Cracking includes surface filiform cracks, microcracks, white top, cracking, and ejection caused by mold sticking or runner sticking. It is divided into demolding cracking and application cracking according to cracking time. Main causes are as follows:
1. Processing:
(1) Excessive processing pressure, too fast speed, too much filling, too long injection and holding time will cause excessive internal stress and cracking.
(2) Adjust mold opening speed and pressure to prevent demolding cracking caused by rapid and strong pulling of parts.
(3) Appropriately increase mold temperature to make parts easier to demold, and appropriately lower material temperature to prevent decomposition.
(4) Prevent cracking due to weld marks and plastic degradation causing low mechanical strength.
(5) Appropriately use mold release agents, pay attention to regularly remove aerosols and other substances attached to mold surface.
(6) Residual stress of workpiece can be eliminated by annealing heat treatment immediately after molding to reduce generation of cracks.
2. Mold:
(1) Ejection should be balanced, such as number of ejector pins and cross-sectional area should be sufficient, demoulding slope should be sufficient, cavity surface should be smooth enough to prevent sticking to front mold, and core surface should ensure that deep bone ribs have sufficient slope, so as to prevent residual stress concentration caused by ejection from cracking due to external force.
(2) Structure of workpiece should not be too thin, and transition part should be made of arc transition as much as possible to avoid stress concentration caused by sharp corners.
(3) Use metal inserts as little as possible to prevent internal stress from increasing due to different shrinkage rates between insert and workpiece or preheating metal insert.
(4) For deep-bottomed workpieces, appropriate demoulding air inlet channels should be set to prevent formation of vacuum negative pressure.
(5) Main channel should be large enough to allow gate material to be demoulded before it is completely solidified, so that it is easy to demould.
(6) Main runner bushing and nozzle joint should prevent cold hard material from being dragged and causing part to stick to fixed mold (hot runner is not applicable).
3. Material:
(1) Recycled material content is too high, resulting in too low strength of part.
(2) Excessive humidity causes some plastics to react chemically with water vapor, reducing strength and causing ejection cracking.
(3) Material itself is not suitable for processing environment or is of poor quality, and contamination will cause cracking.
4. Machine:
Plasticizing capacity of injection molding machine should be appropriate. If it is too small, plasticizing will not be sufficient and cannot be completely mixed, resulting in brittleness. If it is too large, material may degrade.

Gas in bubble (vacuum bubble) is very thin and belongs to vacuum bubble. Generally speaking, if bubbles are found at the moment of mold opening, it is a gas interference problem. Formation of vacuum bubbles is due to insufficient plastic injection or low pressure. Under rapid cooling of mold, pull of corner of cavity causes volume loss.
Solutions:
(1) Increase injection energy: pressure, speed, time and material volume, and increase back pressure to ensure full mold filling.
(2) Increase material temperature to ensure smooth flow. Lower material temperature to reduce shrinkage, and appropriately increase mold temperature, especially local mold temperature where vacuum bubbles are formed.
(3) Set gate at thick part of part, improve flow conditions of nozzle, runner and gate, and reduce pressure consumption.
(4) Ensure smooth exhaust of mold parting surface and improve exhaust conditions of mold at key locations.

Deformation, bending and twisting of injection molded products are mainly due to fact that shrinkage rate in flow direction during plastic molding is greater than that in vertical direction, which causes part to warp due to different shrinkage rates in all directions. In addition, due to inevitable residual internal stress in part during injection filling, warping occurs. These are all manifestations of deformation caused by high stress orientation. Therefore, fundamentally speaking, mold design determines warping tendency of part. It is very difficult to suppress this tendency by changing molding conditions. Ultimate solution to problem must start with mold design and improvement. This phenomenon is mainly caused by following aspects:
1. Mold:
(1) When designing product structure, wall thickness of part should be as uniform as possible.
(2) Design of cooling system should make temperature of each part of mold cavity uniform, pouring system should make material flow symmetrical to avoid warping due to different flow directions and shrinkage rates. Branch channel and main channel near difficult molding position should be appropriately thickened to try to eliminate density difference, pressure difference and temperature difference in cavity.
(3) Transition zone and corners of thick and thin parts should be smooth enough and have good demolding properties, such as increasing demolding slope, improving polishing of mold surface, and keeping ejection system balanced.
(4) Exhaust should be good.
(5) Increase wall thickness of part or increase anti-warping direction, and use reinforcing ribs to enhance anti-warping ability of part.
Plastics:
Crystalline plastics are more likely to warp than amorphous plastics. In addition, crystalline plastics can correct warping by utilizing crystallization process where degree of crystallinity decreases with increasing cooling rate and shrinkage rate decreases.
3. Processing:
(1) If injection pressure is too high, holding time is too long, melt temperature is too low, and speed is too fast, the stress will increase and warping will occur.
(2) If mold temperature is too high and cooling time is too short, part will overheat during demolding and ejection deformation will occur.
(3) While maintaining minimum filling amount, reduce screw speed and back pressure to reduce density to limit generation of internal stress.
(4) If necessary, parts that are prone to warping can be shaped after demolding or annealed after demolding.

This defect is mainly a common problem in plastic parts that are colored with masterbatch. Although masterbatch coloring is superior to dry powder coloring and dye paste coloring in terms of color stability, color purity, and color migration, its distribution, that is, degree of uniform mixing of color particles in diluted plastic, is relatively poor, and finished product naturally has regional color differences.
Main solutions:
(1) Increase temperature of feeding section, especially temperature at rear end of feeding section, so that its temperature is close to or slightly higher than melting section temperature, so that masterbatch melts as soon as it enters melting section, promotes uniform mixing with dilution, and increases chance of liquid mixing.
(2) Under condition of a constant screw speed, increase back pressure to increase melt temperature and shearing effect in barrel.

 

During injection molding process, shrinkage and depression of products are a relatively common phenomenon. Main reasons for this situation are:
1. Machine aspects:
(1) If nozzle hole is too large, melt will flow back and shrink. If it is too small, resistance will be large and material volume will be insufficient, resulting in shrinkage.
(2) Insufficient clamping force will cause flash and shrinkage. Check whether there is a problem with clamping system.
(3) If plasticizing volume is insufficient, a machine with a large plasticizing volume should be selected. Check whether screw and barrel are worn.
2. Mold aspects:
(1) Design of part should ensure uniform wall thickness to ensure consistent shrinkage.
(2) Cooling and heating system of mold should ensure that temperature of each part is consistent.
(3) Pouring system should ensure smooth flow and not too large resistance. For example, size of main channel, branch channel, and gate should be appropriate, finish should be sufficient, and transition zone should be arc-shaped.
(4) For thin parts, temperature should be increased to ensure smooth material flow. For thick-walled parts, mold temperature should be lowered.
(5) Gate should be opened symmetrically and opened as far as possible in thick wall part of workpiece. Volume of cold well should be increased.
3. Plastics:
Crystalline plastics shrink more than amorphous plastics. During processing, amount of material should be increased appropriately, or a nucleating agent should be added to plastic to accelerate crystallization and reduce shrinkage depression.
4. Processing:
(1) Barrel temperature is too high, and volume changes greatly, especially fore furnace temperature. For plastics with poor fluidity, temperature should be increased appropriately to ensure smooth flow.
(2) Injection pressure, speed, back pressure are too low, and injection time is too short, resulting in insufficient material volume or density, and shrinkage pressure, speed, back pressure are too high, and time is too long, causing flash and shrinkage.
(3) When amount of material added, that is, buffer pad is too large, injection pressure is consumed. When it is too small, amount of material is insufficient.
(4) For parts that do not require precision, after injection molding is completed, outer layer is basically solidified, sandwich part is still soft and can be ejected, it is best to remove mold as soon as possible and let it cool slowly in air or hot water. This will make shrinkage depression smooth and less conspicuous, will not affect use.

 

Transparent parts made of polystyrene and plexiglass can sometimes be seen through light with some shimmering, fine silver streaks. These silver streaks are also called scintillation spots or cracks. This is because stress is generated in the vertical direction of tensile stress.
Solutions:
(1) Eliminate interference of gas and other impurities, fully dry plastic.
(2) Reduce material temperature, adjust barrel temperature in stages, and appropriately increase mold temperature.
(3) Increase injection pressure, reduce injection speed.
(4) Increase or decrease pre-molding back pressure, reduce screw speed.
(5) Improve exhaust conditions of runner and cavity.
(6) Clean nozzle, runner and gate for possible blockages.
(7) Shorten molding cycle. After demoulding, annealing can be used to eliminate silver streaks: for polystyrene, keep it at about 80℃ for 15 minutes, or keep it at about 50℃ for 1 hour. For polycarbonate, heat it to above 160℃ and keep it for several minutes.

Main causes and solutions for uneven color of injection molded products are as follows:
(1) Poor diffusion of colorant. This often causes patterns to appear near gate.
(2) Plastic or colorant has poor thermal stability. To stabilize color of product, production conditions must be strictly fixed, especially material temperature, material quantity and production cycle.
(3) For crystalline plastics, try to make cooling rate of each part of product consistent. For products with large differences in wall thickness, colorants can be used to mask color difference. For products with relatively uniform wall thickness, material temperature and mold temperature must be fixed.
(4) Shape and gate form and position of product affect plastic filling situation, causing color difference in some parts of product. Modifications should be made if necessary.

Under normal circumstances, gloss of surface of injection molded parts is mainly determined by type of plastic, colorant and finish of mold surface. However, defects such as surface color and gloss defects, dark surface color, etc. are often caused by other reasons. Causes and solutions are analyzed as follows:
(1) Mold finish is poor, there is rust on cavity surface, etc., and mold venting is poor.
(2) Pouring system of mold is defective. Cold well should be enlarged, runner should be enlarged, main runner, branch runner and gate should be polished.
(3) Material temperature and mold temperature are too low.
(4) Processing pressure is too low, speed is too slow, injection time is insufficient, and back pressure is insufficient, resulting in poor density and dark surface color.
(5) Plastic should be fully plasticized, but degradation of material should be prevented. Heating should be stable and cooling should be sufficient, especially for thick-walled parts.
(6) Prevent cold material from entering part. If necessary, use a self-locking spring nozzle or reduce nozzle temperature.
(7) Too much recycled material is used, quality of plastic or colorant is poor, and it is mixed with water vapor or other impurities.
(8) Clamping force should be sufficient.

Silver streaks in injection molded products include surface bubbles and internal pores. Main cause of defect is interference of gas (mainly water vapor, decomposition gas, solvent gas, and air). Specific reasons are analyzed as follows:
1. Machine:
(1) Barrel and screw are worn or there are dead corners in rubber head and rubber ring, which cause decomposition due to long-term heat.
(2) Heating system is out of control, causing temperature to be too high and decomposition. Check whether there are problems with heating elements such as thermocouples and heating coils. Improper screw design can easily bring in air.
2. Mold:
(1) Poor exhaust.
(2) Friction resistance of runner, gate, and cavity in mold is large, causing local overheating and decomposition.
(3) Unbalanced distribution of gates and cavities, unreasonable cooling systems will cause uneven heat distribution and local overheating or blockage of air passages.
(4) Water leaks into cooling passage into cavity.
3. Plastics:
(1) Plastics are too humid, contain excessive amounts of recycled materials, or contain harmful scraps (scraps are very easy to decompose). Plastics should be fully dried and scraps removed.
(2) If moisture is absorbed from atmosphere or from colorant, colorant should also be dried. It is best to install a dryer on machine.
(3) Amount of lubricant, stabilizer, etc. added to plastic is excessive or unevenly mixed, or plastic itself contains volatile solvents. Decomposition will also occur when mixed plastics are difficult to balance heat level.
(4) Plastic is contaminated and mixed with other plastics.
4. Processing:
(1) Setting temperature, pressure, speed, back pressure, and melt motor speed too high will cause decomposition, or pressure and speed will be too low, injection time and holding pressure will be insufficient, and back pressure will be too low. Due to failure to obtain high pressure, density is insufficient and gas cannot be melted, resulting in silver streaks. Appropriate temperature, pressure, speed and time should be set, multi-stage injection speed should be used.
(2) Low back pressure and fast speed will easily allow air to enter barre, enter mold with molten material. When cycle is too long, molten material is heated for too long in barrel and decomposes.
(3) Insufficient material quantity, too large feeding buffer pad, too low material temperature or too low mold temperature will affect flow of material and molding pressure, and promote formation of bubbles.

When molten plastic meets insert holes, area with discontinuous flow rate, area where the filling material flow is interrupted in cavity and merges in the form of multiple strands, it cannot be completely fused and produces a linear weld seam. In addition, when gate is sprayed and filled, a weld seam will be generated, strength and other properties at weld seam are very poor. Main reasons are analyzed as follows:
1. Processing:
(1) Injection pressure and speed are too low, barrel temperature and mold temperature are too low, causing molten material entering mold to cool prematurely and form a weld seam.
(2) When injection pressure and speed are too high, spraying will occur and a weld seam will appear.
(3) Rotation speed should be increased, and back pressure should be increased to reduce viscosity of plastic and increase density.
(4) Plastic should be dried well, and recycled materials should be used less. Excessive use of release agent or poor quality will also cause weld seams.
(5) Reduce clamping force to facilitate exhaust.
2. Mold:
(1) If there are too many gates in same cavity, gates should be reduced or set symmetrically, or as close to weld seam as possible.
(2) Exhaust at weld seam is poor, and an exhaust system should be opened.
(3) Runner is too large, gating system is inappropriately sized, and gate is opened to avoid melt flowing around insert hole, or insert is used as little as possible.
(4) Wall thickness varies too much, or wall thickness is too thin. Wall thickness of part should be made uniform.
(5) If necessary, a fusion well should be opened at weld seam to separate weld seam from part (overflow block).
3. Plastics:
(1) Lubricants and stabilizers should be added appropriately to plastics with poor fluidity or heat sensitivity.
(2) Plastics contain many impurities and should be replaced with better quality plastics if necessary.

 

Rigid plastic parts such as PS form dense ripples on the surface near gate, centered on gate, which is sometimes called vibration marks. Reason for this is that when melt viscosity is too high and mold is filled in a stagnant flow form, front material quickly condenses and shrinks as soon as it contacts cavity surface, subsequent melt expands and shrunken cold material continues to move forward. Continuous alternation of process causes material flow to form surface vibration marks during forward movement.
Solution:
(1) Increase barrel temperature, especially nozzle temperature, also increase mold temperature.
(2) Increase injection pressure and speed to quickly fill mold cavity.
(3) Improve runner and gate size to prevent excessive resistance.
(4) Mold should have good venting and a sufficiently large cold well should be set.

Some plastic parts may develop swelling or blistering on the back of metal insert or in particularly thick areas soon after demolding. This is caused by release of gases from incompletely cooled and hardened plastic under internal pressure.
Solutions:
1. Effectively cool mold, lower mold temperature, extend mold open time, reduce drying and processing temperatures of material.
2. Reduce mold filling speed, shorten molding cycle, and reduce flow resistance.
3. Increase holding pressure and time.
4. Improve part wall thickness or significant thickness variations.