Common problems and solutions during injection molding
During the injection molding process, various quality issues are prone to occur due to factors such as raw materials, equipment, molds, and processes. Promptly identifying and resolving these issues is crucial to ensuring smooth production. Short shots are a common problem, manifesting as a product that does not fill the mold cavity and has damaged edges. Causes include insufficient injection pressure, slow injection speed, low melt temperature, and poor mold venting. Solutions include: increasing injection pressure and speed to ensure the melt has sufficient momentum to fill the mold cavity; raising barrel and mold temperatures to reduce melt viscosity and improve fluidity; inspecting mold vents, clearing any blockages, and ensuring smooth air evacuation; and enlarging runners or gates if they are too small to reduce flow resistance. For example, when short shots occur when molding thin-walled products, increasing the injection speed and mold temperature is the first step to ensure the melt fills the cavity quickly.
Flash (overflow) is another common defect, manifesting as excess flakes on the edge of the part. It is primarily caused by insufficient clamping force, excessive injection pressure, a loose mold parting surface, or excessively high melt temperature. Solutions include: checking the clamping force to ensure it exceeds the cavity pressure and, if necessary, replacing the injection molding machine with a larger tonnage; reducing injection and holding pressures to reduce the impact of the melt on the parting surface; cleaning the parting surface to remove burrs and impurities. If the parting surface is severely worn, regrind it; and appropriately lowering the barrel temperature to increase the melt viscosity and reduce flash. For example, if flash is primarily found in a specific area of the mold parting surface, this could indicate foreign matter or wear in that area, requiring mold disassembly, cleaning, or repair.
Warping is the distortion of the product’s shape after cooling, which affects assembly and use. The main reasons are uneven cooling of different parts of the product, excessive internal stress, and uneven fiber orientation. Solutions include: optimizing the cooling system to ensure uniform mold temperature, adding cooling water channels for thick-walled areas; adjusting process parameters to reduce injection pressure and holding time to reduce internal stress; for crystalline plastics, controlling mold temperature to ensure uniform crystallization; reasonably designing the product structure to avoid excessive differences in wall thickness, and setting pre-deformation compensation on the mold when necessary. For example, flat products may warp with a bulge in the middle, mostly due to slow cooling in the center. You can add a cooling water well in the center to speed up heat dissipation, or lower the mold temperature to shorten the cooling time.
Silver streaks (silver streaks) appear as white streaks on the surface of a product, primarily caused by moisture, volatiles, or air entrapment from the raw materials. Solutions include: thoroughly drying hygroscopic plastics. For example, PA requires drying at 100-120°C for 4-6 hours; cleaning the barrel and runners to remove degraded plastic impurities; lowering the barrel temperature to prevent plastic degradation; adjusting the injection speed to avoid turbulent air entrapment; and installing venting slots near the gate to exhaust air. For example, silver streaks in PC products are often caused by insufficient drying, requiring extended drying time. Also, check that the hopper is sealed to prevent secondary moisture absorption.
Sink marks are depressions on the surface of a product, primarily appearing in thick-walled areas or at the base of ribs. They are caused by insufficient shrinkage compensation when the melt cools and shrinks. Solutions include: increasing the holding pressure and extending the holding time to ensure continuous replenishment of the melt into the mold cavity; increasing the injection volume to fill the cavity with the melt; optimizing the gate position so that the holding pressure can be effectively transferred to thick-walled areas; and appropriately increasing the mold temperature and slowing the cooling rate for more adequate shrinkage compensation. For example, if a sink mark appears at the base of a rib on a product, the gate can be positioned near the rib, or the transition radius between the rib and the main body can be increased to improve melt flow and shrinkage compensation. Furthermore, for severe sink marks, the wall thickness of the product can be appropriately reduced or the demolding angle can be increased to reduce shrinkage.
