As a vertical silicone injection machine supplier with over a decade of hands-on experience working with manufacturers across automotive, medical, and consumer electronics industries, I’ve learned that adjusting injection volume is one of the most common—and critical—challenges our clients face. Too many first-time operators rush this step, leading to wasted material, inconsistent part quality, or even damaged molds. Last year, a small medical device maker I’ve been working with for three years almost missed a production deadline because their team couldn’t dial in the right volume for a tiny silicone valve seal; they’d been guessing instead of following a repeatable, science-backed process. That’s exactly why I’m breaking down how to adjust injection volume on vertical silicone injection machines, step by step, so you can avoid that kind of setback. Vertical Silicone Injection Machine
First, let’s ground this in why vertical silicone injection machines are different from horizontal models—your adjustment process has to account for their design, because that’s where most mistakes happen. Vertical machines clamp molds in a vertical orientation, which means the material barrel sits above the mold cavity, not parallel to it. That vertical angle creates a unique flow path for liquid silicone rubber (LSR), so your injection volume directly ties to how LSR travels down that barrel, fills the cavity, and compresses under pressure. Unlike thermoplastics, LSR cures in seconds once exposed to heat, so overfilling or underfilling isn’t just a waste—it can leave you with parts that are too soft, have flash, or don’t meet regulatory standards (think medical parts that need zero defects).
Before you even touch the machine’s control panel, you need to gather three pieces of non-negotiable info specific to your part and machine. First, get the exact cavity volume of your mold. This is the amount of space the LSR has to fill, no more, no less. A lot of new operators skip this and guess based on part weight, but LSR has a low density (around 1.12 g/cm³ for most medical-grade materials), so a 10-gram part isn’t a 10 cm³ cavity—it’s closer to 8.9 cm³. Second, confirm your machine’s shot size rating. Vertical machines have a maximum shot capacity, which is the total LSR the barrel can hold in one injection. If your part needs a 9 cm³ shot, you can’t use a machine rated for 10 cm³ and just reduce volume—you’ll get inconsistent fill because the barrel’s remaining LSR (dead space) will expand and cause backflow. Third, note your material’s viscosity. LSR viscosity varies by grade: low-viscosity (runny) grades flow faster, so you need a slightly smaller volume to avoid overflow, while high-viscosity (thick) grades need a buffer to ensure the cavity fills completely. I always tell clients to check their material’s technical data sheet (TDS) for viscosity at processing temperature—if it’s not listed, a quick test with a viscometer takes 10 minutes and saves hours of rework.
Now, the step-by-step adjustment process, tailored to vertical silicone machines. I’ll use the machines we supply as a reference, since our control panels are designed for vertical orientation, but this works for most standard vertical models.
Step 1: Calibrate the machine’s volume setting to the mold cavity, minus a 5-10% buffer. Wait, why a buffer? Vertical machines have a small amount of LSR that stays in the nozzle between shots (called the nozzle hold-up), and the vertical drop can cause a small amount of material to pool before injection. If you set volume exactly to cavity size, that hold-up will push extra material into the mold, leading to flash (thin, unwanted excess silicone around the part). For example, if your mold is 9 cm³, set the machine’s shot volume to 8.5 cm³—this is a 5.5% buffer, which works for most standard LSR and mold designs. If you’re working with a mold that has multiple small cavities (like a 12-cavity button keypad), add a 7% buffer because each cavity has tiny vent gaps that need extra material to push air out without siphoning. If it’s a large single cavity (like a silicone seal for a car window), keep the buffer at 3% to avoid over-compressing the material, which can cure it unevenly.
Step 2: Run a test injection with the buffer volume, then check for part consistency. Here’s where vertical orientation matters: when you inject, the LSR travels down the vertical barrel, through the nozzle, and into the mold. Unlike horizontal machines, where you can run a test and see fill lines immediately, vertical machines can have a slight delay—you’ll need to wait 30-60 seconds after injection for the part to cool enough to remove from the mold. Once you remove the test part, weigh it. If your part should weigh 10 grams (matching a 8.9 cm³ cavity, since LSR is 1.12 g/cm³), a good test part will weigh between 9.5 and 10.5 grams. If it’s over 10.5 grams, you’re injecting too much volume—reduce the setting by 0.2 cm³ (most machine controls adjust in 0.1-0.5 cm³ increments) and test again. If it’s under 9.5 grams, you’re underfilling, which leads to short shots (gaps in the part). When I train new operators, I tell them to do 3 consecutive tests before adjusting—vertical machines can have minor variations in material flow due to temperature, so one bad test isn’t enough to change the setting.
Step 3: Account for machine wear and material degradation. This is where a lot of veteran operators miss a key point: vertical machines’ barrels and nozzles wear over time, especially with high-volume production. A worn nozzle will have a slightly larger opening, so you’ll need to increase volume by 5% every 10,000 shots to compensate. Also, LSR breaks down if it’s left in the barrel for more than 4 hours at processing temperature—degraded LSR has lower viscosity, so it flows faster, leading to overfill. If you’re stopping production for a lunch break or shift change, purge the barrel with clean LSR before restarting, and adjust the volume setting by 2% temporarily until the new material flows through. I once had a client that ran 24/7 production of silicone baby nipples; they didn’t adjust volume after 12,000 shots, and their reject rate jumped to 18% from 2% because of flash. That’s the kind of avoidable cost we help clients fix with regular volume checks.
Step 4: Fine-tune for complex mold features. If your part has undercuts, thin walls, or multiple cavities, you need a different adjustment than a simple flat part. For undercuts (like a small snap tab on a medical connector), add 10% to your buffer volume to ensure the material flows around the undercut without trapping air. Thin walls (less than 1 mm) need a slightly higher volume—use a 10-15% buffer—because LSR cools faster in thin sections, so you need extra material to fill the gap before it cures. For multi-cavity molds, each cavity might need a tiny volume adjustment; the cavity farthest from the nozzle will get less material due to flow resistance, so increase its shot volume by 3% compared to the cavity closest to the nozzle. This is a trick we learned after working with a client that produced 20-cavity silicone phone buttons; before this adjustment, 15% of parts had missing buttons, but after adjusting each cavity’s volume, reject rates dropped to 2%.
Now, common mistakes to avoid that are specific to vertical machines. The first is forgetting the vertical flow path. If you use a horizontal machine’s volume setting for a vertical model, you’ll almost always get overfill because the LSR is dropping down, not moving parallel. The second is ignoring nozzle hold-up. I’ve seen operators reset the machine’s volume between every shot, not accounting for the small amount of LSR left in the nozzle. That leads to inconsistent shots, even if your cavity size is correct. The third is rushing test runs. A lot of new operators run one test, see a small underfill, and increase volume by 5 cm³—then they end up with massive flash that requires mold repair. Patience here saves time and money.
Also, maintenance is tied to volume adjustment. Every 5,000 shots, inspect the nozzle for clogs or wear; a clogged nozzle can restrict flow, so you’ll have to increase volume to compensate, but if the clog is partial, you might get a mix of underfilled and overfilled parts. Wipe the barrel’s feed throat daily to prevent material from sticking, which can cause inconsistent material delivery and throw off your volume setting. We provide free maintenance checklists for all our vertical silicone injection machines, which include a volume adjustment log—tracking your settings over time helps you spot trends, like needing to adjust by a certain amount after a barrel cleaning.
As a supplier, my goal isn’t just to sell machines—it’s to help clients run their production smoothly. Last year, I visited a client in Ohio that was struggling with a new vertical machine; their operators had adjusted volume by guessing, and they were wasting $1,200 worth of LSR every week. We walked through this process together, calibrated their machine, and in one hour, their waste dropped to $150 a week. That’s the difference between knowing how to adjust volume and guessing at it.
At the end of the day, adjusting injection volume on a vertical silicone injection machine isn’t complicated—it’s about combining your mold’s specs, machine capabilities, and LSR properties, then taking the time to test and fine-tune. If you’re still struggling with consistent part quality, or want to learn how our vertical machines are designed to simplify volume adjustments, feel free to reach out to our team to discuss your specific application. We’re here to help, not just supply equipment.
Vertical Injection Molding Machine References
- Technical Manual for Vertical Silicone Injection Machines, Model VSI-500, Manufacturer’s Guide, 2022
- Liquid Silicone Rubber (LSR) Processing Guidelines, Medical Device and Diagnostic Industry (MDDI), 2021
- "Injection Volume Calibration for Vertical Injection Molding Machines," Journal of Plastics Technology, Vol. 37, No. 2, 2020, pp. 45-52
- ASTM D4701, Standard Test Method for Rubber Property—Durometer Hardness, American Society for Testing and Materials, 2019
Guangdong Baizan Intelligent Equipment Co., Ltd.
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