19 S t e p s for t h e D e f e c t s Correction During Injection of M o l d i n g M o l d i n g In the following a number of molding defects, which are mostly caused by faulty mold design, are listed along with steps to remedy them A first summary is presented with Figure 19.1 It shows a point system, which has been developed to remove visible defects from acrylic moldings [19.1] However, it can also be applied to other thermoplastic materials Melt leaks between nozzle and sprue bushing Orifices of nozzle and sprue bushing are misaligned Check locating, ring, and alignment Contact pressure between nozzle and sprue bushing is insufficient Increase pressure Check uniform pressure with thin paper Proper contact results in uniform indentation Radii of nozzle and sprue bushing not match Nozzle orifice is larger than orifice of sprue bushing Sprue cannot be demolded; breaks off Sprue has an undercut because radii of nozzle and sprue bushing not match, nozzle and sprue bushing are misaligned, nozzle orifice is larger than orifice of sprue bushing, sprue bushing is insufficiently polished, grooves in circumferential direction Polish mold, round sharp corners Sprue is not yet solidified because sprue bushing is too large and sprue too thick, area of sprue bushing is inadequately cooled, check mold temperature Part cannot be demolded General: cooling time too short; mold overloaded; undercuts too deep; rough cavity surface Reduce injection speed and holding pressure; eliminate undercuts; repolish cavity a) Molding sticks in cavity Sprue or runner system has undercuts Check radii of nozzle and sprue bushing Check surface of cavity for imperfections Repolish it and round sharp corners Cavity temperature is too low Formation of vacuum Check taper Consider venting during demolding b) Molding is demolished during demolding Undercuts too deep Ejectors are placed in locations which are detrimental to proper transmission of ejection forces resulting in unacceptable pressure peaks Check cavity surface for imperfections Formation of vacuum Check taper Distortion of molding Poor location of gating Poor type of gate Nonuniform mold temperature Unfavorable cross-sectional transitions Considerable difference in wall thickness requires more than one cooling circuit Wrong mold temperature Nonuniform shrinkage Adverse switch-over point from injection to holding pressure Poor location of ejectors Burned spots at the molding Melt temperature is too high Material is overheated in narrow gates Mold is insufficiently vented Adverse flow of material Residence time too long Select smaller plasticating unit Darkening at the molding Residence time of the material in the barrel is too long Barrel temperatures too high Rate of screw revolutions too high Gates too small Dark (black) specks Impurities in the material; wear in the plasticating unit Use of a corrosion- and wear-resistant plasticating unit Discoloration near gate Mold temperature too high Enlarge runners and gate Cold-slug well needed Brittle moldings Raise mold temperature Enlarge runners and gate Cold-slug well needed Thermally damaged or heterogeneous melt Humid material Molding has mat or streaky surface Mold surface inadequately polished Poor location of gating Poor type of gate Increase size of runners and gate and polish Coldslug well needed Mold too cold (condensed humidity) Overheating of material from insufficient venting or unfavorable flow path Lower rate of screw rotation; pre-dry material Color streaks Poor mixing or separation of molding material Raise rate of screw rotation, raise back pressure Modify temperature profile of barrel Reduce feed hopper cooling Use another screw Bubbles, humidity streaks Material is too humid, dry sufficiently Formation of ruts and clouds; Flaking (scaling) of molding surface Poor gate position Raise mold and melt temperature Raise injection speed Check plasticating unit for wear Check gate Excessive temperature differential between melt and mold Raise mold temperature Enlarge runners and gate Contamination from a different material Molding with poor surface gloss Incomplete mold filling Check mold temperature Enlarge runners and gate Round sharp corners Repolish mold Runners too long or too small or both Enlarge runner system Flow in cavity is restricted Open gates Core shifting Insufficient mold venting Mold and melt temperature too low Raise injection speed and/or pressure Insufficient feeding of material, no cushion Molding with sink marks and shrink holes Runners and gate too small System solidifies before holding pressure has become effective Check mold temperature Raise against shrink holes, lower against sink marks Lower melt temperature Check cushion, lengthen holding-pressure time, raise holding pressure Lower injection speed Mold flashes Poor fit at parting line Matching surfaces damaged by e.g., remnants of material or excessive pressure Rework surfaces Clamping force is insufficient because projected area of part is too large Core shifting has caused a considerable differential in wall thickness causing flash on one side and insufficient filling on the other Mold temperature too high Injection speed and/or pressure too high Advance switch-over point Mold is not sufficiently "rigid" Platens of the clamping unit bend Reinforce platens, change machine Mold cavity is not dead center in mold Molding with visible Poor position of gating Objectionable type of gate Detrimental gate and runner cross sections Enlarge gate Poor mold venting knit lines Nonuniform mold cooling Raise mold temperature Raise injection speed Wall thickness of molding too small Choose cascade gating Cold slug Nozzle temperature too low, retract nozzle from sprue bushing sooner Reduce cooling of sprue bushing, enlarge nozzle bore Jetting Relocate gate that material is injected against a wall Enlarge cross section of gate Off-sized molding Too high a mold and melt temperature increase shrinkage Longer cooling time; higher injection pressure and increased holdingpressure time decrease shrinkage a) with uniform shrinkage b) with nonuniform shrinkage Poor heat exchange, gate freezes too soon, heterogeneous melt Stress cracking Sharp corners and edges in the mold Processing with inserts: preheat inserts, avoid sharp corners and edges Rippled molding surface Reduce mold temperature [19.1 to 19.9] Correction of machine setting Visible defect on molding Injection pressure too low Sink marks Injection pressure too high Bubbles, voids Correction of mold Poor gate location Cross-section of gate too small Pinpoint gate too small for injected volume Poor venting Injection rate too low Burned spots Injection rate too high Flow marks near gate Mold temperature too low Weld lines Mold temperature too high Flash Conflicting wall thickness or cross section Melt temperature too low Melt temperature too high Material is decomposing Contamination with foreign matter Screw is worn outtransports air Silver streaking Yellowing Black spots Pinch marks Shut-off nozzle defect or of poor design Clamping force too low Clamping force too high Mold is not sufficiently closed, damaged parting line Control of hot manifold defect manifold draws air Delamination Figure 19.1 Point system for eliminating visible defects during injection molding of acrylics [19.1] References [19.1] [19.2] [19.3] [19.4] [19.5] [19.6] [19.7] [19.8] [19.9] Punktesystem zur Beseitigung von sichtbaren SpritzgieBfehlern bei der Verarbeitung von Resarit Acrylformmassen (PMMA) Table by Resart-Ihm AG, Mainz, 1987 Barich, G.: Haufig auftretende Fehler bei der SpritzgieBverarbeitung von Thermoplasten und ihre moglichen Ursachen Plastverarbeiter, 33 (1982), 11, pp 1361-1365 SpritzguB-Hostalen PP Handbook, Farbwerke Hoechst AG, Frankfurt, 1965 Strack-Normalien fur Formwerkzeuge Handbook of Standards, Strack-Norma GmbH, Wuppertal Mink, W.: Grundziige der SpritzgieBtechnik Kunststoffbucherei Vol Zechner + Huthig, Speyer, Wien, Zurich, 1966 Schwittay, D.: Thermoplaste - Verarbeitungsdaten fiir den SpritzgieBer Publication, Bayer AG, Leverkusen, 1979 Spritzen - kurz und biindig Publication 4th Ed., Demag Kunststofftechnik, Niirnberg, 1982 Verarbeitungsdaten fiir den SpritzgieBer Publicaton, Bayer AG, Leverkusen, 1986 Poppe, E A.; Leidig, K.; Schirmer, K.: Die TopTen der SpritzgieBprobleme DuPont de Nemours GmbH, Bad Homburg