When I first started working with side-sealing bag making machines back in 2012, one of the biggest headaches my team and I dealt with was inconsistent side-seal widths. We’d deliver a machine that was supposed to produce bags with a 12mm side-seal, only to have a customer call a week later saying their seals were off by 2mm. That meant wasted material, rejected batches, and time spent re-calibrating—none of which anyone wants in a production line that’s running 24/7. Over the years, as we’ve grown from a small local supplier to a partner for packaging operations across the globe, we’ve refined our approach to measuring side-seal widths to be precise, repeatable, and easy for our customers to implement on their own. Today, I want to walk you through exactly how we teach our clients to do this, because getting this measurement right isn’t just about technical accuracy—it’s about the strength of every bag you make. Side Sealing Bag Making Machine
First, let’s make sure we’re all on the same page about what the “side-sealing area” actually is. On a side-sealing bag, the side seals are the vertical seams that run along both edges of the bag, joining the front and back film layers together. The width of that seal is the distance from the very edge of the cut film to the inner edge of the heat-sealed line. It might sound simple, but there are a lot of small variables that can throw this measurement off: film type (think thin LDPE vs. thick PP), temperature fluctuations in your machine, roller alignment, even the speed of production. That’s why a one-size-fits-all method never works—you need a consistent, verifiable process.
Let’s start with the tools you’ll need. You don’t need fancy lab equipment to do this, but you do need quality tools to get reliable numbers. First off, a digital caliper is non-negotiable. A cheap plastic caliper might be fine for occasional DIY projects, but for industrial use, a stainless-steel digital caliper with a precision of 0.01mm is the way to go. It’s accurate enough to catch tiny variations that would ruin a seal, and it’s durable enough to hold up in a factory environment. Next, you’ll need at least three sample bags—more on why that number matters in a second. Finally, a clean, flat work surface. If your sample bags are wrinkled or stretched, your measurement will be wrong, so take the extra minute to lay them out smoothly before you start.
Now, let’s get to the actual measurement process, and this is where a lot of people go wrong. They take one measurement from one bag and call it good, but that’s not how consistency works. Side-seal widths can vary slightly along the length of a single bag due to small speed fluctuations in the machine, so you need to measure multiple points on multiple bags to get a true average. Here’s our step-by-step method that we’ve tested hundreds of times with our clients:
First, run your side-sealing bag making machine for at least 10 consecutive bags before you start sampling. That’s key because it lets the machine stabilize—heat settings, roller pressure, and film tension all settle into a consistent state after the initial start-up, and measuring a bag made during start-up will give you a false, unstable reading. Once those 10 bags are done, pick three at random. Don’t grab the first three off the roll; reach a few feet down, then a few more, then another one to avoid any bias from a temporary machine glitch.
Next, mark each bag to ensure you measure the same relative point every time. Take your first bag, hold it flat, and use a fine-tip permanent marker to mark three spots along the side seal: one 5cm from the top of the bag, one in the exact middle, and one 5cm from the bottom. Why 5cm? The very top and bottom of the seal are sometimes affected by the trimmer blades that cut the bag, so measuring at a mid-range point gives you the most accurate picture of the steady-state seal. Repeat this for the other two bags.
Now, take your digital caliper and set it to millimeters (avoid inches for precision work). For each marked point, you’re going to measure two parts of the side-seal area: the total width of the seal itself, and the width of the effective heat-bonded area. Wait—this is a distinction many suppliers and operators miss. The side-sealing area has two components: the “overhang,” which is the extra film left outside the actual seal, and the inner bonded line, which is the part that holds the bag together. The total width is the full distance from the film edge to the inner line, but the bonded width is the actual functional part. Both matter, because a seal that’s 15mm total but only has 5mm of bonded area is just as weak as a 5mm total seal.
So, for each marked point, place the fixed jaw of your caliper exactly on the very edge of the film (the side that would be the outer edge of the finished bag). Then, gently extend the movable jaw until it touches the middle of the inner, crimped line of the heat seal—don’t press hard enough to deform the seal, because that will skew your number. Record that total width. Then, adjust the movable jaw to touch the inner edge of the seal on the front film and the inner edge of the seal on the back film—this is the bonded area, which is the actual strength-bearing part. Write that number down too.
Here’s where the third bag comes in: repeat this entire process for the middle and bottom marks on the second and third bags. Now you have 9 total measurements for total seal width, and 9 for bonded seal width. Add all the total seal numbers together and divide by 9 to get your average, and do the same for the bonded seal numbers. That average is your true side-seal width. If you only measured one point, you might have gotten a reading that’s off by 1 or 2mm, which might not sound like much—but for a bag that’s meant to hold 10kg of dog food or 5 liters of cooking oil, that extra millimeter of bond strength can mean the difference between a sealed bag and a leak that costs you thousands in product returns.
But measuring the width is only half the battle. Even if your measurement is exact, if your process isn’t repeatable, you’ll get inconsistent seals week after week. That’s why we also teach our clients at our training sessions how to verify their measurements over time, and how to adjust their machine if their seal width drifts.
The most common cause of inconsistent side-seal width is misaligned sealing rollers. Over time, the heat and pressure from the rollers can cause them to shift slightly, especially on machines that run 18 or 20 hours a day. If your rollers are even 0.5mm out of alignment, your seal width will be uneven along the bag. To check alignment, we use a straightedge—something as simple as a steel ruler works. Hold the straightedge against the side of the two sealing rollers, and see if there’s a gap between the ruler and the roller. If there is, you can adjust the alignment screws on the roller mounts, one quarter turn at a time, then run a test bag and re-measure the seal width. Make small adjustments—you can always tweak more, but over-adjusting can cause bigger problems.
Another factor that people overlook is film tension. If the film is too tight as it goes through the side-sealing station, it will stretch, which makes the seal look narrower than it actually is once it’s cut. If it’s too loose, the film will wrinkle, and the seal will have uneven widths and weak spots. We recommend setting film tension by running a test roll, stopping the machine mid-bag, and gently tugging the film—if it stretches more than 5mm when you tug it firmly, it’s too loose; if it’s hard to tug at all, it’s too tight. Adjust the tension rollers on your machine until the film runs smoothly without stretching or wrinkling, then re-measure your seal width.
Temperature also plays a big role, especially for thicker films. If your heat sealer is set too low, the seal won’t bond properly, and the crimped line won’t form correctly, making it hard to get an accurate width measurement. If it’s set too high, the film will melt and spread, making the seal wider than intended. Always calibrate your temperature using a thermal strip—you can get these from most packaging supply stores—place it between the sealing rollers when the machine is running, and check the temperature reading against your machine’s display. A 10°C difference might not sound like much, but it can change your seal width by up to 1mm.
Wait, but what if you’re working with a custom bag that has a specific side-seal width requirement? For example, a stand-up pouch that needs a 10mm seal, vs. a heavy-duty industrial bag that needs a 20mm seal. Our advice here is to test three different widths first, not just the one you think you need. Run three small batches of bags with 12mm, 15mm, and 18mm seals, then perform a burst test. To do a burst test, you can take a sealed bag, fill it with water or air, and apply pressure until the seal breaks. The seal that takes the most pressure before breaking is the ideal width for your film and product. We do this test with every new client that has custom bag requirements, because what works for one product often doesn’t work for another. For example, a bag holding light candy might work with a 8mm seal, but a bag holding frozen meat that’s going to be transported cross-country needs at least 12mm to withstand temperature changes and rough handling.
I remember a client from Texas we worked with a few years back who was having major issues with side-seal failures on their 5-gallon feed bags. They were using an 8mm seal, and within a week of production, 10-15% of the bags were leaking. They tried adjusting temperature and pressure, but nothing worked. When we did a site visit, we had them use our measurement method, and found their average seal width was only 6.5mm—way too narrow for the 10kg feed bags they were making. We had them adjust the sealing rollers to get a 12mm average seal width, and after that, the failure rate dropped to less than 0.5%. That’s the difference between accurate measurement and guessing: it solves real, costly problems.
Now, let’s talk about common mistakes people make when measuring side-seal widths, because these are the ones that trip up even experienced operators. The first is measuring the seal when the bag is still on the roll and under tension. Once you cut the bag and remove it from the machine, the film relaxes, so the seal width might change by 1-2mm. Always measure the bag after it’s fully cooled and removed from the roll. The second mistake is not accounting for the crimp on the seal. Some sealing methods create a smooth, flat seal, while others create a crimped, zig-zag seal. If your seal is crimped, take your caliper and measure across the peak of the crimp, not the valley, because the peak is the part that’s bonded. The third mistake is using a ruler instead of a digital caliper. A standard ruler only has a precision of 1mm, which is fine for basic measurements, but if you’re working with narrow seals or tight tolerance requirements, 1mm can be the difference between a good seal and a bad one.
At the end of the day, measuring side-seal width isn’t just a technical step—it’s a way to ensure your bags are strong, consistent, and cost-effective. Wasting film because of inconsistent seals adds up fast: a company making 100,000 bags a week with a 2mm over-seal is throwing away 200 meters of film every week, which translates to hundreds of dollars in unnecessary costs. On the flip side, a seal that’s too narrow will lead to product returns, damaged reputation, and lost revenue.
If you’re struggling with side-seal width consistency on your side-sealing bag making machine, or if you need help calibrating your process, our team is here to help. We’ve worked with packaging operations of all sizes, from small local brands to global Fortune 500 companies, and we tailor our support to your specific needs, whether that’s on-site training, remote troubleshooting, or adjustments to your machine settings. Every production line has its own quirks, and the right measurement method will work with those quirks, not against them. Don’t let inconsistent seals hold your business back—reach out to us today to discuss how we can help you optimize your side-sealing process.
Side Sealing Bag Machine References
- Packaging Machinery Manufacturers Institute. (2020). Testing and Calibration Procedures for Flexible Packaging Equipment.
- Soroka, W. (2019). Fundamentals of Packaging Technology (5th ed.). Institute of Packaging Professionals.
- American Society for Testing and Materials. (2021). Standard Test Method for Seal Strength of Flexible Barrier Materials (ASTM F88/F88M-21).
Zhejiang Ruitai Packaging Machinery Co., Ltd.
Zhejiang Ruitai Packaging Machinery Co., Ltd. is one of the most professional side sealing bag making machine manufacturers and suppliers in China, featured by quality products and good service. Please rest assured to wholesale advanced side sealing bag making machine from our factory. Customized orders are welcome.
Address: D6 Building, No.66 Tingchao Road, Shangwang Town, Ruian, Zhejiang, China.
E-mail: ruiandavid2020@gmail.com
WebSite: https://www.rtbagmachine.com/
