Why packaging machines use serrated blades instead of straight edges

A straight blade cuts film by pressing it against an anvil until the material fails. On BOPP or laminated film only a few tens of microns thick, that demands a very sharp edge and a very tight blade-to-anvil clearance. Let the edge dull slightly and the film stretches and slips away instead of parting.

A serrated blade works differently. The tooth tips punch through the film first and create a row of small tears, then the flanks of the teeth rip through the material left between them. Cutting force drops sharply, the blade becomes far less sensitive to mounting tolerance, and — important in food packaging — the serrated edge lets the end user tear the pack open by hand without scissors.

This is why almost every horizontal flow wrapper, vertical form-fill-seal machine, bag making machine, carton sealer and tape cutting machine uses a toothed blade at the cross-cut station.

Types of serrated blades found in a packaging plant

Long straight serrated knives

The most common type by far. The blade mounts on the cross sealing jaw of a flow wrapper or on the cutting assembly of a VFFS machine. Typical sizes run from about 150 mm to over 600 mm long, 15–40 mm wide, 1.0–2.0 mm thick, with a row of round or slotted holes for fastening and alignment. Sizes seen regularly on the market include 178 × 15 × 1.8 mm with 6.5 × 10 slots, and 500 × 40 × 1.5 mm with a 3 mm pitch.

Circular serrated blades

Used on bag making machines, slitters and film cutting machines wherever a continuous cut is needed in the machine direction. Circular toothed blades also appear in rubber and paper converting when a toothed cut is preferred over a clean shear cut.

Profiled and special-shape serrated blades

This group covers T-shaped knives, triangular knives, curved and offset tooth profiles, corner-cut knives, perforating knives and dotted-line knives. Nearly all are made to drawing, since every machine builder uses its own mounting arrangement. Chinese blade shops specialising in this category commonly quote lengths up to 4,000 mm and thicknesses down to 0.10 mm.

Hot knives and cold knives

A hot knife carries a heating element in the blade body, cutting and sealing the film edge in one action, usually with a non-stick coating on the cutting face. A cold knife cuts purely mechanically and works alongside a separate sealing station. PE bag machines use both arrangements depending on their design.

Reading the specification of a serrated blade

When you talk to a supplier, these six figures decide whether the blade will fit and run:

• Length × width × thickness — measure the old blade with a caliper. Do not take the figure from the machine catalogue, since many plants have already switched to a different size.
• Tooth pitch (P) — the distance between two adjacent tooth tips. Typically 2–3 mm for thin film, 4–6 mm for heavier film and PE bags, and 6–8 mm for carton sealer blades.
• Tooth depth (H) — usually 0.6 to 1.0 times the pitch. Shallow teeth give a cleaner edge; deep teeth tear more aggressively but tend to leave burrs.
• Tooth angle — 30°, 45° or 60° depending on how tough the film is.
• Mounting holes — diameter, centre distance, round or slotted, countersunk or not.
• Hardness — state the HRC you want, because the same steel grade can be tempered to very different levels.

Two more characteristics rarely appear on a purchase order yet directly control cut quality: flatness of the blade body and consistency of tooth height. A blade bowed by a few tenths of a millimetre will cut through at one end and leave film attached at the other, and no amount of machine adjustment will fix it.

Choosing pitch by packaging material

The general rule: the finer the pitch, the cleaner the cut edge — but the more slender the teeth and the faster they wear. A coarse pitch gives a more durable blade, yet the cutting force is spread out and film can be left uncut between teeth.

Steel grades and heat treatment

Industrial blade shops normally work with the following grades for packaging serrated knives:

• SK5 / SK4 (high-carbon steel) — low cost, suited to thin film cutting at moderate output.
• 9CrSi (equivalent to 90CrSi) — alloy tool steel with a good balance of hardness and price, very common on long serrated knives.
• Cr12MoV / SKD-11 / D2 — cold work die steel with strong wear resistance and low distortion in heat treatment. The default choice for blades running continuously.
• W6Mo5Cr4V2 / SKH-51 / M2, W18Cr4V, M42 — high-speed steels that hold hardness at elevated temperature, so they suit hot sealing knives.
• ASP60, K30, tungsten carbide — for abrasive fillers or where very long service life is required, at the cost of brittleness and price.

For SKD-11 serrated blades, hardness is usually set at 58–62 HRC. High-speed steels can reach 62–64 HRC. Worth noting, though: raising hardness is not always an improvement. A serration is a slender structure, and an over-hardened blade will chip the moment it touches the anvil or a product that strays into the cutting zone. On many machines a 58 HRC blade outlasts a 63 HRC one.

Surface coatings

For products containing sugar, oil or adhesive, a PTFE-based non-stick coating markedly reduces film sticking to the edge. TiN and TiAlN coatings target wear resistance rather than release. Keep in mind that every coating has a finite life and will disappear well before the blade body wears out, so if sticking is the problem, address sealing temperature and cleaning schedule as well — the coating alone will not carry it.

Common line problems and how to approach them

One point often overlooked: the knife holder is a wear part too. It works at high speed, and if the material is inadequate or not properly hardened it deforms over time — at which point even a brand-new blade cuts badly. When a blade change fails to improve cut quality, inspect the holder before blaming the blade.

Maintenance and service life

Blade life depends on the film, machine speed and cleaning practice, so any "number of cuts" figure from a supplier is indicative at best. In two-shift operations, blades cutting BOPP are typically replaced or reground after one to three months; tape cutting blades go faster because adhesive builds up and attacks the edge.

A few habits that extend life:

• At the end of each shift, blow out with compressed air and use a soft brush to clear film debris from the tooth roots. Debris build-up lifts the blade and throws off the clearance.
• Never scrape the edge with hard metal tools.
• Leave the sealing jaws open when the machine is stopped, to avoid scorching film and burning off the coating.
• Check fastener tightness weekly; check holder and bearing wear monthly.
• Always keep a spare set. A blade costs far less than an hour of stopped line waiting for delivery.

Lightly worn blades can be reground, but only while the tooth profile is intact. Regrinding reduces tooth height, so after two or three passes replacement is normally the better call. Once a tooth has chipped, regrinding is pointless — the gap will leave an uncut trace on every pack.

What to prepare when ordering non-standard blades

Most serrated blades in packaging are machine-specific. To get an accurate quotation and a correct blade the first time, send:

1. A drawing showing length, width, thickness, pitch, tooth depth, tooth angle, and hole position and size.
2. Photographs of the old blade, both face and edge views, laid against a ruler.
3. The old blade itself where possible. This is the most accurate route and eliminates most errors.
4. Details of the material being cut, machine speed, and whether the cut is hot or cold.
5. Your required steel grade and hardness — or a description of the problem you are having, so the supplier can propose one.

If the current blade performs well and you only need a replacement, keep the specification as it is. If the current blade has a problem, say what the problem is rather than asking for an exact copy, because an exact copy reproduces the fault as well.

Serrated packaging blades from DTS

DTS supplies straight serrated knives, circular toothed blades and profiled blades for flow wrappers, VFFS machines, bag making machines, case sealers and tape cutting machines. Common sizes are held in stock, and we manufacture to drawing or to sample in SK5, 9CrSi, SKD-11, SKH-51 and carbide, with non-stick coating available as an option.

Send a photo or drawing of the blade you are running now and we will confirm the specification and quote.