طراحی با پلاستیک

طراحی با پلاستیک

اسلاید 1: Global Engineering Support Center Designing with plastics 10.09.2004Product & Technology

اسلاید 2: Design guidelinesUniform wall thickness Coring out Shrinkage and warpageRadiusParting linesDraftUndercutsFlow pattern & weld linesGate size and locationStrengthening featuresBossesRibs and GussetsStructural reinforcements Special FeaturesThreads and insertsSnap fitsLiving hingesPress fitsContents

اسلاید 3: Generally range from 2 mm to 4 mm. Thin wall injection molding ~ 0.5 mm Thinner wall faster cooling of part, short cycle times =>lowest part costs Uniform wall thicknessUniform wall thickness eliminates air entrapment.Design guidelines Avoids warpage and build-up of stress Easier to fill in the mold cavity Any transitions should be uniform

اسلاید 4: Coring out aids in eliminating air entrapmentExamples of coring in injection molded partsCoring out Coring out helps maintain uniform wall thickness Leads to lesser usage of materialDesign guidelines Often stronger design Lesser cost

اسلاید 5: Uneven sections cause distortion, warpage, cracks, sinks, and strains because of differences in shrinkage from one section to anotherShrinkage & Warpage Shrinkage affects the dimensional accuracy Shrinkage occurs in two forms - initial and post-moldingShrinkage across the diagonal is higher than along the edges, tension force along the diagonal causes corners to curl up or downDesign guidelines Initial shrinkage occurs when the part cools Post-molding shrinkage is defined as shrinkage occurring after 24h of the molding Non-uniform shrinkage lead to warpage of partsComparison of a normal and a warped part

اسلاید 6: Generally 1xThickness Fillet radiuses provide streamlined flow paths for the molten plastic Sharp corners greatly increase the stress concentration From Graph R/T over 0.5 the stress concentration factor gets lowerRadiusDesign guidelines

اسلاید 7: Example of a series of possibilities for designed mismatches.Parting lines Parting line is the place at which the core and cavity meet More specifically the moving and stationery parts of the mold The part should be designed so as to present a good parting line to the mold makerParting line selection for different partsDesign guidelines A flash is usually produced at the P/L and has to be finished manually or mechanically If the part is aesthetically important the P/L should be concealed on a thin inconspicuous edge of the partParting line selection

اسلاید 8: Excessive wear on mold parting lines cancreate a mismatch on a molded part that will appear greatly exaggeratedWays to avoid flash at P/LTolerances have to relaxed for dimensions across P/LPossibility of parting line mismatchParting linesDesign guidelines

اسلاید 9: Where would you place the parting line?Where would you place the parting line?Exercise

اسلاید 10: Draft is the angle between the direction of ejection of a part from the mold and the surface of the part Facilitates part removal from the moldExample of using multiple drafts to permit ease of part removalUsing parallel drafts with heavy walls.DraftDesign guidelines General draft 0.5 - 2 degrees 1.5° min. per 0.025mm depth of texture

اسلاید 11: Undercuts Undercuts are any design features that inhibits the normal opening of the moldDesign guidelinesUndercuts Mold opening direction

اسلاید 12: Design guidelinesUndercuts Inside undercuts can be made with removable wedges or ejector pins and ejector wedges Undercuts can be avoided by simple changes in design of part Complicated undercuts need special mechanisms in mold Ways to avoid undercutsA) mold knock-out pin through opening in part B) cavity cutoff approaches.Split-mold design eliminates the undercut problem but the part develops a parting line.

اسلاید 13: Design guidelinesUndercutsCreating undercuts with simple tooling.Mold opening directionCreating undercuts with cams

اسلاید 14: Identify the undercuts & parting lineExercise

اسلاید 15: Meld line with parallel flow frontsFlow patterns Incorrect design leads to flow patterns that lead to defects Weld lines caused by opposite flow fronts Meld lines are caused by parallel flow fronts Appropriate changes in design and gate location can lead to better flow patternsOpposite flow fronts produce a weld linethat could also contain entrapped air.Design guidelinesPart failed on a weld line

اسلاید 16: Recessed gate in a hidden area of a handleGates and ejectionDesign guidelinesSome types of gates Gate is the opening through which the melt enters the mold cavity Correct selection of type and position of gate ensures defect free part Aesthetic requirements need to be specified by the designerGate and ejection system designed for a part Ejection of the part is also to be considered during design An ejector pin landing pad can be integrated into the wall of a part design in order to provide material to seat the ejector pinsEjector pin landing pads

اسلاید 17: Bosses are projections on a plastic part designed to add strength facilitate alignment attach fasteners such as screws or accept threaded insertsBossesStrength features Wall thickness < 60 % of nominal wall Isolate bosses to eliminate sinksIsolate bosses from nominal walls

اسلاید 18: Strength featuresBossesDesign guidelines for molding bosses.Guidelines for designing a gusset

اسلاید 19: Ribs Ribs increase the bending stiffness of a part Rib thickness should be ~60% the wall thickness Strength featuresExamples of rib design characteristics.

اسلاید 20: ExercisesIdentify BossesIdentify Strengthening features used

اسلاید 21: Examples of geometric structural reinforcement techniques.Structural reinforcements(A) Circular or diagonal ribbing eliminates twisting(B) Bi-directional ribbing eliminates sagging or bendingA B Strength features

اسلاید 22: Summary

اسلاید 23: Comment on guidelines followedComment on guidelines followed

اسلاید 24: Examples of internal and external threads.Threads and inserts Threads can be molded or tapped into a plastic Internal threads usually need a rotary action in the mold External threads can be molded splitting the moldSpecial features Screw threads should be of the coarse type and have the outside of the thread rounded Metal inserts can be used for quality threads

اسلاید 25: Molded-in countersinks avoids chipping and burrsExamples of self-threading screws.Counter bore are often better than countersinks and avoids breakage due to hoop stress developedSpecial featuresThreads and insertsKnurls and grooves anchor inserts in molded products

اسلاید 26: Hot-roll plastic technique over insertsOther ways to assemble insertsThreads and insertsSpecial features

اسلاید 27: Snap fit design Snap fits rationalizes the assembly of plastic parts Takes over other functions like bearing, spring cushioning, fixing Fixing of the joined parts is weaker than in welding, bonding, and screw joiningSpecial featuresExamples of different snap-fit designs

اسلاید 28: Snap fit designSpecial featuresCore and cavity for creating the snap

اسلاید 29: Example of basic polypropylene integral hinge design (All dimensions are in inches.)Living hinges Materials such as Polypropylene can be molded with a living hinge Direction of flow of material should be perpendicular to hinge Used primarily in packaging products but can have engineering applications tooSpecial featuresSome examples of living hinges.

اسلاید 30: Press-fits Press fit is when a plastic boss accepts a plastic or metal shaft or pin The features should be dimensioned correctly so as to get the right interference Press-fit conditions for two typical situationsSpecial features

اسلاید 31: Press-fitsAlternate press-fit designs for metal pins in a plastic hubSpecial features

اسلاید 32: Joining plastics Ultrasonic welding Hot-plate welding Spin weldingSpecial features

اسلاید 33: Hot stamping Tampo printing Offset printing In-mold decorating Textures and colorsDecorating plasticsSpecial feature