Twelve kinds of injection molding defect cause 360 degree depth perfect analysis

I. Analysis of the causes of cracking of injection molded products

Cracking, including the surface of the parts filamentary cracks, microcrack, top white, cracking and caused by the parts sticky mold, runner sticky mold or trauma crisis, according to the cracking time divided into demold cracking and application cracking. There are mainly the following aspects caused by.

1. Processing.

(1) processing pressure is too large, too fast, the more material filling, injection, holding pressure time is too long, will cause excessive internal stress and cracking.

(2) Adjust the mold opening speed and pressure to prevent the rapid strong pulling parts caused by mold cracking.

(3) Appropriately adjust the mold temperature to make the parts easy to release, and appropriately adjust the material temperature to prevent decomposition.

(4) Prevent cracking due to fusion marks and low mechanical strength caused by plastic degradation.

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

(6) Residual stress in the part can be reduced by annealing heat treatment immediately after molding to eliminate internal stress and reduce the generation of cracks.

2. Mold:

(1) The ejector should be balanced, such as the number of ejector bars, the cross-sectional area should be sufficient, the slope of the mold should be sufficient, the cavity surface should be smooth enough, so as to prevent the ejector residual stress concentration and cracking due to external forces.

(2) The structure of the part cannot be too thin, the transition part should try to use the arc transition, avoid sharp corners, chamfering caused by stress concentration.

(3) minimize the use of metal inserts, to prevent the inserts and parts shrinkage rate is different resulting in increased internal stress.

(4) The deep bottom parts should be set up with appropriate air intake orifices to prevent the formation of negative vacuum pressure.

(5) The main channel is large enough to release the mold when the gate material has not had time to cure, so that it is easy to release the mold.

(6) Mainstream channel bushing and nozzle joint should prevent the dragging of cold hard material and make the parts stick to the fixed mold.

3. Material.

(1) The content of recycled material is too high, causing the strength of the parts to be too low.

(2) The humidity is too high, causing some plastics to react chemically with water vapor, reducing the strength and ejecting cracking.

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

4. Machine aspect.

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

Second, injection molding products bubble analysis of the causes

Air bubble (vacuum bubble) is very thin gas belongs to the vacuum bubble. Generally speaking, if the bubble has been found in the moment of opening the mold is a gas interference problem. The formation of vacuum bubble is due to insufficient filling into the plastic or low pressure. Under the effect of rapid cooling of the mold, the fuel pulling with the cavity joint angle, resulting in volume loss.

Solutions.

(1) Increase the injection energy: pressure, speed, time and material volume, and increase the back pressure to make the mold filling plentiful.

(2) Increase the material temperature to flow smoothly. Lower the material temperature to reduce shrinkage, and increase the mold temperature appropriately, especially the local mold temperature in the vacuum bubble formation area.

(3) Set the gate in the thick part of the part, improve the flow condition of the nozzle, runner and gate, and reduce the consumption of pressure service.

(4) Improve the mold exhaust condition.
 

Three, injection molding products warping deformation of the causes of analysis

The deformation, bending, twisting phenomenon of injection molded products is mainly due to the plastic molding flow direction of the shrinkage rate than the vertical direction of the large, so that the contraction rate of the parts are different and warpage, but also due to injection filling mold inevitably in the internal residual stress of the parts and cause warpage, these are high-stress orientation caused by the deformation of the performance. Therefore, fundamentally speaking, the mold design determines the warpage tendency of the parts, and it is very difficult to suppress this tendency by changing the molding conditions, and the final solution must start from the mold design and improvement. This phenomenon is mainly caused by the following aspects.

1. mold.

(1) the thickness and quality of the parts should be uniform.

(2) the design of the cooling system to make the mold cavity temperature uniform in all parts, the pouring system to make the material flow symmetrical to avoid warpage caused by the flow direction, shrinkage rate is different, appropriate thickening of the more difficult to form part of the manifold, the mainstream, as far as possible to eliminate the cavity density difference, pressure difference, temperature difference.

(3) The transition area of the thickness of the part and the corner should be smooth enough to have good mold release, such as increasing the release margin, improving the polishing of the mold surface, and keeping the balance of the ejector system.

(4) The exhaust should be good.

(5) Increase the wall thickness of the part or increase the anti-warping direction, and enhance the anti-warping ability of the part by reinforcing bars.

(6) The material used in the mold is not strong enough.

2. Plastics.

Crystalline than non-crystalline plastic warpage deformation opportunities, plus crystalline plastic can use the crystallinity with the cooling rate increases and decreases, the shrinkage rate becomes smaller crystallization process to correct warpage deformation.

3. Processing.

(1) injection pressure is too high, the holding time is too long, the melt temperature is too low and the speed is too fast will cause the increase of internal stress and warpage.

(2) The mold temperature is too high and the cooling time is too short, so that the parts are overheated when the mold is released and the ejection deformation occurs.

(3) Reduce the screw speed and back pressure to reduce the density to limit the internal stress while maintaining the minimum filling volume.

(4) If necessary, soft deformation of the mold can be carried out for the parts that are easy to warp and deformation or demoulding after demoulding.

Fourth, injection molding products color strip color line color flower analysis

Although masterbatch coloring is better than dry powder coloring and dyeing paste coloring in terms of color stability, color quality purity and color migration, etc., the distribution, i.e., color particles in diluted plastic in the mixing uniformity is relatively poor, and the finished products naturally have regional color differences.

The main solutions.

(1) increase the temperature of the filling section, especially the temperature of the back end of the filling section, so that its temperature is close to or slightly higher than the temperature of the melting section, so that the color masterbatch into the melting section as soon as possible to melt, promote and dilute uniform mixing, increase the opportunity of liquid mixing.

(2) Under the condition that the screw speed is certain, increase the backpressure so that the temperature of the molten material in the barrel and the shearing effect can be improved.

(3) modify the mold, especially the pouring system, such as the gate is too wide, molten material through, turbulent flow effect is poor, the temperature is not high, so it is not uniform, color band cavity, should be changed to narrow.

V. Analysis of the causes of shrinkage and depression of injection molded products

Injection molding process, product shrinkage depression is a relatively common phenomenon. The main reasons for this situation are.

1. Machine.

(1) the injection nozzle hole is too large caused by the reflux of molten material and shrinkage, too small resistance to a large amount of material is not enough to appear shrinkage.

(2) Insufficient clamping force will also cause shrinkage due to flying edges, and the clamping system should be checked for problems.

(3) The plasticizing volume is not enough, you should use the machine with large plasticizing volume, check whether the screw and barrel are worn.

2. Mold.

(1) The design of the parts should be uniform wall thickness to ensure consistent shrinkage.

(2) The cooling and heating system of the mold should ensure that the temperature of each part is the same.

(3) The pouring system should be smooth and the resistance should not be too large, such as the size of the main flow channel, diversion channel and gate should be appropriate, the finish should be sufficient and the transition area should be rounded.

(4) The temperature should be increased for thin parts to ensure smooth material flow, and the mold temperature should be reduced for thick-walled parts.

(5) The gate should be opened symmetrically, as much as possible in the thick-walled part of the part, and the volume of the cold material well should be increased.

3. Plastics.

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

4. Processing.

(1) The barrel temperature is too high and the volume changes a lot, especially the temperature of the front furnace, the plastic with poor fluidity should be appropriately raised to ensure smoothness.

(2) injection pressure, speed, back pressure is too low, injection time is too short, so that the amount of material or density is not enough and shrinkage pressure, speed, back pressure is too large, too long caused by flying edge and shrinkage.

(3) The amount of material added, that is, the buffer pad is too large when the injection pressure is consumed, too small, the amount of material is not enough.

(4) For the precision of the parts, in the injection pressure is completed, the outer layer of the basic condensation hardening and the sandwich part is still soft and can be ejected parts, early out of the mold, let it cool slowly in the air or hot water, can make the shrinkage depression gentle and less conspicuous and does not affect the use.
 

Six, the analysis of the causes of transparent defects of injection molding products

Melting spots, silver lines, cracks polystyrene, plexiglass transparent parts, sometimes through the light can be seen some shining silver lines like filaments. These silver lines are also called scintillation spots or cracks. This is due to the vertical direction of the tensile stress, the use of the right polymer molecules heavy flow orientation and the unoriented part of the folding rate differences show up.

Solution.

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

(2) lower the material temperature, segmentation adjustment of the barrel temperature, appropriate to increase the mold temperature.

(3) Increase the injection pressure and reduce the injection speed.

(4) Increase or reduce the backpressure of pre-molding and reduce the rotational speed of the screw.

(5) Improve the flow channel and cavity exhaust condition.

(6) Clean up possible clogging of injection nozzles, runners and gates.

(7) shorten the molding cycle, after demolding, annealing methods can be used to eliminate the silver pattern: for polystyrene at 78 ℃ for 15 minutes, or 50 ℃ for 1 hour, for polycarbonate, heated to 160 ℃ or more to maintain a few minutes.
 

Seven, injection molding products color uneven analysis of the causes

The main reasons for uneven color of injection molded products and the solutions are as follows.

(1) poor diffusion of colorant, this situation often makes patterns near the gate.

(2) Poor thermal stability of plastic or colorant, to stabilize the color of the parts, we must strictly fix the production conditions, especially the material temperature, material quantity and production cycle.

(3) For crystalline plastic, try to make the cooling speed of each part of the parts consistent, for the wall thickness difference of the parts, available coloring agent to cover the color difference, for the wall thickness of the more uniform parts to fix the material temperature and mold temperature.

(4) the shape of the part and the form of the gate, the location of the plastic filling situation has an impact on some parts of the part to produce chromatic aberration, if necessary, to modify.

Eight, injection molding products color and luster defects in the cause of analysis

Under normal circumstances, the luster of the surface of the injection molded parts is mainly determined by the type of plastic, coloring agent and the finish of the mold surface. However, there are often other reasons for the surface color and luster defects of the products, surface darkness and other defects. The causes and solutions are analyzed as follows.

(1) Poor mold finish, cavity surface rust, etc., poor mold exhaust.

(2) The mold casting system has defects, should increase the cold material well, increase the flow channel, polish the main flow channel, diversion channel and gate.

(3) The material temperature and mold temperature is low, if necessary, the local heating method of the gate is available.

(4) The processing pressure is too low, the speed is too slow, the injection time is not enough, the backpressure is not enough, which causes poor compactness and makes the surface dark.

(5) The plastic should be fully plasticized, but to prevent the degradation of the material, the heat should be stable, and the cooling should be sufficient, especially for thick walls.

(6) Prevent the cold material from entering the parts, change to a self-locking spring or reduce the nozzle temperature if necessary.

(7) Use too much-recycled material, poor quality of plastic or coloring agent, mixed with water vapor or other impurities, and poor quality of lubricant used.

(8) The clamping force should be sufficient.
 

Nine, injection molding products, the causes of silver pattern analysis

Injection-molded products with silver lines, including surface bubbles and internal pores. The main reason for the defect is the interference of gas (mainly water vapor, decomposition gas, solvent gas, air). Specific causes are analyzed as follows.

1. On the machine.

(1) barrel, screw wear or over glue head, over glue ring there is material flow dead space, long-term heat and decomposition.

(2) heating system out of control, resulting in high temperature and decomposition, should check whether the thermocouple, heating ring and other heating elements have problems. The screw is not designed properly, resulting in individual decomposition or easy to bring in air.

2. Mold aspects.

(1) poor exhaust.

(2) The mold runner, gate, cavity friction resistance, resulting in local overheating and decomposition.

(3) Unbalanced distribution of gates and cavities, the unreasonable cooling system can cause unbalanced heating and local overheating or blocking the passage of air.

(4) Cooling path leakage into the cavity.

3) Plastic.

(1) plastic humidity, add too much proportion of recycled material or contain harmful crumbs (crumbs are very easy to decompose), should be fully dried plastic and eliminate crumbs.

(2) moisture absorption from the atmosphere or from the coloring agent, the coloring agent should also be dried, it is best to install a drier on the machine.

(3) The amount of lubricants, stabilizers, etc. added to the plastic is too much or unevenly mixed, or the plastic itself with volatile solvents. Mixed plastic decomposition will also occur when the degree of heat is difficult to take into account.

(4) The plastic is contaminated, mixed with other plastics.

4. Processing.

(1) Set the temperature, pressure, speed, back pressure, melt motor speed is too high to cause decomposition or pressure, speed is too low, injection time, holding pressure is not sufficient, back pressure is too low, due to failure to obtain high pressure and density is not enough to melt the gas and appear silver pattern, should set the appropriate temperature, pressure, speed and time and the use of multiple injection speed

(2) Low backpressure and fast speed make the air enter the barrel and enter the mold with the molten material, and when the cycle is too long, the molten material is heated too long in the barrel and decomposition occurs.

(3) The amount of material is not enough, the material cushion is too large, the material temperature is too low or the mold temperature is too low to affect the flow of material and molding pressure, prompting the generation of bubbles.
 

Ten, plastic products with fusion seam analysis of the causes

The molten plastic in the cavity due to encounter insert holes, flow rate incoherent area, mold filling material flow interrupted area and in the form of multi-strand convergence, because it can not completely fuse and produce a linear fusion seam. In addition, fusion joints are also generated in the case of gate injection filling, and the strength of the fusion joints is poor. The main reasons are analyzed as follows.

1. Processing.

(1) The injection pressure and speed are too low, and the barrel temperature and mold temperature are too low, resulting in premature cooling of the molten material entering the mold and the appearance of fusion joints.

(2) When the injection pressure and speed are too high, there will be a melt joint due to injection.

(3) The rotational speed should be increased to increase the backpressure so that the plastic viscosity decreases and the density increases.

(4) Plastic should be dried well, recycled material should be used sparingly, too much mold release agent or bad quality will also appear melt seam.

(5) Reduce the clamping force to facilitate exhaust.

2. Mold.

(1) Too many gates in the same cavity, should be reduced or set symmetrically, or set as close to the fusion seam as possible.

(2) Poor exhaust at the fusion joint, an exhaust system should be opened.

(3) The sprue is too large, the casting system is not properly sized, and the gate should be opened to avoid the melt flowing around the insert hole, or to use as few inserts as possible.

(4) The wall thickness changes too much, or the wall thickness is too thin, which should make the wall thickness of the parts even.

(5) If necessary, a fusion well should be opened at the fusion joint to make the fusion joint out of the part.

3. Plastics.

(1) the poor fluidity or heat-sensitive plastic should be added appropriately lubricants and stabilizers.

(2) The plastic contains many impurities, if necessary, to replace the quality of good plastic.

Eleven, the analysis of the causes of injection molding products shock lines

PS and other rigid plastic parts in its gate near the surface, to the gate as the center of the formation of dense ripples, sometimes called shock lines. The reason is that when the melt viscosity is too large and the mold is filled in the form of stagnant flow, the material at the front end quickly condenses and shrinks when it touches the surface of the cavity, while the molten material later expands the cold material that has shrunk and continues to move forward in the process of continuous alternation so that the material flow forms a surface ripple in the advance.

Solution.

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

(2) improve the injection pressure and speed, so that it can quickly fill the cavity.

(3) Improve the flow channel and gate size to prevent excessive resistance.

(4) The mold exhaust should be good, and a large enough cold material well should be set.

(5) Do not design the parts too thin.
 

Twelve, injection molding products swelling and bulging causes of analysis

Some plastic parts in the molding off the mold, soon after the back of the metal insert or in a particularly thick part of the swelling or bulging. This is because the plastic not completely cooled and hardened under the action of the internal pressure penalty release gas expansion caused.

Solution measures.

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

2、Lower the mold filling speed, reduce the molding cycle, reduce the flow resistance.

3、Improve the pressure and time of holding pressure.

4、Improve the condition that the wall of the part is too thick or the thickness varies greatly.