In-depth analysis of the causes of injection molding defects

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

Processing aspect

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

(2) Adjust the mold opening speed and pressure to prevent the rapid and strong pulling of the 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 the frequent elimination of 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.

 

For the mold

(1) The ejection should be balanced, such as the number of ejector bars and 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 ejection caused by external forces.

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

(3) Use metal inserts as little as possible to prevent different shrinkage rates of the inserts and parts caused by increased internal stress.

(4) The deep bottom parts should be set up with an appropriate air intake orifice 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 it is easy to release the mold.

(6) Main flow channel bushing and nozzle joint should prevent the cold hard material from dragging and making the parts stick to the fixed mold.

 

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 causing ejection cracking.

(3) The material itself is not suitable for the environment being processed or the quality is not good, and contamination can cause cracking.

 

Machine aspect

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

Analysis of the causes of air bubbles in injection molded products

Air bubble (vacuum bubble) 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, speed, time, and material volume, and increase the back pressure to fill the mold plentifully.

(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 area of vacuum bubble formation.

(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.


 

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

The deformation, bending, and twisting of injection molded products is mainly due to the contraction rate in the flow direction of plastic molding than the vertical direction, so that the contraction rate of the parts are different and warp, and because of the injection filling mold inevitably in the internal residual stress of the parts and cause warping, these are the performance of deformation caused by high-stress orientation. 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.
 

 

Mold aspect

(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 warping caused by the flow direction, the shrinkage rate is different, appropriate thickening of the more difficult to form part of the manifold, the main flow channel, 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 warp resistance direction, and enhance the warp resistance of the part by reinforcement.

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

 

Plastic

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.

 

Processing aspects

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

(2) The mold temperature is too high and the cooling time is too short so that the part is overheated when it is demolded 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 that are easily warped and deformed.

 

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

 

Main solutions

(1) Increase the temperature of the feeding section, especially the temperature of the back end of the feeding section, so that its temperature is close to or slightly higher than the temperature of the melting section so that the color masterbatch enters the melting section as soon as possible to melt, promote uniform mixing with dilution, and increase the chance of liquid mixing.

(2) Under the condition that the screw speed is certain, increase the backpressure so that the temperature of the melt 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, when the molten material through, the turbulent flow effect is poor, the temperature is not high, so it is not uniform, ribbon mold cavity, should be changed to narrow.


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

 

Processing

(1) The injection 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 shrink.

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

(3) Insufficient plasticizing volume should be chosen to use the machine with large plasticizing volume, check whether the screw and barrel are worn.

 
 

Mold aspect

(1) The design of the parts should make the wall thickness uniform and ensure the shrinkage is consistent.

(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 main flow channel, diversion channel, gate size should be appropriate, the finish should be enough, and the transition area should be rounded transition.

(4) The temperature should be increased for thin parts to ensure the material flow smoothly, 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.

 

For plastics

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

 

Processing

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

(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 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.

 

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

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

 

Solution

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

(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 reduce the pre-molding backpressure and reduce the screw speed.

(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, and use annealing methods to eliminate silver patterns after demolding: for polystyrene hold at 78°C for 15 minutes, or 50°C for 1 hour, and polycarbonate, heat to 160°C or more for several minutes.