AGMA-07FTM03-2007.pdf
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1、07FTM03 Material Integrity in Molded Plastic Gears and its Dependence on Molding Practices by: T. Vale, ABA- -PGT, Inc. TECHNICAL PAPER American Gear Manufacturers Association Copyright American Gear Manufacturers Association Provided by IHS under license with AGMA Licensee=Boeing Co/5910770001 Not
2、for Resale, 07/25/2008 03:51:38 MDTNo reproduction or networking permitted without license from IHS -,-,- Material Integrity in Molded Plastic Gears and its Dependence on Molding Practices Tim Vale, ABA- -PGT, Inc. The statements and opinions contained herein are those of the author and should not b
3、e construed as an official action or opinion of the American Gear Manufacturers Association. Abstract The quality of molded plastic gears is typically judged by dimensional feature measurements only. This practice overlooks potential deficiencies in the plastic injection molding process and its effe
4、ct on the integrity of the plastic material. These deeper issues are often not given proper consideration usually until a related gear failure demands its study and evaluation. This paper identifies some of these oversights in the molding process,theresultanteffectontheplasticmaterialanddiscussesthe
5、irlikelyeffectonshortandlongtermgear performance. Copyright 2007 American Gear Manufacturers Association 500 Montgomery Street, Suite 350 Alexandria, Virginia, 22314 October, 2007 ISBN: 978-1-55589-907-3 Copyright American Gear Manufacturers Association Provided by IHS under license with AGMA Licens
6、ee=Boeing Co/5910770001 Not for Resale, 07/25/2008 03:51:38 MDTNo reproduction or networking permitted without license from IHS -,-,- 1 Material Integrity in Molded Plastic Gears and its Dependence on Molding Practices by Tim Vale, ABA-PGT Inc., Manchester, CT It is standard practice in the gear ind
7、ustry to define the quality of a gear based on physical measure- ment of size and form. This is never more evident than in the AGMA gear quality ratings where mea- sured values of total composite error (TCE) and toothtotootherror(TTE)are usedto neatlycatego- rize and rate the quality of a gear. Beyo
8、nd that, a great deal of effort might also be placed on using more advanced metrology equipment to gather more and more information about the size and form of the gear. This could include using elemental in- spection to get detailed information about the invo- lute and lead of the gear, using optica
9、l or a coordi- nate measuring machine to define the form of the inner diameter (ID), the flatness of thepart, thepro- fileofother featureson thegear orusing aprofilom- eter or other techniqueto quantifythe surfacefinish of the gear teeth.This is all good information to have about your gear. In machi
10、ned metal or plastic gears, if you havestarted witha qualitypiece ofraw material, you can be confident that if physical mea- surements are repeatable and within specification, your gear supplier has done a fine job of supplying you a quality gear. With molded plastic gears, this practice of relying
11、on physical measurements as proof of quality overlooks potential deficiencies in the injection molding process and the effect it may have on the integrity of the plastic material. Thescopeofthispaperistospotlightthesemolding deficiencies and to discuss the hidden effects they can have on the end pro
12、perties of your plastic gear. There is a lot of published data available on the physical properties of every type and grade of plas- tic imaginable. What isless availableand oftendis- regarded by both end users and molders of plastic gears is information on just how these published properties are af
13、fected by processing conditions during injection molding. When published data is compiledthereisacertainamountofcarethatgoes intoassuringthattestspecimenshavebeenmolded using optimum molding conditions which will in turn yieldthebestphysicalproperties. Optimumproper- ties of the plastic material can
14、not be achieved with- outoptimumprocessingconditions. Properpartde- signplaysamajorroleingettingthemostoutofyour plastic material, but with all things equal, if proper care is not taken during molding, all of the up front analysis done by a gear designer can quickly be- comemeaningless. Inadditionto
15、reducingphysical properties, poor molding will also create the condi- tionsforfailuremodesthatcouldnotbepredictedor accounted for by even the most prudent of designers. It is also important to note here that there is no way around the fact that a high quality molded plastic gear starts with the desi
16、gn and construction of a high quality plastic gear mold. This mold shall al- ways have proper cooling channels, venting, prop- erlysized gatesand runners,sufficient coring,suffi- cient ejection capabilities, quality mold surface finish, precision fits and tolerances, concentricity between mold compo
17、nents and proper steel selec- tion. This paper will not focus on why those things are important or how they are achieved. Instead it will be assumed that a very sound mold with all of these considerations has been produced and is be- ing used. This paper will describe the things that can go wrong re
18、gardless of the mold and part de- sign if the gear molder is not disciplined and com- mitted to molding a highquality gearfrom theinside out. Crystallinity and shrinkage Whenstudyingtherelationshipbetweenprocessing and end properties of a molded gear, the two most basic fundamentals that need to be
19、understood are that of crystallinity and shrinkage. For purposes of this paper, it is important to have a basic under- standing of these for three reasons: 1) The amount ofcrystallinityina semi-crystallinepolymerhassig- nificant impact on the end properties of the plastic. 2) Improperly predicted sh
20、rinkage is often a key driver behind why a molder would choose to violate general good molding practice in an effort to get a part that meets the physical size requirements spe- cifiedonthepartdrawing. 3)Shrinkageandcrystal- Copyright American Gear Manufacturers Association Provided by IHS under lic
21、ense with AGMA Licensee=Boeing Co/5910770001 Not for Resale, 07/25/2008 03:51:38 MDTNo reproduction or networking permitted without license from IHS -,-,- 2 linityareboth highlydependent onthe processcon- ditions ultimately controlled by the molder. Most plastics fall neatly into one of two categori
22、es: amorphousorsemi-crystalline. Allplastics,regard- less of their level of crystallinity are comprised of many polymer chains. With an amorphous materi- al,thefulllengthofallthepolymerchainsremainsin a somewhat random state before, during, and after heatingthematerialtotherequiredprocessingtem- per
23、ature. The difference in the polymer at low tem- perature vs high temperature is that at higher tem- perature there is more space between the polymer chainsallowingthemto movemore freely. SeeFig- ure 1. This increased free volume and heat energy is what eventually allows the plastic to flow and be i
24、njection molded. With a semi-crystalline material, you will find these same amorphous (random) bunches of polymer chains. However, in addition you will also find areas of tightly packed, regularly shaped crystalline structures called spherulites. These spherulites are made up of many sections of pol
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