting line by “control” is to disguise it by adding texture or design to the edges of the molds at the parting plane. The best design is a series of thin lines all running parallel to the parting plane of the mold. Although this technique does not completely eliminate a parting line, the parallel lines make it more difficult to detect.

Because the gates enter the mold cavity at the parting plane, the parallel line pattern used to conceal the parting line may be disrupted by bare, rough spots where the gates are broken off. A texturing tool (Fig. 11.2) is used to reengrave the flawed design. The tool is placed in a drill press, and the flawed part of the casting is then held against the side of the shaft of the rotating tool.

Gating: A gate may be made less noticeable by leading it into the cavity at hidden or unimportant parts of the casting. Using tapered ingates and pie-wedge vents also helps minimize marks left on the casting by gating and venting. However, when texture or design cannot be used, it may be necessary to remove the flaws caused by the gating with a rubber wheel mounted in a drill press or with a polishing head. Rubber wheeling the problem area will remove surface flaws quickly. Polishing may also be used. However, these are procedures that require additional labor which, in turn, will add to the cost of the casting.

Cut-outs: The position of the parting lines on a casting may also be controlled by removing mold rubber from the cavity locations before placing the models in the uncured mold. These cut-outs allow models to sink deeper into the rubber of the modified half of the mold set during the compression stage of curing and, thus. effectively reposition the parting line. Unless the models are unusually large or highly modulated, it is important that rubber removed from one mold half be repositioned on the opposite mold half when this technique is used. (See Chapter 13. “Modulated molds”.)

Sectional molds: Parting lines may be controlled by redesigning a mold as a sectional type. However, using this strategy can also complicate the moldmaker’s job, because the sectional design may multiply the number of parting planes. (See Chapter 14, “Sectional molds”.)

Fig. 11.2: Texturing tool.

Self-control: Models with tapered edges actually find their own optimum position in the mold relative to the parting plane without any corrective work on the part of the moldmaker. However, models that have square or rounded edges usually require some kind of control or correction to reposition the parting line.

Flat-back or ‘dummy’ mold: The standard procedure for controlling the parting line on castings that have flat backs such as coins and trophy plaques is to use a ‘flat-back’ mold. A “flat-back” mold is made with a prevulcanized mold half, or “dummy,” that has been cured blank without any impression.

11.3: How to make a flat-back or ‘dummy’ mold for pewter casting

The flat back of a ‘flat-back’ mold is simply a pre-cured mold half laid out with the usual locators and basin former but without any models. It is cured against a smooth metal plate to give it a perfectly flat surface along the parting plane. One half of this cured mold set is then used as either the top or bottom half of another mold. The models and an uncured mold half are cured against it to make a production mold. (Figs. 11.3, 11.4) During curing, the models are compressed entirely into the uncured mold half. The result is a mold in which the parting plane intersects the cavities at the bases or backs of the castings and, so, leaves no parting line.

Fig. 11.3: The flat back (dummy) mold, cured with an uncured mold half (or sections). Because the pre-cured rubber of the dummy mold half will not take any impression during the second curing, it will force the models entirely into the uncured half, so that the parting plane will be at the top of the cavity.

Procedure for making a ‘flat-back’ mold:

1. Lay out an uncured mold set with registration nuts and basin former.

Do not place any models in it. The mold set may be cured with or without the die-cut sprue opening removed and with or without a basin former, depending on whether the dummy will become the top (half with sprue opening) or the bottom (half with basin) of the finished production mold.

Because rubber reproduces all the surface irregularities of whatever material it is cured against, using a metal ‘curing plate’ will ensure a smooth, even surface along the mold’s parting plane. The curing plate can be made from light gauge metal cut in a circle the same diameter as the mold. If the curing plate is made of metal that has texture or a design rolled into it, the dummy that is cured with it will produce castings that have textured backs. If used with a basin former, a hole should be cut in the center of the curing plate to accommodate it. Registration nuts are soldered to both sides of the plate. These will cure concave registration cavities into both dummies which will, in turn, produce rubber registration nuts when they are cured the second time against the uncured mold half that contains the models.

1. Place the dummy mold halves with the curing plate between them in a frame set and cure as usual.

Fig. 11.4: The dummy mold half.

1. After curing, either half of the dummy can be used to make the flat-back mold. The dummy half, the models, and a basin former are positioned in the mold frame with an uncured mold half, compressed, and cured for the length of a cycle. New registration nuts will not be required since those in the dummy will cure either as a male or female in the uncured mold half (AC/DC). The cured mold set that results will have a controlled parting line, since only one half of the mold set, the uncured half, will have taken the impressions of the models.

Note: A mold set with registration nuts made of rubber is a sure sign that a “dummy” mold was used (not that a “dummy” made it).

1. Gate and vent the new mold as required. There is also a shortcut method of making a flat-back mold. It substitutes the paper separators that come in cartons of uncured molds for the metal curing plate. One half of a mold set is laid out with models. registration nuts, and preforms. The paper interleaf is cut out to accommodate nuts, basin, and preforms, but not the models. It is then placed between the top and bottom halves of the mold set, and the entire mold put through a standard curing cycle. The paper prevents the models from making an impression in the one half of the mold by, in effect, functioning in the same way that a curing plate or a pre-cured dummy does. Many moldmakers find that this paper process works well with small castings. With larger castings that should have surfaces that are perfectly smooth and flat, it may not work quite so well.

11.4: How to make a base mold to correct parting lines

The ‘base mold’ corrects rather than controls the parting line. The moldmaker begins with a standard cured mold set in which the parting lines have been controlled as much as possible. One half of the set is selected for correction. This should be the half in which the parting plane intersects the cavity at a part of the casting closer to the part where the moldmaker wants it to be relocated. A new mold set is made with this half. It is important to note that a base mold cannot be made with just any production mold. Rather, the moldmaker must begin with a new mold set prepared specifically for the purpose of producing a base mold to control parting lines.

Procedure for a base mold: See Fig. 11.5 for a graphic illustration of the complete procedure for making a base mold.

1. Lay out a mold with the models positioned so that the parting lines are controlled as much as possible.

2. Cure the mold with the models as usual, but do not use a sprue former.

3. After curing, do not cut the mold. Analyze it to see how the parting lines are positioned and which half of the mold set is the best to use for parting line correction. The best half to use is the half in which the actual parting plane lies closer to a part of the cavity that would be a good place for the parting plane to be repositioned to. This mold half will be used to make the base mold. Experienced moldmakers can position models in a mold set with enough care and foresight that they are able to use both halves of the set as a base. If only one half of the set can be used, discard the other half. (Or, simply lose it! Or give it to a novice moldmaker to practice cutting with, which amounts to the same thing.)

4. Carefully remove the models from the mold half selected to be the base and set them aside to be returned to their cavities after the parting lines have been corrected. (Fig. 11.6) Because there are always slight differences in dimensions among models, it is essential that each model be returned to the same cavity that it was cured in. If they are not, rubber flash and voids will result when the base mold is recured.

Because it is next to impossible to mark a model with its location in a mold, most moldmakers use the following procedure to insure that they return each model to the cavity that it came out of.

a) Before removing the models. make a distinctive locating mark on the perimeter of the mold half that has been chosen to be the “boss” in the base mold. (For an example, see Fig. 11.10. The moldmaker has used an arrow to mark the tie-tac mold.)

b) Place the uncured mold half that will be used later in Step 6 beside the base, and make a locating mark on it that corresponds to the locating mark on the base.

Fig. 11.5: Procedure for making the base mold. Step 1. An uncured mold set “A” and “B” are cured with models only; 2. The mold half “B” with the best parting line control is then selected to be used as a base mold; 3. The models are removed and identified to be replaced later in their original cavities; 4. The parting plane around each cavity is then corrected to the desired parting line. (Mold “B”); 5. The models are returned to their cavities in mold “B”; 6. Mold “B” and an uncured mold half “C” are then cured together; 7. Mold “B” is peeled away from mold “C.” and stored; 8. Mold “C” with models is then cured with mold “D”; 9. Mold set “C” and “D” are then gated and vented and become a production mold that has a corrected parting line. Models are stored with mold “B” for future use.

Fig. 11.6: Models removed from a mold half se- lected as the base must be identified for replace- ment in the same cavities that they were removed from.

Fig. 11.7: Correcting the parting line with a base mold.

c) Beginning at the locating mark, remove the models one by one from the base and place them in sequence on the uncured mold half.

d) After the cavities in the base have been corrected, the models can be taken from the uncured mold half one by one in reverse sequence and replaced in their original cavities.

Fig. 11.8: Correcting the parting line by cutting away rubber from the parting plane around the cavity.

1. Correct the parting plane surrounding each cavity by trimming away rubber around the edge of the cavity down to the place in the cavity at which you want the new “corrected” parting line to be located. Correction may be made with the scalpel, a razor blade, or by abrasion using a rubber wheel mounted on the flexible shaft drill. Cuts should be as few and as smooth as possible. (Figs. 11.7. 11.8. 11.9) This mold half with corrected parting lines is now the ‘base’ of the base mold.

2. Re-heat the base on the vulcanizer platen. This is necessary because the rubber will have cooled and shrunk slightly, which will make it difficult to return the models to their cavities. After re-heating, replace each model in the same cavity in the base that it came out of. (Step 4d.)

Fig. 11.9: The corrected production mold made from a base mold with production casting showing position of corrected parting line.

1. Place an uncured mold half on the base, and cure this assembled mold. The rubber of the uncured mold half will fill the areas around each cavity where the rubber has been trimmed away to reposition the parting plane. It is not necessary to use additional registration nuts in this and following steps because the female cavities and the male nuts that were previously cured in the molds will now switch their sex, curing in the new molds in the opposite gender. (Unlike most sex change operations, all the parts involved in this one remain functional.)

2. After curing, carefully place the mold set on the bench, base up. This is important: slowly peel the base away from the mold half that has just been cured against it. Always work with the base when performing this step, never with the newly cured mold half. The reason for this is that the models will have cured into the new mold half in Step 8, and must remain firmly seated in this half for the next step. If they should inadvertently come out, they are almost impossible to reinsert in their cavities. Thus, care must be taken that the models are not disturbed.

3. Set the base (the mold half just peeled away) aside. It should be labeled and stored with the models for future reusability.

The ‘base’ of a base mold is never actually cast so long as it is being used as a ‘base.” Its only function is to serve as a pattern. As a pattern, it can be re-cured many times without being harmed. However, a new base will eventually have to be made, because, after repeated curings, the rubber will begin to harden and will become difficult to “peel,” as is required in Step 8. The old base can be put in production after a new base is made.

1. Place the mold half (with the models still in their cavities), that was cured against the base in Step 7 on a hot vulcanizer platen to reheat for a few moments. This step is necessary because cured rubber has a different coefficient of expansion from uncured rubber. If the cured mold half is cold when Step 11 is performed, the result could be dimensional changes in the cavities, voids and rubber flash. 11. After the cured mold half is heated thoroughly, place an uncured mold half on it and then cure this assembled mold. This new mold set is a production mold that has prepositioned or “corrected” parting lines. It can now be gated. vented, and placed in production. (Fig. 11.10, 11.11a.b.)

Fig. 11.10: Closeup of cavities in a base mold after parting planes have been corrected. Note how smooth the corrective cuts around each cavity are. Arrow cut in mold perimeter between the two registration nuts is moldmaker’s model locator mark (step 4-a) made to insure that each model is returned to the same cavity from which it was removed. This mold is the original tie-tac mold before it was made into a multi-layer sectional.

Fig. 11.11a: ‘Road-runner mold’. Top mold half.

Fig. 11.11b: ‘Road-runner mold’. Bottom mold half.

Figs. 11.11a,b: Road-runner mold. 1. Production mold was made using the “base-mold” technique. Note the rubber male nuts; 2. Gating is top and bottom; 3. Venting is standard and drilled. Note that vents are near gate entrances; 4. Both mold halves are shown to illustrate the top and bottom gating

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