Processing on injection molding machines

For thin-walled parts, an increase in mold temperature, higher injection speed or increased melt temperature have a positive effect. If the thermal and mechanical requirements to be met by the molding allow it, a grade of molding compound with greater ease of flow can be employed. PLEXIGLAS^® 6N shows particular ease of flow. Among the impact-modified grades, PLEXIGLAS^® Resist zkBR or the PLEXIGLAS^® Resist zkHF series are recommended, for example.

PLEXIGLAS^® molding compound is dry when packaged and can be processed without predrying even after several weeks' storage, thanks to the polyethylene lining. We nevertheless advise our customers to use suitable drying equipment.

PLEXIGLAS^® can be processed on injection-molding machines with 3-zone generel purpose screws for thermoplastics.

Predrying should generally be performed at the highest possible temperatures without causing the pellets to cake. Temperatures of 10 to 15°C below the Vicat softening temperature should therefore be observed. You will find the Vicat softening temperature in our product information leaflets on the respective material or in the CAMPUS database. We recommend the use of desiccant-type driers.

Plasticization should be performed as gently as possible (at low speed). The usual peripheral speed for processing PLEXIGLAS^® is 0.3 m/s.

This fault points to various causes: 
- Mold not optimally filled 
- Shrinkage not adequately compensated for 
- Air entrapment (fluctuations in material feed)

Remedies: 
- Examine material feed 
- Reduce melt temperature 
- Extend hold time 
- Increase holding pressure 
- Reduce injection speed 
- Increase mold temperature 
- Reduce screw speed 
- Raise back pressure 
- Improve predrying 
- Enlarge gates and possibly runners

Possible cause: 
The melt is not optimally pressed against the mold surface.

Remedies: 
- Increase melt temperature 
- Raise mold temperature 
- Increase injection speed 
- Increase injection pressure 
- Exchange material

This fault points to various causes:

1. Material is too damp

Remedies: 
- Predry material 
- Lower melt temperature 
- Raise injection pressure 
- Increase injection speed 
- Increase back pressure

2. Decomposition due to overheating of melt

Remedies:
- Check melt temperature (cylinder nozzle and hot runner) and reduce if necessary
- Lower back pressure
- Reduce screw speed 
- Reduce injection speed 
- Enlarge gates and possibly runners

Possible cause: 
Insufficient pressure / dwell pressure to compensate for shrinkage

Remedies: 
- Check material feed 
- Increase mold temperature 
- Reduce melt temperature 
- Raise holding pressure 
- Prolong hold time 
- Extend injection time 
- Increase injection pressure 
- Reduce injection speed 
- Lower screw speed - Enlarge gates and runners

Possible cause: 
Cold slug that protrudes into the mold

Remedies: 
- Check that nozzles are tight 
- With mechanical shut-off nozzle, check that the latter functions properly- Increase nozzle temperature 
- Raise mold temperature 
- Avoid sharp edges, particularly at the transition from the gate to the mold cavity 
- Round off and gates polish well 
- Polish the nozzle, which should be conical to allow reliable extraction of the cold slug 
- Provide a blind hole in front of the ejector 
- Raise melt temperature

Possible cause: 
Sprue is overloaded or not optimally designed

Remedies: 
- Polish sprue bushing 
- Make sure sprue bushing is conical and oversized in comparison with the nozzle 
- Enlarge ejector or the undercut around it 
- Round off Z-shaped ejector slightly to prevent sprue from breaking at this point 
- Prolong cooling time 
- Reduce injection pressure 
- Reduce holding pressure

Possible cause: 
Unfavorable venting of mold

Remedies: 
- Reduce injection speed 
- Reduce cylinder temperature 
- Install vent 
- Change wall thickness in mold 
- Optimize predrying

Possible cause: 
Irregular shrinkage due to different molding section or contour (surface)

Remedies: 
- Extend cooling time 
- Control temperature of two mold halves separately; cool the "hollow" side, keep the other side warm 
- Install heating / cooling channels just below the mold surface 
- Lower melt temperature by reducing band heater temperature, screw speed and back pressure 
- Decrease mold temperature 
- Use a clamping device for the moldings

This fault points to various possible causes:

1. Recesses

Remedies: 
- Increase mold temperature 
- Raise melt temperature 
- Increase injection pressure 
- Increase injection speed 
- Vent mold more effectively

2. Differences in wall thickness, sharp transitions

Remedy: 
- Increase mold temperature 
- Raise melt temperature 
- Reduce injection speed 
- Change location of gate to improve flow conditions 
- Adapt wall thickness to achieve a uniform melt flow 
- Vent mold more effectively

Possible cause: 
Differences in wall thickness

Remedies: 
- Reduce injection pressure 
- Increase mold temperature 
- Raise melt temperature 
- Relocate gates 
- Round off sharp-edged transitions 
- Reduce depth of lettering and / or round off edges

Possible cause: 
Jetting of melt

Remedies: 
- Reduce injection speed 
- Increase mold temperature 
- Vary melt and nozzle temperature 
- Round off and polish gates well 
- Enlarge gates and possibly runners 
- Provide a blind hole in front of ejector

At optimum draft angles (c. 2°, but at least 1°) and with well-polished molds, PLEXIGLAS^® moldings can usually be demolded without any problems. If there are problems with demolding owing to unfavorable mold or molding design, PLEXIGLAS^® molding compounds can also be supplied in special "le" grades for easy mold release. If the requirements to be met by the part allow it, grades of PLEXIGLAS^® Resist molding compound with higher impact strength can be used instead of standard molding compound.

This faults to various possible causes:

1. Impurities, mixing with other thermoplastics

Remedy: 
- Completely wash out other thermoplastics 
- Clean (purge) the cylinder and screw with our cleaning agent ACRIFIX^® sp 
- Avoid contamination with other thermoplastics in the drier and / or conveyor system 
- Do not add regrind 
- Do not mix different grades of PLEXIGLAS^® molding compound

2. Differences in wall thickness, sharp-edged transitions, poor plasticizing

Remedy: 
- Reduce injection speed 
- Increase mold temperature 
- Vary meld and nozzle temperature 
- Round off and thoroughly polish gate 
- Lower screw speed Increase back pressure 
- Use cylinder with better plasticizing properties

Possible cause: 
Melt is too thin-bodied or foams

Remedies: 
- Reduce back pressure 
- Predry molding compound more effectively 
- Lower melt temperature 
- Decrease nozzle temperature 
- Use shut-off nozzle 
- Return stroke of screw after material feed 
- Fit nozzle closely to mold

Possible cause: 
Contamination with other thermoplastics

Remedies: 
- Completely wash out other thermoplastics 
- Clean (purge) cylinder and screw with our cleaning agent ACRIFIX^® sp 
- Avoid contamination with other thermoplastics in the drier and / or conveyor system

Possible cause: 
Impurities

Remedies: 
- Clean (flush) cylinder and screw with our cleaning agent ACRIFIX^® sp
- Remove the screw in case of stubborn deposits 
- Avoid contamination in the drier and/or conveyor system 
- Do not use regrind 
- Clean degassing zone at the cylinder 
- Remove burnt material at the tip of the nozzle 
- Switch off return unit

This faults point to various possible causes:

1. Air intake, fluctuations in material feed

Remedy: 
- Increase back pressure 
- Vary feed temperature 
- Cool water in feed section 
- Clean cylinder 
- Avoid contamination (other thermoplastics) 
- Add lubricant to the surface of the pellets

2. Fluctuations in material feed due to brief or prolonged interruptions of cycle (screw becomes too hot)

Remedy: 
- Avoid interruption 
- Always retract screw when machine is not in operation

Possible cause: 
Unfavorable mold venting

Remedies: 
- Reduce injection speed 
- Lower cylinder temperature 
- Install venting channel 
- Change wall thicknesses of mold 
- Optimize predrying

Should the supplied molding compound show too high a moisture content for processing after prolonged storage (bubbles or streaking), predrying is recommended. If the recommended drying temperatures and minimum drying times are observed, simple ovens with or without airflow are sufficient, but shorter drying times are obtained with so-called desiccant-type driers. Vacuum cabinets are often preferred in climates with high atmospheric humidity. Small portable units with high performance are available in addition to stationary units.

The minimum wall thickness is 1 mm. We cannot state a maximum wall thickness. Thicknesses of more than 25 - 30 mm are very rare, but quite possible.

The main influencing factors are: 
- melt temperature, 
- mold temperature, 
- holding pressure, 
- hold time, 
- injection speed.

The sprue can best be separated by tweaking it off with heated pliers sawing or milling (with carbide- or diamond-tipped tools) laser cutting.