Problems and Solutions in Products Manufactured by Plastic Injection Method
Burn Marks
Symptoms: Stains, brown spots, black spots, burns, burnt material, black and brown stripes, and bronzing.
Cause: Contamination in the material mix. Stains on the part surface are considered defects. The material may be mixed with burnt material or there might be a hot spot in the process that locally burns the plastic. Old screw and barrel machines can cause such defects due to the presence of burnt residues inside the machine.
Solution: The only way to prevent this is to find and clean the components causing the error in the screw and barrel.
Gas Bubbles, Voids
Symptoms: Small bubbles observed on the outer surface of the part.
Cause: The presence of substances in the melt that can quickly transition to a gas phase during injection. These defects can occur on the surface or inside the structure of the part.
Solution: Material stored in a cold storage or silo can cause vaporization of moisture when processed in a hot or warm pressing environment. Therefore, the material should be dried before production. Hot air dryers for materials like PP, PS and dehumidifying dryers for PET, PC, NA, POM, PMMA should be used.
Poor Color Distribution
Symptoms: Uneven color concentration throughout the part. Different color distributions observed on the part.
Cause: The melted raw material may not have been completely homogenously mixed with the color concentrate in the screw.
Solution: All melting and mixing occur due to the rotation of the screw. Adjustment in mixing can only be achieved by using mixing screws and by reducing barrel temperatures and increasing back pressure. This indicates that more mechanical action is needed as the material melts. Also, dosing systems should be used for homogeneous mixing.
Silver Streaks
Symptoms: Silver-like shiny streaks on the surface of the part.
Cause: The material is overheated. Condensed gas remains trapped on the mold surface.
Solution: Cooling equipment should be checked. Ensure equipment provides steady flow and constant cooling temperature. Using chillers or thermo-converters can be effective.
Delamination – Shrinkage and Orange Peel Effect
Symptoms: The part appears to be made of separate layers that easily peel off. The part may shrink more than calculated.
Cause: Generally in slow filling, especially in NA group materials, cold melt layers form, and shrinkage can be observed.
Solution: Excessive mold spray can also cause this. The material nearly freezes as it enters the mold. More material entering freezes as a non-cohesive layer. Increasing injection speed and using mold conditioners will solve this problem.
Stringing
Symptoms: When the mold opens, plastic extensions that look and stretch like threads are seen dangling downwards.
Cause: Occurs due to slow flow of thick material from the sprue bushing. The pressure in the barrel has not been efficiently relieved.
Solution: To prevent this, shot-off nozzles are placed in the molding machine. They are specifically designed to prevent stringing. In hot runner molds, if the channels are not functioning properly, this can lead to stringing. Stringing can be controlled to some extent with standard nozzles.
Ejector Pin Marks
Symptoms: Marks of ejector pins on the part.
Cause: The pressure of the ejectors can leave marks on the part.
Solution: This defect can be eliminated by balancing the temperature of the part and the force of the ejector pins. Additionally, robotic automation systems can be used in the mold for product removal.
Flash
Symptoms: Excess material, usually at the parting line of the mold.
Cause: The pressure of the material filling the mold cavity overcomes the pressure keeping the mold closed, causing excess material to spill out and form flash.
Solution: If proper loading control is not maintained, the mold cavity can be overfilled, leading to this defect. The screw should advance and retract consistently in each cycle, injecting the same volume of material into the mold.
Warping
Symptoms: The finished part is out of shape, warped.
Cause: Internal stresses in the part cause warping, reflecting on the geometry of the part.
Solution: Warping can occur in molds with improper runner entrance design, runner entrance geometry, designs with high stress, and inadequate or incorrect runner entrance location. Increasing cooling time will stop the stresses.
Slip
Symptoms: Despite the screw turning, the machine fails to capture material for the next shot.
Cause: The machine fails to catch the added material.
Solution: This error could have been caused by emptying or malfunctioning of the hopper. Using a vacuum loader will eliminate these issues.