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Design and material advice from the pros, Part 3: Insert molding essentials
September 1, 2005
4 Min Read
.svg?width=850&auto=webp&quality=95&format=jpg&disable=upscale "Design and material advice from the pros, Part 3:Insert molding essentials")
Insert molded parts from Illinois Precision (Bicknell, IN) run the gamut from electrical to medical applications. The company also markets its own line of rotary insert molding machines and produces tools for insert molded parts. Insert molder Repro Parts (Elmhurst, IL) specializes in parts that require integral circuitry in custom made-to-specification components and assemblies for prototype through production quantities. |
In the final installment of this series, IMM examines the pros, cons, and design guidelines for hybrid metal-plastic parts produced by insert molding.
DuPont’s Alan C. Miller offers the following caveats in his paper, “Molded-In Inserts: Precautions and Guidelines.â€
Molding metal inserts into a part, often considered a last resort because of the difficulties associated with it, may pay off with some outstanding advantages when adequate safeguards are taken in the design stage.
Advantages
The three principal reasons for using metal inserts are:
To provide threads that are serviceable under continuous stress or permit frequent part disassembly.
To meet close tolerances on female threads.
To provide a permanent means of attaching two high-load-bearing parts, such as a gear to a shaft.
In most cases, it is possible to secure the insert in a premolded part via press- or snapfitting or ultrasonic insertion. These and other options, as well as the possible disadvantages of insert molding listed below, should be carefully considered before the final design decision is made.
Disadvantages
Inserts can “float†or be dislocated in the mold, or cause damage to it.
Plastic may flash internal threads of an insert and require cleaning.
Inserts are often difficult to load and can prolong the molding cycle.
Inserts require preheating to expand.
Inserts in rejected parts are costly to salvage.
When plastic shrinks around the insert, hoop stress is molded into the part and cracking may result.
Determining Stress
Of all the complaints associated with molding inserts, delayed cracking of the surrounding plastic because of molded-in hoop stress is the most common. In order to estimate hoop stress, assume that the strain in the material surrounding the insert is equivalent to the mold shrinkage. An indication of the stress can be gained by checking a stress-strain chart for the specific material. If such data are not readily available, multiply the mold shrinkage by the flexural modulus of the material (shrinkage x modulus = stress).
Materials
Most engineering plastics can be used in insert molding. Lower shrinkage rates for some materials, however, give them an advantage in these applications. For example, one nylon resin has a mold shrinkage rate of .016 mm/mm, while an acetal has a shrinkage rate of .025 mm/mm. Both would be equally well suited for certain applications requiring a molded-in insert, but the nylon would have a clear advantage.
The higher shrinkage for the acetal yields a stress of approximately 52.4 MPa (7600 psi), which is about 75% of the ultimate tensile strength of the material. The thickness of the boss material surrounding an insert must be adequate to withstand this stress, however, and as thickness increases, mold shrinkage goes up.
If we assume part life of 100,000 hours, the 52.4 MPa stress will relax to approximately 14.8 MPa (2150 psi). That’s not bad, but long-term creep rupture data (derived from data for plastic pipe, a natural example of hoop stress) suggest the possibility that a constant stress of 17.9 MPa (2600 psi) for 100,000 hours will lead to a creep rupture failure. A part exposed to elevated temperatures, additional stress, stress risers, or an adverse environment could easily fracture under such conditions.
Because glass- and mineral-reinforced resins have the advantage of even lower mold shrinkages than their base resins, they too have been used with success. Their lower elongation is offset by a typical mold shrinkage range of .005 to .010 mm/mm.
Though weldlines of heavily loaded glass or mineral resins may have only 60% of the strength of the unreinforced material, the addition of a rib at that point can substantially increase the support provided by the insert boss.
Design Guidelines
Inserts should be rounded, or have rounded knurling, and there should be no sharp corners. An undercut should be provided for pull-out strength.
The insert should protrude at least .4 mm (.016 inch) into the mold cavity. Depth of molding beneath it should equal at least one-sixth of the insert’s diameter to avoid sink marks (see drawing, above right).
Boss diameter should be 1.5 times the insert diameter except for inserts with a diameter greater than 12.9 mm (.5 inch; see drawing, above left). For the latter, the boss wall should be derived with the overall part thickness and specific grade of material in mind. Keep the metal insert small relative to the plastic surrounding it.
Toughened grades of resin should be considered. These have higher elongation than standard grades and a greater resistance to cracking.
Inserts should be preheated before molding. This minimizes post-mold shrinkage, pre-expands the insert, and improves weldline strength.
Conduct a thorough end-use test program to detect problems in the prototype stage of development. Testing should include temperature cycling over the range to which the application may be exposed.
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