The advantages of plastic injection molding in parts production include fast production speed, high efficiency, automation of operations, and shape and size flexibility. Furthermore, injection molded products are dimensionally accurate, easily interchangeable, and molded parts can be molded into complex shapes. This environmentally friendly process is suitable for mold processing areas such as mass production and products with complex shapes.
Although operators are familiar with the specific machines and operating skills in the plastic parts molding process, various rapid injection molding defects in mold design and materials are inevitable. The article mainly deals with plastic injection molding problems and solutions:
Analyze the causes of these problems caused by raw materials, plastic parts or mold design, molding processes, etc., and propose corresponding solutions.
1. Short photos
Short shots are a phenomenon where the mold cavity cannot be completely filled.
Causes:
(1) Nozzle temperature, material or injection pressure and speed are too low
(2) Uneven plasticization of raw materials
(3) Poor exhaust performance
(4) Insufficient liquidity of raw materials
(5) The part is too thin or the sprue opening is too small (6) The molten polymer hardens prematurely due to unfavorable design
Remedy:
Material: Use more liquid materials
Mold Design:
(1) Fill in the thick wall before filling in the thin wall to avoid the retention phenomenon.
(2) Increase the number of ports and channel size accordingly to reduce process aspect ratio and flow resistance.
(3) The position and size of the exhaust port should be adjusted appropriately to avoid the phenomenon of poor ventilation.
Machine:
(1) Check whether the check valve and the inner wall of the material cylinder are severely worn
(2) Check whether there is material in the feed opening or bridging.
Procedure:
(1) Increase injection pressure and injection speed to increase shear heat
(2) Increase the injection amount
(3) Increase material cylinder temperature and mold temperature
2. Air traps
Air trapping is air trapped in the cavity that creates bubbles in the part.
Causes: This is caused by the fact that when the two melt fronts meet, air cannot escape from the interface, the plunger or the ventilation opening.
Remedy: Structural design: Reduce thickness inconsistency and try to ensure uniform wall thickness
Shape Drawing :
(1) Install a vent at the last full point
(2) Redesign of inlet channel and channel system
Law Suit :
(1) Reduce the last stage injection speed
(2) Increase the mold temperature
3. Fragility
Brittleness means that the plastic part cracks or breaks easily in one place
Causes:
(1) Dry conditions are not suitable; excessive use of recycled material
(2) The injection temperature setting is incorrect
(3) The input channel and channel system settings are not suitable
(4) The stain resistance of enamel is not high
Remedy:
Material:
(1) Set suitable drying conditions before injection molding
(2) Reduce the use of recycled materials and increase the proportion of raw materials
(3) Use high-strength plastic. Shape design: Increase the size of main channel, branch channel and gate
Machine: Choose a well-designed screw to make the temperature distribution more uniform during laminating
Procedure:
(1) Reduce the temperature of the material cylinder and nozzle
(2) Reduce back pressure, screw speed and injection speed
(3) Increase material temperature and injection pressure, improve casting strength
4. Burn marks
Burn marks arise because the gas in the cavity cannot be removed in a timely manner, which leads to a black color at the end of the flow.
Causes:
(1) The air in the cavity cannot be removed in time.
(2) Material degradation: very high melting temperature; very high screw speed; incorrect design of the gate system
Remedy:
Mold design:
(1) Install an exhaust system in a location where exhaust is easily generated
(2) Increase the size of the gate system
Procedure:
(1) Reduce injection pressure and speed
(2) Reduce cylinder temperature
(3) Check whether the heater and thermocouple are working properly.
5. Lightning
This means there is excess plastic in the mold divider or ejector part.
Causes:
(1) Insufficient clamping force
(2) The form has defects
(3) Form conditions are inadequate
(4) Improper exhaust system design
Remedy:
Mold design:
(1) Design the mold appropriately to ensure that the mold can be closed when the mold is closed
(2) Check the size of the outlet opening
(3) Mold surface cleaning
Machine: Set up the machine for the appropriate size process:
(1) Increase injection time and decrease injection speed
(2) Reduce cylinder temperature and nozzle temperature
(3) Reduce injection pressure and pressure
6. Delamination
Refers to the surface of the part, which can be removed layer by layer.
Causes:
(1) Mixture of other incompatible polymers
(2) Too much release agent during molding
(3) Resin temperature is inconsistent
(4) Excessive humidity
(5) The gate and flow path have acute angles
Remedy:
Material: Avoid incompatible contaminants or contaminated recycled materials mixed with raw materials
Shaping: Chamfer any sprues or doors with sharp angles
Procedure:
(1) Increase the temperature of the cylinder and mold
(2) Proper drying of material before molding
(3) Avoid using too much release agent
7. Rays
A spatter mark is caused by melt flowing too quickly, usually in a serpentine shape.
Causes:
(1) The size of the door is very small and has a large cross-sectional area on the surface of the product.
(2) Filling speed is too fast
Remedy:
Mold design:
(1) Increase the gate size
(2) Change the side gate to the back gate
(3) Raise the stop pin in front of the gate
8. Simplify
Flow lines are wavy-shaped defects on the surface of the product, called frog jumping type, caused by the slow flow of melt.
Causes:
(1) Mold temperature and material temperature are too low
(2) Injection speed and pressure are too low
(3) The size of the flow channel and gate is too small
(4) Due to the structure of the product, the acceleration during the filling flow is very large.
Remedy: Mold design:
(1) Increase the size of the cold well in the flow channel
(2) Expansion of channels and floodgates
(3) Shorten the main channel or switch to a hot runner
Procedure:
(1) Increase injection speed
(2) Increase injection pressure and pressure
(3) prolong pressure retention time
(4) Increase mold temperature and material temperature
9. Fog
There is a cloud-like discoloration in the sprue area. The cause is fracture of the enamel.
Causes: If the entry points are too small or the cavities in the adhesive are too thin, the melt flow rate is too high, small breaking points and high shear rates often lead to increased shear stresses, which leads to casting cracking and mist displacement.
Remedy: Mold flow simulation. It is possible to predict the temperature, shear rate, and shear stress of the melt flowing through the narrow zone above. Moldflow generally provides an upper limit for the temperature, shear rate, and shear stress of various plastic materials. Moldflow engineers can make adjustments based on the analysis results to find the appropriate door size and cavity wall thickness at the glue inlet to prevent fogging.
10. Stripes
Streaks are those in which water, air or carbon are distributed along the surface of the part in the direction of flow.
Causes:
(1) The moisture content of raw materials is too high
(2) Air is trapped in the raw material
(3) Polymer degradation: The material is contaminated, the cylinder temperature is too high; insufficient injection volume
Remedy:
Mold design:
Check whether the exhaust air position is sufficient. Procedure:
(1) Choose the right injection molding machine and mold
(2) When changing the material, completely remove the old material from the pipe
(3) Exhaust system improvement
(4)Reduce melting temperature, injection pressure or injection speed
11. Sink marks
It is a phenomenon in which the surface of the part is concave at the level of the wall thickness.
Causes:
(1) The injection pressure or holding pressure is too low
(2) The retention time or cooling time is too short
(3) The melting temperature or molding temperature is too high
(4) Inadequate design of parts structure
Remedy:
Project:
(1) Corrugated surface on the surface where dents can easily occur
(2) Reduce the wall thickness of the workpiece, minimize the length/width ratio, and keep the adjacent wall thickness ratio at 1.5 to 2. Try to create a smooth transition. Redraw the thickness of the ribs, countersinks and ribs. It is generally recommended that its thickness is 40 to 80% of the base wall thickness.
Procedure:
(1) Increase the injection pressure and pressure
(2) Increase the gate size or change the gate position
12. Binding lines
Weld lines are surface defects caused by two-stream welding.
Causes: If manufactured parts have holes, inserts, multiport injection molding processes, or if the wall thickness of parts is uneven, weld lines may occur.
Remedy:
Material: Increase the fluidity of molten plastic
Mold design:
(1) Change the position of the gate
(2) Add a ventilation opening
Procedure:
(1) Increase melting temperature
(2) Reduce the amount of release agent
13. Pattern
The most difficult problem to solve in the design and manufacture of plastic parts is warpage.
Causes:
(1)Mold structure: casting, cooling system and ejection system
(2) Product structure: The wall thickness of plastic parts changes, with curved or asymmetrical geometry, ribs and BOSS column design are not useful
(3) Production process: Plastic parts are not completely cooled, and the injection and pressure retention curves are inadequate
(4) Plastic materials: Plastic materials do not contain any additional fillers and the amount of shrinkage is the same.
Remedy:
(1) The mold temperature is unstable. Ensure balanced mold cooling/heating
(2) Irregular transverse thickness. Redesign product shape and size according to resin properties
