Hey there! I’m a materials engineer with Linn, and I’ve been knee-deep in the world of TPE (thermoplastic elastomer) soft rubber for over a decade. If you’re reading this, chances are you’ve hit a snag with flow marks messing up your TPE parts during injection molding. Those annoying streaks or ripples on the surface can make a perfectly good product look cheap, and nobody wants a client sending back parts because of that. I’ve been there, wrestled with this issue more times than I can count, and learned a ton along the way. Today, I’m spilling all my tips and tricks to help you smooth things out—literally! Let’s dive in and figure out how to kick those flow marks to the curb.
What Are Flow Marks, Anyway?
Before we start tweaking machines or molds, let’s get on the same page about what flow marks are. Picture this: you’re molding a TPE part, and when it pops out, the surface has these weird lines or waves, like someone dragged a paintbrush across wet paint. That’s a flow mark. It happens when the molten TPE doesn’t flow evenly in the mold, usually because of uneven cooling, pressure, or speed. TPE is a fantastic material—flexible, durable, and versatile—but it’s also picky as heck about processing conditions. One wrong setting, and bam, you’ve got streaks.
Flow marks come in a few flavors:
Wavy ripples: Think water flowing over a rocky stream. These often show up when the material moves too fast or the mold’s too cold.
Radial streaks: Lines fanning out from the gate like sunrays, usually tied to low pressure or a poorly placed gate.
Hazy patches: A foggy or cloudy look on the surface, often caused by moisture or additives vaporizing during molding.
Once you know what you’re dealing with, fixing it feels less like guesswork and more like a plan. So, let’s roll up our sleeves and get to it.
My Go-To Fixes for TPE Flow Marks
Over the years, I’ve learned that flow marks aren’t just a one-and-done problem—they’re a puzzle with pieces scattered across materials, molds, machines, and settings. I’m going to walk you through my step-by-step approach, broken down into manageable chunks. Follow these, and you’ll be well on your way to smoother parts.
1. Start with the Material
The TPE itself is often where the trouble begins. A bad batch or improper handling can lead to flow marks before you even touch the machine.
Dry, dry, dry: TPE loves to soak up moisture like a sponge, and wet material is a flow mark magnet. When water vaporizes during molding, it leaves streaks or haze. I always dry my TPE at 175-210°F (80-100°C) for 2-4 hours, depending on the grade. Check your material’s spec sheet for exact numbers.
Additive issues: Some TPEs come loaded with lubricants or plasticizers that can volatilize at high temperatures, creating a foggy surface. If you suspect this, talk to your supplier about a low-volatility formula. I’ve had luck switching to cleaner grades when haze was a problem.
Melt flow index (MFI): The MFI tells you how easily the TPE flows. Too high, and it races through the mold, causing ripples. Too low, and it struggles to fill, leaving streaks. Pick a grade that matches your part’s thickness and mold design—your supplier can help here.
Pro tip: Always store TPE in a dry, sealed environment. I once had a batch sit in a humid warehouse for a week, and let me tell you, the flow marks were a nightmare until we dried it properly.
2. Fine-Tune Your Injection Molding Process
If the material checks out, the next suspect is your process settings. This is where most flow mark fixes live, and it’s also where you can make the biggest difference without breaking the bank.
Crank up the mold temperature: A cold mold makes TPE solidify too fast, locking in flow marks before the surface can smooth out. I usually set my mold to 100-140°F (40-60°C) for TPE, adjusting based on hardness and part thickness. Warmer molds help the material flow and settle nicely.
Play with injection speed: Too fast, and the TPE splashes around, creating waves. Too slow, and it doesn’t fill evenly. My trick is to start slow for the first 20-30% of the shot, then ramp up once it passes the gate. This cuts down on ripples and gives a cleaner finish.
Boost holding pressure: If you’re seeing radial streaks, low holding pressure might be letting the material shrink unevenly. Try bumping it up by 10-20%, but watch out for flash (those pesky extra bits around the edges). A little extra pressure can fill gaps and smooth things out.
Dial in the melt temperature: TPE usually runs at 350-430°F (180-220°C), but the sweet spot depends on your grade. Too hot, and you risk degrading the material or vaporizing additives. Too cool, and it’s sluggish, causing uneven flow. Start low and nudge up in 10°F (5°C) increments until the marks fade.
I can’t stress enough how much trial and error helps here. Every machine and mold combo is a bit different, so don’t be afraid to tweak and test.
3. Take a Hard Look at the Mold
Molds are like the unsung heroes of injection molding—when they’re off, everything’s off. Here’s what to check:
Gate placement: If the gate’s too close to the part’s surface, the TPE shoots in like a fire hose, leaving streaks. I’ve fixed this by moving the gate to an edge or switching to a fan gate for smoother flow. It’s a bit of work, but worth it.
Venting: Poor venting traps air in the mold, which can cause hazy or uneven surfaces. Adding vent slots (about 0.001-0.002 inches or 0.02-0.05mm deep) along the parting line can let air escape without leaking material.
Surface finish: A mold that’s too rough drags on the TPE, while one that’s too slick can cause sticking. I aim for a polish of Ra 0.4-0.8 micrometers—smooth enough to flow well but not so glossy it grabs the part.
If your mold’s old or worn, it might be time for a refresh. I’ve seen molds with scratched-up cavities throw flow marks like nobody’s business until they were repolished.
4. Test Small, Track Everything
Here’s a habit that’s saved my bacon more times than I can count: run small batches after every change and log the results. Adjust one thing—say, mold temp—mold a few parts, and note what happened. Then move to the next tweak. This keeps you from getting lost in a sea of variables and builds a roadmap for what works. I use a simple spreadsheet with columns for temp, pressure, speed, and part quality. It’s old-school, but it works.
Quick Reference Table for Flow Mark Fixes
To make things easier, I’ve put together a table summarizing common flow mark issues, their causes, and how to fix them. Keep this handy next time you’re at the machine.
| Flow Mark Type | Likely Cause | Fixes to Try | Watch Out For |
|---|---|---|---|
| Wavy Ripples | Injection speed too high, mold too cold | Lower speed, raise mold temp to 120°F (50°C) | Don’t slow too much—risk short shots |
| Radial Streaks | Low holding pressure, bad gate design | Increase holding pressure, relocate gate | Avoid over-pressuring (causes flash) |
| Hazy Patches | Wet material, additive off-gassing | Dry at 175°F (80°C) for 3 hours, lower melt temp | Check dryer function |
| Uneven Fill | Wrong MFI, low melt temp | Switch to matching MFI grade, raise melt to 390°F (200°C) | Don’t overheat—degrades material |
A Real-Life Win I’m Proud Of
Let me share a story from last year that still makes me smile. A client came to us at Linn with a TPE phone case project—50A hardness, super soft and grippy. Problem was, every part had these ugly radial streaks fanning out from the gate, like a bad abstract painting. We checked the material first: bone-dry, good MFI, no issues there. Then I noticed their mold had a tiny gate—1.2mm diameter, way too small for the flow needed. I suggested bumping it to 1.8mm and tweaking the holding pressure from 80 bar to 100 bar. We ran a test batch, and holy cow, the parts came out mirror-smooth. The client was thrilled, and we locked in a big order. Moments like that remind me why I love this work—cracking a tough problem feels like winning a prize.
Preventing Flow Marks Before They Start
Fixing flow marks is great, but stopping them before they happen is even better. Here are some habits I swear by:
Keep your dryer in tip-top shape. A busted dryer can leave moisture in your TPE, and you’ll be chasing your tail with flow marks. Test it monthly.
Clean molds after every run. Leftover TPE or gunk can mess up the next batch. A quick wipe-down goes a long way.
Talk to your supplier regularly. A consistent TPE formula saves headaches. I check in with mine every few months to make sure nothing’s changed.
FAQ: Your Flow Mark Questions Answered
To wrap things up, here are some common questions I hear about flow marks, with answers straight from my experience:
What’s the difference between flow marks and splay?
Flow marks are surface streaks from uneven flow or cooling. Splay is those silvery, web-like marks caused by moisture or trapped gas. Fix flow marks with process tweaks; fix splay by drying material thoroughly.
Can high mold temps cause problems?
Yup—if it’s too hot, say above 160°F (70°C), you might get sticking or longer cycle times. Check your TPE’s datasheet for the safe range.
Will switching TPE grades eliminate flow marks?
It can help, especially if the MFI or additives are off, but it’s pricey. I’d exhaust process and mold fixes first unless the material’s clearly wrong for the job.
Do flow marks affect part strength?
Usually no—they’re mostly cosmetic. But if they come with uneven filling, you might see slight weaknesses in those areas. Better to fix them either way.
Final Thoughts
Flow marks can be a real pain, but they’re not unbeatable. With a little patience and some methodical tweaks—material prep, process settings, mold design—you can get those TPE parts looking flawless. I’ve shared everything I’ve learned from years of trial and error, and I hope it gives you a head start. Next time you spot those pesky streaks, take a deep breath, try these steps, and you’ll be golden. Got more questions or a tricky case? Drop a comment below—I’d love to help you troubleshoot!
