The molding process accommodates thread forming directly in a part, avoiding the expense of secondary, thread-cutting steps. The cost and complexity of the tooling usually determines the feasibility of molding threads. Always compare this cost to the cost of alternative attachment options, such as self-tapping screws. Easily molded in both mold halves,
external threads centered on the mold
parting line add little to the molding cost. Typically, threads that do not lie on the parting line require slides or side actions that could add to molding costs. All threads molded in two halves are prone to parting line flash or mismatch.
Thread designs requiring unscrewing devices add the most cost to the mold. Most of the mechanisms for molding internal threads — such as collapsible and unscrewing cores — significantly increase the mold’s cost and complexity. Occasionally, threads in parts made of flexible plastics, such as unfilled polyamide 6 or polyurethane elastomers, can be stripped from the mold without special mechanisms. Rarely suited to filled resins or stiff plastics such as polycarbonate, this option usually requires generously rounded threads and a diameter-to- wall-thickness ratio greater than 0 to . Usually, molding threads on removable cores reduces mold cost and complexity but adds substantially to the costs of molding and secondary operations. For this reason, limit this option to low-production quantities or designs that would be prohibitively complex to mold otherwise.
Thread profiles for metal screws often have sharp edges and corners that can reduce the part’s mechanical performance and create molding problems in plastic designs. Rounding the thread’s crests and roots lessens these effects. Figure -4 shows common thread profiles used in plastics. Although less common than the American National (Unified) thread, Acme and Buttress threads generally work better in plastic assemblies. Consider the following when specifying
molded-in threads:
• Use the maximum allowable radius at the thread’s crest and root; • Stop threads short of the end
to avoid making thin, feathered threads that can easily cross-thread (see figure -5);
• Limit thread pitch to no more than threads per inch for ease of molding and protection from cross threading; and
• Avoid tapered threads unless you can provide a positive stop that limits hoop stresses to safe limits for the material.
Page 6 of 68: This document contains important information and must be read in its entirety. Threads Figure 2-35
Design guidelines to avoid cross threading.
Pipe Threads Figure 2-36
Standard NPT tapered pipe threads can cause excessive hoop stresses in the plastic fitting.
Tapered pipe threads, common in
plumbing for fluid-tight connections, are slightly conical and tapered and can place excessive hoop stresses on the internal threads of a plastic part. When mating plastic and metal tapered threads, design the external threads on the plastic component to avoid hoop stress in plastic or use straight threads and an “O” ring to produce the seal (see figure -6). Also, assure that any thread dopes or thread lockers are compatible with your selected plastic resin. Polycarbonate resins, in particular, are susceptible to chemical attack from many of these compounds.
For best performance, use threads designed specifically for plastics. Parts that do not have to mate with standard metal threads can have unique threads that meet the specific application and material requirements. The medical industry, for example, has developed special, plastic-thread designs for Luer-lock tubing connectors (see figure -7). Thread designs can also be simplified for ease of molding as shown in figure -8.
GENERAL DESIGN
Page 7 of 68: This document contains important information and must be read in its entirety. Molded Threads Figure 2-38
Luer-lock thread used in medical applications.
Medical Connectors Figure 2-37
Page 8 of 68: This document contains important information and must be read in its entirety. Lettering Figure 2-39
Deep, sharp lettering can cause teardrop defects as shown on top photo. The bottom shows the improvement with
rounded, shallow lettering. Lettering Figure 2-40
Design suggestions for the cross-sectional profile of lettering.
LETTERING
The molding process adapts easily for molding-in logos, labels, warnings, diagrams, and instructions, saving the expense of stick-on or painted labels, and enhancing recyclability. Deep, sharp lettering is prone to cosmetic problems, such as streaks and tear drops, particularly when near the gate (see figure -9). To address these cosmetic issues, consider the following: • Limit the depth or height of
lettering into or out of the part surface to approximately 0.00 inch; and
• Angle or round the side walls of the letters as shown in figure -40.
GENERAL DESIGN
Page 9 of 68: This document contains important information and must be read in its entirety.
TOLERANCES
Many variables contribute to the dimensional stability and achievable
tolerances in molded parts, including
processing variability, mold
construction, material characteristics, and part geometry. To improve your ability to maintain specified tolerances in production:
• Use low-shrinkage materials in parts with tight tolerances; • Avoid tight tolerances in
dimensions affected by the alignment of the mold halves or moving mold components such as slides;
• Design parts and assemblies to avoid tight tolerances in areas prone to warpage or distortion, and
• Adjust the mold to produce dimensions in the middle of tolerance range at optimum processing conditions for the material.
To avoid unnecessary molding costs, specify tight tolerances only when needed. Generally, the size and variability of other part features determine the actual tolerance required for any one component or feature within an assembly. Rather than dividing the allowable variability equally over the various features that govern fit and function, allot a greater portion of the total tolerance range to features that are difficult to control. Reserve tight tolerances for features that can accommodate them reasonably.
Tolerances Figure 2-41
Geometric tolerancing methods can
expand the effective molding tolerance by better defining the size and position requirements for the assembly. Rather than define the position and size of features separately, geometric tolerancing defines a tolerance envelope in which size and position are considered simultaneously.
0 Page 40 of 68: This document contains important information and must be read in its entirety. Tolerances Figure 2-42
As the hole size increases, the position tolerance can increase without restricting the through-hole clearance.
Figure -4 shows the size and position of a hole specified in both standard and geometric tolerances. The standard tolerances hold the position and size of the hole to ±0.00. The geometric tolerances specify a hole size tolerance of ±0.00 but allow the position tolerance to vary within a 0.006 tolerance zone when the hole is at its smallest diameter (maximum material condition). When the hole is larger than the minimum size, the difference between the actual hole size and the minimum hole size can be added to the tolerance zone for the position tolerance. At the maximum hole size, 0.50, the position tolerance zone for the center of the hole is 0.0 or ±0.006 from the stated vertical and horizontal positions. As the hole becomes larger, the position can vary more without restricting the required through-hole for the post or screw that passes through the hole (see figure -4).
GENERAL DESIGN
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