--- title: "Sink Marks on Injection Molded Parts: What's Really Going On" url: "https://www.imspares.com/blog/sink-marks-injection-molding-causes-solutions/" author: "IMSPARES Technical Team" date: "2026-10-08" last_modified: "2026-10-31" category: "Troubleshooting" tags: ["sink marks","molding defects","packing pressure","wall thickness","part design"] --- # Sink Marks on Injection Molded Parts: What's Really Going On Sink marks are depressions on the surface of molded parts. Learn what causes them and the practical solutions that actually work in production. [Sink marks](https://www.imspares.com/troubleshooting/#sink-marks). Every molder has dealt with them. Those annoying depressions on the surface, usually opposite ribs, bosses, or thick sections. They look like someone pressed a thumb into the plastic before it hardened. And in a sense, that's exactly what happened — the inside of the part shrunk more than the outside, pulling the surface inward. ## The Physics in Plain Language Plastic shrinks as it cools. More volume of plastic = more total shrinkage. When you have a thick section behind a visible surface — like a rib intersection or a boss — the thick area takes longer to cool and shrinks more than the surrounding thinner walls. The surface solidifies first (it's against the cold mold wall). Then the interior continues cooling and shrinking. As it shrinks, it pulls the already-solidified surface inward. That's your sink mark. The key insight: **sink marks are a geometry problem first, and a process problem second.** You can sometimes eliminate them with process adjustments, but if the part design has a massive rib-to-wall ratio, no amount of packing pressure will fully compensate. ## The Part Design Factor This is where the conversation gets uncomfortable, because fixing part design is harder than adjusting machine settings. But it's the truth: **The #1 rule:** Rib thickness should be 50-60% of the adjacent wall thickness. A 2mm wall should have ribs no thicker than 1.0-1.2mm at the base. When ribs are thicker than this guideline, you get a thick zone at the rib-wall junction. That thick zone is the sink mark waiting to happen. **Bosses** create the same problem. The ring of material where a boss meets the wall is essentially a thick section. If you have control over part design, this is where to focus. If you don't (you're molding someone else's part), then process adjustments are your only option. ## Process Solutions ### Packing Pressure and Time Packing (holding) pressure is the primary process control for sink marks. After injection fills the cavity, the screw maintains pressure to push additional material into the cavity as it shrinks. Higher packing pressure forces more material in, compensating for shrinkage. Longer packing time maintains that pressure until the gate freezes and seals off. But there are limits: - If the gate freezes before the thick section has cooled enough, packing pressure can't reach the problem area - Excessive packing pressure can cause [flash](https://www.imspares.com/troubleshooting/#flash), sticking, or overpacking (its own set of problems) - On older machines with worn [check rings](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/check-ring/), maintaining consistent packing pressure is harder because the check valve leaks back ### Injection Speed Faster injection fills the cavity before much cooling happens. The surface layer is thinner at the time packing begins, making packing more effective. However, too-fast injection can cause jetting, burn marks, or other issues. It's a balance. ### Mold Temperature Higher mold temperature keeps the surface pliable longer, allowing packing pressure to work more effectively. This is particularly helpful for semi-crystalline materials (nylon, POM, PP) which shrink more than amorphous materials. The trade-off: higher mold temperature = longer cooling time = longer cycle. ### Melt Temperature Higher melt temperature reduces viscosity, allowing pack pressure to transmit further through the part. But excessively high melt temperature increases overall shrinkage (the plastic starts from a hotter, more expanded state and has further to shrink). Usually, melt temperature adjustments have a smaller effect on sink marks compared to pack pressure and mold temperature. ## Material Considerations Some materials are more prone to sink marks than others: **High shrinkage materials** (PP, PE, nylon, POM): More prone, especially crystalline grades. These materials can shrink 1.5-2.5% and form distinct sink marks. **Low shrinkage materials** (PS, ABS, PC, PC/ABS): Less prone, but still possible with thick sections. Shrinkage is typically 0.4-0.7%. **Glass-filled grades**: Less sink because the glass fiber reduces shrinkage and stiffens the part. A 30% glass-filled nylon rarely shows sink marks that would be visible on unfilled nylon. ## When You Can't Fix It Sometimes you just can't fully eliminate sink marks with process alone. The part design demands it, and the customer won't accept design changes. In those cases: - **Texture the visible surface** — a textured surface hides sink marks incredibly well. Mold texturing (EDM, chemical etch, laser) is a legitimate engineering solution, not a hack. - **Move the gate** — gating closer to the thick section allows packing pressure to reach the problem area more effectively. - **Use gas-assist injection** — nitrogen gas injected into the thick section creates a hollow channel, eliminating the thick mass that causes sinking. Requires equipment and mold modifications. - **Accept cosmetic limits** — communicate honestly with the customer about what's physically achievable with the current design. ## Machine Condition Matters On older machines, inconsistent sink marks (some shots fine, others not) often point to: - **Worn check ring** — inconsistent shot volume and pack pressure delivery. Check ring replacement is relatively inexpensive and can dramatically improve consistency. - **Worn [proportional valves](https://www.imspares.com/injection-molding-machine-parts/hydraulics/proportional-valve/)** — inconsistent pressure control means inconsistent packing. - **[Hydraulic issues](https://www.imspares.com/injection-molding-machine-parts/hydraulics/)** — pressure spikes or drops during pack phase. Check [hydraulic pump](https://www.imspares.com/parts/hy-pw-06/) condition and [oil filter](https://www.imspares.com/parts/o-tc-02/) cleanliness. Before chasing process parameters, make sure your machine can actually deliver consistent injection and packing. If the machine is inconsistent, process optimization is futile — you're chasing a moving target.