expensive injection molding defects!

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


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


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


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


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 inhomogeneous 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


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

CADMOULD® 3D-F

CADMOULD® 3D-F ADVANTAGES IN THE WORK PROCESS

— Integrated CAD independent adaptation of wall thicknesses of molded parts

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compensation of shrinkage and warpage — Measurement of part properties like dimensions, flatnesses etc.

with integrated measurement devices — Modular design of the software for customized adaption to user

requirements

The Cadmould® 3D-F product family has been developed successfully for more than 30 years in Germany. Cadmould® 3D-F uses the highly precise 3D-F method especially developed for injection molding simulation, making Cadmould® 3D-F the superior simulation solution. Cadmould® 3D-F is a high-tech software product

which is however, easy to use, very fast and precise. Simcon participates continuously in national and European R&D projects, to enhance the simulation quality and to find new fields of application for virtu-al engineering. Our goal is to find technical solutions that supply our customers with reliable solutions during the development of complex plastics parts.


THE EXPERT SOFTWARE FOR OPTIMIZATION OF PART, MOLD AND PROCESS

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CADMOULD® 3D-F PERFORMANCE FEATURES — Generates geometry and mesh automatically — Uses the unique Cadmould® 3D-F framework method — Provides very high resolution in areas of high gradients (e.g. temperatures and shear velocities across

the wall thickness) — Provides very exact simulation results for plastic parts

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expensive injection molding defects!

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