1、07FTM03Material Integrity in Molded Plastic Gears and itsDependence on Molding Practicesby: T. Vale, ABA-PGT, Inc.TECHNICAL PAPERAmerican Gear Manufacturers AssociationMaterial Integrity in Molded Plastic Gears and itsDependence on Molding PracticesTim Vale, 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.AbstractThe quality of molded plastic gears is typically judged by dimensional feature measurements only. Thispracticeoverlookspotentialdeficienciesinth
3、eplasticinjectionmoldingprocessanditseffectontheintegrityof the plastic material. These deeper issues are often not given proper consideration usually until a relatedgear failure demands its study and evaluation. This paper identifies some of these oversights in the moldingprocess,theresultanteffect
4、ontheplasticmaterialanddiscussestheirlikelyeffectonshortandlongtermgearperformance.Copyright 2007American Gear Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314October, 2007ISBN: 978-1-55589-907-31Material Integrity in Molded Plastic Gears and its Dependenceon Mold
5、ing Practicesby Tim Vale, ABA-PGT Inc., Manchester, CTIt is standard practice in the gear industry to definethe quality of a gear based on physical measure-ment of size and form. This is never more evidentthan in the AGMA gear quality ratings where mea-sured values of total composite error (TCE) and
6、toothtotootherror(TTE)are usedto neatlycatego-rize and rate the quality of a gear. Beyond that, agreat deal of effort might also be placed on usingmore advanced metrology equipment to gathermore and more information about the size and formof the gear. This could include using elemental in-spection t
7、o get detailed information about the invo-lute and lead of the gear, using optical or a coordi-nate measuring machine to define the form of theinner diameter (ID), the flatness of thepart, thepro-fileofother featureson thegear orusing aprofilom-eter or other techniqueto quantifythe surfacefinishof t
8、he gear teeth. This is all good information tohave about your gear. In machined metal or plasticgears, if you havestarted witha qualitypiece ofrawmaterial, you can be confident that if physical mea-surements are repeatable and within specification,your gear supplier has done a fine job of supplyingy
9、ou a quality gear. With molded plastic gears, thispractice of relying on physical measurements asproof of quality overlooks potential deficiencies inthe injection molding process and the effect it mayhave on the integrity of the plastic material.Thescopeofthispaperistospotlightthesemoldingdeficienci
10、es and to discuss the hidden effects theycan have on the end properties of your plastic gear.There is a lot of published data available on thephysical properties of every type and grade of plas-tic imaginable. What isless availableand oftendis-regarded by both end users and molders of plasticgears i
11、s information on just how these publishedproperties are affected by processing conditionsduring injection molding. When published data iscompiledthereisacertainamountofcarethatgoesintoassuringthattestspecimenshavebeenmoldedusing optimum molding conditions which will in turnyieldthebestphysicalproper
12、ties. Optimumproper-ties of the plastic material cannot be achieved with-outoptimumprocessingconditions. Properpartde-signplaysamajorroleingettingthemostoutofyourplastic material, but with all things equal, if propercare is not taken during molding, all of the up frontanalysis done by a gear designe
13、r can quickly be-comemeaningless. Inadditiontoreducingphysicalproperties, poor molding will also create the condi-tionsforfailuremodesthatcouldnotbepredictedoraccounted for by even the most prudent ofdesigners.It is also important to note here that there is no wayaround the fact that a high quality
14、molded plasticgear starts with the design and construction of ahigh 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 surfacefinish, precision fits and tolerances
15、, concentricitybetween mold components and proper steel selec-tion. This paper will not focus on why those thingsare important or how they are achieved. Instead itwill be assumed that a very sound mold with all ofthese considerations has been produced and is be-ing used. This paper will describe the
16、 things thatcan go wrong regardless of the mold and part de-sign if the gear molder is not disciplined and com-mitted to molding a highquality gearfrom theinsideout.Crystallinity and shrinkageWhenstudyingtherelationshipbetweenprocessingand end properties of a molded gear, the two mostbasic fundament
17、als that need to be understood arethat of crystallinity and shrinkage. For purposes ofthis paper, it is important to have a basic under-standing of these for three reasons: 1) The amountofcrystallinityina semi-crystallinepolymerhassig-nificant impact on the end properties of the plastic.2) Improperl
18、y predicted shrinkage is often a keydriver behind why a molder would choose to violategeneral good molding practice in an effort to get apart that meets the physical size requirements spe-cifiedonthepartdrawing. 3)Shrinkageandcrystal-2linityareboth highlydependent onthe processcon-ditions ultimately
19、 controlled by the molder.Most plastics fall neatly into one of two categories:amorphousorsemi-crystalline. Allplastics,regard-less of their level of crystallinity are comprised ofmany polymer chains. With an amorphous materi-al,thefulllengthofallthepolymerchainsremainsina somewhat random state befo
20、re, during, and afterheatingthematerialtotherequiredprocessingtem-perature. The difference in the polymer at low tem-perature vs high temperature is that at higher tem-perature there is more space between the polymerchainsallowingthemto movemore freely. SeeFig-ure 1. This increased free volume and h
21、eat energyis what eventually allows the plastic to flow and beinjection molded. With a semi-crystalline material,you will find these same amorphous (random)bunches of polymer chains. However, in additionyou will also find areas of tightly packed, regularlyshaped crystalline structures called spherul
22、ites.These spherulites are made up of many sections ofpolymer chains called lamellae that have folded upupon themselves and are held tightly together dueto intermolecular forces acting between the foldedsections of polymer chain. See Figure 1.This tight packing and intermolecular forces arewhat give
23、s the semi-crystalline plastic (such asacetal or nylon) the properties that are desirable forgear applications. These include friction and wearproperties, chemical resistance, and strength.Heating of a semi-crystalline material through itsmelt temperature effectively melts any crystallinestructureth
24、atmayhaveexistedintheplasticpelletsbeforeprocessingcausingthepolymerchains togobacktoawidelyspacedrandomstateverysimilartoan amorphous material at its processing tempera-ture. It is upon cooling of the material back throughits crystallization temperature that crystalline struc-tures are formed once
25、again. How quickly the poly-mer is taken from its melt temperature downthrough this crystallization temperature will deter-mine both size and quantity of crystallinestructuresformed. So the resultant crystallinity of any moldedgear is very much determined by the gear molder.Figure 1. Amorphous vs se
26、mi-crystallinepolymer chains(courtesy DSM)Mold shrinkage is the difference in size between amoldcavityandthemoldedproductthatisproducedinthatcavity. Most simply,shrinkage isa productofthermalexpansion. Like mostall materials,plasticswillexpandwhenheatedandcontractwhencooled.Given the relatively high
27、 CTE of plastics (typically 525xthatofsteel)andthehighprocessingtemper-atures used during injection molding (typically350F - 700 F above room temperature), properlypredictingand managingthis heatingand coolingofthe plastic during molding is critical in getting a highquality part. Plastic (with no fi
28、llers) shrinkagesrange from approximately 0.4% to 4% of the cavitydimensions depending on processing conditionsand part geometry.In addition to the shrinkage caused by thermal ex-pansion, semi-crystalline materials exhibit addi-tional shrinkage as the dense crystalline structures3are formed during c
29、ooling. This is the reason whysemi-crystalline materials generally have highershrink rates than amorphous materials. In Figure 2the general way in which these two types of materi-als expand during heating and contract during cool-ing can be seen. For both amorphous and semi-crystalline materials the
30、re is a temperature abovewhich the rate of expansionwill increase. Thishap-pens at the point where there is sufficient free vol-ume between polymer chains and the intermolecu-lar forces between them becomesless ofa factorinholding the chains close together. This graph isshowingtheglasstransitiontemp
31、erature(Tg)oftheamorphous material and the crystalline melt tem-perature (Tm) of the semi-crystalline material.Note: the very high spike (almost vertical line) forthe semi-crystalline material shows the rapid ex-pansion as crystals melt (left to right on the graph)and the rapid contraction as they a
32、re formed (righttoleftonthegraph). Thisspikeisexactlywhysemi-crystalline materials shrink more than amorphousmaterials. Graphs like these for plastics are gener-atedusingveryslowratesof heatingand coolingona small sample of material. In the injection moldingof a typical gear, the part will have a co
33、mparativelylargecrosssectionandexperiencesacomparative-lyfastcoolrate. Becauseofthis,allormostareasofthe part will not behave exactly as shown.Shrinkage becomes variable and quality defectscan result if care is not taken by the molder to mini-mize the amount of variation both across the lengthand wi
34、dth of the part and through the thickness ofthe part. Also note in the graph that the amount ofshrinkage is also dependent on pressure. This isanother variable controlled by the molder. Note:with machined plastic or metal gears, this processofexpansionandcontractionoftherawmaterialhasalready happene
35、d in production of thegear blankit-selfandisthereforenotavariablethatrequirescon-sideration by the gear manufacturer. On the otherhand, a plastics mold maker and molder have tohave considerable experience with making plasticgearstounderstandfullyhowmuchshrinkageisgo-ing to occur and more specificall
36、y understand howshrinkagerateswillvaryacrossthelengthandwidthof a part as a result of the various geometries andwall thicknesses in the part.Figure 2. Specific volume vs temperature for amorphous and semi-crystalline polymers(courtesy DSM)4Material handlingMaterial suppliers are doing an increasingl
37、y goodjobatsupplyingaconsistenthighqualityrawmateri-al to the gear molder for use in plastic gears. Thematerial type, grade, supplier andpercentage ofre-grind allowed are spelled out in black and white bythe end user on their part drawing. What is notspelled out on the part drawing is exactly how th
38、emolder should handle this material once the baggets opened. If molding resin is not handled in acleanorganizedfashion,thereisriskofforeignmat-ter other than the specified plastic making its wayinto the molded gear. This foreign matter could in-clude tiny or not so tiny pieces of dirt, dust, fiber,c
39、ardboard, metal or plastic. Different and often in-compatible plastic material can get introduced intothe molding process either by cross contaminationof the plastic pellets before molding or by residueleftinthemoldingmachinescrewandbarrelassem-bly from a prior molding run. If a substantial sizedpie
40、ceofthisforeignmattermakesitswayintoahighstressareaofthegearitcanact asa sitefor acrackto develop, propagate and ultimately fail.Agoodgearmoldershallalwayshaveproperproce-dures in place to keep the chance of this happeningto an absolute minimum. These include generalgood housekeeping practices aroun
41、d the moldingmachines and material handling areas along withregular cleaning of screw and barrel assemblies.Rigorous procedures for identifying materials andfortrackingmateriallotnumbersareveryimportant.Use of a closed central material feeding system forfeeding material tothe moldingmachines willgre
42、at-ly minimize the risk of contaminating molding mate-rials. This eliminateshaving multiplebags orboxesofmaterialopenednearthemoldingmachineitisin-tended to run in as well as the surrounding moldingmachineswhereitisnotintendedtorunin. It isalsoimportant to note that the contamination is not al-ways
43、going to be a different color than the moldingmaterial, so it may not get detected through normalvisualinspection. Thisfactmakesitallthemoreim-portant that gears are sourced witha molderwhosestandard practices can be trusted to protect yourproduct.RegrindBy definition, a thermoplastic material is on
44、e thatcanbemelted,formed(through cooling),re-meltedandre-formedagain. Thischaracteristicofplasticsis taken advantage of by molders and end users byallowing molded parts and runners to be ground upand recycled into new molded products. Using thisrecycled material (regrind) is a very good thing forrea
45、sonsof costand conservation. Andif thequalityof the regrind is controlled properly by the molder,there is very little reason for not using some per-centage of regrind in your molded gears.When someone in the molding industry speaks ofregrind quality, they are usually referring to thecon-sistency of
46、the granules that are produced afterpartsorrunnersarereground. Aconsistentgranulesize with a minimum amount of fines (small dust orflake like particles) or excessively long granules isdesiredforafewreasons. Aconsistentgranulesizewill help to ensure uniform melting in the moldingprocess the next time
47、 the regrind gets used. Non-uniform melting results in the molten plastic havinginconsistent viscosity which in turn can result in in-consistentdimensionsinyourmoldedpart. Finesinthe regrind can lead to black specking in a light col-ored material due to the fact that the small particlesmelt sooner a
48、nd may degrade during processing.Since dimensions and black specking can be de-tected through physical andvisual inspection,thereis no need to discuss any further in this paper.Of even greater importance than granule size is theregrind cleanliness (lack of contaminants) and thecontrolled exposure to
49、 heat and shear. It is duringthe process of creating and using regrind that con-tamination has the best chance of being introducedinto the molding process. For this reason, every-thing stated in the earlier section on material han-dling applies in particular to the regrind. Also manymoldersdonotkeepsufficienttabsonthenumberofheat histories that the regrind has seen and will notdiscard material after an acceptable number hasbeenreached. Thiscanresultinmaterialpropertiesthat are greatly reduced due to the excessiveamounts of shear and heat seen by the plastic. Agea
