Finding a sink mark in injection moulding on an otherwise perfect part is one of those things that can drive a production manager crazy. You've got your cycle times dialed in, the finish looks great, but then you catch the light hitting a flat surface and there it is—a small, shallow depression that makes the whole component look cheap. These little craters don't just mess with the aesthetics; they can also signal internal stresses that might compromise the part's integrity down the line.
If you're staring at a bin of rejected parts right now, don't sweat it. Dealing with sink marks is a rite of passage in the moulding world. It's rarely a "one and done" fix, but once you understand the physics of what's happening inside the cavity, you can usually troubleshoot your way out of the problem fairly quickly.
Why Do These Dips Actually Happen?
To fix the issue, we have to look at what's going on while the plastic is cooling. Essentially, a sink mark is caused by thermal contraction. When plastic is in its molten state, it takes up more volume than when it's a solid. As it cools down inside the tool, it shrinks.
Usually, the outer skin of the part cools first because it's touching the cold steel of the mould. This creates a solid "shell." However, if the section of the part is particularly thick, the material in the very center stays hot and molten longer than the surface. As that inner core finally cools and shrinks, it pulls on the outer skin. Since the skin is still a bit soft or the internal vacuum is strong enough, it gets sucked inward, creating that dreaded dimple on the surface.
Think of it like a loaf of bread that collapses in the middle because the outside crust set before the inside was fully baked. It's all about the timing of the cooling process.
The Design Culprits
Most of the time, a sink mark in injection moulding can be traced back to the original part design. It's tempting to add thick sections for strength, but that often backfires.
Ribs and Bosses
The most common place you'll see sinking is directly opposite a rib or a boss. If you have a 2mm wall and you attach a 2mm thick rib to the back of it, the "T-junction" where they meet is now significantly thicker than the rest of the part. That extra mass of plastic takes much longer to cool.
A good rule of thumb is to keep your rib thickness between 40% and 60% of the main wall thickness. If you go much thicker than that, you're almost guaranteed to see a shadow or a sink on the "show" side of the part. If you're working with high-gloss materials, you might even need to drop that down to 30% because shiny surfaces show every single imperfection.
Sharp Corners and Thick Walls
Heavy wall sections are the enemy of a clean finish. If you can, try to maintain a uniform wall thickness throughout the part. When you hit a corner, if the outer radius isn't proportional to the inner radius, you end up with a "thick" spot in the bend. Coring out these heavy areas is usually the best move. It saves material, speeds up your cycle time, and kills the sink marks all in one go.
Tweaking the Process
Sometimes you can't change the design because the mould is already cut and the customer isn't budging. In that case, you have to play with your machine settings. This is where the "art" of moulding comes in.
Pressure is Your Best Friend
The most direct way to fight a sink mark in injection moulding is to shove more plastic into the cavity to compensate for the shrinkage. This is where holding pressure and holding time come into play. After the initial injection, you want to maintain pressure so that as the plastic cools and shrinks, more molten material is forced in to fill the gaps.
If your holding pressure is too low, or if you're letting off the pressure too soon, the plastic will pull away from the walls. However, there's a limit. If you over-pack the part, you might end up with flash or parts that get stuck in the tool.
Temperature Control
It sounds counterintuitive, but sometimes lowering the melt temperature can help. If the plastic is slightly cooler when it enters the mould, it has less "shrinking" to do as it solidifies.
On the flip side, you also need to look at your mould temperature. If the mould is too hot, the skin stays soft for too long, making it easier for the internal shrinkage to pull the surface inward. By dropping the tool temperature, you help that outer skin freeze faster and stay rigid enough to resist the pull from the cooling core.
Gate Location and Size
If your gate is too small, it might freeze off (solidify) before the part is fully packed. Once the gate is solid, no amount of holding pressure from the machine will reach the cavity. If you're seeing sink marks and your gate is tiny, you might need to open it up a bit.
Also, you generally want to gate into the thickest section of the part. This ensures that the areas most prone to sinking get the most pressure for the longest amount of time.
Material Matters
Not all plastics are created equal. Some are just naturally "sinkier" than others. Semi-crystalline resins (like Polypropylene or Polyethylene) have a much higher shrinkage rate compared to amorphous resins (like ABS or Polycarbonate).
If you're using PP, you're going to have to be much more disciplined with your wall thicknesses and packing pressures. If the project allows for it, switching to a material with a lower shrinkage rate or one that's glass-filled can sometimes solve the problem without a single tool change. Glass fibers don't shrink, so they act like a "skeleton" that holds the shape of the part while the resin cools around them.
Quick Troubleshooting Checklist
If you're standing at the press and the parts look like they've got a bad case of the dimples, try this sequence:
- Increase Holding Pressure: Give it more "pack" to fill the void.
- Increase Hold Time: Make sure the gate hasn't frozen before the sink is gone.
- Check Your Cushion: If your screw is bottoming out (cushion is zero), you aren't actually applying any holding pressure to the part.
- Lower Melt Temp: Reduce the total thermal energy the tool has to remove.
- Cooling Time: Give the part more time in the tool so the skin is "rock hard" before it's ejected.
- Gate Size: If all else fails, look at the gate. If it's too small, you're fighting a losing battle.
Does it Always Matter?
Let's be real for a second—sometimes a sink mark in injection moulding isn't a dealbreaker. If the mark is on an internal component that no one will ever see, it might be better to just leave it. Trying to pack out a sink mark often increases your cycle time, which means your parts cost more to make.
However, if you're making consumer electronics, automotive interiors, or high-end medical devices, "good enough" usually isn't. In those cases, it's about finding that sweet spot between design, material, and process.
It's often a balancing act. You want a part that looks perfect, but you also want it to be cheap and fast to produce. By understanding that sink is just the result of the laws of thermodynamics, you can stop guessing and start fixing. Keep your walls thin, your ribs thinner, and your packing pressure high, and you'll be well on your way to a sink-free production run.