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What to Consider When Converting Parts from Machined to Molded

Now to address the “all other things being equal” assumption. (Hint: All of the other things are not equal.)

Stable design. The cost to revise a mold is generally greater than the cost to adjust a machining program. This is especially true
   if the added or revised feature is not in the mold’s line of draw. Prototype tooling is a great option when the part design is still in
   development. A “steel safe” approach leaves selected part features small in the mold so that revisions to thicken them require
   removing steel—a much less expensive proposition than adding steel.
Validation costs. Since validation and submission costs are significant, they should be considered early in the process. A valid
   strategy may include machining parts for development and mechanical testing, and molding parts for design validation,
   submittal and market launch. In some cases, the best solution is to mold a near-net shape and machine any part details that
   would increase the mold cost.
Surface finish. While it is true that material in bar-stock form has typically lower residual stress than a molded part, machining
   operations remove the smooth resin-rich surface and add stress risers in the form of micro cracks—cracks that are not present
   in molded parts.
Knit lines. Molded parts will have knit lines (sometimes called meld or weld lines) whenever the material flows around a hole
   and re-joins on the opposite side. With proper part design, mold design and scientific molding process, knit line strength will
   approach the strength of a non-knitted feature. Additives such as glass fiber may compromise knit line strength as the fibers do
   not efficiently cross the flow fronts, leaving a resin-rich (glass-starved) zone. In many cases, revisions to the part design or the
   addition of an overflow feature can minimize this effect. This issue must be thoughtfully considered if your strategy includes
   machining parts for testing and molding parts for production.
Cleanliness. Machined parts are susceptible to burrs and particulate—requiring secondary deburring and washing operations.
   Packaging a molded part right out of the press avoids these potential sources of contamination.
Multi-material options. It is becoming commonplace to combine multiple materials into one part without the need for assembly.
   This molding technology provides the flexibility of multiple material properties (such as strength plus a resilient seal) without
   leaving the cleanroom.
Lead time. Parts can be machined in a matter of days, but prototype molds can also be produced in a short lead time, especially
   if the part geometry is not too complex. Molded parts, however, win the leadtime challenge as quantities increase.

Machining parts might have been an effective way to work out the bugs in your product design. But when the pressure is on to increase part quality, cleanliness, lead time and profitability, it’s time to consider molding.

Jeff Randall, PE, is Vice President of Engineering at MRPC, a manufacturer of silicone, medical rubber and thermoplastic components and assemblies for the medical device industry. He received degrees in mechanical engineering and business administration and a Master’s certificate in project management from the University of Wisconsin-Madison. He can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..