Although designers should always provide generous tolerances whenever possible, there are many times tight tolerances must be maintained for fit, function or appearance. These images illustrate design details in a set of injection molded parts that were required to comply with reasonable, but tight-fitting, tolerances to attain cosmetic and functional requirements. The molder was included in the design reviews to interject his comments and commitment to maintain the specifications.
One of the greatest challenges for any designer faced with designing an injection molded part is providing enough clearance in the design for tolerance variation. Tolerance variation depends upon several variables, including materials, process control and tool design. Acceptable tolerance ranges in a design will vary greatly from one molder to another. It’s imperative that designers discuss reasonable critical tolerance specifications with a molder and consider options for possible mold revisions, if required. This may require certain design features to be intentionally designed with extra clearance, which will later be tightened by removing steel from the mold. No one wants to add steel with welding to remedy interference problems. Molders may offer a number of suggestions for maintaining tight tolerance control, including post machining, fixturing and gate locations.
Surface finish affects part quality, mold cost, mold cycle, and delivery time. Surface finishing is used to enhance surface clarity for appearance of the molded product. The standard steel finishes range from a number one (mirror finish) to a number six (grit blast finish). Specifying the mold surface finish does not necessarily produce the expected result on the finished molded product. Although a requirement for a part with a high-gloss finish requires a high-gloss finish on the mold cavity, other factors, such as resin, gating, melt and mold temperature, injection speed, and mold venting affect the surface finishing of the part. For extremely high-gloss finishing, the types of steel used in the cavities may need to be specified to ensure reasonable life of the polished cavity in production.
Heat treatment is part of the mold making process. It is carried out to achieve satisfactory combinations of abrasion resistance, strength, and toughness in both carburizing and deep hardening types of mold steels. When a mold is to be heat treated, precautions should be taken to prevent surface carburization or decarburization as well as distortion and cracking. The injection mold cavities need to be heat treated by methods that introduce residual stresses; such a condition may result in distortion during subsequent heat treatment. Therefore, a stress relief treatment at about 1200 °F is recommended if considerable machining has been done. To allow for distortion on subsequent heat treatment, injection mold components are generally rough machined within 0.125 to 0.25 in of final dimensions before stress relieving.
Electrical erosion is metal removal process using a master electrode that is electrically conductive. Copper alloys are generally used to make the master electrode because hardness is not a requirement. Cast zinc and machined graphite are also used in some instances. The principle of spark erosion is used by the mold making industry. The gap between the master and the cavity insert is quite uniform and small. As the master descends, small intense sparks are generated wherever the gap is reduced. Erosion occurs on both master and cavity inserts, the master’s negative erodes only at 1/4 to 1/10 of the speed at which the cavity insert erodes at positive polarity. The cavity insert may be hardened before the EDM process begins so that distortion due to heat treatment is eliminated. The dielectric fluid must circulate continuously to remove the minute particles that are formed between the master and the cavity insert. EDM is slow compared to mechanical cutting of soft steels, but for certain conditions, such as narrow deep slots, it offers great advantages.