Mold Design and Materials
Mold design is closely interrelated to part design, and will depend on the following factors:
- Part design
- Process and forming equipment
- Length of production run (projected product life)
- Material used for mold
- Cooling rates and uniformity required
- Draft angle (acceptable vs required)
Various kinds of materials have been used successfully in making molds for vacuum forming. For experimental or short runs, wood and plaster are the most commonly used materials. Cast phenolic and epoxy resin molds work well in short to medium runs (recent advances are making them useful in some long run applications as well). Long production runs generally require a metal mold. Both ferrous and non-ferrous metals have been used extensively for this purpose, with advantages and disadvantages of different types fairly well balanced depending on the nature of the part to be formed, the volume of production, type of equipment and numerous other variables.
The improved thermal conductivity of metal molds explains their ability to control the mold surface temperature, creating greater uniformity part-to-part, and reduced cycle times. This, of course, must be balanced against mold cost. Aluminum is the most widely used material for thermoformed tooling because of favorable fabrication costs plus superior thermal conductivity.
The following is a brief description of the properties and characteristics of various mold materials.
Wood
The use of hard wood for experimental or short run is a common practice. Caution should be exercised in the type of adhesive used for joining sections. A thermosetting glue is satisfactory. The wood should be kiln dried and glued with the grain in parallel direction since wood has different shrinkage rates across grain versus with grain. For improved surface finish and wear resistance, wooden molds can be coated with an epoxy resin and then sanded, buffed, and polished. Coating the entire mold with epoxy will improve stability by preventing the absorption of moisture by the wood.
Plaster
Plaster molds are usually constructed of such materials as Densite or Hydrocal. The primary advantages of plaster as a mold material are: 1) low in cost; 2) easily shaped; 3) sets at room temperature. Disadvantages are inability to control surface temperature and durability.
Plastic
Cast phenolic, cast filled epoxy and furan resins have the same general advantages as mold materials, i.e., excellent dimensional stability, good abrasion resistance, and a smooth, non-porous surface. Plastic molds may be prepared and patched when necessary at very little expense. For added strength, the bottom of a cast plastic mold may be reinforced with resin-impregnated fiberglass.
Aluminum
Aluminum molds can be made in two basic ways. They can be fabricated from aluminum plate stock and machined to proper dimensions, or they can be made by casting the aluminum, then machining and finishing.
Sprayed Metal
The mold consists of a sprayed metal shell, reinforced with resin impregnated backing for proper rigidity. For all practical purposes, sprayed metal molds of ferrous or non-ferrous metals are classified as permanent. Some molds which have been sprayed with aluminum, copper, nickel, low carbon steel, tin and zinc have made as many as half a million pieces with no evidence of deterioration. Accurate detail can be reproduced with molds of this type.
Mold Finish
Generally, the finish on the pattern for casting will influence the quality of the finish of the mold, whether it is cast or sprayed. Since it is often easier to polish a male than a female mold cavity, the cost factor might affect the decision of whether to use the male drape or straight female vacuum method, all other factors being equal.
On many applications, the finish of the mold is unimportant because only the non-contact side of the formed part is seen.
The use of sealers, lacquers or hard paste waxes are helpful in obtaining a good finish on the pattern used for the casting of plaster or plastic resin molds.
In machined aluminum female cavity molds, texture can be sandblasted or etched into the mold surface to create finished part texture. Texture depth, and mold draft angle must be considered to prevent scuffing when the part is extracted from the mold.
Vacuum
Depending on part design, vacuum must be supplied through strategically placed holes or slots in the mold to facilitate proper forming. The amount, size, and location of the vacuum holes will depend on part design and desired detail. Vacuum hole size is less critical in male drape form applications, as vacuum hole marks will only be visible on the inside of the part. In female tool applications, where the part is not intended to be painted, vacuum holes and slots must be sized and positioned so they leave no visible marks on the surface of the part.
Cooling and Heating
Precise mold temperature control is essential to creating parts of consistent quality with minimum cycle times. For most applications, cooling coils placed strategically within the mold allow the circulation of hot water at the beginning of the job (to bring the mold up to the required forming temperature), and cold water through the production run (to bleed off heat retained by the mold from the forming process). Ideally the mold should be designed to provide a temperature differential between inlet and exit water of no more than 5 degrees F. Certain materials, like polycarbonate, require electrically heated molds to maintain the required forming temperatures.