Furthermore, the shrinkage of the material in the direction of the flow will
be different from that perpendicular
to the flow As a result, a slide-gated gear or rotating part will be somewhat
elliptical rather than round.
In order to eliminate this problem, "diaphragm gating" can be used
which will cause the injection of material in all directions at the same time (see
Figure 1.66). The disadvantage of this method is the presence of a burr at the hub and no means of
support of the core pin because
of the presence of the sprue.
The best, but most elaborate way is
"multiple-pin-gating" (Figure 1.67). In this case the plastic injected
at several places symmetrically located. This will assure reasonable viscosity
of plastic when the material welds, as well as create uniform shrinkage in all
directions. The problem is the elaborate nature of the mold arrangement- so called
3-plate molds (Figure 1.68) accompanied by high costs. If precision is a
requirement, this way of molding is a must, particularly it the gears are of a
larger diameter.
To compare the complexity of a 3-plate mold with a 2plate mold
which is used for edge gating, Figure 1.69 can serve as an illustration.
20.19 Conclusion
In this section we have attempted to highlight the procedure
for proper design of plastic gears as well as illustrate the difficulties and
complexities involved in the production of molded plastic gears for technical
applications.
It is a fact that many gears are produced by molders for whom
a gear is not any different from a
"fancy door-knob". In many instances
taking this position is justified - such as toys etc. However, if technical
applications and stringent requirements are involved, it is imperative that a
thorough knowledge of disciplines such as: gear design, mold design,
tool-making, molding and machining (for secondary operation) is used in order to
produce a superior or even an acceptable product.
ACKNOWLEDGEMENT OF REPRINTS
References:
1. Earle Buckingham, Manual of Gear Design",3 Vols.,
Industrial Press, New York, 1935
2. Chironis, N.P.(Editor): "Gear Design and
Application", McGraw-Hill Book Co., Inc. New York, NY., 1967
3. D.W. Dudley, "Gear Handbook", McGraw-Hill,
NewYork,1962
4. Knut 0. Kverneland (Editor), "World Metric
Standards for Engineering", Industrial Press, New York, NY. 1978
5. G.W. Michalec,"Precision Gearing: Theory and
Practice", John Wiley & Sons, New York, 1966
6. J.E. Shigley,"Mechanical Engineering Design",
McGraw-Hill, New York, 1963
7. W. Steeds,"Involute Gears, Longmans, Green and Co.,
London, 1948
8. El. DuPont de Nemeurs and Co., "Gears of DELRIN and
ZYTEL’", Wilmington, Delaware.
9. Celanese Plastics and Specialties Co., "Design and
Production of Gears in CELCON Acetal Copolymer", Chatham, New Jersey, 1979
10. The Polymer Corp, "Nylatron Nylon Gear Design Manual’, Reading, Pa.
11. L.D. Martin, "Injection Molded Plastic
Gears",
Plastic Design and Processing Magazine , Pt.1 ,pp. 38-45. August 1968.
12. E.l. DuPont de Nemours and Co.. DELRIN Design Handbook", Willinggton. Delaware, 1967
13. Clifford E. Adams, "Plastie Gearing" Marcel Dekker, Inc. New YorK
1986 -
Literature of general interest:
R.W.Woodbury: "History of Gear Cutting
Machines" M.I.T. Technology Press
Cambridge, Mass, 1958
D.W.Dudley: "The Evolution of the Gear Art" AGMA Paper No. 990.14
(Published in book form by AGMA Jan. 1969)
T-158