Contact pressure between nozzle and sprue bushing is insufficient.. Nozzle orifice is larger than orifice of sprue bushing.. area of sprue bushing is inadequately cooled, check mold temp
Trang 1Melt leaks between
nozzle and sprue
bushing.
Sprue cannot be
demolded;
breaks off.
Part cannot be
demolded.
a) Molding sticks
in cavity.
b) Molding is
demolished during
demolding.
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 do not match Nozzle orifice is larger than orifice of sprue bushing
Sprue has an undercut because
1 radii of nozzle and sprue bushing do not match,
2 nozzle and sprue bushing are misaligned,
3 nozzle orifice is larger than orifice of sprue bushing,
4 sprue bushing is insufficiently polished, grooves in circum-ferential direction Polish mold, round sharp corners
Sprue is not yet solidified because
1 sprue bushing is too large and sprue too thick,
2 area of sprue bushing is inadequately cooled, check mold temperature
General: cooling time too short; mold overloaded; undercuts too deep; rough cavity surface Reduce injection speed and holding pressure; eliminate undercuts; repolish 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
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
1 9 S t e p s f o r t h e C o r r e c t i o n o f M o l d i n g
D e f e c t s D u r i n g I n j e c t i o n 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
Trang 2Distortion of
molding.
Burned spots at
the molding.
Darkening at the
molding.
Dark (black) specks.
Discoloration near
gate.
Brittle moldings.
Molding has mat
or streaky surface.
Color streaks.
Bubbles, humidity
streaks.
Formation of ruts
and clouds;
Flaking (scaling)
of molding surface.
Molding with poor
surface gloss.
Incomplete mold
filling.
Poor location of gating Poor type of gate Nonuniform mold temperature Unfavorable cross-sectional transitions Consider-able 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
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
Residence time of the material in the barrel is too long Barrel temperatures too high Rate of screw revolutions too high Gates too small
Impurities in the material; wear in the plasticating unit Use of a corrosion- and wear-resistant plasticating unit
Mold temperature too high Enlarge runners and gate Cold-slug well needed
Raise mold temperature Enlarge runners and gate Cold-slug well needed Thermally damaged or heterogeneous melt Humid material Mold surface inadequately polished Poor location of gating Poor type of gate Increase size of runners and gate and polish Cold-slug 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 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
Material is too humid, dry sufficiently
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
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
Trang 3Molding with sink
marks and shrink
holes.
Mold flashes.
Molding with visible
knit lines.
Cold slug
Jetting
Off-sized molding.
a) with uniform
shrinkage
b) with nonuniform
shrinkage
Stress cracking.
Rippled molding
surface.
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
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
Poor position of gating Objectionable type of gate Detrimental gate and runner cross sections Enlarge gate Poor mold venting Nonuniform mold cooling Raise mold temperature Raise injection speed Wall thickness of molding too small Choose cascade gating
Nozzle temperature too low, retract nozzle from sprue bushing sooner Reduce cooling of sprue bushing, enlarge nozzle bore Relocate gate that material is injected against a wall Enlarge cross section of gate
Too high a mold and melt temperature increase shrinkage Longer cooling time; higher injection pressure and increased holding-pressure time decrease shrinkage
Poor heat exchange, gate freezes too soon, heterogeneous melt
Sharp corners and edges in the mold Processing with inserts: preheat inserts, avoid sharp corners and edges
Reduce mold temperature [19.1 to 19.9]
Trang 4Figure 19.1 Point system for eliminating visible defects during injection molding of acrylics [19.1]
R e f e r e n c e s
[19.1] Punktesystem zur Beseitigung von sichtbaren SpritzgieBfehlern bei der Verarbeitung von
Resarit Acrylformmassen (PMMA) Table by Resart-Ihm AG, Mainz, 1987.
[19.2] Barich, G.: Haufig auftretende Fehler bei der SpritzgieBverarbeitung von Thermoplasten
und ihre moglichen Ursachen Plastverarbeiter, 33 (1982), 11, pp 1361-1365.
[19.3] SpritzguB-Hostalen PP Handbook, Farbwerke Hoechst AG, Frankfurt, 1965.
[19.4] Strack-Normalien fur Formwerkzeuge Handbook of Standards, Strack-Norma GmbH,
Wuppertal.
[19.5] Mink, W.: Grundziige der SpritzgieBtechnik Kunststoffbucherei Vol 2 Zechner +
Huthig, Speyer, Wien, Zurich, 1966.
[19.6] Schwittay, D.: Thermoplaste - Verarbeitungsdaten fiir den SpritzgieBer Publication,
Bayer AG, Leverkusen, 1979.
[19.7] Spritzen - kurz und biindig Publication 4th Ed., Demag Kunststofftechnik, Niirnberg,
1982.
[19.8] Verarbeitungsdaten fiir den SpritzgieBer Publicaton, Bayer AG, Leverkusen, 1986 [19.9] Poppe, E A.; Leidig, K.; Schirmer, K.: Die TopTen der SpritzgieBprobleme DuPont de
Nemours GmbH, Bad Homburg.
Correction of machine setting Visible defect on molding Correction of mold Injection pressure too low
Injection pressure too high
Injection rate too low
Injection rate too high
Melt temperature too low
Melt temperature too high
Material is decomposing
Contamination with foreign
matter
Screw is worn
out-transports air
Shut-off nozzle defect or
of poor design
Sink marks Bubbles, voids
Burned spots
Flow marks near gate Weld lines Flash Silver streaking Yellowing Black spots Pinch marks Delamination
Poor gate location Cross-section of gate too small
Pinpoint gate too small for injected volume Poor venting
Mold temperature too low Mold temperature too high
Conflicting wall thickness
or cross section Clamping force too low Clamping force too high Mold is not sufficiently closed, damaged parting line Control of hot manifold defect manifold draws air