Processing on injection molding machines

Frequently Asked Questions

About PLEXIGLAS® Molding Compounds
How can demolding cracks be avoided?
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® molding compound with higher impact strength can be used instead of standard molding compound. 

How can mold filling be improved?

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.

Does PLEXIGLAS® molding compound require drying prior to processing?
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. 

Are special injection-molding machines (screws) required for processing PLEXIGLAS® molding compound?
PLEXIGLAS® can be processed on injection-molding machines with 3-zone generel purpose screws for thermoplastics. 

At which temperatures can PLEXIGLAS® molding compounds be predried?
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. 

Which screw speeds are specified for processing PLEXIGLAS® molding compound?
Plasticization should be performed as gently as possible (at low speed). The usual peripheral speed for processing PLEXIGLAS® is 0.3 m/s. 

How can bubbles be avoided inside a thick-walled molding?
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

How can a finely grooved surface (orange-peel effect) be avoided on moldings?
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

How can a chipped surface and striation be avoided in moldings?
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

How can sink marks be avoided on moldings?
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

How can dull spots be avoided behind sharp edges or in the gate area?
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

How can the sprue breakage be avoided when the mold is opened?
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

How can air bubbles be prevented from becoming entrapped at the edges of a molding?
Possible cause:  
Unfavorable venting of mold
Remedies:
- Reduce injection speed
- Reduce cylinder temperature
- Install vent
- Change wall thickness in mold
- Optimize predrying

How can warpage of the moldings after ejection be prevented?
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

How can deformation of the moldings after ejection be prevented?
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

How can weld lines be prevented behind inserted recasses and letters?
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

How can creases be prevented in thick-walled moldings?
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

How can moldings be prevented from breaking apart on ejection?
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. 

How can streaking, haze and other irregularities be prevented inside the molding?
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

How can PLEXIGLAS® molding compound be prevented from drooling at the nozzle?
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

What can cause milky streaks?
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

How can brown or black particles be prevented in moldings?
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

How can black streaks and dark clouding and / or milky bubbles be avoided?
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

How can burns be prevented in moldings?
Possible cause:  
Unfavorable mold venting
Remedies:
- Reduce injection speed
- Lower cylinder temperature
- Install venting channel
- Change wall thicknesses of mold
- Optimize predrying

Which driers are suitable for drying PLEXIGLAS® molding compounds?
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. 

What wall thicknesses are possible for moldings made of PLEXIGLAS®?
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. 

How can shrinkage during processing be influenced?
The main influencing factors are:  
- melt temperature,
- mold temperature,
- holding pressure,
- hold time,
- injection speed.

What is an economical way of separating the sprue?
The sprue can best be separated by tweaking it off with heated pliers sawing or milling (with carbide- or diamond-tipped tools) laser cutting.

Quicklinks

back to FAQs
Disclaimer
This information and all further technical advice is based on our present knowledge and experience. However, it implies no liability or other legal responsibility on our part, including with regard to existing third party intellectual property rights, especially patent rights. In particular, no warranty, whether express or implied, or guarantee of product properties in the legal sense is intended or implied. We reserve the right to make any changes according to technological progress or further developments. The customer is not released from the obligation to conduct careful inspection and testing of incoming goods. Performance of the product described herein should be verified by testing, which should be carried out only by qualified experts in the sole responsibility of a customer. Reference to trade names used by other companies is neither a recommendation, nor does it imply that similar products could not be used.
The Business Unit Performance Polymers of Evonik is a worldwide manufacturer of PMMA molding compounds sold under the PLEXIGLAS® trademark on the European, Asian, African and Australian Continent and under the trademark ACRYLITE® in the Americas.