A typical molded part may have literally dozens, if not hundreds, of specifications. A few common end-use part requirements are provided at the left of the center of Fig. 2.2. Some of these requirements drive the geometry, material selection, and other design details about which the mold design engineer may seem to have little control. Even so, the mold designer should be aware of these requirements, as they can influence selection of the mold materials, surface finish and treatments, mold design, and performance evaluation of the finished mold.
Manufacturers are generally required to use design standards to ensure that the product being designed and manufactured will perform as intended after com- mercial sale. In many segments of the plastics industry, such as medical devices, regulatory agencies have developed extensive standards governing the design, manufacturing, and testing of plastic products. A detailed discussion of regulatory compliance is beyond the scope of this book. However, the mold designer should be aware of any regulatory compliance issues that may affect the mold engineering. A few common regulatory agencies and their compliance programs are provided at the center of Fig. 2.2. These include the American National Standards Institute (ANSI, http://www.ansi.org/), ASTM International (http://www.astm.org/), the U.S. Food and Drug Administration (FDA, http://www.fda.gov/), the International Electrotechnical Commission (IEC, www.iec.ch/), the U.S. Department of Defense Index of Specifications and Standards (MIL-SPEC, http://stinet.dtic. mil/), the In- ternational Standards Organization (ISO, http://www.iso. org/), the Underwriters Laboratories (UL, http://www.ul.com/), and many others.
The mold design engineer does not usually need to know every detail of these specifications since they generally pertain to the use of the molded product and not specifically to the injection mold. However, the mold designer should inquire about any governing regulations that may affect the mold design. Ideally, the cus- tomer should provide a copy of any such regulations and highlight the specific re- quirements related to the molded product design.
The specification of dimensions and tolerances is of critical importance to both the mold designer and injection molder. Dimensions in product designs are typically specified with absolute tolerances. Figure 2.3 provides an example of four different methods for specifying tolerances. The most common method is the general toler- ance, typically specified in the signature block, which is applied to any dimension without an explicitly specified tolerance. In this case, the width offset of 25 mm does not have a specified tolerance and so would be governed by the general toler- ance of ± 0.2 mm. The height offset of 20.0 mm has a specified bilateral tolerance of ± 0.1 mm, which is actually redundant with the general specified tolerance of ± 0.1 mm for dimensions specified with one decimal place. If the height offset was specified to a different tolerance, this explicit specification would override the general tolerance.
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2.2 Design Requirements
Figure 2.3 Tolerance specification methods
The hole with a diameter of 18 mm is specified with limit dimensions, such that the hole diameter must be between 17.90 mm and 18.00 mm. The two decimals of precision do not reflect any additional precision on the tolerance but rather the absolute range of acceptable dimensions for the hole diameter. Finally, the shaft diameter is specified with a unilateral tolerance, meaning that the diameter of the shaft must be between 18 and 18.10 mm. The product and mold designer would both understand from these tolerance specifications that the shaft is meant to nominally fit into the hole with an interference fit such that there is no diametral clearance between the two.
While product designers will usually consider tolerances in absolute terms as de- scribed with respect to Fig. 2.3, plastics molders will tend to consider tolerances in relative terms. The reason is that molded plastics will shrink as a percentage of their length, so tight absolute tolerances will become more difficult to achieve as the part’s length dimensions increase. For instance, a typical tolerance may be considered as ± 0.4 % of the nominal dimension, such that a 100 mm length would be specified as 100 ± 0.4 mm. A tight tolerance may be considered as ± 0.1 %, such that a 10 mm diameter may be specified as 10 ± 0.01 mm.
The achievement of very tight tolerances requires careful mold design, process engineering, and consistent material properties. For this reason, product design- ers are encouraged to specify a single general tolerance governing most dimen- sions along with only a few tighter tolerances on specific dimensions that are crit- ical to product. Just because a tolerance is specified does not mean that it is achievable. In fact, it is not uncommon for product designers to over-specify the tolerances on many dimensions [10]. Mold designers should discuss tight toler- ance specifications with the product development team, and communicate that such specifications may require prototype molding to characterize the shrinkage behavior, nonuniform profiling of shrinkage rates in different areas of the mold, and mold modifications during mold commissioning.
Product designers will often provide specifications on the aesthetics, including re- quirements on color, color matching across multiple components, and gloss levels. It is common for the product design to specify the mold surface finish and mold
surface texture, which may add significant cost to the injection mold. Also, the mold design engineer should be made aware of critical aesthetic surfaces in which aesthetic defects (such as from knit-lines, gate blemish, sink, witness marks, etc.) should be avoided.