Page 123 - Engineering Plastics Handbook

Polyacetals 97

has automated controls for the debinding time required and debinding
completion time [17]. Automation reduces the debinding cost and time
and simplifies the process [17]. Contamination from residual binder due
to carbonization during sintering is eliminated [17]. Heated-chamber sin-
tering furnace walls are heated up to 180°C (359°F) to prevent condensa-
tion which occurs on cold chamber walls [17].
By using GERO-Hochtemperaturoefen (high-temperature) equipment,
following catalytic debinding, load stacks are placed into the PDS 120
sintering furnace followed by residual debinding and sintering of “brown
pieces.” Low-alloy steels such as carbonyl iron and 42CrMo ,toolsteels such
4
as 100Cr , stainless steels such as 316L and 17-4PH, and titanium and tita-
6
nium alloys are sintered in the PDS 120 sintering furnace [17]. According
to GERO, the company’s PDS 120 sintering furnace has the following ben-
efits compared with conventional sintering equipment:

1. N , H , or Ar partial-pressure operation for effective residual debinding
2
2
2. Heat exchanger and circulation of process gas that provide cycle
times less than 12 h (cold/cold)
3. Special rotary vane pump for vacuum applications
4. T max up to 1450°C (2642°F)
5. Hot wall furnace with a metallic heater
6. Low carbon content, perfect part surface
7. Very little finishing needed, allowing applications such as watch com-
ponents, glasses, and other jewelry parts

Hot air staking is the method of choice for assembling polyacetal pneu-
matic valves for seat-height adjuster systems in commercial vehicles [20].
Phasa Developments Type 30/10 plastic hot air staking machine was
finally selected over ultrasonic welding because staking was found to pro-
vide consistent assembling and high throughput, independent of operator
skill level [20]. Further, hot staking was determined to be quieter and
cleaner than ultrasonic welding. Hot air staking is used by KV Ltd. (Milton-
Keyes, UK), which designs, develops, and manufactures pneumatic and
fluid control systems [20].
The polyacetal pneumatic valves replaced more expensive metal valves
which were either machined or cast. The polyacetal pneumatic valve meets
several operating requirements, including reliable performance for several
years (in trucks that must endure the harsh environment of road-to-vehicle
vibration), design life of 100,000 operating cycles, and cost-effectiveness
[20]. Hot staking assembly satisfied two operational requirements of the
in-place pneumatic valves: using the vehicle’s air supply and installing a
microswitch to bring in an external air supply as needed [20]. Consequently,