Guide to Managing Sink Marks in Injection Molding Production
Sink marks are among the commonest cosmetic defects of injection molded parts. A characteristic is surface depression often seen in thick wall sections. Examples of features that use thick material include ribs or bosses. They cause uneven shrinkage while the part cools. It happens that the thicker sections pull apart more when compared to the thinner sections. This leaves divots on the opposite side of the part. Additionally, a greater difference in material thickness results in more shrinkage. The defect is a major limiting factor in designing plastic molded parts. Especially thin-walled parts such as display monitors and TV bezels. Customers ideally want electronics that are virtually free from sink mark. Unlike toys that clearly have visible sink marks on their surfaces.
How Improved The Injection Molding Parameters Lessens The Sinking Defect
Make The Outer Walls Uniform
The design strategy helps decrease sink size. It involves making ribs that are 40% to 60% that of the outer thicker walls. Ribs are important since they offer added rigidity and strength to a part. This is even without the help of extra material. However, ribs of similar thickness to that of the part run the risk of showing more prominent sink marks. Therefore, keeping them at 40% to 60% the size of the outer wall thickness is best. At Creatingway, we aim at keeping the ribs thinner than the outer walls. This will ensure your part remains more stable while still reducing the sink mark size when we will do injection molding production for your projects.
Remove Some Material Away from Boss Features
A boss is a feature made from an extruded-out cylinder. It is normally perpendicular to the surface with an interior hole. The hole can accommodate an insert, a thread, or screw needed for assembly. This neat trick removes visible rings present on the cosmetic surface of a part. The rule is to have a boss with a lesser thickness than the outer wall. We have determined the sweet spot ratio ranging from 40% to 60%. Also, the triangular groove found at the base of the boss should be 30% wall thickness deep when we will do injection mold production. It should then merge back with the outer wall at a tapering angle of 30-degrees. These additional cuts on the boss prevent the pulling of material during cooling. Thus, it mitigates sink marks development. It is also vital to separate the bosses from the side walls. This prevents constructing a thick section to merge the sidewall and boss. It would have been the ideal site for the later formation of a sink mark. Preferably, use ribs as support between wall and boss. Another option is coring the wall section then joining parts later during assembly using screws.
Changing Packing Pressure Values
At Creatingway, we subject molten plastic under pressure while inside a mold to make parts. The pressure used has a great impact on the mechanical and thermal stresses of the part. We could take advantage of this phenomenon and carefully control it while aiming to reduce sink marks development. Increasing both the packing pressure and holding time reduces sink marks. We do this by adding more material into a cavity after already being filled. However, it also results in slower injection speeds. Thus, the creation of flow marks is possible resulting in more defects. Therefore, a balance needs to be found to maximize the elimination of all sink marks. Further, sink marks occur due to uneven cooling of the wall and tool parts. Therefore, changing the coolant material from the cavity side to the core may change the direction of pull. However, it may result in part distortion if not properly controlled.
Altering The Gate Size and Location
The gate an injection molding production process allows molten plastic to enter the mold. A cooling channel design may affect sink mark development. For instance, a tunnel gate freezes the molten plastic quicker than when compared to an edge gate. The fast cooling rate reduces the packing time available in the mold. Thus, it increases the chances of your part getting sink marks. It also does not allow enough time for areas within the cavity to fill up effectively. This means the process is not taking advantage of the packing pressure way of remedying the sink marks. A large gate to prolong freezing time is also not a great option. it significantly slows the injection resulting in the formation of flow marks.
Adding Texture On Cosmetic Surfaces
Even with the best design for an injection mold part, sink marks may remain visible. Surfaces finish that is glossy and shiny quickly betray your part to even the faintest of sink marks. At Creatingway, we advise you to consider having your part get a matte surface finish. Mold texturing of MT-11020 or MT-11010 are great options to hide those unsightly sink marks. It works by reducing glare that exposes the minor imperfections present on the part surface.
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