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Explain The Reasons For Problems With Injection Molds
Jun 11, 2018

Core Tip: Analysis of Cracking Causes of Injection Molded Products. Cracking, including surface cracking, microcracking, whitening, cracking, and due to sticking of parts or runners, or traumatic crisis, cracking and cracking according to the cracking time. There are mainly the following reasons:

 

1. Processing:

(1) Excessive processing pressure, too fast speed, more charging, too long time for injection and holding pressure will cause excessive internal stress and cracking.

(2) Adjust the mold opening speed and pressure to prevent the mold from cracking due to rapid and strong drawing.

(3) Appropriately increase the temperature of the mold to make the parts easy to release, and appropriately lower the temperature to prevent decomposition.

(4) Prevention of cracking due to welding marks, degradation of the plastic due to plastic degradation.

(5) Appropriate use of mold release agents, pay attention to the elimination of aerosols and other substances that adhere to the mold surface.

(6) The residual stress of the part can be reduced by reducing the internal stress by annealing heat treatment immediately after molding.

2. Mold aspect:

(1) Ejection should be balanced. For example, the number of ejector pins and the cross-sectional area should be sufficient. The draft should be sufficient, and the cavity surface should be sufficiently smooth so as to prevent cracking due to the concentration of residual stress caused by the external force.

(2) The structure of the parts shall not be too thin, and the transitional part shall be rounded as far as possible to avoid stress concentration caused by sharp corners and chamfers.

(3) Minimize the use of metal inserts to prevent internal stress from increasing due to the difference in shrinkage between inserts and parts.

(4) Proper mold release air inlets shall be provided for deep-bottom parts to prevent the formation of vacuum negative pressure.

(5) The sprue of the main channel is sufficient enough for the ambassador to be demolded in the future, so that it is easy to demold.

(6) The joining of the main runner bushing to the nozzle should prevent the chilling material from dragging and sticking the workpiece to the fixed mold.

 

3. Materials:

(1) The content of recycled material is too high, resulting in too low intensity of the part.

(2) Excessive humidity causes chemical reactions between some plastics and water vapor, reducing strength and causing cracking.

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

4. Machine side:

Injection molding machine plasticizing capacity should be appropriate, too small plasticization can not fully mix and become brittle, will degrade when too large.

 

Analysis of the Causes of Bubbles in Injection Moulding Products

Bubbles (vacuum bubbles) are very thin gases are vacuum bubbles. In general, if a bubble is found at the moment of mold opening, it is a gas interference problem. Vacuum bubbles are formed due to insufficient plastic injection or low pressure. Under the rapid cooling of the mold, the fuel with the corners of the cavity pulls, resulting in a loss of volume.

 

Solution:

(1) Increase the injection energy: pressure, speed, time, and amount of material, and increase the back pressure to make the mold full.

(2) Increased material temperature flow smoothly. Reduce the material temperature to reduce the shrinkage, and properly increase the mold temperature, especially the local mold temperature at the vacuum bubble site.

(3) The gate is set at the thick part of the part to improve the flow conditions of the nozzle, runner and gate, and reduce the consumption of the press service.

(4) Improve mold exhaust conditions.

Analysis of the Causes of Warpage and Deformation of Injection Molded Products

The phenomenon of deformation, bending, and distortion of injection-molded products is mainly due to the fact that the shrinkage in the direction of flow during the plastic molding is larger than that in the vertical direction, which causes warping of the workpiece due to the different shrinkage rates, and is inevitably due to injection molding. There is a large amount of internal stress remaining in the part causing warpage. These are all manifestations of deformation due to high stress orientation. So fundamentally, the mold design determines the warpage tendency of the parts. It is very difficult to suppress this tendency by changing the molding conditions. The final solution to the problem must begin with the mold design and improvement. This phenomenon is mainly caused by the following aspects:

1. Mold aspect:

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

(2) The design of the cooling system should make the temperature of each part of the mold cavity uniform, and the pouring system should make the material flow symmetrical to avoid warpage caused by the different flow direction and shrinkage rate, and appropriately thicken the shunts and mainstreams that are difficult to form. Road, as far as possible to eliminate the cavity density difference, pressure difference, temperature difference.

(3) The transitional area and corners of the parts should be sufficiently rounded to have good mold release properties, such as increasing the mold release margin and improving the polishing of the mold surface. The ejector system must be balanced.

(4) The exhaust should be good.

(5) Increase the wall thickness of the parts or increase the warpage resistance direction, and strengthen the ribs to strengthen the warpage resistance of the parts.

(6) The strength of the material used for the mold is insufficient.

2. Plastics:

The crystal type has more warping deformation opportunities than the non-crystalline plastic, and the crystal-type plastic can use the crystallization process in which the degree of crystallinity decreases as the cooling rate increases and the shrinkage becomes smaller to correct the warpage.

3. Processing:

(1) The injection pressure is too high, the pressure holding time is too long, too low melting temperature and too fast speed will cause the internal stress to increase and warp.

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

(3) Reducing the screw rotation speed and back pressure reducing density while maintaining the minimum charge amount limits the generation of internal stress.

(4) If necessary, soft-setting or stripping of the easily warped and deformed parts can be performed.

Injection products color stripe color analysis

The appearance of this kind of defect is mainly the problem that plastic parts colored with masterbatch often appear. Although the masterbatch coloring is superior to the dry powder coloring and dyeing in the aspects of color stability, color purity and color migration. Coloring, but distributive, that is, color particles in diluted plastics is relatively poor in the degree of mixing, and finished products naturally have regional color differences. The main solution:

(1) Increase the temperature of the feed section, especially the temperature at the rear end of the feed section, so that the temperature is close to or slightly higher than the temperature of the melting section, so that the masterbatch melts as soon as possible when it enters the melting section, promotes uniform mixing with the dilution, and increases liquid mixing opportunities.

(2) When the screw speed is fixed, increase the back pressure so that the temperature and shearing of the melt in the barrel are improved.

(3) modify the mold, especially the pouring system, such as the gate is too wide, the melt through, the turbulent flow is poor, the temperature is not high, so uneven, ribbon mold cavity, should be changed to narrow.

Analysis of Causes for Shrinkage of Injection Molding Products

In the injection molding process, shrinkage of the product is a common phenomenon. The main reasons for this are:

1. Machine side:

(1) The nozzle hole is too large to cause the melt to flow back and shrink. If the hour resistance is too large, there is insufficient material to shrink.

(2) Insufficient clamping force will cause shrinkage of flash. Check if the clamping system has any problems.

(3) Insufficient amount of plasticizing machine should use a large amount of plastic, check the screw and the barrel is worn.

2. Mold aspect:

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

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

(3) The gating system should be smooth and the resistance should not be too large. For example, the sizes of main channels, runners, and gates should be adequate, the degree of finish should be sufficient, and the transition zone should have a circular transition.

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

(5) The gate should be symmetrically opened, as far as possible in the thick parts of the workpiece, the volume of the cold well should be increased.

3. Plastics:

Crystalline plastics shrink more than non-crystalline plastics. During processing, it is necessary to increase the amount of material, or add a change agent in plastics, in order to speed up crystallization and reduce shrinkage dents.

4. Processing:

(1) The barrel temperature is too high and the volume change is large, especially the temperature of the forehearth, and the plastic with poor fluidity should be properly increased in temperature to ensure smooth flow.

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

(3) The amount of material to be added is the consumption of injection pressure when the cushion is too large. If it is too small, the amount of material is insufficient.

(4) For parts that do not require precision, after the injection pressure is maintained, the outer layer is basically condense hardened and the sandwich part is still soft and can be ejected. The mold is released early and allowed to slowly cool in air or hot water. , Can make the contraction depression flat and not so conspicuous and does not affect the use.

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

Melting spots, silver streaks, cracked polystyrene, transparent plexiglass parts, and sometimes you can see some glistening filaments of silver. These silver lines are also called bright spots or cracks. This is due to the stress in the vertical direction of the tensile stress, and the difference in the rate of collapse of the right-to-weight polymer molecules with respect to the non-oriented portion is manifested.

Solution:

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

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

(3) Increase injection pressure and reduce injection speed.

(4) Increase or decrease pre-plastic back pressure to reduce screw speed.

(5) Improve the flow path and cavity exhaust conditions.

(6) Clear up possible blockages in the nozzles, runners, and gates.

(7) Shortening the molding cycle, after annealing, the annealing method can be used to eliminate the silver streaks: For polystyrene at 78°C for 15 minutes, or 50°C for 1 hour, for polycarbonate, heat up to 160°C for several minutes .

Analysis of the causes of uneven color in injection molded products

The main causes of color unevenness in injection molding products and solutions are as follows:

(1) Poor diffusion of the colorant, which often causes patterns near the gate.

(2) The thermal stability of plastics or colorants is poor. To stabilize the color of the parts, the production conditions must be strictly fixed, especially the material temperature, material quantity, and production cycle.

(3) For crystal-type plastics, try to make the cooling rate of all parts of the parts consistent. For parts with large differences in wall thickness, coloring agents can be used to mask the color difference. For parts with uniform wall thickness, the material temperature and mold temperature should be fixed. .

(4) The shape of the parts and the form of the gate, the position has an influence on the plastic filling situation, causes some parts of the parts to have a color difference, and must be modified when necessary.

Analysis of the causes of color and gloss defects in injection molded products

Under normal circumstances, the gloss of the surface of an injection molded part is mainly determined by the type of plastic, the colorant, and the finish of the mold surface. However, due to some other reasons, there are often defects such as the surface color of products, gloss defects, and dark surfaces. The reasons for this and the solutions are as follows:

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

(2) The casting system of the mold is defective. The cold material well should be enlarged, the flow path should be enlarged, the main flow path should be polished, the runner and the gate should be polished.

(3) The temperature of the material and the temperature of the mold are low. If necessary, local heating of the gate can be used.

(4) The processing pressure is too low, the speed is too slow, the injection time is insufficient, and the back pressure is insufficient, resulting in poor compaction and dark surfaces.

(5) The plastic should be fully plasticized, but the degradation of the material must be prevented, the heat must be stable, and the cooling must be sufficient, especially for thick-walled ones.

(6) Prevent the cold material from entering the part. If necessary, use a self-locking spring or reduce the nozzle temperature.

(7) Excessive use of recycled materials, poor quality of plastics or colorants, mixed water vapor or other impurities, and poor quality lubricants.

(8) The clamping force should be sufficient.

Analysis of Causes of Silver Patterns in Injection Molded Products

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

1. Machine side:

(1) There is dead material flow in the material barrel, screw wear or the rubber head, and in the aprons, and it decomposes when heated for a long time.

(2) The heating system is out of control, resulting in decomposition due to excessive temperature. Check whether there are any problems with the heating elements such as thermocouples and heating coils. Incorrect design of the screw leads to a solution or easy air.

2. Mold:

(1) Bad exhaust.

(2) The frictional resistance of runners, gates, and cavities in the mold is large, causing partial overheating and decomposition.

(3) The unbalanced distribution of gates and cavities and the unreasonable cooling system can cause unbalanced heat and local overheating or blocked air passages.

(4) The cooling passage leaks into the cavity.

3. Plastics:

(1) The humidity of the plastic is too large, and the proportion of the added regenerated material is too much or contains harmful crumb material (the crumb material is easily decomposed). The plastic should be fully dried and the chips should be eliminated.

(2) Absorb moisture from the atmosphere or absorb moisture from the colorant. Also, the colorant should be dried. It is best to install a desiccator on the table.

(3) The amount of lubricants, stabilizers, etc. added to the plastic is excessive or mixed, or the plastic itself contains volatile solvents. Mixed plastics can also be decomposed when the heat is difficult to reach.

(4) The plastic is contaminated 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, the injection time, pressure is not enough, back pressure is too low, due to the failure to obtain high pressure and density Insufficiently unable to melt the gas and appear silver lines, should set the appropriate temperature, pressure, speed and time and use multiple injection speeds.

(2) The back pressure is low, and the speed is fast and easy to make the air enter the cylinder, and the molten material enters the mold. When the cycle is too long, the melt is heated in the cylinder for a long time and decomposition occurs.

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

Analysis of Causes of Welded Joints in Plastic Products

When melted plastic meets multiple strands in the cavity due to the presence of insert holes, inconsistent flow speeds, and areas where the charge flow is interrupted, a linear weld seam may not be produced due to complete fusion. In addition, weld spouts are also generated in gate injection molding, and the weld seams have poor strength and other properties. The main reasons are as follows:

1. Processing:

(1) injection pressure, speed is too low, cylinder temperature, mold temperature is too low, resulting in molten material entering the mold premature cooling and weld joints.

(2) When the injection pressure or velocity is too high, spray occurs and the weld seam appears.

(3) The rotational speed should be increased, and the pressure of the back pressure should be increased so that the viscosity of plastics will decrease and the density will increase.

(4) Plastics should be dried well. Recycled materials should be used sparingly. Excessive amounts of mold release agents or poor quality can also cause weld seams.

(5) Reduce the clamping force to facilitate exhaust.

2. Mold aspect:

(1) Too many gates in the same cavity should be reduced in gate or symmetrical setting, or set as close to the weld joint as possible.

(2) Exhaust is not good at the weld joint. An exhaust system should be opened.

(3) The runner is too large, the size of the gating system is improper, and the gate opening is designed to avoid the flow of melt around the insert hole, or to minimize the use of inserts.

(4) If the wall thickness changes too much or if the wall thickness is too thin, the wall thickness of the part shall be uniform.

(5) If necessary, a fusion well should be set up at the weld joint to make the weld joint separate from the workpiece.

3. Plastics:

(1) Lubricants and stabilizers should be properly added to poorly fluid or heat-sensitive plastics.

(2) The plastic contains many impurities, and it is necessary to replace the plastic with good quality when necessary.

Analysis of the Causes of Shock Pattern in Injection Moulding Products

The surface of a rigid plastic part such as PS near its gate is densely corrugated centered on the gate, sometimes referred to as a grooving pattern. The reason is that if the melt viscosity is too large and the mold is filled with stagnant flow, the material at the front end will quickly condense and shrink as soon as it comes into contact with the surface of the cavity, while the subsequent melt expands and expands the shrinkage of the cold material to continue the process. Alternately, the surface of the stream has been formed by alternating flow patterns.

Solution:

(1) Increase the temperature of the barrel, especially the nozzle temperature, and increase the mold temperature.

(2) increase injection pressure and