Injection Molding

For transparent molding compounds, ensure a dust-free production environment (clean filter of the drying system, material containers, machine hoppers, etc.), because dust particles can cause part defects.

TROGAMID® molding compounds are not miscible with most other polymers. Even minor contaminations cause clouding. Therefore, the plasticating units and all melt channels should be cleaned carefully prior to production.

Coloring TROGAMID® is possible and can be done using TROGAMID®-based color concentrates or through compounding at the plant. Dry coloring using powder colorants is also possible, but due to handling efforts not recommended; pneumatic conveying is then precluded. The use of color pastes on a “neutral basis” can lead to incompatibilities and thus to a negative impact on the mechanical properties of TROGAMID® (such as reduced weld line strength or reduced transparency based on the formation of clouds) and should therefore be checked for each individual case.

TROGAMID® molding compounds are delivered with a moisture content less than 0.10 wt.% in moisture-proof packaging. For high-quality molded parts, we recommend additional drying in a dry-air dryer.

Drying temperature: 80 °C - 90 °C
Drying time: 8 - 12 hours
Recommended residual moisture for additional drying 0.02 < 0.06 %

Drying recommendadtions:

• Type: dry-air dryer
• Dew point of the dryer should be < -30 °C
• Granulate conveying with dried air is recommended
• Air flow rate: ~ 2.2 m³/h/kg

For intense contamination, mechanical cleaning is recommended using one of the following methods:

• Ultrasounic bath
• Oven cleaning / pyrolysis and subsequent brushing, sandblasting, dry ice cleaning, polishing, etc.

For minor contamination, or when changing the molding compound, the following procedure is recommended:

1. Providing high-viscosity PP (MFI < 1g/10 min)
2. Increase the temperature settings of the heating zones by 30 °C - 40 °C, but not more than 310 °C
3. Plasticize the PP with an increased metering stroke and back pressure and then discharge at high speed
4. Purge until no more contamination can be detected in the melt

As an alternative to high-viscosity PP, it is also possible to use a cleaning granulate, such as ACRIFIX®, ASACLEAN® NewE. Special manufacturer specifications on the use of cleaning granulate must be obeyed!

Mold temperature

The optimal mold temperature for TROGAMID® is 60 °C – 80 °C. Generally, a lower mold temperature, or a bigger difference between mold temperature and melt temperature, leads to higher internal stresses.

Melt temperature

As a rule of thumb, the optimal melt temperature for TROGAMID® is 280 °C – 300 °C. For short cycle times and short residence times in the plasticating unit, higher temperatures can be set; in such a case, temperature stability of colorants, additives and polymer must be checked.Shear heating during plastication and injection phase can raise melt temperature significantly above desired temperature levels.

• For brief production interruptions up to ten minutes, several purging cycles are recommended prior to production restart.
• For production interruptions of up to one hour, the plastication unit should be discharged. The screw should be moved to the foremost position, barrel temperatures should be lowered to 150 °C.
• For longer production interruptions, purging with PP or a cleaning granulate, followed by discharging of the plastication unit is recommended. Barrel heating should be switched off and remaining material in the hopper should be stored protected against moisture.

Temperature settings for plastication unit

• Increasing temperature profile is recommended, rising (from the hopper to the nozzle) in steps of 10 °C).
• Feeding zone temperature should be in the range of 40 °C - 90 °C.

Peripheral screw speed

• Between 3 and 12 m/min; optimal: 10 m/min
• Higher speeds, (such as > 18 m/min) are possible, but may lead to processing problems .

Decompression

• Decompression can adversely affect the surface quality (e.g. due to entrapped air and should be minimized.

Back pressure

• Back pressure: 40 bar - 100 bar

Metering delay

• In order to minimize the residence time, metering time delay - utilizing the entire cooling phase - is recommended for production. For larger units, metering delay decreases thermal inhomogeneity.

Injection speed

• Material shearing should be considered setting injection speed. In thin wall areas - such as gates - shear rate should be less than 40000 1/s. Shear heating effects can be evaluated utilizing simulation, and injection speed limited accordingly. For fiber-reinforced molding compounds shear rates should stay below 25,000 1/s. In addition to the temperature increase, fiber damage due to shear stress should also be evaluated.
• Typical injection pressures range between 800 bar and 1600 bar.

Packing/holding pressure

• As a rule of thumb, the holding pressure should be about 70 - 100 % of the maximum injection pressure.
• Typical holding pressure: up to 1400 bar.

The holding pressure compensates the volume contraction after cavity filling and melt packing. The time in which the melt can continue to flow is determined directly by the pressure level, the gate sealing, and the part wall thickness. Mold and melt temperature have little influence on this. After optimization of part cavity, gate and runner geometry the holding pressure should be high enough to avoid sink marks, but as low as possible (in order to avoid high inner stresses). In general, internal stresses increase with increasing packing pressure.

Cooling time

Cooling time can be estimated using the rule of thumb t = 2...3 * s² where t is the cooling time in seconds and s the wall thickness in mm. For more precise cooling time calculations and for hot spot determination in part and mold, an injection molding process simulation is recommended.

• In general, a mold release agent is not required. Some analog molding compounds are available with a mold release agent.
• Proper mold and molding process design avoids demolding problems. Evonik offers technical support in this area.

As a rule of thumb, oven tempering reduces internal stress level of molded parts. Inner stresses decreases during storage after injection molding, too.

In general, when processing thermoplastics, it is necessary to ensure adequate shop floor ventilation. An extraction system above the machine nozzle is recommended. Additional information is available in the respective safety data sheet.