Analysis Of The Problems And Causes Of Injection Molds

- Jun 24, 2017-

Analysis on the Cracking of Injection Molding


Cracking, including the surface of the pieces of filamentous cracks, micro-cracking, top white, cracking and due to sticky sticky mold, flow channel sticky mold caused by or traumatic crisis, according to the cracking time mined mold cracking and application cracking. Mainly due to the following reasons:


Processing:


(1) processing pressure is too large, too fast, more charge, injection, packing time is too long, will cause excessive stress and cracking.


(2) to adjust the mold speed and pressure to prevent rapid tensile parts caused by stripping cracking.


(3) appropriate to increase the mold temperature, so easy to mold parts, appropriate to reduce the temperature to prevent decomposition.


(4) to prevent due to welding marks, plastic degradation caused by low mechanical strength and cracking.


(5) appropriate use of release agent, pay attention to often eliminate the mold surface attached to the gas mist and other substances.


(6) residual stress of the workpiece, can be immediately after the formation of annealing heat treatment to eliminate the internal stress and reduce the formation of cracks.


2. Mold:


(1) the top to balance, such as the number of mandrels, cross-sectional area should be sufficient, stripping slope should be sufficient, the cavity surface should be smooth enough, so as to prevent the external force caused by the residual stress concentration and cracking.


(2) parts structure can not be too thin, the transition part should try to use circular transition, to avoid sharp corners, chamfer caused by stress concentration.


(3) as little as possible with metal inserts, to prevent the insert and the workpiece shrinkage caused by different internal stress increases.


(4) on the deep bottom parts should be set to the appropriate stripping into the air intake, to prevent the formation of vacuum negative pressure.


(5) the main channel is enough to make the cast material ready to cure when the mold, so easy to stripping.


(6) the main channel bushing and nozzle joint should be to prevent the cold material drag and so that the workpiece on the fixed mold.


3. Material:


(1) the raw material content is too high, resulting in the strength of the workpiece is too low.


(2) humidity is too large, resulting in some plastic and water vapor chemical reaction, reduce the strength and the emergence of the top cracking.


(3) the material itself is not suitable for the processing of the environment or poor quality, pollution will cause cracking.


4. Machine side:


Injection molding machine plasticization capacity should be appropriate, too small plasticization is not fully mixed and become brittle, too large will be degraded.


Cause Analysis of Bubble in Injection Molding


Bubble (Vacuum Bubble) The gas is very thin and belongs to the vacuum bubble. In general, if the presence of bubbles in the moment of open molds is a matter of gas interference. The formation of the vacuum bubble is due to insufficient filling or low pressure. In the mold under the effect of rapid cooling, and the cavity angle of the fuel pull, resulting in volume loss results.


Solution:

The same time as the above-

(1) to improve the injection of energy: pressure, speed, time and volume, and improve the back pressure, so that filling fullness.


(2) to increase the temperature flow smoothly. Reduce the temperature to reduce shrinkage, appropriate to improve the mold temperature, especially the formation of the vacuum bubble part of the local mold temperature.


(3) the gate is set in the thick part of the workpiece to improve the nozzle, runner and gate flow, reduce the consumption of pressure.


(4) to improve the mold exhaust conditions.


Cause Analysis of Warping Deformation of Injection Molding


The deformation, bending and twisting of the injection molded product are mainly due to the fact that the shrinkage of the flow direction in the plastic molding is larger than that in the vertical direction, so that the shrinkage rate of the workpiece is different and warped, and the injection molding is inevitable The internal residual material has a large internal stress caused by warping, these are high stress orientation caused by the deformation of the performance. So fundamentally speaking, the mold design determines the warping tendency of parts, to change the molding conditions to suppress this tendency is very difficult, and ultimately solve the problem must be from the mold design and improvement. This phenomenon is mainly caused by the following aspects:


1. Mold:


(1) the thickness of the workpiece, the quality should be uniform.


(2) the cooling system is designed to make the mold cavity part of the temperature uniformity, the casting system to make the flow symmetry to avoid the flow direction, the shrinkage of different caused by warping, appropriate thicker part of the more difficult to shape the mainstream, Road, try to eliminate the cavity of the density difference, the pressure difference, the temperature difference.


(3) the thickness of the transition zone and the corner should be smooth enough, to have a good mold release, such as increased demoulding margin, to improve the surface of the polishing, the top of the system to maintain balance.


(4) exhaust to be good.


(5) to increase the wall thickness of the workpiece or increase the direction of anti-warping, reinforced by the reinforcement to enhance the anti-warping ability.


(6) mold material used by the lack of strength.


2. Plastic:


Crystalline type than the non-crystalline plastic warping deformation opportunities, coupled with the crystalline plastic can be used with the cooling rate increases with the cooling rate decreases, the shrinkage of the crystallization process to correct the warping deformation.


3. Processing:


(1) injection pressure is too high, the holding time is too long, the melt temperature is too low too fast will cause internal stress increases and warping deformation.


(2) mold temperature is too high, the cooling time is too short, so that when the mold part of the overheating and the emergence of deformation.


(3) to reduce the screw speed and back pressure to reduce the density to limit the generation of internal stress while maintaining a minimum charge.


(4) if necessary, easy to warp deformation of the parts of the mold after the soft shape or demoulding after the treatment of rice.


Injection molding products color bar color line color flower analysis


The appearance of this defect is mainly the use of masterbatch colored plastic parts more often the problem, although the masterbatch coloring in the color stability, color purity and color migration and other aspects are better than dry powder coloring, dyeing Coloring, but the distribution, that is, the color of the pigment in the mixing of the degree of uniformity is relatively poor, the finished product naturally with a regional color difference. The main solution:


(1) to increase the temperature of the feeding section, in particular the temperature of the back end of the feeding section, so that the temperature close to or slightly higher than the melting section of the temperature, so that the masterbatch into the melting section as soon as possible after melting to promote uniform mixing with the dilution to increase the liquid mixing opportunities.


(2) in the screw speed is constant, increase the back pressure to the barrel of the melt temperature, shear effect has been improved.


(3) modify the mold, especially the pouring system, such as the gate is too wide, when the melt through the turbulence effect is poor, the temperature is not high, so uneven, ribbon cavity, should be narrowed.


Analysis on the Cause of Shrinkage of Injection Molding


Injection molding process, the product shrinkage is a more common phenomenon. The main reasons for this are:


1. Machine side:


(1) nozzle hole is too large to cause the flow of reflow and shrinkage, too small resistance to excessive shrinkage.


(2) lack of clamping force caused by flash will shrink, should check the clamping system is a problem.


(3) plasticization should be selected insufficient plasticization of the machine, check the screw and the barrel is worn.


2. Mold:


(1) parts designed to make the wall thickness uniform, to ensure consistency.


(2) mold cooling, heating system to ensure that the temperature of the various parts of the same.


(3) pouring system to ensure smooth, the resistance can not be too large, such as the main channel, shunt, gate size should be appropriate, smooth enough to transition zone to circular transition.


(4) the thickness of the thin pieces should be to ensure smooth flow of materials, thick-walled parts should reduce the mold temperature.


(5) gate to be symmetrical set, as far as possible in the thick parts of the workpiece, should increase the volume of cold wells.


3. Plastic:


Crystalline plastic than non-crystalline plastic contraction calendar damage, processing should be appropriate to increase the amount of material, or in the plastic addition agent, to speed up the crystallization, reduce the shrinkage depression.


4. Processing:


(1) barrel temperature is too high, the volume change, especially the furnace temperature, poor mobility of plastic should be appropriate to improve the temperature, to ensure smooth.


(2) injection pressure, speed, back pressure is too low, the injection time is too short, so that the amount of material or density and shrinkage pressure, speed, back pressure is too large, too long caused by flash and shrink.


(3) feeding capacity that is too large when the buffer pressure injection pressure, too small, lack of material.


(4) for the accuracy of the parts do not require the completion of the injection pressure, the outer layer of the basic condensation of hardening and sandwich parts are still soft and ejected parts, early die, let it in the air or hot water slowly cooling , Can make the contraction of the depression gently and not so conspicuous and does not affect the use.


Analysis on the Causes of Transparent Defects in Injection Molding

Melting, polished, polystyrene, plexiglass transparent parts, and sometimes through the light can see some sparkling filament-like crayon. These crayons are also called scratches or cracks. This is due to the stress in the vertical direction of the tensile stress, and the use of the polymer molecules in the heavy flow direction and the non-orientation partial fold rate difference is shown.


Solution:


(1) to eliminate the interference of gases and other impurities, the plastic fully dry.


(2) to reduce the material temperature, sub-regulation of the cylinder temperature, the appropriate increase in mold temperature.


(3) increase the injection pressure, reduce the injection speed.


(4) increase or decrease the pre-molded back pressure and reduce the screw speed.


(5) to improve the flow channel and cavity exhaust conditions.


(6) clean the nozzle, the flow channel and the gate may be blocked.


(7) to shorten the molding cycle, after stripping can be used to eliminate the removal of the cue method: polystyrene at 78 ℃ for 15 minutes, or 50 ℃ for 1 hour, the polycarbonate, heated to 160 ℃ above to maintain a few minutes The


Analysis on the Uneven Color of Injection Molding


The main reasons for the uneven color of injection molding products and solutions are as follows:


(1) poor diffusion of colorants, this situation is often the gate near the pattern.


(2) plastic or colorant thermal stability is poor, to stabilize the color of parts, must be strictly fixed production conditions, especially the material temperature, material and production cycle.


(3) on the crystalline plastic, try to make parts of the parts of the cooling rate of the same, for the wall thickness difference between the larger parts, colorants can be used to mask the color difference, the wall thickness is more uniform parts to be fixed material temperature mold temperature The


(4) the shape of the parts and gate form, the location of the plastic filling the impact of some parts of the parts to produce color, if necessary, to be modified.

Analysis on the Causes of Color and Gloss Defects in Injection Molding



Under normal circumstances, the surface of the injection molding products are mainly composed of the type of plastic, colorants and mold surface finish determined. But often because of some other causes of the product surface color and gloss defects, dark surface and other defects. Cause this and the solution is analyzed as follows:


(1) mold finish is poor, the cavity surface rust, mold exhaust bad.


(2) mold pouring system is flawed, should increase the cold material wells, increase the flow channel, polished main channel, shunt and gate.


(3) temperature and mold temperature is low, if necessary, can be used local heating gate.


(4) processing pressure is too low, the speed is too slow, lack of injection time, back pressure is insufficient, resulting in poor density and make the surface dark.


(5) plastic to fully plastic, but to prevent the degradation of materials, heat to be stable, the cooling should be sufficient, especially thick-walled.


(6) to prevent the cold material into the parts, if necessary, use self-locking spring or reduce the nozzle temperature.


(7) the use of excessive raw materials, plastic or colorant poor quality, mixed with water vapor or other impurities, the use of poor quality lubricant.


(8) clamping force should be enough.

Cause Analysis of Crazing of Injection Molding

Injection molded products, including surface bubbles and internal pores. The main cause of the defect is the interference of the gas (mainly water vapor, decomposition gas, solvent gas, air). The specific reasons are as follows:


1. Machine side:


(1) barrel, screw wear or plastic head, the existence of plastic tape dead ends, long-term heat and decomposition.


(2) heating system out of control, resulting in high temperature decomposition, should check the thermocouple, heating rings and other heating elements are problematic. Improper screw design, resulting in a solution or easy to bring into the air.


2. Mold:


(1) poor exhaust.


(2) mold in the flow channel, gate, cavity friction resistance, resulting in local overheating and decomposition.


(3) gate, cavity distribution is uneven, unreasonable cooling system will cause the heat imbalance and local overheating or blocking the air channel.


(4) The cooling passage leaks into the cavity.


3. Plastic:


(1) plastic humidity, add more than the proportion of recycled materials or contain harmful scraps (broken material easily decomposed), should be fully dry plastic and eliminate scraps.


(2) from the atmosphere moisture absorption or moisture from the colorant, the colorants should also be dry, it is best to install the dryer on the machine.


(3) the amount of lubricant, stabilizer, etc. added in the plastic or mixed with the uneven, or the plastic itself with a volatile solvent. Mixed plastic heat degree is difficult to take into account when there will be decomposition.


(4) plastic contaminated, mixed with other plastic.


4. Processing:


(1) set the temperature, pressure, speed, back pressure, melt motor speed is too high to cause decomposition, or pressure, the speed is too low, injection time, pressure is not sufficient, back pressure is too low, due to failure to obtain high pressure and density Insufficient can not melt the gas and appear crazing, should be set to the appropriate temperature, pressure, speed and time and the use of multi-stage injection speed.


(2) back pressure is low, fast speed so that the air into the barrel, with the melt into the mold, the cycle is too long when the heat in the barrel is too long and decomposition.


(3) lack of material, feeding buffer pad is too large, too low temperature or mold temperature is too low affect the material flow and molding pressure, to promote the formation of bubbles.



Analysis of the Causes of Weld Seam in Injection Molding


Fused plastic in the cavity due to encounter insert holes, flow rate is not coherent area, filling material flow interrupted area and multi-stranded form of convergence, because it can not be completely fused to produce a linear weld. In addition, in the occurrence of gate injection mold will also generate weld joints, weld joints, such as the strength of poor performance. The main reasons are as follows:


1. Processing:


(1) injection pressure, the speed is too low, the barrel temperature, mold temperature is too low, resulting in the melt into the mold premature cooling and welding seam.


(2) injection pressure, the speed is too high, there will be jet and the emergence of welding seam.


(3) should increase the speed, increase the back pressure pressure to reduce the viscosity of plastic, the density increases.


(4) plastic to dry good, renewable materials should be less, the amount of release agent too much or the quality of the weld will appear bad.


(5) to reduce the clamping force, easy exhaust.


2. Mold:


(1) the same type of cavity gate too much, should reduce the gate or symmetrical settings, or as close as possible to the weld seam settings.


(2) bad at the weld, should be set up exhaust system.


(3) the runner is too large, the gauging system is not properly sized, the gate is opened as far as possible to avoid the melt flow around the insert hole, or as little as possible with the insert.


(4) wall thickness change is too large, or wall thickness is too thin, should make the workpiece wall thickness uniformity.


(5) if necessary, should be opened in the welding seam fusion welding joints from the parts.


3. Plastic:


(1) for poor mobility or heat of the plastic should be appropriate to add lubricant and stabilizer.


(2) plastic containing impurities, if necessary, for the quality of good plastic.

Cause Analysis of Seismic Pattern of Injection Molding

PS and other rigid plastic parts in the vicinity of its gate surface to the gate as the center of the formation of dense ripples, sometimes called the shock pattern. The reason is that the melt viscosity is too large and stagnant form of filling, the front of a material contact with the cavity surface will soon shrink condensation, and then the melt and expand the contraction of the cold material to continue the process Constantly alternating to make the material in the forward formation of the surface shock pattern.


Solution:


(1) to improve the barrel temperature, especially the nozzle temperature, but also improve the mold temperature.


(2) to improve the injection pressure and speed, so that the rapid filling mold cavity.


(3) to improve the flow channel, gate size, to prevent excessive resistance.


(4) mold exhaust to be good, to set a large enough cold wells.


(5) parts do not design too thin.


Analysis of the Causes of Swelling and Bubble of Injection Molding


Some plastic parts in the mold after stripping, and soon on the back of the metal insert or in the special thick parts of the swelling or bubble. This is because the plastic is not completely cooled hardening of the plastic pressure caused by the release of gas caused by expansion.


solution:


1. Effective cooling. Reduce the mold temperature, extend the mold time, reduce the material drying and processing temperature.


2. Reduce the filling speed, reduce the forming cycle, reduce the flow resistance.


3. Increase the packing pressure and time.


4. Improve the workpiece wall is too thick or thick changes in the situation.



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