Principles of Centrifugal Rubber Mold Casting : Chapter 8
Chapter 8: Placement
8.1: Positioning models in the mold
The positions in which models are placed in a mold set depend on many factors: the size of the models; the size and type of mold; where and how the cavities are to be gated. The most important considerations in the layout of any mold are the minimum allowable distances between: models; models and locators; models and the mold perimeter; models and the mold basin; models and the mold backsides; locators; locators and the mold perimeter. Models should be placed at least 2 ¾” out from the mold center, and no less than ⅝” from the mold perimeter. However, molds can be made by experienced moldmakers in which models have been positioned as close as ¼” away from the center of the mold and as little as ⅜” from the mold perimeter. However, the smaller the clearance between cavities and basin, the more difficult it will be for molten metal to distribute evenly to all the mold cavities, and consequently, the greater the likelihood that some cavities will not fill. Because runners and longer gates help to clean metal before it reaches the cavities, their length, size and configuration may prove crucial to the production of high quality castings. For the novice, it is better to allow the maximum possible clearance around each cavity.
Fig. 8.1: Scribing lines to insure a symmetrical mold.
Fig. 8.2: Layout of a squash mold using a preform runner.
a. Space between model and mold basin.
b. Space between models
c. Space between model and mold perimeter
d. Space between model and a registration nut
e. Space between registration nuts
f. Space between registration nut and mold perimeter
g. Space between a mold cavity and the backsides of a mold
Fig. 8.3: Minimum clearances in a typical squash mold.
Fig. 8.4: ‘Copper College’ plaque (model mold). 1. Gating is top and bottom, side and direct; 2. Venting is standard design; 3. Note the registration nuts surrounding the cavity.
When planning a mold layout the following points should be considered:
Balance: Models should be arranged in any mold in as balanced and symmetrical a pattern as possible. Heavier pieces should be placed opposite each other and as close to the mold center as possible. Balanced layouts are important in preventing distortion of the cavities during the spin cycle, and also in preventing vibration of the casting machine when it is running. Vibration is a strong suspect in defective castings since it can affect metal delivery and solidification.
Testing: It is better to try out experimental gating and venting configurations as well as layout in model molds rather than in production molds, since any problems and failures are not as likely to tie up production should they occur during the model mold stages.
Fig. 8.5: ‘Copper College’ plaque (production mold). 1. Models have been rotated 90° from their position in the model mold; 2. Gating is direct from the basin to front and back of the cavity with a slight appendix’; 3. Venting is standard. Note that it is cut in an ingate shape; 4. Although the mold would have lent itself to the use of a flat back or dummy’ the parting line was not critical.
Fig. 8.6: “Bement’ buckle mold (1st model mold). 1. Original model was made from flat brass. To make the necessary modifications, it was cast in white metal using this mold; 2. Gating was extremely large to compensate for shrinkage; 3. Only one model was used, since only one was available.
Fig. 8.7: ‘Bement’ buckle mold (modified). 1. Model has been given a slight curvature, and a ‘dog leg and ears’ have been soldered on the back of the model to accommodate a belt. (Fig. 10.3); 2. The model has been rotated 90” from its position in Fig. 8.6; 3. Gating is as large as in the 1st model mold, top and bottom: 4. Venting is both top and bottom. ‘Dog leg’ and ‘ears’ have been drilled in the cavities and channeled to the bottom mold backside. (not visible).
Figure 8.1 shows how to scribe an uncured mold with a compass to ensure that the layout is symmetrical. Figure 8.2 is an exploded view of a typical squash mold showing standard placement of models, locators, sprue and basin former.
8.2: Spacing allowances in molds
The dimensions given in Fig. 8.3 are minimum clearances between the cavities and the registration nuts, basin, mold perimeter, and the backs of the mold. Dimensions may vary with the design of the model, the mold type, the rubber compound, and any of the other variables in the CRMC process. They are, however, given only as suggestions for typical molds.
8.3: Distortion
During spin casting, centrifugal force causes the mold cavities to distort slightly as molten metal enters them. This distortion is in addition to those caused by rubber shrinkage and by the differences between the alloy used to cast the model and the alloy used in production. Distortion caused by centrifugal forces is especially severe in large, thin, flat designs.
Fig. 8.8: ‘Bement’ buckle mold (production). 1. Production models have been rotated 90° from their position in the modified mold, both for spacing and to control their shape; 2. Gating is top and bottom direct to the side and back. Note the ‘appendix’; 3. Venting is again drilled into the ‘dog leg’ and ‘ears’ Because of space limitations set by the gating, vents are drilled where they enter the cavities.
To help compensate for this distortion, models should be positioned slightly different in each model mold that is made between the prototype and the production mold. Each succeeding mold should have the models rotated a full 90° from the position it had in the preceding mold. Figures 8.4 and 8.6 show an original model mold. In Figures 8.5, 8.7 and 8.8, the models have been rotated 90° to compensate for possible distortion.
However, simply rotating the models may not be sufficient in preventing distortion. Each casting -from the model mold to the production mold- must be carefully examined before it is used since factors other than rotation, such as the amount of air pressure being used to miter the molds in the CRMC machine, the pouring temperature of the alloy and the machine speed can all affect the ultimate shape of a casting. When modulated models such as cubes, spheres, ovoids, and other complex shapes and forms are being used, they should be rotated at least 180° from their position in each succeeding mold made. Again, it is important to inspect each casting produced before placing it in a succeeding mold to ensure that distortion caused by other factors in casting has not happened.
Fig. 8.9: Layout template.
8.4: Mold layout guide
Because most production molds are very similar in layout, as far as placement and number of models being used, a simple template can be made to facilitate any production mold layout. The template illustrated in Fig. 8.9 shows one that is used for 8 or 4 models. A series of templates can be made for various size molds and quantities of models using either a light gauge metal or cardboard.
Fig. 8.10: ‘Pen holder and base (model mold). 1. Mold was used to cast out production models for a paperweight pen holder for the Arconium Corporation of America, one of the Oster Group of companies; 2. The unusual basin and runner were made by using the principles of modulated molds. It serves as an excellent ‘solidification shrinkage feeder head’ to the larger casting cavity, 3. Venting is standard design, top and bottom; 4. Because of the two different size models used in the model mold, a large quantity of registration nuts have been used to miter the larger casting cavity; 5. The large amount of “flash’ was caused by improper air pressure setting while casting out this mold for purposes of illustrating this chapter. The porosity in the castings was caused by a cold mold and incorrect alloy selection for a casting of this size.
Fig. 8.11: ‘Eagle’ (model mold). 1. The runner was made from a preform ring: 2. Note the heavy gating top and bottom (3); 3. Venting is standard; 4. Detail faces down into the mold; 5. Basin is concave; 6. Ideally, the model should have had an opposing one to balance the mold.
PREVIOUS CHAPTER:
Chapter 7: Models
NEXT CHAPTER: