Editing Polyurethane (section)

==Manufacturing==
The methods of manufacturing polyurethane finished goods range from small, hand pour piece-part operations to large, high-volume [[bunstock]] and [[boardstock]] production lines. Regardless of the end-product, the manufacturing principle is the same: to meter the liquid isocyanate and resin blend at a specified stoichiometric ratio, mix them together until a homogeneous blend is obtained, dispense the reacting liquid into a mold or on to a surface, wait until it cures, then demold the finished part.

===Dispensing equipment===
Although the capital outlay can be high, it is desirable to use a meter-mix or dispense unit for even low-volume production operations that require a steady output of finished parts. Dispense equipment consists of material holding (day) tanks, metering pumps, a mix head, and a control unit. Often, a conditioning or heater–chiller unit is added to control material temperature in order to improve mix efficiency, cure rate, and to reduce process variability. Choice of dispense equipment components depends on shot size, throughput, material characteristics such as [[viscosity]] and filler content, and [[process control]]. Material day tanks may be single to hundreds of gallons in size and may be supplied directly from drums, IBCs ([[intermediate bulk container]]s, such as [[caged IBC tote]]s), or bulk [[storage tank]]s. They may incorporate level sensors, conditioning jackets, and mixers. Pumps can be sized to meter in single grams per second up to hundreds of pounds per minute. They can be rotary, gear, or piston pumps, or can be specially hardened [[lance pump]]s to meter liquids containing highly abrasive fillers such as chopped or hammer-milled [[glass fiber]] and [[wollastonite]].{{citation needed|date=November 2020}}

<gallery>
Image:HighPressureDispenseUnit800x600.png|A high-pressure polyurethane dispense unit, showing control panel, high-pressure pump, integral day tanks, and hydraulic drive unit
Image:HighPressureLHeadFront600x800.png|A high-pressure mix head, showing simple controls (front view)
Image:HighPressureLHeadRear600x800.png|A high-pressure mix head, showing material supply and hydraulic actuator lines (rear view)
</gallery>

The pumps can drive low-pressure (10 to 30&nbsp;bar, 1 to 3&nbsp;MPa) or high-pressure (125 to 250&nbsp;bar, 12.5 to 25.0&nbsp;MPa) dispense systems. Mix heads can be simple static mix tubes, rotary-element mixers, low-pressure dynamic mixers, or high-pressure hydraulically actuated direct [[Impinging mixer|impingement mixers]]. Control units may have basic on/off and dispense/stop switches, and analogue pressure and temperature gauges, or may be computer-controlled with flow meters to electronically calibrate mix ratio, digital temperature and level sensors, and a full suite of statistical process control software. Add-ons to dispense equipment include nucleation or gas injection units, and third or fourth stream capability for adding pigments or metering in supplemental additive packages.

<gallery>
Image:LowPressureMixHead600x800.png|A low-pressure mix head with calibration chamber installed, showing material supply and air actuator lines
Image:LowPressureMixChamberComponents800x600.png|Low-pressure mix head components, including mix chambers, conical mixers, and mounting plates
Image:LowPressure5GallonDayTanks800x600.png|5-gallon (20-liter) material day tanks for supplying a low-pressure dispense unit
</gallery>

===Tooling===
Distinct from pour-in-place, bun and boardstock, and coating applications, the production of piece parts requires tooling to contain and form the reacting liquid.
The choice of mold-making material is dependent on the expected number of uses to end-of-life (EOL), molding pressure, flexibility, and heat transfer characteristics.

RTV silicone is used for tooling that has an EOL in the thousands of parts. It is typically used for molding rigid foam parts, where the ability to stretch and peel the mold around undercuts is needed.
The heat transfer characteristic of RTV silicone tooling is poor. High-performance, flexible polyurethane elastomers are also used in this way.

Epoxy, metal-filled epoxy, and metal-coated epoxy is used for tooling that has an EOL in the tens of thousands of parts. It is typically used for molding flexible foam cushions and seating, integral skin and microcellular foam padding, and shallow-draft RIM bezels and fascia. The heat transfer characteristic of epoxy tooling is fair; the heat transfer characteristic of metal-filled and metal-coated epoxy is good. Copper tubing can be incorporated into the body of the tool, allowing hot water to circulate and heat the mold surface.

Aluminum is used for tooling that has an EOL in the hundreds of thousands of parts. It is typically used for molding microcellular foam gasketing and cast elastomer parts, and is milled or extruded into shape.

Mirror-finish [[stainless steel]] is used for tooling that imparts a glossy appearance to the finished part. The heat transfer characteristic of metal tooling is excellent.

Finally, molded or milled [[polypropylene]] is used to create low-volume tooling for molded gasket applications. Instead of many expensive metal molds, low-cost plastic tooling can be formed from a single metal master, which also allows greater design flexibility. The heat transfer characteristic of polypropylene tooling is poor, which must be taken into consideration during the formulation process.