1、07FTM04Applying Elemental Gear Measurement to MoldModification of Molded Plastic Gearsby: G. Ellis, ABA-PGT, Inc.TECHNICAL PAPERAmerican Gear Manufacturers AssociationApplying Elemental Gear Measurement to MoldModification of Molded Plastic GearsGlenn Ellis, ABA-PGT, Inc.The statements and opinions
2、contained herein are those of the author and should not be construed as anofficial action or opinion of the American Gear Manufacturers Association.AbstractAlthoughelementalgearinspectionisrarelyspecifiedformoldedplasticgears,themeasurementequipmentand practices can be valuable in advancing the mold
3、ing processes and improving the quality of the moldedgears. After a brief description of plastic gear tooling and molding, this paper gives examples of specificelementalmeasurementsandrelatesthemtoprocesschangesandqualityimprovements.Suchexamplesforspur and helical gears include: profile measurement
4、s leading to gear mesh noise reduction; leadmeasurementleadingtoincreasedfacewidthloaddistributionand,continuingfromthat,eventothemoldingof crowned gears; and index measurement leading to improved roundness of gears molded from fiberreinforced plastic materials.Copyright 2007American Gear Manufactur
5、ers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314October, 2007ISBN: 978-1-55589-908-01Applying Elemental Gear Measurement to Mold Modificationof Molded Plastic GearsGlenn Ellis, ABA-PGT Inc.Elemental inspection of molded plastic gears hasnot been practiced until recent years
6、. Its use hasbeen limited to a few plastic molders specializing ingears,possiblybecauseofthecostoftheelementalinspection machine or the unfamiliarity with itsbenefits. Theyarenotcommonlyusedforeverydayinspection of molded plastic gears. They are, how-ever, used very successfully as a diagnostic tool
7、during the advanced development of the mold andmolding process. This paper will present examplesof such use.Thegearmoldingprocessincludesthemoldingma-chineandthemold. Theplasticmaterial,ingranularform,isloadedinto themachine wherethe plasticisheatedandmelted. Atthestartofthemoldingcycle,after the mo
8、ld is closed, the molten plastic is in-jected into the mold at a controlled temperature,flowrateandpressure. Inthemold,themoltenplas-tic flows through a runner system and enters eachgearcavitythroughagatingsystem. Aftersufficientcooling and solidification, the mold is opened andthe gears are ejected
9、 with the help of knockout(ejector) pins. After the ejected gear is furthercooled and the plastic adequately conditioned, it isready for inspection (figure 1).All parts of this process determine the accuracy ofthemoldedgear,startingwiththechoiceoftheplas-tic material and whether it is filled or unfi
10、lled. Somefillersmayincludeglassorcarbonfibersforadditionstrength or PTFE for lubrication. The influencescontinuewiththedesignofthegear,includingprovi-sion for gate location and shape, features, such asribs and posts, out-of-round bore, and wall thick-ness size. All these features can introduce vary
11、ingcross sections and lead to varying degrees of moldshrinkage throughout the gear. Ideally, designchange can be made to the gear to make it more“molderfriendly”. Uniformwallthicknessisnormallythe best. There are some exceptions including ta-per for ejection. With uniform walls there is a betterchan
12、ce of similar shrinkage through out the part.Ribsmayberequiredforstructuralreasonsbuttheycause irregular wall sections leading to varyingshrinkage. Posts and holes will also disrupt theshrinkage. Some of these features also alter theflow of material. This is particularly a problem withfilledmaterial
13、s. Whentwomaterialmeltflowscometogetherthefiberorientationwillchangealteringtheshrinkage. This isknown asa knit(weld) line. Allofthesewillhaveaninfluenceonthefinalshapewhichwill have to be compensated for so that an accept-able gear can be molded. Proper and experiencedmolding can further help impro
14、ve the part outcomebut in some cases actual steel rework is required.Close OpenFigure 1. Plastic gear injection mold2Mold design can also have an influence on the finalshapeofthe gear. Thefirst itemsselected mightbethe location and design of the gates and knockoutpins. The gear design then has a shr
15、inkage allow-ance added to the gear tooth form to arrive at thecavitydesign. Inmanycases,withasimplepartandwith an unfilled plastic material, this cavity designwillleadtoagearmoldedtoprintwithoutfurtherstu-dy. In many other cases, further evaluation, includ-ing the elemental inspection, is required
16、to upgradethe quality of the gear.Double-flank gear checkers are typically used onthe initial inspection to compare gear accuracy topart specifications. This will indicate whether themolded gear is acceptable without further effortIn the double-flank check, the plastic gear is rolledtogetherinclosem
17、eshwith amaster gear. Aspringload or its equivalent is used to maintain the closemesh condition. The recorded center distance willtypicallyvaryoverthefullrotationoftheplasticgear.This plotted record compares the gear accuracy tothe gear part specifications pointing out any exces-sive deviations. The
18、re are times when a solutioncan be found right away. Other times are when thediagnostic service of the elemental inspection isneeded. Thedoubleflankinspectionwillcontinuetobe used for in-process inspection where the ele-mental inspection is used for initial evaluation andtrouble shooting.It is commo
19、n practice to make a full set of the ele-mental measurements. This includes profile, lead,index, and tooth thickness. The printouts of the in-spection results include both a plot and numericalvalues. The plot supplies an immediate under-standing of the gear geometry and helps to identifythe potentia
20、l trouble spots. The numerical data willbe used in deciding which of these trouble spotsrequireprocesschangesormoldchangesandwhatshould be the size of those changes (figure 2).In some cases the error is symmetrical around thewholepart. Thismaybecausedbyanincorrectma-terial shrinkage. This is the mos
21、t common and isgenerally easy to correct. Once the error is identi-fied by use of the elemental inspection a new cavitycan be built with a modified shape so that the nextparts molded will shrink within specifications.The error may be in the profile or the lead. In somecasesitmaybeboth. Typicallyfour
22、teethareevalu-ated for these features. More or less teeth can becheckedasneeded. Whencheckingtheprofileandleadtheoutputsareslope,crownandhollow. Limitsare set for the range that you want evaluated. Theslope is the amount of change within the set limits.The crown is the maximum measure of any convexc
23、onditionalongthesetlimit. Hollowisthemaximummeasure of any concave condition along the setlimit.Theprofileischecked fromthe formdiameter outtothe tip. There are times that the tooth profile variesas you go around the gear. This is usually causedby a filled material or an odd feature on the part. Ift
24、his occurs the mold cavity must have varying toothprofilestocompensateforthe error. Anotherprofileproblem may be caused by taper. This is when theprofile is different from one end to the other. It isgood practice to check the profile in more then onelocation. Additional compensation in the mold cav-
25、ity will be needed for this problem.The lead is checked in a similar way to the profile.This should be checked whether it is a spur or heli-cal gear. When checked on a spur gear a tooth ta-per can be detected. On a helical gear the lead canbeverifiedandalsoanytoothtapercanbeseen. Aswith the profile,
26、 the lead can vary around the gearbecause of different shrinkages due to a filledmaterial or odd features.The index and tooth thickness is measured at thesame time. All of the teeth are checked around thegearforthismeasurement. Basedonthismeasure-ment the index, pitch and spacing is calculated.Addit
27、ional outputs from this measurement is aver-age tooth thickness, the tooth thickness variationand the runout. Once again the index and tooththickness may vary due to the material shrinkage.3Figure 2. Elemental inspection chartsExample #1This is a 7 tooth spur gear of 24 diametral pitch and20 degree
28、pressure angle. The material is an un-filled nylon, a medium shrinkage material. Theproblem appeared to be caused by a “D” shapedcentral hole. The double flank inspection chart re-vealed the resulting out of round condition. Fromthepartsmoldedintheinitialgearcavity design,theelemental inspection sho
29、wed very good profile andlead. The problem was revealed in the indexmeasurement, see figure 3. The upper part of the4index plot shows that the left flank had a total varia-tion of 0.0020 with the right flank showing 0.0019.The lower part of the figure shows the tooth thick-ness variation of 0.0004 a
30、nd a runout of 0.0018.The tooth indexing was shifted to compensate on anew cavity. The results of the mold cavity changeareshowninfigure4,with theindex valuesreducedto 0.0005 and 0.0006. The runout is now 0.0004.Figure 3. Index/tooth thickness charts with high index variation before rework5Figure 4.
31、 Index/tooth thickness charts with low index variation after reworkExample #2Thisisa47toothspurgear,withamoduleof1anda20 degree pressure angle. The part was moldedwith five gates with knit lines leading to varyingshrinkagearoundthegear. Figure5showed there-sults. Thetooththicknessvariationwas0.0029,
32、butwith a runout of only 0.0008. A new cavity wasmade with compensation to each individual tooththickness. Figure 6 shows the results after modi-fications. Thetooththicknessvariationhasbeenre-duced to 0.0010 with the runout of 0.0007 evenslightly smaller than before.6Figure 5. Index/tooth thickness
33、charts with high tooth thickness variation before rework7Figure 6. Index/tooth thickness charts with low tooth thickness variation after reworkExample #3This is a 24 tooth helical gear. It is a 24 diametralpitchand9degreehelixangle. TheupperportionofFigure 7 shows that the helix angle was close to9.
34、19 and the involute slope is high. A new cavitywas made with compensation to both the lead andslope. After the cavity was altered the helix anglewas improved to 9.02 and the involute was greatlyimproved (figure 8).8Figure 7. Lead/involute charts with high lead and involute slope error before rework9
35、Figure 8. Lead/involute charts with low lead and involute slope error after rework10ConclusionsInjection molding, with the attempts to predict partshrinkage, is still not an exact science. Even thegear of simplest design may not meet print specifi-cationsonthefirsttrial. This willoften leadto asec-o
36、ndtrialbasedoninformationlearnedfrom thefirst.With elemental measurement equipment, the prob-lems can be defined and quantified, taking theguess work out of making corrections. After thecorrections are made, the elemental inspectionserves to verify the improvements.Another advantage an elemental ins
37、pection hasover other measuring systems is that it can checkspecial features. One of these features would becrowning. Figure9showssomecrowningalongthelead. This is an attractive modification when axialalignment may be an issue. The elemental inspec-tion can verify the crown amount and location.Figure 9. Lead chart showing designed crown
