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Delrin® | Technical Guide
Delrin® Acetal Homopolymer
Thermoplastic Resin Molding Guide
Delrin® | Technical Guide
Table of Contents
1. Processing guideline summary................................................ 5
Drying considerations .............................................................. 5
Melt temperatures.....................................................................5
Operating conditions.................................................................5
Hot runner recommendations.................................................. 5
Mold temperatures................................................................... 5
Shrinkage considerations......................................................... 5
2. Safe handling information.........................................................6
Safety precautions.................................................................... 6
3. Drying guidelines....................................................................... 8
Effects of moisture................................................................... 8
Moisture absorption..................................................................8
Dryer conditions........................................................................ 8
4. Molding equipment—mold........................................................ 9
Mold temperature control.......................................................10
Basic recommendations........................................................ 10
Mechanical structure.............................................................. 10
Conventional runner system and gate layout....................... 11
Runner system........................................................................ 11
Gate location........................................................................... 11
Gate design............................................................................. 12
Hot runner systems................................................................ 13
Thermal control of hot runner molds.....................................14
Venting.....................................................................................15
Undercuts................................................................................ 16
Ribs.......................................................................................... 17
Sharp corners.......................................................................... 17
Draft angles............................................................................. 17
Weld lines................................................................................ 17
Mold cleaning and maintenance............................................18
Delrin® | Technical Guide
Table of Contents (continued)
5. Molding equipment—injection unit.........................................18
Screw....................................................................................... 18
Screw design........................................................................... 18
Screw size............................................................................... 19
Screw design for the use of color concentrate..................... 19
ELCee™ screw..........................................................................19
Check ring or backflow valve................................................. 19
Screw corrosion/abrasion...................................................... 20
Nozzles.................................................................................... 20
Accumulator for thin wall applications..................................21
6. Molding parameters—start-up and shutdown procedures...21
General information on purging and start-up........................ 21
Purging.................................................................................... 21
Start-up.................................................................................... 21
Start-up and shutdown procedures for specific situations.... 22
Start-up with resin change..................................................... 22
Start-up from a cylinder containing Delrin®........................... 22
Start-up after emergency shutdown...................................... 22
Shutdown.................................................................................22
Shutdown when a restart with Delrin is planned.................. 23
Shutdown when a restart with another resin is planned...... 23
Interruptions............................................................................ 23
Action to follow when the nozzle heater band breaks down......... 23
7. Molding parameters................................................................ 23
Clamping force........................................................................ 23
Melt and cylinder temperature............................................... 23
Nozzle temperature................................................................ 24
Mold temperature................................................................... 24
Molding cycle.......................................................................... 25
Injection phase........................................................................ 25
Pack or hold pressure phase..................................................26
Cooling time............................................................................ 27
Screw retraction phase........................................................... 27
Evaluation of melt quality....................................................... 27
Foaming test.................................................................................. 28
Unmelt test................................................................................. 28
Delrin® | Technical Guide
Table of Contents (continued)
8. Material behavior.......................................................................... 28
Thermal stability during processing......................................... 28
Low emission molding................................................................ 29
Viscosity and rheological behavior...........................................29
Influence of temperature............................................................ 29
Influence of shear rate................................................................ 29
Influence of molecular weight................................................... 30
Ability to fill .................................................................................. 30
Dimensional considerations...................................................... 31
Fundamentals of dimensional control..................................... 31
Mold shrinkage............................................................................ 31
Factors affecting mold shrinkage............................................ 32
Hold pressure............................................................................... 32
Hold (pressure) time (HPT)........................................................32
Mold temperature........................................................................ 33
Part thickness.............................................................................. 33
Gate dimensions.......................................................................... 33
Effect of pigments....................................................................... 33
Post molding shrinkage.............................................................. 34
Annealing...................................................................................... 35
Environmental changes.............................................................. 35
Dimensional tolerances.............................................................. 35
Molds............................................................................................. 35
Molding conditions...................................................................... 35
9. Auxiliary operations.................................................................... 36
Regrind.......................................................................................... 36
Effect on mechanical properties............................................... 36
Coloring......................................................................................... 36
Material handling......................................................................... 37
10.Troubleshooting guide............................................................... 37
Delrin® | Technical Guide
1. Processing guideline summary
Hot runner recommendations
Delrin® is used successfully in hot runner tools at many
customers for different applications. To avoid problems during
the production in hot runner tools, the following guidance should
be followed:
Delrin® POM thermoplastic resins may be processed on
conventional injection molding machines using standard industry
practices. Specific attention to processing details will enhance
quality and productivity. This summary represents a key subset
of the detailed molding information found in the remainder of this
molding guide.
• No hold up spots can be tolerated in the hot runner
• The use of self-insulating tips is not recommended
• Temperatures in the hot runner system should not exceed
190°C (374°F)
Drying considerations
As a general rule, Delrin® standard grades do not require drying.
However, drying for 2–4 hours at 80°C (176°F) in a dehumidified
dryer is necessary under these conditions:
Mold temperatures
Using a lower mold temperature leads to shorter crystallization
time (HPT), therefore, a shorter cycle time, lower mold shrinkage
but higher post mold shrinkage (especially if parts are then
exposed to elevated temperatures). A balance should be
found depending on the temperature in use and the required
dimensional precision of the molded part short and long term.
• If moisture content is above 0.2%
• When a resin container is damaged
• When the material is not properly stored in a dry place at room
temperature
• When packaging stays open for a significant time
• For Delrin® standard grades, a mold temperature of 80–100°C
(176–212°F) is recommended as good compromise
• For high precision parts, a higher mold temperature up to 120°C
(248°F) might be necessary
For toughened Delrin®, moisture content should be <0.05%. Dry at
4–8 hours at 80°C (176°F) in a dehumidified dryer.
Melt temperatures
• For Delrin® standard grades, the optimum melt temperature
is at 215±5°C (419±10°F)
• For parts that require a very low emission behavior, it is
recommended to use a melt temperature of 205±5°C
(401±10°F)
• For toughened grades, it is recommended to use a melt
temperature of 205±5°C (401±10°F)
• Processing temperatures should be matched to part size
• Excessive residence (or hold-up) times (HUT) and/or
temperatures may cause degradation
Shrinkage considerations
Operating conditions
Note: High mold temperatures and thick part sections may
increase shrinkage. Shrinkage in reinforced resins is controlled
by glass fiber orientation, which results in different shrink rates in
parallel to and perpendicular to flow directions.
Shrinkage in semi-crystalline resins such as Delrin® is due to:
• Crystallization of the polymer
• Thermal contraction of the part as it cools to room temperature
Causes of part distortion include:
• A high level of glass orientation
• Poor mold temperature uniformity
• Large changes in wall thickness of the part
• Keep back pressure as low as possible.
• It is crucial to check that the applied screw rotation speed is
low enough to avoid over-shearing. Depending on the grade,
the limit for the screw rotation speed is 0.3 m/s or lower.
• Hold pressure is key for part properties. Depending on the
grade, a hold pressure range of 60–110 MPa (8.7–16.0 kpsi) is
recommended. As a general rule, hold pressure time should be
8 sec per mm (0.04 in) of part thickness (based on 3 mm (0.12
in) part thickness).
Delrin® | Technical Guide
possibility. Similarly, for resins requiring hot mold temperatures,
pay special attention to inadvertent contact with mold surfaces and
cooling fluid transfer lines, as these may also present a burn risk.
2. Safe handling information
Safety precautions
Processing thermoplastic resins is a generally safe operation
when one complies with the following processing
recommendations:
Small amounts of gases and particulate matter (i.e. low molecular
weight modifiers) may be released during the molding, purging,
drying, regrinding, and clean-up of thermoplastic resins. We
recommend that adequate local exhaust ventilation be provided
during the processing of Delrin® thermoplastic resins in order to
meet regional requirements. It is necessary to consider the removal
of dust and particles created during the distinct processes of
drying, molding, purging and regrinding all while considering the
maximum residence (hold-up) times and processing temperatures.
A. Delrin® thermoplastic resins are molded at high temperatures
and contact with molten resin can inflict severe burns. At
temperatures above the melting point, moisture and other gases
may generate pressure which, if suddenly released, can cause the
molten polymer to be violently ejected from the machine nozzle
or the hot runner system.
To minimize the chance of an accident, the instructions given in
this text should be followed carefully. Potential hazards must be
anticipated and either eliminated or guarded against by following
established procedures including the use of proper protective
equipment and clothing.
• Thermoplastic polymers can form gaseous decomposition
products during long residence times (Hold-Up Times, HUT) at
the maximum recommended melt temperatures
• Adequate local exhaust ventilation should also be provided
during the regrind operation
• Adequate local exhaust ventilation must be provided during the
burnout of any equipment that contains thermoplastic resin,
e.g. nozzles, etc.
Be particularly alert during purging and whenever the resin is
held in the machine or hot runner system at higher than usual
temperatures or for longer than usual periods of time–as in a
cycle interruption. Please read and insure understanding of the
sections on molding parameters thoroughly.
Refer to Proper Use of Local Exhaust Ventilation During
Processing of Plastics for details.
In purging, be sure that the high volume (booster) pump is off
and that a purge shield is in place. Reduce the injection pressure
and jog the injection forward button a few times to minimize the
possibility that trapped gas in the cylinder will cause splattering
of the resin.