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Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Product & Technology Global Engineering Support Center Designing with plastics 10.09.2004 H&GT Organization - 06/25/2004 1 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Contents Design guidelines • Uniform wall thickness • Coring out • Shrinkage and warpage • Radius • Parting lines • Draft • Undercuts • Flow pattern & weld lines • Gate size and location Strengthening features • Bosses • Ribs and Gussets • Structural reinforcements Special Features • Threads and inserts • Snap fits • Living hinges • Press fits H&GT Organization - 06/25/2004 2 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Uniform wall thickness • 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 • Avoids warpage and build-up of stress Design guidelines • Easier to fill in the mold cavity • Any transitions should be uniform H&GT Organization - 06/25/2004 Uniform wall thickness eliminates air entrapment. 3 Product & Technology Quality • Excellence • Support H&GT GESC Bangalore Coring out • Coring out helps maintain uniform wall thickness • Leads to lesser usage of material • Often stronger design Coring out aids in eliminating air entrapment Design guidelines • Lesser cost Examples of coring in injection molded parts H&GT Organization - 06/25/2004 4 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Shrinkage & Warpage • Shrinkage affects the dimensional accuracy • Shrinkage occurs in two forms - initial and post-molding • Initial shrinkage occurs when the part cools Design guidelines • Post-molding shrinkage is defined as shrinkage occurring after 24h of the molding Shrinkage across the diagonal is higher than along the edges, tension force along the diagonal causes corners to curl up or down • Non-uniform shrinkage lead to warpage of parts H&GT Organization - 06/25/2004 Uneven sections cause distortion, warpage, cracks, sinks, and strains because of differences in shrinkage from one section to another Comparison of a normal and a warped part 5 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Radius • Generally 1xThickness • Fillet radiuses provide streamlined flow paths for the molten plastic • Sharp corners greatly increase the stress concentration Design guidelines • From Graph R/T over 0.5 the stress concentration factor gets lower H&GT Organization - 06/25/2004 6 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support 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 maker Design 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 part Parting line selection Parting line selection for different parts H&GT Organization - 06/25/2004 7 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Parting lines Design guidelines Tolerances have to relaxed for dimensions across P/L Excessive wear on mold parting lines can create a mismatch on a molded part that will appear greatly exaggerated Possibility of parting line mismatch Ways to avoid flash at P/L H&GT Organization - 06/25/2004 8 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Exercise Where would you place the parting line? Where would you place the parting line? H&GT Organization - 06/25/2004 9 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Draft • 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 mold • General draft 0.5 - 2 degrees draft_eject_anim.gif Design guidelines • 1.5° min. per 0.025mm depth of texture H&GT Organization - 06/25/2004 Using parallel drafts with heavy walls. Example of using multiple drafts to permit ease of part removal 10 Product & Technology Quality • Excellence • Support H&GT GESC Bangalore Undercuts • Undercuts are any design features that inhibits the normal opening of the mold Design guidelines Mold opening direction H&GT Organization - 06/25/2004 Undercuts 11 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Undercuts • 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 Design guidelines • Complicated undercuts need special mechanisms in mold Ways to avoid undercuts A) mold knock-out pin through opening in part B) cavity cutoff approaches. H&GT Organization - 06/25/2004 Split-mold design eliminates the undercut problem but the part develops a parting line. 12 Product & Technology Quality • Excellence • Support H&GT GESC Bangalore Undercuts Design guidelines Mold opening direction Creating undercuts with simple tooling. shutoff_cam_anim.gif Creating undercuts with cams H&GT Organization - 06/25/2004 13 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Exercise Identify the undercuts & parting line H&GT Organization - 06/25/2004 14 Product & Technology Quality • Excellence • Support H&GT GESC Bangalore Flow 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 fillpattern_anim.gif Design guidelines • Appropriate changes in design and gate location can lead to better flow patterns Opposite flow fronts produce a weld line that could also contain entrapped air. Meld line with parallel flow fronts Part failed on a weld line weldline_anim.gif H&GT Organization - 06/25/2004 15 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Gates and ejection • 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 designer Some types of gates Design guidelines • 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 pins Recessed gate in a hidden area of a handle Gate and ejection system designed for a part Ejector pin "landing pads H&GT Organization - 06/25/2004 16 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Bosses • Bosses are projections on a plastic part designed to – add strength – facilitate alignment – attach fasteners such as screws or accept threaded inserts Strength features • Wall thickness < 60 % of nominal wall • Isolate bosses to eliminate sinks H&GT Organization - 06/25/2004 Isolate bosses from nominal walls 17 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Strength features Bosses Design guidelines for molding bosses. H&GT Organization - 06/25/2004 Guidelines for designing a gusset 18 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Ribs • Ribs increase the bending stiffness of a part Strength features • Rib thickness should be ~60% the wall thickness H&GT Organization - 06/25/2004 Examples of rib design characteristics. 19 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Exercises Identify Bosses Identify Strengthening features used H&GT Organization - 06/25/2004 20 Product & Technology Quality • Excellence • Support H&GT GESC Bangalore Strength features Structural reinforcements Examples of geometric structural reinforcement techniques. A B (A) Circular or diagonal ribbing eliminates twisting (B) Bi-directional ribbing eliminates sagging or bending H&GT Organization - 06/25/2004 21 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Summary H&GT Organization - 06/25/2004 22 Product & Technology H&GT GESC Bangalore Comment on guidelines followed H&GT Organization - 06/25/2004 Quality • Excellence • Support Comment on guidelines followed 23 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support 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 mold • Screw threads should be of the coarse type and have the outside of the thread rounded Examples of internal and external threads. Special features • Metal inserts can be used for quality threads H&GT Organization - 06/25/2004 24 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Threads and inserts Special features Knurls and grooves anchor inserts in molded products Examples of self-threading screws. Molded-in countersinks avoids chipping and burrs H&GT Organization - 06/25/2004 Counter bore are often better than countersinks and avoids breakage due to hoop stress developed 25 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Threads and inserts Special features Other ways to assemble inserts Hot-roll plastic technique over inserts H&GT Organization - 06/25/2004 26 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Snap fit design • Snap fits rationalizes the assembly of plastic parts • Takes over other functions like bearing, spring cushioning, fixing Special features • Fixing of the joined parts is weaker than in welding, bonding, and screw joining Examples of different snap-fit designs H&GT Organization - 06/25/2004 27 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Special features Snap fit design Core and cavity for creating the snap H&GT Organization - 06/25/2004 28 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Living hinges • Materials such as Polypropylene can be molded with a living hinge • Direction of flow of material should be perpendicular to hinge Special features • Used primarily in packaging products but can have engineering applications too H&GT Organization - 06/25/2004 Some examples of living hinges. Example of basic polypropylene integral hinge design (All dimensions are in inches.) 29 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Press-fits • Press fit is when a plastic boss accepts a plastic or metal shaft or pin Special features • The features should be dimensioned correctly so as to get the right interference Press-fit conditions for two typical situations H&GT Organization - 06/25/2004 30 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Special features Press-fits Alternate press-fit designs for metal pins in a plastic hub H&GT Organization - 06/25/2004 31 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Joining plastics • Ultrasonic welding • Hot-plate welding Special features • Spin welding H&GT Organization - 06/25/2004 32 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Decorating plastics • Hot stamping • Tampo printing • Offset printing • In-mold decorating Special features • Textures and colors H&GT Organization - 06/25/2004 33 Product & Technology H&GT GESC Bangalore Quality • Excellence • Support Building a New El ect ri c W o rl d H&GT Organization - 06/25/2004 34