How to Improve Injection Molding Visual Quality?
When it comes to injection molding, there are a lot of room for costly mistakes. Injection molding visual quality issues ranges from tiny surface defect to big problems which affect the safety, performance and function of the injection molded parts.Injection molding visual quality issues are caused by molding process, materials, tooling design or a combination of all.
Understanding common injection visual quality defects and how to avoid them can help to deduce the unsellable good and products return rate. Here are the most common injection molding visual quality issues. And how to improve injection molding visual quality during rapid manufacturing and mass production?
Injection molding visual quality issues caused by process problems
It is difficult to adjust some injection molding visual quality issues. Some injection molding visual quality issues can be avoided by adjusting the molding process without redesigning the mold or replace production equipment. Some injection visual quality defects can be avoided easily and simply by adjusting the flow rate, temperature and pressure.
Flow lines
Flow lines is a wavy pattern in different color than the surrounding area and on narrower section of the injection molded parts. Flow lines also like a ring-shaped band on the surface near the entry points of the mold which the molten material flows through. Flow lines affect the integrity of the parts. In certain cases, flow lines are unacceptable. For example, high quality sunglasses.
Flow lines are caused by the cooling speed of the material as it flows in different directions throughout the mold. Different wall thickness also causes the material to cool at different rates. For example, molten plastic cools fast during the injection molding process. The flow lines are evident when the speed of the injection is slow. The plastic is solid and gummy. It is filling the mold and cause the wave pattern.
There are some ways to improve the flow lines. For example, increasing the injection molding speed, pressure and material temperature to make sure the material fills the mold before cooling. Making the corners of the mold round to increase the wall thickness. This will keep the flow rate consistent and avoid flow lines. Relocating the mold gates to create more distance between mold gates and mold coolant which prevents the material from cooling early during flow. Increasing the nozzle diameter to increase the flow speed and avoid early cooling.
Burn marks
Burn marks are black or rust colored discoloration on an edges or surface of a plastic injection molded part. Brun marks won’t affect injection molded part integrity.
Burn marks are caused by trapped air, resin itself, o-0verhearting in the mold cavity during injection molding process.
There are some ways to prevent the burn marks. Lowering the melt and mold temperature to avoid overheating, reducing the injection speed to limit the risk of trapping. Enlarging gas vents and gates to let the trapped air to escape the mold. Shortening the cycle time of the mold so the trapped air and resin will not have a chance to overheat.
Deformation
Deformation is caused when different parts of a component shrink unevenly. Like wood will warp when it dries unevenly. Plastic will wrap in cooling process when uneven shrinkage puts undue stress on different areas of the injection molded parts. The undue stress will bend and twist the completed parts as it cools.
Deformation is cause by too quick cooling. And excessive temperature, low thermal conductive of the molten material will make the deformation problem worse. The uniform thickness of the mold will also cause these problems.
There are some ways to prevent deformation. Make sure the cooling process is gradual and long to prevent uneven stresses. Lower the mold and material temperature. Switch to a material which shrinks less in cooling process. Design the mold with uniform wall thickness and part symmetry to make sure great stability during cooling.
Air bubbles
Air pockets are trapped air bubbles in completed injection molded parts. Air bubbles is considered to be a minor defect. In some certain cases, larger or numerous air bubbles will weaken the injection molded parts.
Air bubbles are caused by inadequate molded pressure to force tapped air out of the mold cavity.
There are some ways to prevent the air bubbles. Raise the injection molding pressure to force out trapped air bubbles. Choose a material with lower viscosity to limit the risk of air pockets. Place gates close to the thickest parts of the mold to avoid premature cooling.
Sink marks
Sink marks are small recesses in a flat and consistent surface of injection molded parts. Sink parts happen when the inner parts of the molded parts shrink.
Sink marks are similar to vacuum voids. Sink marks is caused by too slowly cools. The shrinkage pulls the outside material inward before it cools, leading to a depression. Sink marks are likely happen in thicker parts of a component.
There are some ways to prevent sink marks. Increase the holding pressure and time to let the material near to the surface of the parts to cool. Increase the cooling time to lower the risk of shrinkage. Design the mold with thin wall for quicker cooling near the surface.
Weld lines
Weld lines are on the surface of the injection molded parts. The molten material converged after splitting off into two or more directions of a mold. The weld lines are like hair. The weld lines weak the material bonding and will lower the strength of the injection molded parts.
When two or more fronts of molten plastic material collide, a certain temperature is needed. If not, the two or more fronts of material will solidify and will not bond, so there are weld lines.
There are some ways to prevents weld lines. Increase the material temperature to avoid partial solidification. Before the materials fills the mold, increase the injection speed and pressure to limit cooling. Choose a material with a lower melting temperature of viscosity to have quicker flow and avoid early cooling.
Jetting
Jetting is kind of deformation of an injection molded parts. Jetting is caused by excessive injection. To prevent jetting, reduce the injection pressure to prevent quick squirting of the material into the mold cavity. Increase the material and mold temperature to keep the jet of material from solidifying. Design the mold with the injection gate located so the material is directed across the mold.
Discoloration
Discoloration happens when an injection molded part is a different color than desired. Discoloration is caused by the leftover pellets in the hopper in the nozzle or mold from a previous production run. There are some ways to prevent discoloration. Clean the hopper, nozzle and mold between production runs. Use a purging compound to remove excess color. Make sure material supplier uses color agents with adequate thermal stability.
Plastic injection molding need significant initial investment in tooling. It is important to design the right mold at the beginning. Defects of the injection process or material are easily to and cheaply to fix. But injection molding visual quality will damage the bottom line. If you understand the common injection molding visual quality, you will know how to avoid them and make sure your injection molded parts meet your desired quality standards.
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