In-plant nitrogen generators put the 'gas' in gas-assist molding.

What is the best source of nitrogen for gas-assist injection molding? According to recent cost comparisons from two leading nitrogen suppliers, on-site nitrogen generation often beats compressed-gas cylinders and liquid nitrogen tanks by a long shot. The word on nitrogen generation's economic benefits comes from Air Products and Chemicals Inc., Allentown, Pa., and Air Liquide America Corp. of Houston, both of which supply nitrogen generators as well as nitrogen in cylinders and tanks.

Choosing the optimum nitrogen source can be tricky business, since molders must consider not just their gas-volume requirements but also less apparent cost factors like electric-power rates and expected duration of a gas-assist molding job. "Unfortunately, there are no hard-and-fast rules of thumb for choosing a nitrogen source," says Timothy Boland, an Air Products engineer.

Over the past two years, Air Products and Air Liquide have seen an upswing in use of nitrogen generators as the gas-assist market matures and molders move into longer-term, higher-volume jobs. "Taking all factors into account, on-site nitrogen technology is the best method of supply for the gas-assist process," says Robert Watkins, marketing manager for Air Liquide. (To see how nitrogen generation makes sense, refer to the sidebar on p. 23 and to Tables 1 and 2.)

TABLE 1 - AIR PRODUCTS' COST COMPARISON OF TREE [N.sub.2]-SUPPLY
SYSTEMS

No. of Presses                      One        One         Two
Operating Time,
hr/day                              8          24          24
Resin Savings, $/hr                 15.20      15.20       30.40

Nitrogen Gas from Liquid Nitrogen
Gas Cost, $/hr                      2.44       2.44        4.88
Equipment Cost
(Tank & Vaporizer),
$/hr                                3.13       0.86        0.86
Total Cost, $/hr                    5.57       3.30        5.74

Nitrogen Gas from a Cylinders
Cylinders/hr                        1.8        1.8         3.6
Total Cost (@$4 per
100 cyl.), $/hr                     17.72      17.72       36.48

Nitrogen as from a Membrane System

Generating Equipment
Cost, $/r                           5.00       1.39        1.39
Electrical Costs
(@ 2 kwh/100 scf),                  0.44-      0.44-       0.89-
$/hr                                0.80       0.80        1.60

Total Cost, $/hr
$0.05/kwh                          5.44       1.83        2.28
$0.06/kwh                          5.53       1.92        2.45
$0.07/kwh                          5.62       2.01        2.63
$0.08/kwh                          5.71       2.10        2.81
$0.09/kwh                          5.80       2.19        2.99

DO-IT-YOURSELF NITROGEN

In contrast to other nitrogen sources, which require regular deliveries of either gas-filled cylinders or liquid nitrogen, on-site generation systems create a steady supply of nitrogen gas right on the shop floor. According to Watkins, the non-cryogenic generators used by injection molders utilize a polymer membrane to separate dry, filtered, compressed air into its constituent gases, including the nitrogen that accounts for about 78% of the total. The resulting nitrogen gas is typically around 95-99% pure. Molding applications should generally use at least 98% pure nitrogen, according to Watkins. He says that number was developed through joint research with GE Plastics.

Like other supply methods, nitrogen generators, which produce gas at about 150 psig, need some sort of supplementary pressure booster and storage buffer in order reach and consistently supply the 1200-6000 psig needed for the gas-assist process, Watkins notes.

TABLE 2 - AIR LIQUIDE'S COST COMPARISON

Inlet Pressure
to Control
Unit, psig                        1200       800        500

Nitrogen Volume
per Cycle, scf                    2.29       1.52       0.95

Cycle Time, sec                   60         60         60

Operation Time,
hr/month                          576        576        576

Total Nitrogen Use,
scf/month                         78,994     52,663     32,914

Number of Cylinders
per month
(@230 scf per cylinder)           675        338        178

Cylinder cost,
$/month                           4220       2110       1110

Nitrogen Generator
Cost, $/month                     1445       1445       1445

Liquid Nitrogen
Cost, $/month                     1592       1500       1313

Installing the unit is said to be simple. "Installation of a membrane nitrogen generator is similar to the installation of an air compressor," says Boland.

Expenses involved in nitrogen generation include the equipment cost and the power to run the generator's air compressor. Typically, a molder would lease the generator for around $400 to $1500/month, depending on the unit's capacity. An $800/month generator capable of serving two presses 24 hours a day was used in Air Products' analysis.

HIDDEN COSTS OF GAS

Nitrogen cylinders remain a viable and cost-effective choice for short-run gas-assist jobs, say both Watkins and Boland. And since cylinders can supply gas at high pressure - up to 6000 psig - they can also spare molders the cost of compression equipment.

But in the great majority of cases, cylinders are the most costly option because they don't offer the economy of liquid nitrogen or the combined convenience and economy of on-site generation. "The cost of nitrogen from cylinders often "barely breaks even with the resin savings associated with gas assist," says Boland. What's more, because they contain a fixed volume of gas, the cylinders' pressure drops as they empty. "Unless the molder has supplemental compression available, the gas in the cylinder will be useless once the cylinder pressure drops below a certain level," Boland says.

Watkins adds a list of "hidden" costs weighing against cylinders - delivery concerns, distribution costs, downtime, and safety/labor considerations for changing cylinders. These costs do not apply to self-generation, which places the nitrogen source at the point of use, he says.

Liquid-nitrogen tanks may be cheaper than gas cylinders, but tanks still require delivery trucks, take up space outdoors, and carry a monthly equipment charge for the vaporizers and compressors needed to turn the liquid into a pressurized gas suitable for molding. "Before the emergence of on-site technology, liquid nitrogen was the only cost-effective way to supply medium to large volumes of nitrogen," says Watkins.

Two Views of Gas-Supply Economics

The tables on pp. 24 and 26 show different nitrogen-costing scenarios. Table 1, from Air Products and Chemicals Inc., is based on a real gas-assist job - a 25-g vent cover made from a 25% glass-filled polyester. Air Products made several assumptions as a starting point for that analysis:

* Gas assist saves 8 g/part.

* Each part has a 5-cc gas void formed using 5000-psia nitrogen pressure.

* Estimated average nitrogen consumption is 2.1 lb or 28.98 scf/hr.

* Cycle time is 25 sec, and the parts are produced in a four-cavity tool at a rate of 576 parts/hr.

* Resin price is $1.50/lb.

* Bottled nitrogen gas costs $4/100 scf, while liquid nitrogen costs $0.55/100 scf plus a monthly service charge of $500 for a 3000-gal tank and vaporizer.

* The nitrogen generator used is an Air Products PRISM system with 1000 scf/hr capacity, capable of serving two presses at a monthly leasing fee of roughly $800. It typically uses between 1.6 and 2.1 kwh/100 scf of nitrogen gas.

* Cost of compression needed by all three systems is not included.

Table 2, from Air Liquide, assumes a longer cycle time (60 sec) and 24-hr operation in the Midwest. Equipment pricing in this example is the total cost: It takes into account all monthly fees such as power and leasing costs.