Sign up for the PlasticsToday NewsFeed newsletter.
Sponsored By
Practical Remedies for Thermoset Surface Defects
April 1, 1997
9 Min Read
.svg?width=850&auto=webp&quality=95&format=jpg&disable=upscale "Practical Remedies for Thermoset Surface Defects")
Does this scene sound familiar? A noticeable surface defect occurs duringmoulding or trials. The problem has to be solved, and so it is, often bytrying various options until something clicks. Afterwards, the solutionand the steps needed to reach it are generally forgotten - unless the sameproblem occurs again, and the same people happen to be there to solve it.If the problem solvers are not there, or if it's a different problem, thewhole process is repeated. This kind of wasteful repetitious problem solvingwould not occur if previous solutions were saved, organized, and easy tofind when needed.
The professionals at the Kunststoff-Institut für die MittelständischeWirtschaft (KIMW) in Lüdenscheid, Germany, have researched and cataloguedthe 25 most common surface defects found in injection and compression mouldingof thermoset materials. They have also summarized the most probable causesfor each problem and created systematic troubleshooting guides for injectionand compression moulding.
Following are five of the most common surface defects, followed bytheir causes and the troubleshooting guide for injection moulding.
Porosity. Individual grainsare
|
POROSITY
Description: Uncompressed areas appearing mostly at theend of a flow path or around weld lines, also at domes, ribs, and thin-wallareas. The surface is rough, there are gloss differences, and melting isincomplete, possibly with colour changes.
Cause: Mostly insufficient compacting of the mouldingcompound. Possible reasons include low shot volume, insufficient pressuretransfer, insufficient venting to allow volumetric filling (possible dieseleffect).
Troubleshooting:
Residual melt cushion too small?
If Yes...
Increase backpressure.
Check steadiness of plasticizing time (bad feeding, no material inhopper).
Check mould temperatures (control fluctuation).
Run process with melt cushion.
Other possibilities:
Minimize temperature fluctuations by better thermal design (positionof temperature sensors/cartridge heaters, better control).
Avoid restricted flow areas in gating/feed system.
If No...
Sink marks directly after moulding?
If Yes...
Increase metering stroke.
Increase injection pressure.
Increase holding pressure.
Increase holding time
(check part weight).
Reduce injection rate.
Adopt ventilation program.
Other possibilities:
Check moulding compound
iscosity, reactivity.
Install ventilation.
Enlarge gates.
Check screw wear.
If No...
Check ventilation, clean if needed.
Check temperature distribution in mould and adapt.
Other possibilities:
Avoid restricted flow areas inside mould.
Check gating/feed system geometry for evenness, change if needed.
Blister. Swollen surface ofa moulded part. |
BLISTERS
Description: Blisters are gas inclusions that can causedeformations on the surface of the moulded part. They can appear as largeareas spread over the entire moulded part (undercuring), or as small blisters(overcuring, pimples, fish eyes) that can be either local or spread overthe part (outer layer cures too fast). Not to be confused with elevations,which can be the result of mould damage, blisters do not always appearin one place.
Small blisters result from the
|
Cause: Small Blisters. Pimples or fish eyes, as they areoften called, occur when the peripheral layer cures too fast compared tothe rest of the part and volatiles in the compound cannot escape duringcuring. After demoulding, the pressure gradient forces the gas incursionstoward the outer layer but the high crosslinking permits only slight swellingor bulges.
Cause: Large Blisters. The peripheral layer of the partdoes not completely cure and remains soft. When gas pressure forces volatilesin the compound to the outer layer, lack of counter pressure allows large-areaswelling or cracks.
Troubleshooting:
Do small blisters appear (pimples, fish eyes)?
If Yes...
Reduce mould wall temperature.
Check temperature distribution in mould.
Reduce curing time.
Reduce injection rate.
Increase holding pressure.
Increase holding time.
Apply venting cycle/check venting.
Increase cylinder/nozzle temperature above 100¡C (212¡F)only if using polycondensating moulding compounds.
Optimize back pressure.
Optimize screw speed.
Other possibilities:
Use drier compounds, possibly pre-dried.
Optimize evenness of mould heating.
If No...
Do large-area blisters appear?
If Yes...
Increase mould wall temperature.
Increase curing time.
Check temperature distribution and adapt if needed.
Optimize injection rate.
Increase backpressure.
Increase screw speed.
Increase cylinder/nozzle temperature above 100¡C (212¡F)only if using polycondensating moulding compounds.
Other possibilities:
Use drier moulding compounds, possibly pre-dried.
Decrease nozzle diameter.
CRACKS
Description: Small gaps that often appear near gates, wallthickness variations, weld lines, restricted flow areas, blisters, inserts,or sharp edges.
Cracks at the gate of an
|
Causes: Anisotropic shrinkage, varying temperatures, andprevented or obstructed shrinkage of gas incursions can cause internalstress. External stress can come from demoulding forces, undercuts forexample, or from mechanical forces. The internal stress caused by shrinkageonto the core during curing can result in stress peaks in areas such ascorners and thickness variations.
Cracks at the gate can result from the stress differences caused bythe pressure gradient between the gate and the end of the flow path duringfilling. Since the material has cured at the cavity wall, cracks can appearin areas with the highest pressure. In a filled cavity, the holding pressurecan exert enough force on the already cured gate area to cause stress cracks.
Stress cracks can also be caused by strong force during the demouldingof parts with tunnel gates or by a pressure gradient in the feed systemcaused by material backflow. The cause of the latter is holding pressurethat is too low, too short or nonexistent.
Before troubleshooting, the following inspections shouldbe carried out:
Check mould for temperature differences, which should be less than5°C (9°F).
Are cracks caused by undercuts?
Draws should be large enough.
Check if ejectors are causing large deformations.
Optimize injector speed.
Troubleshooting:
Do cracks appear near gate and is part completely compressed?
If Yes...
Lower holding pressure.
Decrease holding pressure time.
Decrease injection rate.
Optimize changeover time.
Other possibilities:
Use material with less shrinkage.
Enlarge gate.
If No...
Stress crack caused when |
Do cracks appear near sharp edges, bypasses, cores, or restricted flowareas?
If Yes...
Decrease injection rate.
Optimize holding pressure.
Optimize holding pressure time.
Increase backpressure.
Increase screw speed.
Other possibilities:
Avoid sharp edges and by-passes.
Optimize gate type and position.
If No...
Do cracks seem due to shrinkage differences caused by material accumulationssuch as near wall thickness variations, domes and ribs?
If Yes...
Increase holding pressure.
Increase holding pressure time.
Increase injection rate.
Increase cylinder nozzle temperature to >100°C (212°F) forpolycondensating moulding compounds.
Increase backpressure.
Use smaller machine nozzle.
Optimize mould wall temperature.
Other possibilities:
Pre-dry material or use compound with less shrinkage.
Enlarge gates.
Smooth mould wall if too rough in direction of demould.
If No...
Are inserts used?
If Yes...
Preheat inserts.
Increase holding pressure.
Increase holding pressure time.
Decrease injection rate.
Decrease mould temperature.
Cracks. Restricted flow area |
If No...
Apart from the cracks, is there excessive flash?
If Yes...
Consider stiffening mould.
Provide center support.
Insert intermediate plate, etc.
If None of the remedies has helped...
Other possibilities:
Injection-compression moulding.
Compression moulding.
Restricted flow zones include in front of cores and domes,around ribs, etc. For example, moulding compound flows farther in frontof a core, but next to the core the cavity fills faster and high temperaturesoccur. The compound in these restricted flow zones cures faster than inflow areas. The different levels of crosslinking in the resulting phaseboundaries result in shrinkage and cracks.
CLOUDS
Clouds are created according |
Description: An area of the part with boundaries transverseto the direction of flow that appears matte compared to the rest of thesurface. This defect often occurs around gates and after wall thicknessvariations. Often these clouds can be wiped off.
Causes: Lubricant content in the moulding compound istoo high. Short pressure releases - for example, the switchover from injectionto holding pressure or a wall thickness variation - cause lubricant depositsto settle at the flow front. During injection, the pressure drop allowsthis excess lubricant to penetrate into the moulding compound. At firstit surrounds the injection point and then settles, in the case of a frontallyflowing moulding compound, on the mould wall.
Troubleshooting
Does the defect appear near the gate?
If Yes...
Avoid pressure relief during changeover from injection to holding pressure.
Cloud formation at a pin gate
Reduce injection speed.
Optimize changeover point.
Reduce mould wall temperature.
Other possibility:
Reduce lubricant content in moulding compound.
If No...
Reduce injection speed during cavity filling.
Reduce mould temperature.
Other possibility:
Reduce lubricant in moulding compound.
COLOUR STREAKS
Color streaks clearly show |
Description: Streaks visible on the part surface as discolourations.They are oriented either as oblong stripes (e.g., in restricted flow areas)or transverse to the flow direction (at the flow front).
Causes: Colour streaks are the result of uneven distributionof the material components (small or still highly viscous) or of differentorientation of the colour pigments on the surface of the part. Thermaldamage of the moulding compound can also cause colour changes.
Troubleshooting:
Is the moulding compound contaminated or has the batch been changed?
If Yes...
Use cleaning material to clean the hopper.
This cloud along the boundary |
Other possibilities:
Check moulding compound for contamination.
Reduce dust content in surrounding air (grayness).
If No...
Are mould or screw worn?
If Yes...
Other possibilities:
Change screw or rework mould.
Possibly coat mould.
Adapt moulding compound, e.g. round off glass fibers.
If No...
Colour streaks. Shearing ofthe |
Are colour differences a result of thermal change?
If Yes...
Reduce injection speed.
Increase machine nozzle diameter.
Reduce backpressure.
Reduce rotational speed.
Reduce mould temperature.
Check temperature distribution.
Colour streaks. Stretching effect |
Other possibility:
Avoid sharp edges and transitions.
If No...
Increase backpressure.
Increase injection rate.
Increase cylinder/nozzle temperature.
Increase screw speed.
All 25 problems and solutions are in a compact handbook: Guide to SurfaceDefects on Thermoset Compression and Injection Moulded Parts, availablein English from the IMM Book Club. Call (1) 303-321-2322 or fax (1) 303-321-3552for ordering information, or write Deb Golanty, IMM Book Club, 55 MadisonSt., Suite 770, Denver, Colorado 80206, USA.
Contact Information: Kunststoff-Institut Mrs. S.Köhler or Mr. G.Oussios Karolinenstraße 1 58507 Lüdenscheid,Germany. Phone: (49) 2351 1064 191 Fax: (49) 2351 1064 190
You May Also Like
