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injection_molding_fault_and_prevention.pdf

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INJECTION_MOLDING_FAULT_AND_PREVENTION
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Injection molding faults in styrene copolymers and their preventionI. Injection molding faults 1. Streaks 1.1 Moisture streaks/splay marks 1.2 Burning streaks/silver streaks 1.3 Dark streaks due to indrawn air 1.3 Color streaks 2. Peeling/delamination 3. Weld lines 4. Air entrapments/bubble formation 5. Sink marks 6. Voids 7. Glossy spots or differences in gloss/mat spots 8. Microcracks, crazing, stress whitening 9. Diesel effect 10. Demolding problems 11. Push marks (“tiger stripes/tiger lines”) 12. Record effect 13. Short shot 14. Flash formation 15. Jetting 16. Cold slug 17. Warpage II. Overview: Control of injection molding faults by changing the processing parameters 4 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36Introduction Plastic moldings produced by the injection molding process are usually long-lived consumer goods whose suitability for the purpose in question depends not only on the properties of the finished part but also highly on the surface quality. The complex inter- play among molding and mold design, processing conditions for the raw material and the parameters for the actual process require a great deal of experi- ence for optimum results especially when focusing on the short-term removal of processing faults. On the basis of an individual case a decision has to be taken as to whether simple and rapid fault correction (by modifying the processing parameters for example) is possible or whether intervention in the design of the parts or the layout of the mold and gating is nec- essary. In conclusion the surface faults occurring most com- monly in styrene copolymers are noted and described and recommendations for their removal are set out. 31. Use material adequately predried to a residual mois- ture content of < 0.1 %. 2. Increase back pressure. Silvery or dark surface streaks and in exceptional cases black discolorations. 1. Reduction of the injection speed. 2. Avoidance of undersized gates and sharp changes in direction (shear intensive areas) in the mold. 3. Check the controller of the hot runner system and the barrel heater. 4. Reduction of melt temperature, screw speed (rpm), residence time (if necessary use a smaller plasticizing unit) and back pressure. Appearance Cause Correction Appearance Cause Correction 1. Damage to the plastic melt due to excessively high temperatures or overlong residence times resulting from gaseous decomposition products. 2. High shear heating due to small gate cross sections or by sharp changes of direction in the mold. 1. Streaks 1.1 Moisture streaks/splay marks 1.2 Burning streaks/silver streaks Usually oblong, silvery surface streaks which are open in the shape of a U counter to the direction of flow. The material pumped out of the cylinder is foamed and shows blisters on the surface. Moisture content too high; water vapor is produced dur- ing melting and this results in the surface of the molding being torn open. I. Injection molding faults 41.1 Moisture streaks 1.2 Burning streaks Fig. 1.1.1: Streaks due to an exces- sively high mois- ture content in the granules Fig. 1.1.2: Section through a torn streak; transmitted light, magnification 80:1 Fig. 1.2.1: Silvery surface streaks Fig. 1.2.2: Black discolorations on a transparent part Sprue 5Appearance Cause Correction 1.3 Dark streaks Streaks ranging in color from dark to black 1. Processing using a screw which is too deeply flighted in the feed section (air intake). 2. “Dead spots” in the plasticising unit or hot runner system. 3. Defective nonreturn valve. 4. Screw decompression is too great or too fast. 1. Raise temperature in feed section so that melting is earlier; use more suitable screw. 2. Check plasticizing unit and hot runners for zones of impeded flow and if necessary correct them. 3. Replace defective nonreturn valve. 4. Shorten the path for screw decompression or decompress at a reduced rate. 6 The cause for the formation of streaks can be deter- mined in many cases only after costly investigations, especially as the appearance of burning and moisture streaks is similar. Extensive knowledge of the plastic, the mold design and the processing are indispensable for overcoming the problem. Before launching expen- sive and time-consuming investigations the following points should be checked in the sequence given and if necessary optimized: Appearance Cause Correction 1.4 Color streaks Color differences 1. Inhomogeneous distribution of colorant; accumulations of colorant; unsuitable color batch; alignment/ orientation of the usually inorganic pigments by flow processes; thermal pigment damage. 2. “Dead spots” in the plasticising unit or hot runner system. 3. Contamination 1. Use of suitable colorants and batches; ensure good homogenization and dispersion; avoid thermal over- loading. 2. Avoid dead spots in the plasticising unit and in the hot runner system. 3. Make sure a good cleaning of the plasticising unit. Melt temperature Injection speed Residence time in the cylinder Back pressure Moisture content of granules Mold venting Cleaning of the plasticising unit Screw recovery speed7 1.3 Dark streaks Fig. 1.4.3: Color streaks in transmitted light; magnification 7:1 Fig. 1.4.1: Color streaks (inhomogeneous distribution of colorant) Fig. 1.4.2: Color streaks due to deposits of material due to “dead spots” in a hot runner system Fig. 1.3.1: Dark streaks caused by deposits of thermally damaged material from the hot runner 1.4 Color streaks8 Notches, hairlines Change in apparent color; espe- cially when inorganic decorative effect pigments are used the weld line appears as a dark line; conspicuous in dark, brilliant or transparent moldings having smooth surfaces polished to a high gloss. Appearance Cause Correction Appearance Cause Correction As far as possible, position weld lines where they have no visual or mechanical impact (flow promoters, flow inhibitors); check mold engineering: if necessary enlarge sprue channel, gate and machine nozzle; avoidance of aprupt changes in wall thickness and nonuniform mold filling; provide effective mold venting. Processing: Optimization of melt temperature, mold sur- face temperature and injection speed; new color formula- tion (organic or inorganic pigments, higher pigmentation). Use lower viscosity material. Appear in multiphase systems such as ABS or ASA. Flow fronts having an already cooled peripheral layer encounter one another and no longer allow fusion without marks. Decorative effect pigments and, e.g., glass fibers tend to straighten up in the region of the weld line (adversely affecting appearance and usually mechanical properties also). 3. Weld line 2. Peeling/Delamination Detached, slate-like surface layers, e.g. due to cross cutting; usually not easy to identify since the surface is flawless; a “skin” can often be pulled off when the surface is scratched with a blade. The molded part exhibits bubbling after warm storage. 1. High shear stresses result in the formation of layers; even in compatible multiphase systems. 2. Contamination with an incompatible thermoplastic or master batch. 1. Increase the melt temperature and reduce the injection speed. 2. Avoid contamination due to extraneous material or due to an unsuitable master batch (specifically the carrier).9 Fig. 3.3: Weld line (metallic effect coloring) Fig. 3.4: Color marking in the weld line; vertical illumination, dark field, magnification 52:1 Fig. 3.2: Weld line notch; transmitted light, polished, magnification 560:1 Fig. 3.1: Weld line notch; vertical illumination, magnification 11:1 Fig. 2.1: Peeling and delamination caused by strange material in the granules Fig. 2.2: Partial cross section through Fig. 2.1: (sample thickness 1.5 mm) Fig. 2.3: Position 1 Fig. 2.4: Position 2 Position 1 Position 2 1.5 mm 3. Weld line 2. Peeling/DelaminationAppearance Cause Correction The air entrapped during injec- tion of the melt is visible as a cavity (air bubble) in the molded part. Air entrapments should not be confused with voids. 1. During mold filling air is entrapped on account of an inopportune shape of the molding and if occluded in the peripheral region close to the surface this can give rise to bubbles. 1. As far as necessary optimize the geometry of the molding with the aid of a mold flow calculation. 2. Check design and condition of mold vents. 4. Air entrapments/bubble formation 10Fig. 4.1: Air entrapment Fig. 4.2: Air bubble cut open Air bubble 4. Air entrapments (bubble formation) (front elevation and section) 1112 Appearance Cause Correction Sink marks are always produced in regions of accumula- tion of material when the contraction in volume arising during the cooling phase cannot be adequately compen- sated for by the holding pressure. Avoid large differences in wall thickness and accumula- tions of material (e.g. by means of rib structures having large radii, fixing domes, etc.); a rib thickness of 0.5 - 0.7 times the basic wall thickness is advantageous. Optimize melt and mold temperature; set the holding pressure, holding pressure time and melt cushion to ade- quate dimensions (increase values). As far as possible gate in the thickwalled area; design the sprue and gate cross sections in keeping with the material and molded part. Cool thick-walled parts in cold water (freezing of the peripheral layer) in order to shorten the setting time. 5. Sink marks Depressions on the surface of the molded part. Appearance Cause Correction As for Item 5. Exception: When the outer skin is thick enough to absorb the shrinkage stresses. The melt in the interior is pulled towards the outside (molding surface) so that vacuum cavities are produced in the region which is still plastic. This occurs only when the molding has cooled for long enough. Avoid big differences in wall thickness and accumulations of material (e.g. by means of rib structures having large radii, fixing domes, etc.). Pay heed to correct temperature control; set the holding pressure, holding pressure time and melt cushion to adequate dimensions. As far as possible gate in the thickwalled area; design the sprue and gate cross sections in keeping with the material and molded part. Usually not discernible from the outside except in the case of transparent materials. Parts, usually thick-walled, which have been cut open reveal cavities. 6. Voids13 Sink mark Fig. 5.1: Sink mark on the visible side (Cause: Material accumulation as shown in Fig. 5.2) Fig. 5.2: Material accumulation (avoidable by suitable recessing) Melt accumulation 5. Sink marks (front elevation and section) Fig. 6.1: Voids at the end of the flow path in a cup 6. Voids14 Appearance Cause Correction Injected parts generally having a gloss which is too low or too high. Nonuniform apparent gloss or color at certain areas. 1. As a result of design related material accumulations, such as aprupt changes in wall thickness, ribs and fixing bosses sink marks are produced which give rise to glossy spots in textured surfaces. 2. Mat spots often arise on glossy parts having complex geometry (aprupt changes in wall thickness, ribs, openings) when at the same time the mold filling process is unsatisfactory. 3. On weld lines due e.g. to alignments and changed flow conditions. 4. Sprue and gate cross sections are too small. 5. Mold wall temperature, melt temperature and injection speed are not favorable. 6. Holding pressure and holding pressure time are too low. 1. Avoid material accumulations and aprupt changes in wall thickness; gate the part in the thickwalled area. 2. Optimize the part and mold filling, e.g. injection profile (multistage injection). Polish the finished part. 3. As far as possible position weld lines where they are not visible (flow promoters, flow inhibitors); mold flow studies also provide guidance. 4. Have adequately sized sprue and gate cross sections. 5. Optimize the processing temperature. Check the mold cooling and coolant flow. 6. Adjust holding pressure and holding pressure time. 7. Glossy spots or differences in gloss/mat spotsFig. 7.2: High reproduction accuracy (mat); scan- ning electron micrograph, magnification 50:1 Fig. 7.4: Poor reproduction accuracy (glossy); scan- ning electron micrograph, magnification 50:1 Fig. 7.1: Differences in gloss on account of good (mat) or poor (glossy) mold reproduction Fig. 7.3: High reproduction accuracy (mat); scan- ning electron micrograph, magnification 200:1 glossy mat Fig. 7.5: Poor reproduction accuracy (glossy); scan- ning electron micrograph, magnification 200:1 7. Glossy spots or differences in gloss/mat spots (Automobile mirror mounting) 1516 Appearance Cause Correction Due to exceeding the maximum permissible yield point as a result of the following for example. 1. Action of external force, arising for example from forcible demolding of undercuts. 2. Overstressing the component. 3. Internal stresses in the part due to inappropriate processing conditions. 4. Use of mold release sprays which may, for example, cause stress cracking. 1. Reduce the force acting on the molding from the out side or employ thermoplastics having lower susceptibility to stress whitening e.g. Terluran 877T or Luran S; optimize the mold design. 2. Improvement of the part design. 3. Increase mold surface and melt temperatures, reduce holding pressure and setting times and adjust injection speed; aim for low-stress processing in line with the requirements of the material; do not demold under residual pressure; select the ejector mechanisms and demolding drafts in such a way that troublefree demolding without relatively great force is ensured; alteration of sprue and gate conditions; modification of part design. 4. Employ suitable demolding aids. Milky to whitish cloudiness (incident light is scattered dif- fusely). 8. Microcracks, crazing and stress whitening17 Fig. 8.1: Microcracks produced by overloading and deforming under residual pressure Fig. 8.2: a) Ballpoint pen with stress whitening in the thread region (undercut) made from pure polybutadiene ABS b) The same ballpoint pen without stress whitening made from Terluran 877 T a) a b) b 8. Microcracks, crazing and stress whitening Fig. 8.3: Terluran 900-series (967 K) Terluran 877 T18 Appearance Cause Correction 1. Poor venting of mold at flow path ends or 2. Confluence of several melt fronts. In both cases the air to be displaced from the mold cavity is highly compressed and so overheated that the plastic melt chars locally. 1. Provide effective venting in the critical regions; reduce injection speed and melt temperature. 2. Identify critical points at the planning stage using a moldflow simulation for example and correct the shape, gate location and wall thickness-distribution of the part by modifications. 3. Decrease clamp force to provide a temporary solution. Scorching or black colorations at the end of the flow path or at points of confluence of melt streams (entrapped air). 9. Diesel effect19 Fig. 9.1: Scorch marks due to poor mol
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