Steps for the Correction of MoldingDefects During Injection Molding

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 do not
match. Nozzle orifice is larger than orifice of sprue bushing.

Sprue cannot be
demolded;
breaks off.

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 circumferential 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.

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. 2. 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.