RTP Company: The Global Leader in Specialty Compounding

End-Use Case Study —

Knife Sharpener

This handsome high tech machine was designed to put a precision, long lasting edge on all kinds of knives for the professional chef and the cook in the home kitchen. Its success was dependent on critical parts meeting strict requirements.These were the requirements for this critical part of an innovative new knife sharpener designed and manufactured by Edgecraft Corporation, Wilmington, Delaware.

The machine has 3 sets of angled slots that guide the knife blade over rotating surfaces embedded with tiny diamond particles. The first set of stones presharpens, the second and third progressively finer hone the blade. The rotating, or orbiting member, is the part shown above.

This complex shaped part, with the honing surfaces inserted in molded-in-slots, moves at a linear speed of 50 feet/minute in an orbiting fashion to hone the knife blade. The requirement for tight dimensional stability is because the clearance is on the order of 1/1000 inch between this orbiting member and the stationary plastic pedestal in which it rides. The part must be precision molded with no variance on any axis. A warped part will cause galling between the member and the pedestal and can affect the precision of the sharpening angle.

The part is subjected to some lateral pressure. Wear on the part could diminish the machine's precision by altering the sharpening angle -- thus "built in" lubrication to prevent wear is an important requirement.

The OEM requires cost effective material and processing without sacrificing physical and thermal properties or strict control of tolerances.

Material selection synopsis:

1. Polycarbonate was eliminated because of its poor wear characteristics against the mating PBT stationary plastic part.
2. Die cast and extruded aluminum were evaluated but eliminated because of material and processing costs.
3. Nylon 6/6 with glass fiber and PTFE exhibited warpage.
4. Nylon 6/6 compounded by RTP Company with glass beads and PTFE exhibited high molding accuracy and extremely tight dimensional stability. In addition, the material, RTP 299 X 51410, resisted wear and was very cost effective. Final tailoring of the formula included adjusting the percentage of ingredients for optimum performance and improved mold flow for easy processing.

Other parts of the knife sharpener that are molded of RTP Company compounded materials include the base and fan shroud. Dimensional accuracy and stability was again a highly critical requirement and the maintenance of structural integrity in a 55 C environment was necessary. A UL flammability rating of HB was also important. RTP 503 HB (SAN with glass fiber) was selected for these parts.

This case history illustrates plastic's ability to cost effectively meet material requirements. This has been accomplished because of the advantage as a compounder of access to and unbiased selection of base resins and the ability to tailor a formulation to exact specification.

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