Introduction

A hot melt system on a packaging line has one job that everybody notices and several that nobody notices until they go wrong. The visible job is holding a carton flap shut. The invisible ones are holding it shut after eight hours in a cold store, using the same amount of adhesive on carton number 40,000 as on carton number one, and not shutting the line down because somebody has to scrape char out of the tank.

Most hot melt purchases in India are made on tank capacity and price. Those are the two least useful numbers to buy on.

Start with adhesive consumption, not tank size

Work out grams per carton first. A typical corrugated case seal uses somewhere between 0.8 and 2.5 grams of adhesive depending on bead count, bead length and board weight. Multiply by cartons per hour.

A line running 1,200 cartons an hour at 1.5 grams per carton burns about 1.8 kg an hour. That number, not the tank size, tells you what melt rate you need.

Melt rate is the specification people skip. A 10 kg tank with a 4 kg per hour melt rate cannot sustain a line consuming 6 kg per hour, no matter how big the reservoir is. You will run fine for the first ninety minutes and then the tank temperature will sag as cold pellets absorb heat faster than the elements can replace it. Bead quality drops, bonds fail, and everyone blames the adhesive.

Ask for melt rate in kg per hour and check it against your consumption with at least 30 percent headroom.

Temperature control is where cheap systems fail

Hot melt adhesive has a working window, usually somewhere between 150 and 190 degrees C depending on formulation. Run below it and the adhesive is too viscous to wet the substrate, so you get bonds that look fine and peel apart in transit. Run above it and the adhesive oxidises and chars.

Three things to check on any system you are evaluating:

Control accuracy. Look for plus or minus 1 degree C or better on the tank, hose and gun. Systems using simple bimetallic thermostats can swing 10 degrees or more, which puts you outside the working window in both directions during a single cycle. RTD sensors with PID control are what you want.

Independent zone control. Tank, hose and applicator should each have their own setpoint. Hoses lose heat along their length, so the hose usually needs to run slightly hotter than the tank to deliver adhesive at the right temperature at the nozzle. One shared setpoint for everything means at least one zone is wrong.

Over-temperature cutout. A failed sensor that lets a tank run to 250 degrees C will destroy a full charge of adhesive and coat the tank interior in carbon.

The charring problem nobody mentions in the quotation

Hot melt adhesive degrades when it sits hot. Leave a tank at 180 degrees C over a weekend and you get char: hard carbon flakes that break loose, travel down the hose and block filters and nozzles.

Three design features reduce it, and you should ask about all three.

Tank geometry matters. A tank with sharp internal corners and a large surface-to-volume ratio holds stagnant adhesive in the corners where it cooks. Sloped, smooth-walled tanks with a first-in-first-out flow path keep the material moving.

Non-stick tank lining, usually PTFE, stops adhesive bonding to the wall in the first place.

A standby temperature function drops the tank to around 120 degrees C after a set idle period. That keeps material molten enough to come back up in a couple of minutes but slow enough that degradation almost stops. On a line with breaks and shift changes, this single feature can cut adhesive waste noticeably.

Match the system to your adhesive chemistry

EVA-based hot melts are the standard for general carton sealing. They are inexpensive, bond well to corrugated board, and work between roughly 150 and 180 degrees C. They also have poor cold resistance. Below about 5 degrees C the bond turns brittle, which is a problem for frozen food and cold chain distribution.

Metallocene and polyolefin hot melts run cleaner, char far less, and hold up better at low temperature. They cost more per kilo. Because they char less, some users find the total cost lands close to EVA once you count adhesive wasted to degradation and the labour spent cleaning tanks.

Pressure-sensitive hot melts stay tacky after cooling and are used for resealable packs and product attachment rather than permanent seals.

The point is that the machine has to suit the chemistry. Some polyolefins need specific seal materials, and a system built purely for EVA may not tolerate them. Tell your supplier which adhesive you are running before you get a quotation, not after.

Applicator choice

Handheld guns suit low volume, repair work and irregular pack formats. Fine for a few hundred cartons a day.

Automatic applicator guns triggered by a photo eye and a timing controller are the standard for anything above roughly 500 cartons an hour. Bead placement stays consistent regardless of operator, and adhesive consumption typically drops 20 to 30 percent against manual application, simply because nobody over-applies to be safe.

Bead versus spray depends on the substrate. Beads work on plain corrugated. Spray or swirl patterns spread the same mass of adhesive over a wider area, which helps on coated or laminated board where a single bead has too little contact area to hold.

Adofil builds hot melt glue dispensers with automatic gun control for packaging applications, and the applicator is usually where we spend the most time getting a specification right.

Running cost, not purchase price

Compare systems over three years using four figures.

Adhesive is the largest number by far. A line consuming 1.8 kg per hour across two shifts uses roughly 7,200 kg a year. A 15 percent consumption difference between a well-controlled system and a poorly controlled one is over 1,000 kg a year of adhesive.

Adhesive waste from charring is separate, and it is usually invisible in the accounts because it gets booked as ordinary consumption.

Power draw varies with tank insulation quality. A poorly insulated tank running 24 hours a day costs real money at Karnataka industrial tariffs.

Downtime is the one that hurts most and is hardest to quantify in advance. Ask any supplier what the mean time between nozzle blockages looks like on their systems with your adhesive, and be suspicious of anybody who will not give you a number.

Pre-purchase checklist

Before you sign anything, confirm:

• Melt rate in kg per hour, with at least 30 percent headroom over your peak consumption
• Independent PID temperature control for tank, hose and gun, accurate to plus or minus 1 degree C
• Standby temperature mode with a configurable idle timer
• PTFE-lined tank with a first-in-first-out flow design
• Confirmed compatibility with your specific adhesive grade, in writing
• Filter locations and how long a filter change takes
• Spares availability in India and typical lead time
• Hose length matched to your actual layout, since every extra metre is a heat loss and a cost

That last one catches people. Ordering a 3-metre hose because it is the standard option, then discovering the tank cannot sit where you assumed, means either a rushed hose order or a compromised machine position.

Adofil manufactures hot melt dispensing systems in Bangalore and supports them from branches in Coimbatore, Delhi and Pune. If you want a system sized against your actual line rather than a catalogue category, send us your carton throughput, adhesive grade and bead pattern through the contact page. You can also browse the full product range first.