Twelve kinds of injection molding defect causes

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

Cracking, including filiform cracks on the surface of the part. Microcrack. Top white. Cracking and due to mold sticking. Flow channel sticky mold or trauma crisis, according to the cracking time is divided into cracking and application cracking. These are mainly caused by the following aspects.

1. processing aspects.

(1). Processing pressure is too large. Speed is too fast. The more material filling. Injection. Too long holding time will cause excessive internal stress and cracking.

(2). Adjust the mold opening speed and pressure to prevent the fast and strong pulling parts from cracking.

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

(4). Prevent the cracking due to the degradation of the plastic caused by the fusion trace and the mechanical strength.

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

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

2. For mold:

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

(2). The structure of the part should not be too thin, the transition part should try to use rounded transition, avoid sharp corners. Chamfering causes stress concentration.

(3). Use metal inserts as little as possible, 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 flow channel is enough to release the mold before the big sprues have time to cure so that it is easy to release the mold.

(6). Main runner 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). Humidity is too large, causing some plastics and water vapor chemical reactions, reducing the strength and ejection cracking.

(3). The material itself is not suitable for the environment being processed or the quality is not good, 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.

II. Analysis of the causes of air bubbles in injection molded products

Air bubble (vacuum bubble). The gas is very thin belongs to the vacuum bubble. Generally speaking, if the bubble is found at the moment of mold opening, it is a gas interference problem. The formation of vacuum bubbles 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. Velocity. Time and material volume, and increase the back pressure to make the mold filling plentiful.

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

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

(4). Improve the mold exhaust condition.

III. Analysis of the causes of warping and deformation of injection molded products

Deformation of injection molded products. Bending. The warping phenomenon is mainly due to the shrinkage rate in the flow direction of the plastic molding being larger than the vertical direction, so that the shrinkage rate of the parts in different directions and warping, and because of the injection filling mold inevitably in the internal residual stress of the parts caused by warping, 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 aspects.

(1). The thickness of the manufactured parts. Quality should be uniform.

(2). The cooling system should be designed to take the temperature of all parts of the mold cavity uniform, the pouring system should make the material flow symmetrical to avoid the flow direction. Shrinkage rate is different and causes warpage, appropriate thickening of the more difficult to form part of the manifold. The main runner of the more difficult part should be thickened, and the density difference in the cavity should be eliminated as much as possible. 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). Exhaust air should be good.

(5). Increase the wall thickness of the part or increase the warp resistance direction, and enhance the warp resistance of the part by reinforcement.

(6). Insufficient strength of the material used in the mold.

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 internal stress to increase and warpage deformation.

(2). The mold temperature is too high, the cooling time is too short so 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, perform soft mold deformation or demoulding after demoulding for parts prone to warpage and deformation.

IV. Analysis of injection molding products color stripes color lines color flowers

This defect is mainly a common problem of plastic parts colored with masterbatches. Although masterbatch coloring is better than dry powder coloring in terms of color stability, color purity and color migration, etc., the distribution, distribution, and color migration of masterbatch coloring are better than dry powder coloring. Dyeing paste coloring, but the distribution, that is, color particles in the dilution of plastic in the mixing uniformity is relatively poor, the finished products naturally with regional color differences.

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 masterbatch enters the melting section as soon as possible to melt, promote uniform mixing with dilution, and increase the chance of liquid mixing.

(2). In the case of a certain screw speed, increase the back pressure to make the temperature of the melt in the barrel. Shearing actions are improved.

(3). Modify the mold, especially the pouring system, if the gate is too wide, when the molten material passes through, the turbulent flow effect is poor, the temperature is not raised high, so it is not uniform, the color band mold cavity, should be narrowed.

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 aspects.

(1). Nozzle hole is too large to cause molten material reflux and shrinkage, too small resistance to a large amount of material is not enough to appear shrinkage.

(2). Insufficient clamping force will cause shrinkage due to flying edge, check the clamping system for problems.

(3). Insufficient plasticizing volume should be used to plasticize a large amount of machine, check whether the screw and barrel wear.

2. For mold.

(1). Design the parts to make the wall thickness uniform, to ensure that the shrinkage is consistent.

(2). Cooling of the mold. Heating system to 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 main flow channel. Shunt channel. The size of the 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 the smooth flow of material, and the mold temperature should be reduced for thick-walled parts.

(5). The gate should be opened symmetrically, as far as possible in the thick-walled parts of the parts, 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, to speed up the crystallization, reduce shrinkage depression.

4. Processing.

(1). Barrel temperature is too high, volume changes, especially the temperature of the former furnace, the poor flow of plastic should be appropriate to increase the temperature. Ensure smoothness.

(2). Injection pressure. Speed. Backpressure is too low. The injection time is too short so the material volume or density is not enough and the shrinkage pressure. Speed. Too much backpressure. Too long time causes shrinkage due to flying edge.

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

(4). For the parts that do not require precision, 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.

VI. Analysis of the causes of transparent defects of injection molded products

Melting spot. Silver stripe. Cracked polystyrene. In the transparent parts of Plexiglas, some shiny silver lines can be seen through the light. These silver lines are also called scintillation spots or cracks. This is due to the stress in the vertical direction of the tensile stress, which is caused by the heavy flow orientation of the polymer molecules and the difference in the folding rate of the unoriented part.

Solution.

(1). Eliminate the interference of gas and other impurities, and dry the plastic adequately.

(2). Reduce the material temperature, adjust the barrel temperature in sections, and increase the mold temperature appropriately.

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

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

(5). Improve the flow path and cavity venting condition.

(6). Clear the injection nozzle. Possible blockage of runners and gates.

(7). Shorten the molding cycle and eliminate the silver pattern by annealing method after demolding: for polystyrene, hold at 78℃ for 15 minutes or 50℃ for 1 hour, for polycarbonate, heat to 160℃ or more for several minutes.

VII. Analysis of the causes of the uneven color of injection molded products

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

(1). Poor diffusion of colorant, this situation often makes the pattern near the gate.

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

(3). For crystalline plastics, try to make the cooling speed of all parts 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 position of the plastic filling has an impact on the situation, so that some parts of the part produce a color difference, if necessary, to modify.

VIII. Analysis of the causes of color and gloss defects of injection molded products

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

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

(2). The casting system of the mold has defects, should increase the cold material well, increase the flow channel. Polishing the main flow channel. Shunt and sprue.

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

(4). Processing pressure is too low. Speed is too slow. Insufficient injection time. Insufficient back pressure, resulting in poor compactness and dark surface.

(5). Plastic should be fully plasticized, but to prevent the degradation of the material, the heat should be stable, cooling should be sufficient, especially for thick-walled.

(6). Prevent the cold material from entering the parts, change to a self-locking spring, or lower 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, poor quality of lubricant used.

(8). The clamping force should be sufficient.

IX. Analysis of the causes of the silver pattern of injection molded products

The silver pattern of injection molded products includes surface bubbles and internal pores. The main reason for the defect is gas (mainly water vapor. Decomposition gas. Solvent gas. Air). The interference of. Specific causes are analyzed as follows.

1. machine aspects.

(1). Barrel. Screw wear or over rubber head. Over the rubber ring, there is a dead corner of the material flow, long-term heat, and decomposition.

(2). Heating system out of control, resulting in high temperature and decomposition, should check the thermocouple. Heating element such as heating ring is not a problem. Improper design of the screw, resulting in a solution or easy to bring in air.

2. Mold aspects.

(1). Poor air exhaust.

(2). Runner in the mold. Gates. High frictional resistance of the cavity, resulting in local overheating and decomposition.

(3). Gates. Unbalanced distribution of cavities and unreasonable cooling system will cause unbalanced heating and local overheating or obstruction of the air passage.

(4). Cooling pathway leakage into the cavity.

3. Plastics.

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

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

(3). The lubricant is added to the plastic. The amount of stabilizer, etc. is too much or unevenly mixed, or the plastic itself with volatile solvents. Decomposition may also occur when the mixed plastics are subjected to a degree of heat that is difficult to balance.

(4). Plastic is contaminated, mixed with other plastics.

4. Processing.

(1). Setting temperature. Pressure. Speed. Backpressure. Melt motor speed is too high to cause disintegration or pressure. Low speed, injection time. Insufficient holding pressure. If the back pressure is too low, the density is not high enough to melt the gas and the silver pattern appears. Pressure. Speed and time and use of multi-stage injection speed

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

(3). Insufficient amount of material, too big cushion, too low material temperature, or too low mold temperature all affect the flow of material and molding pressure and promote the generation of air bubbles.

X. Analysis of the causes of melt seams in plastic products

Molten plastic in the cavity due to encounter insert holes. Inconsistent flow rate area. When the mold filling material flow is interrupted in the area and converges in multiple strands, a linear fusion joint is created because it cannot be completely fused. 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). Injection pressure. Speed is too low, the barrel temperature. Mold temperature is too low, resulting in premature cooling of the molten material entering the mold and the appearance of fusion joints.

(2). Injection pressure. If the injection speed is too high, there will be a fusion 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 fusion seam.

(5). Reduce the clamping force to facilitate exhaust.

2. Moulds.

(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 seam, an exhaust system should be opened.

(3). The sprue is too big. Improperly sized pouring system, open the gate to avoid the melt flowing around the hole of the insert, or 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). For poor flowability or heat-sensitive plastics should be added to the appropriate lubricants and stabilizers.

(2). Plastic contains a lot of impurities, if necessary, to change the quality of plastic.

XI. 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.

Solution.

(1). Improve the barrel temperature, especially the injection nozzle temperature, and also improve the mold temperature.

(2). Increase the injection pressure and speed to make it fill the cavity quickly.

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

(4). Exhaust the mold well and set a large enough cold material well.

(5). Do not design the parts too thin.

XII. Analysis of the causes of swelling and bulging of injection molded products

Some plastic parts swell or bulge on the back of the metal insert or in particularly thick parts soon after molding and demolding. This is because the plastic is not completely cooled and hardened under the action of 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.