I believe that a short history of injection molding will help in the understanding of what is required from a mold designer. After the Second World War, when plastics technology was beginning, there were no "mold designers." When a mold was needed, it was produced by artisans in tool and die maker shops, who were trying to expand into new fields. They were skilled in building accurate steel tools and dies, and the boss of such shops often worked closely with the molder, who understood better what was required. The molder sketched, often crudely, how the mold should look, and the boss, by closely supervising the machinists as they built the mold components, then by assembling and testing the molds himself (at the molder), built well-functioning molds. These were usually suitable for the, at that time, few existing plastics molding materials, and quite satisfactory for the (by today's standards, low) productivity expected from such molds.
The only prerequisite for the beginner is some knowledge of mechanical drawing delineation, whether it is done electronically on a computer (with programs like AutoCad) or on the drawing board with pencil. Of course, the designer must also be familiar with some areas of basic arithmetic and trigonometry; both are required to put dimensions on the mold parts so they can be machined. Some of the advantages of electronic drafting are the following:
(1)
Designs of entire, or portions, of earlier built molds can be easily used again by simply copying or modifying some existing design features from the program's memory, without the need for tedious redrawing.
(2)
An up-to-date library of standard mold components and hardware can be established, which can be easily and quickly accessed and reproduced in new designs without the need for redrawing them every time they are needed.
(3)
The quality of the drawings produced by a computer printout does not depend on the skilled hand of the designer.
(4)
The computer permits easy transmission of designs to other locations, such as in-house manufacturing centers or manufacturers at other addresses.
(See also IMT, which contains much basic information on injection molding, molding machines, and molds.) The injection molding machine (Fig. 3.1) provides
A safe support for the mold
The opening and closing motion of the mold halves
The clamping force to keep the mold closed while injecting
There are many rules for designing molds. These rules and standard practices are based on logic, past experience, convenience, and economy. For designing, mold making, and molding, it is usually of advantage to follow the rules. But occasionally, it may work out better if a rule is ignored and an alternative way is selected. In this text, the most common rules are noted, but the designer will learn only from experience which way to go. The designer must ever be open to new ideas and methods, to new molding and mold materials that may affect these rules.
After the drawings and the final BoM have been released for production, there is usually a quiet time for the designer, as far as this mold is concerned. Hopefully, there are no problems in buying and machining. If there are problems in the shop, for example errors in machining or—heaven forbid—errors in the drawings, any corrective action must be approved and recorded by the designer or his delegate. There is always the possibility that the same mold will be required again maybe in a year, or much later, and it would be embarrassing if the same errors would then be repeated. After the mold is finally ready for testing, the designer must be present and see that the installation and setup procedures are in accordance with the specifications on the assembly drawings. The designer must also approve any changes required to make the mold work as expected and record what was done to make the mold work before it is shipped. A complete report, specifying the test machine, all temperatures, times, pressure settings, and plastics specifications should be supplied to the customer, together with the mold.
A few examples are provided of typical molded products and how they should be approached. These examples are used to illustrate material discussed earlier in the text.
This section is not meant to be a guide for the actual machining operations, but gives some descriptions of the evolution of machining in mold making. Earliest mold components and plates were produced by first sawing the raw blanks from steel plates of the appropriate thickness bought from the steel mill, with reciprocating or (endless) band saws. The next step was then squaring and/or rough machining these blanks—mostly plates, but also blanks for cavities—on shapers, with the blank held solidly and a single cutting tool moving back and forth over the surface. This slow method was abandoned in favor of rough grinding with special, large grinding machines or milling with large cutting heads in vertical or horizontal milling machines. Both these methods are now used extensively. Since this requires large, expensive machines, which smaller mold makers cannot usually justify economically, a service industry developed, specializing in the machining of the—often large—plates; this was the origin of the mold supply houses such as DME, National, Hasco, and others.
