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Injection Molded Part Problems – Sink Marks

Molder Gets “Owned” by His Sink Marks

Don’t let this happen to you!

injection molding sink mark


When you mold parts that have defects and want to know the cause it’s best to talk to your plastic.

But you have to talk in their very limited vocabulary because to the plastic molecules there are only four basic variables, plastic pressure, temperature, flow and cooling.

That’s all the plastic knows or feels.




Here is a conversation a molder recently had with his plastic over a sink mark problem.


Molder:Why are you causing sink marks in my molded parts?

Molecule (speaking knowingly): “Because there are not enough of us molecules in here

Molder: “But I filled and packed the injection mold”

Molecule (speaking condescendingly): “Sure, but at molding temperatures, us molecules move further apart. You packed more of us in but it wasn’t enough. When we cool we moved closer together. Those of us who cooled last pulled on our neighbors trying to get closer. I guess we pulled so hard that we collapsed the wall. You call that a sink mark. To us, well, it’s just what we do

Molder:How can I go about fixing sink marks?

Molecule: (patronizing tone): “Isn’t that obvious? You’re the molder. Take whatever steps you can to put more of us molecules into the mold cavities. Or strengthen the wall so we can’t cave it in. (Ha!)

Molder: (after a little thought): “OK. So the problem is too few molecules, what can I do? I could…”

  1. Increase pack and hold pressure to get more molecules in the cavity. (If the pack/hold time is allowing discharge, increase the hold time.)
  2. Fill a little slower to the keep the gates open longer and let more molecules enter before the gates freeze (I could even increase the gate size to accomplish the same result).
  3. Reduce the mold temperature so instead of adding more plastic I strengthen the plastic surface.

If you have molding problems ask the molecules. They only know four variables:

  1. Heat
  2. Pressure
  3. Flow rate
  4. Cooling rate

This Post Has 25 Comments

  1. Excellent source of info for newbies and experienced personnel . I will be using Paulson training in the near future to help with the training of new operators in the blown film industry.

  2. Your explanation was good but would be even more effective if we had a simple graphic images of the molecules before and after. Very useful and simple to understand

  3. I don’t get how a slow injection speed could make the gates open for a longer time. To me it seems totally the opposite.

    1. By slowing the injection speed, the cavities take longer to fill. Therefore, the gates do not freeze off as quickly as if the cavities were filled faster. Gates remain open and more plastic fills the cavities which reduces the sink mark problem Thanks for your feedback.

  4. Surely the gate will only stay live if the gate diameter is big enough.if its a small gate it will freeze off quickly and the slow injection speed will only make matters worse. Or have I got this wrong.

    1. As long as the plastic is flowing, the gate should stay open. The goal of the slower fill rate is to let the part begin to cool (and shrink) for a longer period of time during injection. This allows you to put more molecules into the cavities. That solves the fundamental problem that the plastic is experiencing. Thank you for the comment.

  5. very useful information thank you. Could we also increase the melt temperature ? This will allow more plastic to enter in the cavity in less time and will provide extra compression if we are in need of more holding pressure to fix the sinks.I guess now we will be in the position of allowing additional holding time as the gates would be staying open slightly longer due to increased melt temp.Please correct me if I am wrong.

    1. Yes. Increasing melt temperature would also work. Keep in mind though that there is usually only 1 best (i.e. most profitable) solution to any molding problem. As you correctly point out, any heat you add to the melt has to be taken out at some point. So you’ve got a trade-off here. I usually look for the solution that gives me the fastest cycle time. That is almost always the most profitable solution.

    1. An increased cycle time might be a by-product of how you fix your sink marks. One method to try is to lower mold temperature so that the molecules at the surface of the part freeze before the sink mark can form. From the plastics point of view, they would say “There are not enough of us plastic molecules in here.” So an increase in packing is also a possible fix.

  6. Mmmm – top of the page is only half the answer. If free volume was explained in a clearer more concise way it would make the machine corrections more logical. For example a given polymeric mass occupies a different cubic volume depending on its temperature. This is because of free volume forming between the polymer chains. Vf is the space that is not occupied by molecules. The difference in free volume between when it’s at room temperature and it melt temperature is what mold techs can understand to be shrink . Any process corrections such as holding time and pressure are replacing this volume loss between the polymer chains as it cools.

    1. Yes, your explanation is correct. The explanation we’ve given is more on the level of our intended audience. We certainly understand all of the physics behind injection molding, but Paulson Training’s mission to to train as many production floor personnel as possible. Your explanation, while technically exactly right, would be out of reach for most of our target audience. Thank you for sharing your expertise.

  7. Awesome answers but to me reducing the injection speed could reduce my cycle time. Hold time also reduce or increase cycle time depending your settings. Well i will give a try.

  8. However, some polymers with low viscosity can also contribute to sink marks, i ve’ just changed my plastic material with high viscosity in the hopper with same injection speed and now the new molded parts show no sign of sink marks whereas the previous material produces molded parts with sink marks even after trying to reduce injection speed, increased holding pressure and temperature as some of you suggested. Material matters most to my experience.

    1. Good point. From the plastic’s point of view…Lower viscosity materials (lower resistance to flow) will allow more plastic molecules into the mold for a given injection fill speed. Enough additional plastic in the mold would eliminate sink marks (depending on their severity).

  9. AS my experienced concern a position of sink mark is a product also gives an Idea . If sink mark at location where air is also escaping from mould Than it will be solved by slow Injection speed, High melt temperature. , If sink mark at thicker position it will be solved by high speed, more packing of material

  10. I have sink where a 1.8 mm rib meets a 3.0 mm wall in PA66 GF33. If the wall thickness were increased from 3 to 4 mm could I expect reduced/eliminated sink marks?

  11. I’m confused by the comment from Paulson Training regarding viscosity: “…Higher viscosity materials (lower resistance to flow) will allow more plastic molecules into the mold…”. My understanding is that lower viscosity materials have lower resistance to flow. They are more ‘runny’. I can visualize how ‘runny’ materials can fill more nooks and crannys in the mold, thus allowing more molecules in the mold. Am I misguided?

  12. If you try the above and you still have sink marks, then reduce the melt temperature. As everyone knows the more heat that gets to molecules the more they move away from each other which means expanding. When they cool down they move back together which means shrinkage. A Sink mark is itself a kind of shrinkage.

  13. All this is great as I had sink problems with GFN2 PPE+PS, my problem is that when I get the sink out the color of the product is not as good. Being that I am new to the injection molding industry I am probably missing something. From what I have read I need a higher temperature on both the mold and the material to make the glass less visible on the outside. When I do this I get sink.

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