Pelletizer die plate: design, materials, wear and refurbishing

A pelletizer die plate is the component at the discharge of the extruder or melt pump that forms the polymer melt into many individual strands through its die holes. It is also called a pelletizing die, die plate or simply the die. In underwater pelletizing, rotating pelletizer knives sweep directly across the hardened die face and cut the strands into pellets the moment they emerge. Process water cools the pellets and carries them to the dryer.

In strand pelletizing, a die also forms strands, but they are first cooled in a water bath and only then cut in the strand pelletizer. The knives never touch the die, so mechanical wear on the die is much lower.

The die plate sets the basic pellet diameter, the uniformity of the cut and the achievable throughput. It is also one of the most heavily loaded wear parts on the line: hot on the melt side, cold on the water side and in constant contact with the knives on the cutting face.

An underwater pelletizer die plate combines several functional zones. The steel die body withstands the melt pressure and is bolted to the diverter valve or die adapter. Melt channels run through the body and end in die holes arranged on one or more hole circles. Hole diameter, number of holes and hole spacing determine pellet size and the flow rate per hole.

To stop the melt from freezing in the holes, the plate is heated, usually with thermal oil, steam or electric heaters. Thermal insulation between the hot body and the cold process water keeps heat loss to the water as low as possible. On the water side sits the wear-resistant die face in the knife path, either as a hard-faced layer or as inserts or segments.

• Die body with melt channels and mounting
• Die holes on one or more hole circles
• Heating by thermal oil, steam or electric heaters
• Insulation between the heated body and the process water
• Wear-resistant die face in the knife path
• Centring to the cutter hub so the knives sit flat on the face

The die face has to meet several requirements at once. It must be hard and abrasion resistant, withstand the process water, conduct heat poorly and survive thermal cycling without cracking. Ideally it can also be repaired.

Titanium carbide materials consist of titanium carbide grains in a hardenable steel matrix. According to the manufacturer's data sheet, Nikro 128 contains around 30 per cent titanium carbide by weight. This material group is regarded as corrosion resistant in process water and conducts heat considerably less than tungsten carbide, which helps keep the die holes hot. The pairing with the knives is also considered favourable, because the rounded titanium carbide grains are less abrasive on the knife edge.

Tungsten carbide is very hard and wear resistant, but it conducts heat more readily and is less resistant to corrosion in water. Hardened tool steel dies cost less but wear faster with abrasive or filled polymers, so they tend to be used for less demanding duties.

For titanium carbide, the segmented design has proven itself: the die face is made of individual segments brazed onto the die body. A complete, void-free braze joint is critical. If a gap remains, melt can creep under a segment and lift it off during operation. LEMCO manufactures titanium carbide die plates in segmented design using its own special brazing process.

Every die plate is matched to the polymer and the process. Polyolefins such as HDPE, LDPE, LLDPE and PP behave differently from polyamides (PA 6, PA 6.6), polyesters (PBT, PET), polycarbonate (PC) or styrenics (PS, ABS). Low-viscosity melts are more prone to pellets sticking together, while semi-crystalline polymers with a narrow melting range are more prone to die hole freeze-off.

• Polymer, melt viscosity and melting range
• Fillers and glass fibres (abrasion in the holes and on the die face)
• Total throughput and throughput per hole
• Target pellet size and shape
• Water temperature and water flow
• Pelletizer type, cutter hub and number of knives

Die plate wear usually shows up in the pellets first: pellets with tails, more fines, uneven pellet sizes and rising knife consumption. Behind these symptoms lie a handful of recurring causes.

• Grooves in the die face: excessive knife pressure or damaged knives running in the same track
• Uneven, dished or wavy wear: cutter hub out of alignment, knives at different heights
• Chipped segment edges: thermal shock, hard foreign particles, start-up with a cold die
• Corrosion pitting: aggressive or poorly treated process water, unsuitable material
• Eroded, enlarged holes: highly filled or glass-fibre reinforced compounds
• Frozen holes: insufficient heating, water too cold, throughput per hole too low
• Detached segments: incomplete braze joint, melt undercutting the hard facing
• Blocked holes: contamination in the melt, inadequate melt filtration

Much of the wear is caused not during steady running but at start-up and shutdown. If the die is not fully heated at start-up, some holes freeze off. The remaining holes then carry more melt, the pellets become uneven and the die face is loaded unevenly. If knives run across the plate without melt, the lubricating film of melt and water is missing and wear on knives and die face rises.

In operation, knife pressure should be as high as necessary and as low as possible. A constant water temperature keeps the holes open evenly. Treated process water, free of solids and with controlled chemistry, reduces corrosion and deposits. Effective melt filtration upstream of the die stops foreign particles from blocking holes or chipping the die face.

• Bring the die fully up to temperature before start-up
• Engage the knives only once melt is flowing
• Set knife pressure to the OEM value and keep it as low as possible
• Keep water temperature and flow constant
• Filter the process water and check it regularly

Inspect the die plate at least every time you change knives. That way you catch grooves and chips before they spoil the cut and damage a new set of knives.

• Check the die face for grooves, wear tracks and chips
• Measure flatness of the die face
• Check every hole for free passage and enlargement
• Inspect segments and braze joints for gaps or lifting
• Check the heating circuit or heaters
• Read the knife track: is the contact even across its full width?
• Record findings and operating hours to derive service intervals

A worn die plate does not automatically need replacing. Light grooves are removed by surface grinding and lapping with as little material removal as possible. If the hard facing is too thin or damaged, segments or the complete die face can be renewed. Refurbishing also includes cleaning and clearing the holes and checking the dimensions.

A new plate usually makes sense when the die body is cracked or distorted, or the hole geometry no longer fits the application. Many plants keep a spare: one plate in production, one being refurbished. This keeps the line available during planned maintenance.

LEMCO refurbishes die plates up to 1250 mm in diameter regardless of the original manufacturer, to European quality standards and with the shortest possible turnaround. We manufacture new die plates to drawing or OEM part number, designed for your polymer and delivered extremely quickly.