expensive injection molding defects!
TRANSCRIPT
kunststofftechnische Software GmbH
EXPENSIVE INJECTION MOLDING DEFECTS! How to eliminate and avoid the most frequent defects in plastics injection molding
REASON Air cannot escape from the mold and is compressed at the end of the flow path. The compression causes the air to heat up to such an extent that burn marks result.
Elimination of the defect via the material Use materials or additives that set free less gas
Elimination of the defect via the process parameters Reduce the injection speed, if possible reduce the clamping force
Elimination of the defect via the mold Add venting channels and check for depositions
Avoid defects Identify the missing venting numerically with CADMOULD beforehand.
1.DIESEL EFFECT Burn marks at the end of the flow path
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REASON Nonuniform shrinkage
Elimination of the defect via the material Use a material with less shrinkage
Elimination of the defect via the process parameters Higher packing pressure, longer packing pressure time, lower mold temperature, lower injection speed, later switchover time
Elimination of the defect via the mold Move the injection point to shorten the flow paths or avoid the weld line, employ cascade filling, add venting channels
Avoid defects Sink marks can be determined with CADMOULD beforehand.
2.SINK MARKS Recesses or depressions on the surface
REASON As the melt does not adhere suffi-ciently to the wall, no uniform flow front develops, leading to a jet of melt instead
Elimination of the defect via the process parameters Reduce the injection speed, increase the melt tem-perature, increase the mold temperature
Elimination of the defect via the mold Move the runner, optimize the gate and runner.
3.JETTING A strand of melt emerges from the gate, generally in the shape of a snake
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Source: Bakelite AG
Elimination of the defect via the material Set a later switchover time, check the shot volume and residual melt cushion, increase the injection pressure or injection speed
Elimination of the defect via the mold Add venting, increase wall thicknesses, insert flow aids, balance the runner system
Avoid defects CADMOULD shows you the unfilled areas so that you can adjust the parameters prior to starting on mold construction.
4.INCOMPLETE FILLING The mold is not completely filled at the end of the flow path or at thin areas
REASON Thin areas or points at the end of the flow path cannot be filled on account of the melt freezing prematurely or insuffi-cient venting. It is also possible that the shot volume or residual melt cushion is too small.
REASON Surface gloss depends on the roughness of the surface, which affects reflectivity. The surface of the injection mold-ed part depends on the nature of the mold surface and how this is reproduced. Surface reproduction can vary on account of the material or the process parameters.
Elimination of the defect via the process parameters Increase the injection speed, increase the mold temperature, increase the melt temperature
Elimination of the defect via the mold Check cooling homogeneity with CADMOULD
5.DIFFERENCES IN GLOSS The surface gloss of the part varies or does not meet up to expectations
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REASON Melt cools rapidly on the mold wall, reducing its flowability and causing the flow front to stop at the mold wall. Following this, incoming melt flows over the rapidly-cooled areas of the outer layer, thus giving rise to a grooved structure.
6.GROOVE EFFECT The surface has a grooved structure
Elimination of the defect via the material Use material with a lower viscosity
Elimination of the defect via the process parameters Increase the injection rate, delay switchover, increase the mold temperature, in-crease the melt temperature
Elimination of the defect via the mold Shorten the flow paths (e.g. by having several injection points), increase wall thicknesses
Source: Bakelite AG
Elimination of the defect via the material Use a higher-viscosity material
Elimination of the defect via the process parameters Increase the clamping force, reduce the injection speed, reduce the packing pressure, reduce the melt temperature, bring the switchover forward
Elimination of the defect via the mold Check the parting plane for damage/deposition, balance the filling through other/additional injection points (=reduce the pressure required), increase the rigidity of the mold
Avoid defects CADMOULD shows you the clamping force that is required. You can then estimate whether the mold is sufficiently rigid
REASON The mold or mold components undergo deformation on account of the high cavi-ty pressure, causing a gap to form in the parting plane. Melt can penetrate this gap.
7.FLASH FORMATION Flash develops in the parting plane of the injection molded parts
Source: Bakelite AG
REASON When two flow fronts meet up, they do not weld together suffi-ciently, because the melt has already cooled down too much.
Elimination of the defect via the material Use a material with a lower viscosity
Elimination of the defect via the process parameters Increase the injection speed, increase the packing pressure, increase the melt temperature, increase the mold temperature.
Elimination of the defect via the mold Move the gate to reduce the flow paths, add venting channels
Avoid defects CADMOULD helps balancing the filling and thus reduce the clamping force in general and the local penetration pressure
8.VISIBLE WELD LINE Weld lines can be clearly seen on account of their shape and/or color
REASON High shrinkage causes the melt to contract as it cools, but no further melt flows into the mold, causing holes to develop.
Elimination of the defect via the material Use a material with a lower viscosity or less shrink-age
Elimination of the defect via the process parameters Increase the packing pressure time or level, re-duce the mold temperature, reduce the injection speed, increase the residual melt cushion
Elimination of the defect via the mold Reduce mass accumulations and changes in wall thickness, optimize the cooling, increase the sprue/gate cross-section
Avoid defects While holes/bubbles cannot be depicted numeric, they can be determined from the simulation at points with increased shrinkage that cannot be eliminated through the packing pressure.
9.VACUUM VOIDS – HOLES/BUBBLES
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REASON There is air inside the melt. This can be caused by non-melted pellets or a leaky nozzle.
Elimination of the defect via the material Use finer pellets
Elimination of the defect via the process parameters Reduce the injection speed, increase the melt temperature, increase the mold temperature
Elimination of the defect via the mold Make sure that the nozzle is not leaking
10.AIR POCKETS Air pockets inside the injection molded part
Source: Bakelite AG
REASON Such high orientations and strains develop close to the gate that the surface layer tears open and the surface becomes matt.
Elimination of the defect via the material Use material with a lower viscosity
Elimination of the defect via the process parameters Reduce the injection speed, increase the melt temperature, increase the mold temperature
Elimination of the defect via the mold Increase the size of the gate, reposition the runner, avoid sharp-edged deflections (e.g. at the runner).
REASON Color pigments are not distributed homogeneously. It is also possible that the color pigment and plastic are incompatible.
Elimination of the defect via the material Ensure the compatibility of the components
Elimination of the defect via the process parameters Increase the back pressure, increase the injection speed
12.COLOR STREAKS Part has an inhomo- geneous color
11.MATT AREAS CLOSE TO THE GATE
Alignment of metallic effect pigments through the flow process
REASON If the pellets are moist, the water in the melt will evap-orate. The bubbles can reach the surface and generate streaks there.
13.SILVER STREAKS/ MOISTURE STREAKS U-shaped streaks pointing in the direction of flow
Elimination of the defect via the material Dry the material
Elimination of the defect via the process parameters Increase the back pressure
Elimination of the defect via the mold Ensure that cooling channels are not leaking
REASON Molecule chains degrade as a result of excessively high thermal loading
Elimination of the defect via the material Shorten the drying time or reduce the temperature, thermal stabilization
Elimination of the defect via the process parameters Reduce the melt temperature, reduce the temperature of the hot runner, reduce the injection speed, reduce the melt cushion, reduce the dwell time of the melt in the plasticizing unit/hot run-ner, check plasticizing unit/hot runner for dead spots, reduce screw speed.
Elimination of the defect via the mold Increase flow cross-sections
Avoid defects Through temperature calculations in CADMOULD you can estimate whether the material will suffer thermal damage.
14.CHARRED STREAKS Yellow, brown, black or silver discoloration
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REASON A part becomes deformed through dissimilar shrinkage as internal stresses develop. Dissimilar shrink-age is caused by inhomogeneous temperature control, different cooling
rates, orientation during filling, wall thickness differences or non-uniform packing pressure. If difficulties are encountered during demolding, the part can undergo further deformation.
Elimination of the defect via the material Differences exist in the shrinkage behavior of amorphous, semi-crystalline and filled or fiber-re-inforced materials
Elimination of the defect via the process parameters Adjust the melt and mold temperature, and also the packing pressure time and level, possibly ad-just the injection speed; also increase the cooling time in the event of deformation during demolding
Elimination of the defect via the mold Avoid differences in wall thickness and accumu-lations of mass, ensure uniform cooling, select ribs, injection points and gate in such a way that as uniform as possible a packing pressure can act for a long time; incorporate draft angles in order to avoid deformation during demolding.
Avoid defects Calculate the shrinkage and warpage in CADMOULD.
15.WARPAGE The part deforms
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