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How RPM Castings are Made   |   Design Tips          

A.L. Johnson offers several advantages with its Rubber Plaster Mold production process.

RPM Prototype toolings can be used for production
In more instances than not, prototypes turn into production. With the Rubber Plaster Mold process, the same tooling that produced prototype castings is 100% ready and capable of supporting production requirements. With the Rubber Plaster Mold process, master tooling is permanent (easily re-workable in the event of design changes), and production molds can be duplicated to accelerate output to support volume requirements.

Rubber Plaster Mold castings reduce time from prototype to production
Other casting processes require long lead times for production dies and create large time gaps from prototype to full production. This gap is greatly reduced with Rubber Plaster Mold castings from the Original prototype tooling.

No Additional Tooling Costs
Paying for expensive dies up front (even though amortized over the life of a product's expected life) drains cash and risks financial set-backs. Instead, the use of Rubber Plaster Mold castings requires no additional tooling costs and keeps cash flow in balance with current production requirements.

RPM reduces risk of new product production
In many instances, industrial companies find themselves in the situation where the market potential for their products is not sufficient to warrant the cost of expensive molds or die tooling, yet they require the advantages of castings, such as shielding, heat dissipation, strength, style, cast lettering, etc. Year in and year out they rely on Rubber Plaster Mold (RPM) and A. L. Johnson Company for their casting requirements.


Design Tips

DIMENSIONS/TOLERANCES
Data contained in the following table is in inches. Drawings with metric dimensions present no problems and can be used by A. L. Johnson Co. without conversion to inches. For convenience conversion factors are:

1mm = .03937008 in. 1 in. = 25.4mm

Rounding off a dimension to express a more liberal tolerance should be done with care to avoid misapplication. For example, if a symmetrical area is dimensioned 8.25" and an intended center feature at 4.12", the undimensioned difference of 4.13" will be so tooled. It would be clearer to specify 8.250" :t .030" and 4.125" :t .015" for nominal symmetry.

NOMINAL TOLERANCES FOR NORMAL CONDITIONS
(Closer tolerances are possible under favorable conditions, but more liberal tolerances are always economically desirable and in some instances may be necessary.)

DIMENSION WITHIN ONE MOLD SECTION BETWEEN TWO MOLD SECTIONS
Up to 3.000 +/- .0l0 +/- .015
3.001- 5.000 +/- .015 +/- .020
5.001- 8.000 +/- .020 +/- .030
8.001-12.000 +/- .030 +/- .040
12.001-16.000 +/- .040 +/- .050

Repeatability (that is: uniformity of any measurement from casting to casting) is of course closer, but even more difficult to predict accurately. A rough approximation is two-thirds of the given tolerance. Flatness of .005 per inch is normal and over large areas usually can be held within .015 total by straightening. Further refinement is possible on areas that are fully accessible to a secondary operation.

RADII FOR CORNERS/FILLETS
The seemingly insignificant details usually covered by casual notation can be very important for RPM castings. The metal pattern often consists of several pieces, some machined in negative form. Consequently, even an innocent little radius callout can be a source of an inordinate amount of tooling expense unless the drawing allows sufficient latitude to permit tooling either nearly sharp or with a full cutter radius. Generally the main outside configuration is tooled as a facsimile of the casting and some cavities are tooled by machining a separate piece in reverse, like a core. However, this is by no means an infallible rule. it is therefore best to allow maximum leeway on all unimportant corners and fillets. This will also allow omitting edge radii along the parting line, which is another way of reducing time and cost of RPM tooling and castings.

HOLES AND CAVITIES
The option of tooling in reverse is an advantage in producing holes and cavities, particularly those with odd shapes or sharp fillets. Small holes are limited to those over .100 diameter, with depth not exceeding the diameter.

Small holes through thicker sections can often be accomplished by including countersinks or counter bores to help support the delicate plaster.

Cast pilot holes or shallow dimples to locate subsequent drilling would involve additional loss of accuracy and are therefore seldom used. Where dimples are desired, the preferred design is a hemispherical depression.

Openings cannot be formed by sliding one section of mold over another, as in die casting, because of the likelihood of damage to the plaster mold. These lateral openings will require special attention. Options are: machining the castings, coring the opening, or changing the design to eliminate the slide-by condition.

CORES
Undercuts should be avoided if the resultant casting is equally acceptable. Where an undercut is desired, two choices are available: (1) using an additional section of mold (a core) which has been cast at right angles to the main top and bottom halves or (2rmachining the desired feature into each casting. If quantities are small and machining is simple, it may be best to machine the RPM castings in the interest of faster delivery and lower cost. RPM castings are better suited to machining because of more uniform metal density.

TYPICAL SPECIFICATIONS
356 T51 Aluminum Alloy per ASTM B 26

CAST SURFACES:125 micro inch

SURFACES MARKED ': MACH STOCK IS TO BE (or) or HAS BEEN ADDED

DRAFT 1 MAX

TOLERANCES: .XXX +/- .0l0, .XX +/- .03
WALL THICKNESS .075 +/- .015
CORNERS & FILLETS .12R MAX
CAST PART NO APPROX. WHERE SHOWN, IN .12 CHARACTERS RAISED .02 APPROX.

MATERIAL SPECIFICATIONS
Most frequently used material is aluminum alloy 356.0, heat treated to either -T51, -T6 or -T61.
Standard heat treatment is rigidly controlled within the A. L. Johnson plant.

Applicable specifications are: QQ-A-596, ASTM B 26, AMS 4218, AMS 4260, AMS 4261, MIL-C-11866 and MIL-A-21180.

QUALITY CONTROL
Conformance to customer requirements is assured through non-destructive testing by independent laboratories and appropriate certification, as applicable. All castings are processed under closely supervised Quality Control compatible with the wide range of conditions specified by customers in aerospace, electronics, computers, optics, instruments, etc.

QUOTATIONS
When requesting a quotation, it is best to supply two prints. Please include your name, address, phone number and any information that may be helpful in quoting. For example, a drawing that shows how the part is to function within an assembly may be more useful than one that shows how the part is to be machined.

A. L. Johnson Company Sales Engineers and field representatives are available upon request for consultation.

The quotation will include one print marked to show what we propose to produce, the price of tooling, the price of each casting in all possible quantities, and delivery schedule for first article and approved production.

ELECTRONIC DATA EXCHANGE
Our engineering department has the capability of receiving, reading and editing electronic drawing files transmitted via The Internet.