apple GE Plasics GE Engineering Thermoplasics DESIGN GUIDE Wall Thickness Paring Lines Ejecion Appearance Pars Ribs/Gusses Bosses Holes Depressions Radii, Filles and Corners Molding Design Injecion
s for Injecion Molded Design A successful applicaion of an engineering hermoplasic requires more han idenifying a specific produc or grade. Three areas design, produc, process are all inerrelaed and he appropriae rules in each area mus be followed o ensure a successful applicaion. In mos cases, he process mus be deermined before a specific resin grade can be seleced. During his review, designers also need o consider wheher he process is capable of meeing he design requiremens such as size, shape, deail and olerance. 7-2 s for Injecion Molded Design
Conens Wall Thickness......................7-4 Wall Thickness Consideraions...................7-5 Paring Line and Ejecion..............7-5 Appearance Pars....................7-7 Ribs................................7-9 Gusses............................7-10 Bosses............................7-11 Boss Design for Faseners......................7-13 Holes and Depressions...............7-14 Radii, Filles and Corners.............7-15 s for Injecion Molded Design 7-3
s for Injecion Molded Design Wall Thickness The ypical plasic par may be considered o have a shell ype configuraion wih a basic surface and feaures which are aached o i o mee funcional requiremens. From a moldabiliy sandpoin, he following are commonly regarded guidelines. Sink Area Core Ou Corner The basic wall of he par should be kep uniform. This provides for even flow of he mel during injecion. Even cooling and shrinkage ha conrols par warpage and reduces molded in sress. Solid Projecions Cored Projecions Coring should be employed where possible o eliminae maerial masses in he par. Coring resuls in more efficien designs and faser more producive cycle imes. I also provides more uniform shrink and avoids sink marks. Poor Good Bes When wall hickness ransiions canno be avoided, he ransiion should be made gradually, on he order of 3 o 1. The gradual ransiion avoids sress concenraions and abrup cooling differences. The par and he gaing should be designed so he mel flows from he hicker secion o he hinner secion. This avoids a resriced flow and reduces molded in sress. I also allows for more uniform packing. 7-4 s for Injecion Molded Design
Wall Thickness/Paring Line and Ejecion Wall Thickness Consideraions The acual deerminaion of he wall hickness is based on a number of consideraions. These include: Applicaion Requiremens. Srucural requiremens including srengh, impac, faigue or deflecion will be influenced by he wall hickness seleced. Elecrical loads may also impac on he wall hickness. Moldabiliy. The size of he par and he abiliy of he maerial o fill he furhes poin can deermine he minimum wall. The maximum flow lengh is also a funcion of ool design wih gae locaion and number of gaes used. Agency requiremens. For some agency properies, he raing is based on a minimum wall hickness which he par design mus mee or exceed o saisfy an agency requiremen. This would be he case for UL flammabiliy or RTI. Wall Thickness The wall hickness specified ypically should mee all he consideraions noed. From a cos sandpoin, he hinnes wall uilizes he leas maerial and resuls in he fases molding cycles. Paring Line and Ejecion The designer needs o consider how he mold will par and design in appropriae draf and shuoff. Ofen design changes o a feaure can eliminae he need for acion in he mold, saving ooling cos and mainenance coss laer on. s relaing o draf, shuoff and paring lines are offered below. POOR BETTER 0 2 5 Poor Nominal Ample On surfaces in he draw of he die, a minimum of 1/2 should be specified. Typical draf is 1. More draf aids ejecion bu may generae a maerial mass on secions conained in one side of he mold. Draf is required for release. STEPPED PARTING LINE Shu Off Angle 7 Type 5 Min. DETAIL IN PARTING LINE Keep feaures in he paring plane o simplify he par. When a sepped paring line is required, allow 7 for shu off. Minimum shu off angle is 5. Drag a he shu off will wear over ime and develop flash. Mainenance o resore mold o flash free pars will be more frequen. s for Injecion Molded Design 7-5
s for Injecion Molded Design Paring Line and Ejecion Possible Mismach Specify mismach on he paring line. Noe esablishes wha is accepable o he engineer for he molder. Mismach A Can be molded Requires in draw of die side acion Redesign holes in he side walls so ha he feaure can be obained wih shu off, hus reducing he need for side acion in he mold. Taking advanage of design flexibiliy can simplify he mold, reducing iniial coss and minimizing mainenance hroughou producion. 7-5 7 o 5 min. Core Caviy Plasic Par A Direcion Mold Opening 2 Secion A-A A For H>3 Balde Knockou Cored Ou Knockou a Inersecion For deep ribs and prorusions, allow for knockous on he ops of he ribs or a inersecions. Knockous for ejecion need o be incorporaed ino design as hey are frequenly greaer han he secion and mus be free o ravel during he ejecion sroke. Min Radius o Reduce Inersecion Thickness 7-6 s for Injecion Molded Design
Appearance Pars Appearance Pars To mainain a Class A surface on a molding, he side behind he appearance surface mus be free of projecions and depressions. If a projecion can no be avoided, hen he maximum hickness a he inersecion is half he nominal wall hickness. Very suble changes in he wall secion may read hrough on a high gloss, high qualiy surface. Even ooling lines made from sloppy fiing lifers may read hrough noiceably on he appearance surface. Minimum Draf Texured Angle Surface Draf 1 0 0 + D 001" Where D = Texure Deph (in.) Some relief may be available o locae a srucural rib opposie a Class A surface if a syling line runs direcly opposie he rib. The read hrough is masked by he syling line. Consider he use of exure on he appearance surface o mask read hrough of any deail on he opposie side. The exure breaks up a glossy surface and minor read hrough is no noiceable. Allow 1 addiional draf for each 0.001 in (0.025 mm) deph in exure. Increase in draf angle is needed o avoid scuffing and obain proper release. s for Injecion Molded Design 7-7
s for Injecion Molded Design Appearance Pars Texured Ends.060 In. Suggesed.040 In. Min. Texured surfaces sop a preferred 0.060 in (1.524 mm) or a minimum of 0.040 in (1.016 mm) from any paring line. Terminaing he exure improves he durabiliy of he paring line. r =.010 Min. Raised or recessed leers on appearance surfaces have a minimum radius of 0.010 in (0.254 mm). The breaking of he sharp edge helps he appearance of he leering. r =.010 Min. Prin noes for appearance pars. Locae knockous, gaes and inser lines away from idenified appearance surfaces. Specificaions include color number, gloss number or exure specificaion. All appearance and pos finish surfaces are idenified. To communicae clearly on he par drawing informaion wih regard o appearance. 7-8 s for Injecion Molded Design
Ribs Ribs The hickness of he rib a he inersecion wih he nominal wall should be 50 o 60% of he nominal wall. Maximum rib heigh: h=3 x nominal wall hickness. The inersecion can develop a mass of maerial if rib hickness ges oo grea. This can affec he fill paern wihin he mold and can resul in sink on he wall opposie he rib. Deep ribs become difficul o fill, may sick in he mold on ejecion, and wih draf hey can generae a maerial mass a heir base. W.75h h max 3 Typical draf for ribs is 1 o 1.5. Minimum draf should be 1/2 per side. Draf is necessary o aid ejecion of he par. Sink? T 1/2" o 1 1/2" R = 0.030-0.060 In. = 0.6T For < 1/8 In. = 0.4T For > 1/8 In. Max. Rib Thickness The inersecion a he base of he rib should radii. 25 o 50% of he wall hickness. A minimum radius of 0.015 in (0.381 mm) is suggesed. The radius eliminaes a sharp corner and sress concenraion. Flow and cooling are also improved. Spacing beween wo parallel ribs should be a minimum of 2 x wall hickness. This keeps he mold from developing a ho blade and cooling problems. The preferred flow of he mel in he mold is down he lengh of he ribs. The flow across he ribs resuls in a branched flow and can rap gas or hesiae because of he hinner secion. Hesiaion can resul in sress and hinder fill. s for Injecion Molded Design 7-9
s for Injecion Molded Design Gusses Gusses may be considered a subse of ribs and he guidelines for ribs apply o gusses. The hickness of a gusse a he inersecion wih he nominal wall should be 50% of he nominal wall. This will keep he gusse from reading hrough he nominal wall as sink. 2 2 PL The heigh of he gusse can be 95% of he heigh of he boss i aaches o. Generally he heigh will be less han 4 imes he nominal wall hickness and he preferred heigh is 2 imes he nominal wall. This has o do wih he effeciveness of he gusses and he ease of molding wih respec o fill and ejecion. 3 Max. 1.5 The lengh of he gusse may vary from 30 o 100% of he heigh of he gusse. This addresses he effeciveness of he gusse. 4 min..50% Boss Heigh 2 5 The inersecion of he gusse wih he feaure or he nominal wall should have a fille wih a radius of 25% of he nominal wall. The radii ge rid of sharp corners which can inroduce sress concenraions and adversely affec he molding process. The spacing beween gusses should be a leas wice he nominal wall hickness. This is he same guideline for ribs and i perains o he srengh and cooling of he mold. 7-10 s for Injecion Molded Design
Gusses/Bosses Bosses Typically he boss OD = 2 ID. This is he general rule of humb which allows he wall o increase as he size of he boss increases. The wall hickness a he base of he boss should remain less han 60% of he nominal wall hickness. Wall hickness greaer han his guideline will resul in a maerial mass which can produce sink and possible voids. This may also exend he cycle ime..030r Draf 1/2 Min..6.015R Draf 1/4 Min. O.D. = 2 I.D. I.D. 2.5 Max. +.6 Max. The boss heigh should be less han 3 OD. A all boss wih he included draf will generae a maerial mass a he base. In addiion, he core pin will be difficul o cool and can exend he cycle ime and affec he cored hole dimensionally. The boss should be radiused a he base. Radii a he base should be 25 o 50% of he nominal wall hickness. A minimum radii of 0.015 in (0.381 mm) is suggesed. Bosses are ofen an aachmen poin and carry significan loads. The inersecion of he base of he boss wih he nominal wall is ypically sressed and his is magnified by a sress concenraion if no radii are used. In addiion, radii help in molding. The end of a cored hole in he boss should have a minimum radius of 0.010 in (0.254 mm) A radius on he core pin avoids a sharp corner which aides molding (fill and cooling), and diminishes sress concenraion. s for Injecion Molded Design 7-11
s for Injecion Molded Design Bosses Draf on he OD is 1/2 minimum. Draf is needed for release from he mold on ejecion..030r Draf 1/2 Min. Draf 1/4 Min. O.D. = 2 I.D. I.D. 2.5 Max. + Draf on he ID is 1/4 minimum. Designs may require minimum aper o ge proper engagemen wih a fasener. Wih proper ejecion and polishing on he mold, hesmall draf angle can be accommodaed..6.015r.6 Max. Bosses adjacen o an exernal wall should be placed inboard a minimum of 0.125 in (3.175 mm) o he edge of he boss OD. This locaion allows he designer o ie he boss o he wall wih ribs and avoids creaing a maerial mass which would sink and lenghen cycle imes. Keep he minimum disance of wice he nominal wall hickness beween 2 bosses. When feaures are locaed oo close o each oher, hin hard o cool areas in he mold will develop and can affec par qualiy and produciviy. 7-12 s for Injecion Molded Design
Bosses Boss Design for Faseners The ID should be.8 nominal screw diameer. This is he proper dimension for a screw o ap hreads in a boss. Screw engagemen should be a minimum of 2-1/2 imes he screw diameer. The deph of he cored hole should be 0.032 in (0.813 mm) greaer hen he screw lengh when fully engaged. A chamfer a he op of he boss is a good lead in for he fasener. Shorer engagemen lenghs risk sripping he hreads during assembly and pull ou srengh may be reduced. This avoids booming he screw causing undo sress and allows room for displaced maerial from he self apping screws, primarily hread cuing screws. The designer can speed he assembly by adding lead in o his design. s for Injecion Molded Design 7-13
s for Injecion Molded Design Holes and Depressions Injecion molding is a high pressure process and he viscous mel can deflec or even bend core pins in he mold. As a resul, design guidelines address hese occasions o help avoid he problem. For blind holes, he lengh over diameer raio should remain below 2. As he diameer of he hole increases above 3/16 inch, he lengh over diameer raio can increase o 3. The srengh of he core pin o wihsand he pressures applied by he advancing mel are bes addressed by relaing he cross secion of he pin o he lengh of he pin. THROUGH CORES D < 3/16 h = 2D 3/8 D > 3/16 BLIND h = 3D CORES D < 3/16 h = 4D D 3/16 h = 6D For hrough holes, suppor of he core pin can be obained on boh ends or he pins can mee in he cener which allows he diameer o lengh raio o increase o 4. For diameers greaer han 3/16 inch, he lengh o diameer raio should remain below 6. Because of he increased suppor, a hrough hole can be wice as long as a blind hole wihou deflecing on he injecion cycle. Excepion The lengh of a core pin may exceed he guideline if a fill paern is such ha a balanced force is developed on he core pin. A verical sep of approximaely 0.015 in (0.381 mm) may be considered around he open end of a cored hole. This allows for sligh mismach in he mold and he core pin o be par of a larger diameer doll needed for mold design. A noe idenifies areas where weld lines are unaccepable because of he high loads and inadequae srengh. Warning o a mold designer or processor abou srucural requiremens as hey relae o molding. 7-14 s for Injecion Molded Design
Holes and Depressions/Radii, Filles and Corners Holes and Depressions The mold design should direc he mel flow down he lengh of slos or depressions and aemp o locae kni lines in hicker secions. A flow as described in he guideline will resul in he leas molded in sress and kni lines are bolsered by he increased secion. 2 Min. 2 Min. 2 Min. The disance beween wo holes or one hole o he edge of he surface should be a leas 2 he nominal wall hickness or 2 he hole diameer if ha dimension is larger. Adequae maerial around a hole is needed for srengh paricularly when kni lines are likely o be presen. The edges and corners of a depression should have a minimum radius of 0.015 in (0.381 mm) The preferred radii is 50% of he nominal wall hickness. The incorporaion of radii aids in he molding and srengh of he par. Radii, Filles and Corners R r R = r + : A fille radius should be beween 25 o 60% he nominal wall hickness. The larger fille radius is suggesed for load carrying feaures. A minimum radius of 0.020 in (0.508 mm) is suggesed. Break any sharp corner wih a leas a 0.005 in (0.127 mm) radius. The ouside corner radius should be equal o he inside radii plus he wall hickness (R = r + ). Proper use of radii avoids sress concenraions in a par. This pracice keeps a uniform wall hickness a he corner and reduces sress concenraions. s for Injecion Molded Design 7-15