There are many different ways to create details for the complete injection mold. Some of the components can be purchased in a finished or semifinished state from mold base suppliers, such as DME, or Mold Base Industries. But the final cavity and core sections, and details specific to a particular product design, must be made using equipment designed to produce accurate and precise components. Moldmakers have at their disposal a variety of methods and equipment to accomplish this feat.
Standard equipment includes:
- milling machines (horizontal and vertical),
- surface grinders,
- cylindrical grinders,
- and drilling equipment (horizontal and vertical).
If a moldmaker does not possess any of these pieces of equipment, the work can be subcontracted to a company that does. Hole drilling for waterlines is an example of this. The process usually requires a horizontal gun drill (or boring machine) for accuracy, and some machine shops specialize in providing this service to moldmakers who do not wish to tie up an investment in the special drilling equipment that might only be used once in a while.
In most cases, the cavity and core sections are fabricated using a model, either actual or virtual. An actual model is created (in wood, plaster, epoxy, or other easily machined or cast material) with the exact shape and contours of the expected product design, sometimes with shrinkage factors applied. In other cases, the shrinkage factors are determined separately during the machining phase and the model is created to the exact required finished dimensions instead.
A virtual model is a computerized three-dimensional drawing. The details of the drawing are down-loaded as computer files to a machining center computer that converts the files to cutting paths used by the moldmaking equipment. In this case, the shrinkage factors are usually calculated by the machining center computer. When utilizing either type of model, it is a good practice not to include shrinkage factors in the original model. This is because the final material selection for molding may change (or another could be added) and it will probably have a different shrinkage factor. If the shrinkage factor is developed during the machining phase instead of the design phase, the same model can be used for any plastic material.
The mold designer should be familiar with the machining methods used to produce mold components. The feeds and speeds of such equipment are detailed in many machining reference books and will not be covered here. The mold designer (and/or novice moldmaker) is prompted to refer to these information sources for a detailed analysis of how to machine mold components. However, in the following section, we will take a cursory look at some other methods of producing cavity and core components.
Other Fabrication Methods
This section will discuss fabrication methods such as casting (ceramic), hobbing, electrical discharge machining (EDM), electrochemical machining (ECM), chemical etching, electroforming (electrolytic deposition), vapor-forming nickel shells, and stereolithography apparatus (SLA).
Cast tooling is usually reserved for product designs having complex shapes and contoured configurations, as well as for surfaces that are decorative (textured or grained) and difficult to attain using the more conventional fabrication methods. Moldings that must reproduce anatomical features or flora detail benefit by utilizing the duplicating accuracy of cast molds.
The ceramic casting process requires a “natural” master model, usually created in an easily machined material such as plaster, wax, soft wood, or low-melting plastic. This model must incorporate proper shrinkage factors for the plastic material to be molded. A negative pattern is then taken from this master using a cold- curing silicon rubber (or similar material). This pattern is then placed in a mold box into which liquid ceramic is poured. The ceramic compound consists primarily of zirconium sand powder mixed with a liquid thermosetting bonding agent that causes the compound to cure and harden. After a post-cure process of baking in an oven, the pattern is ready to use in a conventional metal casting process.
Hobbing is a method of forming molds or cavity sets in metal without removing material. A hardened and polished metal hob (master) built to the external contour of the molding, is slowly forced into a blank of soft steel at a rate of from .031 in. (0.79 mm) up to .375 in. (9.5 mm) per minute. The pattern then becomes a negative image of the hob. Preheating the blank will result in an increase in hobbing depth. Normally, the hobbing depth is limited to one times the diameter of the hob, but this changes to 1:1.5 for the cross-sectional area if the hob is not cylindrical.
As the hob is forced into the blank, a strain hardening of the blank material occurs and anneals the block, one area at a time. The surface of both the hob and the blank must be kept clean at all times or a scale will build up and interfere with hobbing. However, a lubricant should be used to allow consistent flow of the blank material during hobbing. Oil does not provide adequate pressure resistance and molybdenum disulfide is more effective. To reduce friction further, the hob is frequently copper-plated after being highly polished.
Hobbing is usually reserved for applications requiring small cavities with little height and can be more cost-efficient than casting. In addition, a single hob can be used to make several cavity sets within a very short period of time.
Electrical Discharge Machining (EDM)
The EDM process evolved from creative problem solving in the early automotive industry. A machine was developed to create an electrical spark that caused broken bolt studs to erode while still stuck in their surrounding engine blocks. This helped reduce downtime on assembly lines and the machine became known as a “sparker.” The process was refined until the moldmaking industry began using it to remove the bulk of the metal during mold-building exercises.
A conductive material (usually carbon or graphite) is used to fabricate an electrode. This electrode is shaped to duplicate the product that will eventually be molded, and must include proper shrinkage factors and draft allowance. When the electrode is brought close to the metal of the mold cavity, a sparking condition occurs between the two objects because they are both connected to an electrical power source. The power is provided in a pulsing action that creates the spark at an average on-off frequency of approximately 40,000 cycles per second. This results in a metal removal rate ranging from .03 to .1 in.3 (19 to 64 mm3) per minute. The slower the removal rate, the better the final finish of the cavity set. In fact, EDM can be used to create a finish just short of that required for lenses. The average relative cost of using EDM is 75% that of fabricating the same cavity set using conventional machining methods.
Wire EDM is a method of metal removal utilizing the EDM process, but having a continuous wire (.001- to .012-in. [0.03- to 0.3-mm] diameter) that acts as the electrode. This wire feeds through a hole in the steel to be fabricated. As the wire moves through the hole, it is drawn away from the hole in the direction needed to form the shape or contour desired. In some cases, the lead hole can be produced using modern EDM equipment.
Electrochemical Machining (ECM)
This process is based on using the principle of electrolytic action to dissolve the conductive material of a workpiece. The dissolving is caused by an exchange of electrical charges (and materials) between the workpiece anode and the tooling cathode. This process is rarely used for creating mold cavity sets because of the expense of producing the anodes, and the lack of accuracy available for close- tolerance parts.
The etching process has become a popular method for applying textured surfaces to mold cavity sets. These surfaces produce a finish on the molded part that ranges from a soft satin appearance to one of rough alligator hide. There are literally thousands of grades in between.
These textures are desired for a variety of reasons, including changing the appearance of an otherwise dull finish, adding a feeling of warmth to the touch of a plastic part, and hiding imperfections, such as weld lines and blush marks due to processing difficulties.
The success of the process is based on the fact that metals dissolve in acids, bases, and salt solutions. Metal atoms are forced to emit electrons and are discharged as ions from the metal lattice. These free ions are used up by reducing processes with cations and anions that are present in the etching agent. The re moved metal combines with anions to form an insoluble metal salt, which is then removed from the etching agent by filtering.
The exact formulas for etching agents are well-kept secrets of the processor, and almost all mold steels and most nonferrous metals can be chemically etched. The two primary methods of etching (spray and dip) are capable of removing metal at a rate ranging from .0004 to .0035 in. (0.010 to 0.089 mm) per minute, which can be increased by heating.
It is important to understand that textured surfaces (especially on sidewalls) act as undercuts. If the texture is present on a sidewall, an additional draft allowance must be made that amounts to an extra 1° (per side) for each .001 in. (0.03 mm) of texture depth. This ensures that the molded part will be able to be ejected from the mold cavity set.
Electroforming (Electrolytic Deposition)
Cavity sets can be created by electroplating a metal shell (usually nickel) onto the surface of a conductive pattern. The thickness of the shell is in the range of only .125 to .25 in. (3.2 to 6.4 mm), but the process is slow and can take up to 4 weeks to complete this deposition.
After the shell is created, it is removed from the pattern (which can be reused) and placed on a thick backing. The backing material shape and size depends on how the finished mold will be used. For injection molding, the backing is usually a regular steel mold insert placed directly into a mold base. Aluminum also can be used if compression forces are not too high.
Electroforming is time-consuming but very inexpensive (most of the time is spent waiting for electrolytic action to be completed). Waterlines can be incorporated in the backing material if production runs are expected to be longer than a few cycles. The major advantage of electroforming is that it can be used economically to produce parts with complicated parting lines and unusual surface features (such as doll faces). Other methods of achieving these goals are too cost- prohibitive in most cases.
Vapor-forming Nickel Shells
Another method of producing nickel shells for molds is by chemical vapor deposition. While this process is over 100 years old, recent demands for inexpensive molds for the reaction injection molding (RIM) process have brought it to the foreground again.
Chemical vapor forming is a deposition process in which nickel is plated by allowing the vapors of a nickel carbonyl gas to come in contact with a heated mandrel (the plating object) that receives a uniform layer of nickel from the decomposed vapors. After a relatively short period of time (it plates at up to 0.010 in. [0.25 mm] of thickness per hour) a shell is created that accurately reproduces the finish and shape of the mandrel. The shell is then bonded to a backing similar to that used in the electroforming process mentioned earlier.
Stereolithography Apparatus (SLA)
Stereolithography apparatus utilizes the heat from an ultraviolet (UV) radiating laser to cure the surface of a pool of liquid polymer that is sensitive to UV radiation. It is similar to the process of printing on a paper surface by a laser printer. The cured portion of the plastic liquid is only .005-in. (0.13-mm) thick. This layer is then submerged .005 in. (0.13-mm) and another layer is fused to it. This process continues until a finished part is made. The finished part can then be used as a model or pattern for casting or machining a mold cavity set. The cavity set is then placed in a standard mold for the standard injection mold process.
SLA is also utilized with a special epoxy material that is used as the liquid plastic pool. A shell is formed and then cured into a very hard material. It can be back-filled with aluminum-filled epoxy and used as is for a short-run (100 to 500 pieces) cavity set, or plated with nickel to produce a high hardness for long runs (1,000 pieces or so). Most common plastic materials can be used for molding in these epoxy molds and standard gating and runner designs can be utilized. In addition, cycle times, pressures, and melt temperatures can be very close to standard injection molding parameters. This system allows molds to be built in approximately three days from the finish of a product design concept using standard computer-aided design (CAD) systems.