Annular dies (blown film dies)

01

Understand how a spiral mandrel die distributes melt

A Compuplast paper published by TAPPI explains that the spiral mandrel distribution system has become the standard design for blown (tubular) film dies. The paper describes ports that feed the melt to helical grooves cut into the mandrel; a small gap between the body bore and the mandrel lets polymer leak out of the spirals, and because channel depth falls along each spiral, more and more polymer passes into the annular gap until the flow becomes axial. The melt then passes a relaxation chamber and the die lips. The gradual leakage layers material from different ports, adding mixing that smooths colour variation and temperature gradients from the extruder; but a spiral system not designed for the polymer and flow rate can distribute poorly or create a hidden stagnation point that shows only as a line in the film and can build up degraded material. The paper shows how 3D finite-element flow simulation can expose such flaws. [1]

Editorial note: ask the die supplier for flow-simulation results for your resins and output — distribution uniformity and residence time — rather than relying on general claims, since the paper shows design faults that are hard to diagnose on the line. [1]

Give the supplier the resins and their melt properties, number of layers and layer ratios, output rate, film width, thickness and blow-up target, lip diameter and die gap, feed arrangement (bottom or side entry), materials and plating, lip heaters or automatic profile control, internal bubble cooling, mounting to your line, spare lipsets, and delivery.

02

Size the die gap, lip diameter and layers

Davis-Standard, a blown film equipment maker, describes its Centrex dies as concentric spiral designs fed from a nested block with a central flow and layer distributor, computer-modelled for melt-flow uniformity, short residence time and low gel formation. It lists a die gap of 0.030–0.12 in (0.76–3 mm) (typ.); monolayer dies from 4 in (100 mm) to 86 in (2,185 mm), three-layer from 150 mm to 2,185 mm, five-layer from 180 mm to 1,220 mm, seven-layer from 180 mm to 610 mm, and 5-, 7- and 9-layer stationary coextrusion dies from 50 mm to 300 mm. Components are AISI 4140 stress-relieved, pre-hardened forgings (4340 optional), with polished flow surfaces, hard-chrome lips and electroless-nickel mandrels and bodies, taper-lock fits, large internal bubble cooling (IBC) holes and a recessed exit; options include automatic die heaters for profile control and die-lip heaters. [2] Its PE die series for LDPE and LLDPE blends lists lip diameters by body size: 100–200 mm on a 150 mm body, 200–300 mm on 250 mm, 250–350 mm on 300 mm and 350–460 mm on 400 mm. [3]

Editorial calculation: the melt leaves the die through a ring whose circumference is π times the lip diameter, so a 200 mm lip delivers a tube about 628 mm round and a 300 mm lip about 942 mm; at a 0.76–3 mm gap the annular exit area for a 200 mm lip is roughly 480–1,880 mm². [3] [2]

03

Rate the die by output per unit of circumference

Blown film die output is stated per unit of die circumference, so a die's rating scales with its lip diameter. Union Carbide's European patent application EP 0 773 258 (1997) states that LLDPE is particularly vulnerable to melt fracture at die rates above about 4 lb per hour per inch of die circumference with die gaps of about 0.020 to 0.075 in, and claims film from an LLDPE blend extruded at about 6 to 20 lb/h per inch of circumference, preferably 8 to 15, with a die gap of about 0.020 to 0.075 in, preferably 0.035 to 0.050 in. It gives extrusion temperatures of about 150 to 280 °C, preferably 190 to 250 °C, and film gauges of about 0.5 to 6 mils. [4]

Editorial calculation: 1 lb/h per inch is 0.4536 kg / 2.54 cm ≈ 0.179 kg/h per cm, so 8 to 15 lb/h per inch is about 1.43 to 2.68 kg/h per cm; the preferred gap of 0.035 to 0.050 in is about 0.89 to 1.27 mm, inside Davis-Standard's 0.76–3 mm range. A 250 mm die has a circumference of π × 250 = 785 mm (30.9 in), which at 8 to 15 lb/h per inch gives about 247 to 464 lb/h, or about 112 to 210 kg/h. Ask the die supplier for the rated output of the die you are buying for your resin, since the patent's figures apply to its LLDPE blend. [4] [2]

04

Simulation results and plating specs to request from each die maker

When requesting quotations, ask each supplier for flow-simulation results, material and plating specifications, heater options and reference lines running similar resins and layer counts.

05

Ports, spiral mandrel, relaxation chamber and lips

06

Gap, diameter, rated output and steel grade

Davis-Standard and the Union Carbide patent supply the diameter, gap, output, temperature and material rows; pressure and weight are not stated in the reviewed sources.

07

Send the film, resins and line together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Film: [width; thickness; layers; ratios]
Resins: [types; MFI]
Output: [kg/h]
Die: [lip diameter; gap; entry]
Build: [steel; plating; heaters; IBC]
Line: [mounting; existing air ring]
Quantity: [dies; spare lipsets]

See manufacturer records Send an enquiry through CNFX