Material Properties Accuracy and Nonstandard Mechanical Properties for Elastomeric Materials

Material Properties Accuracy and Nonstandard Mechanical Properties for Elastomeric Materials

Best Practice

p/n 33-D274 Rev A

 

One of many great strengths of 3D Systems MultiJet Printing (MJP) technology is in its hands-free post processing capability.  In this technology, the supports are automatically created regardless of part geometry and/or complexity and are easily removed with heat free from any surface defects from even the smallest of features and/or entrapped cavities.  Also, the support wax solidifies quickly as the part is created and this enables extremely high-quality, high-resolution, and true-to-CAD capability as well as very good surface quality.  This capability helps to enable world-class feature fidelity for both general part needs such as rapid prototyping plastic injection molded designs and jigs and fixtures as well as enabling complex applications like flow visualization, medical modeling, RTV molding, injection molding, and even complex bioreactors, etc. 

The EasyClean system was specially designed to offer convenient, fast, and automatic turnkey removal of support material from plastic and elastomeric parts produced on ProJet MJP 2500 3D printers.

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MJP EasyClean System with fast “Steam Mode” and robust “Oven Mode” Capability

The EasyClean system utilizes two warmer units (bulk wax removal and fine wax removal) that use heat or steam to melt wax supports away, without manual labor.  It is intended to operate in a typical office environment, resting on a table, without objectionable noise or odor, and without requiring special ventilation or equipment, only requiring access to a sink.  The system can hold a part as large as the full build size and it can be left active throughout the day for simple and high-volume post processing of parts.  It has a “Steam Mode” that uses condensed steam to quickly remove the support material.  It also has an “Oven Mode” that provides the best mechanical properties for Engineering and Elastomeric materials.

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Steam Mode Lids and Settings

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Oven Mode Lids and Settings

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The EasyClean system has two warmer units and is able to both remove the bulk support wax and fine wax from the part.

It is also standard practice to use a normal convection oven and/or an ultrasonic bath for high-volume, batch processing of parts or to process very large parts. 

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MJP oven finisher and heated ultrasonic bath used in standard MJP post processing for large parts and/or high-volume, batch processing

 

 

There are two key reference documents for MJP post processing. 

  1. The “MJP EasyClean Hardware Setup & Setting Guide” is guide is typically used just once during the unboxing and set up an EasyClean system. 
  2. The “Post Processing Guide” explains the basic process to process a part including detailed instructions related to the EasyClean system and all other general best practices.

These standard setup and post processing papers can be found on the 3D Systems Infocenter.  The processing recommendations for different MJP printers and material families are summarized in this chart. 

image 8.pngThe goal of this paper is to provide more detailed information and examples on best practices for the processing of parts for each different MJP material family to achieve the highest material properties accuracy with respect to the values documented by 3D Systems. Additionally, there are some special processes that can be used to create nonstandard mechanical properties. Finally, there are some special use-cases that can be value-added to the customer (reducing warp, removing yellowing discoloration, and processing parts with entrapped cavities and/or shells and lattice infill). The information in this document are for “super users” and will not be needed by most customers. However, it could be of value for certain needs and/or specialty applications.

Post Process Overview

As represented by the chart above, there are four main steps to post processing.  These are well documented in the 3D Systems Post Processing Guide.  The steps will be reviewed and then each material family will be discussed focusing on special processes required for each material family at each step.

  1. Remove the part(s) from the build plate.  This is done with a freezer or a heat gun / hot plate.  Removal with a freezer can take as little as a few minutes or the parts can be left in the freezer for hrs/days with no negative impact to the mechanical properties.  An automotive or industrial “heat gun” or consumer “hair dryer” or a “hot plate” can be used to heat the build plate holding the parts.  It takes a few minutes.  The bottom layer of support wax will melt first and allow the parts to be removed from the plate.
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Left:  Part Removal Freezer Process, Right:  Part Removal Heat Gun Process

 

  1. Bulk wax removal.  This process removes most of the support material from the part(s) and is done with an “Oven” or a “Steam” process.  After this step, only a thin film of support wax is left on the surface of the parts.  The recommended oven temperature is 65C and Steam set point is level 9 on the EasyClean System.  For either process, the part must be held at temperature for a given amount of time (larger for larger parts or entrapped cavities).  To achieve the highest materials properties accuracy, each material family has an optimal window of processing time and is shown below for each material family.
  2. Fine wax removal.  This process removes the final thin film of support wax from the part.  The parts are placed into a solvent and the support is dissolved off the part.  Either standard, low-viscosity Mineral Oil or 3D Systems EZ Rinse-C is used.  Mineral oil can be used with all materials.  EZ Rinse-C can only be used with Rigid and Specialty materials.
  3. Soapy Wash and Rinse.  Warm soap followed by warm water rinse is used to finish the parts.  The parts should never be moved from hot to cold baths.  Hot parts should not be placed on cold surfaces.  Keep all the temperatures approximately equal to the extent possible.

 

Rigid Materials:  M2R-CL, M2R-WT, M2R-GRY, M2R-BK, M2R-TN

The Rigid materials follow the standard process as outlined in the Post Processing Guide.

  1. Part Removal from Plate:  Freezer or heat gun / hot plate.
  2. Bulk Wax Removal:  Both the steam and oven process work well with all the rigid materials over an extended range of processing times.  The only special recommendation for bulk wax removal is to avoid keeping thin parts in the steamer for more than about 4 hrs.  Thicker parts are more robust to extended processing times in the steam.  Since the steam mode is very quick, any part from the printer should be able to be processed in <4hrs - so there is never a reason to keep parts in the steam for longer.  Rigid materials can stay for an extended time in an oven.  It is even possible for Rigid materials to be processed overnight in the oven.  This can be an efficient processing time saver.   

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  1. Fine Wax Removal:  Mineral Oil or EZ Rinse-C.
  2. Soapy Wash and Rinse:  Warm soapy wash and warm rinse.  The use of extreme temperature differences (hot fine wax removal to cold soapy wash, or hot soapy wash to cold rinse) can cause part cracking.  Keep all the bath temperatures approximately the same during processing.

 

Specialty Materials:  M2S-HT90, M2S-HT250

  1. Part Removal from Plate*: M2S-HT250 parts can crack during the post processing if the parts are removed from the build plate with a freezer. 
  • M2S-HT90:  Freezer or heat gun / hot plate
  • M2S-HT250:  Heat Gun only to avoid cracking
  1. Bulk Wax Removal*:  Both the steam and oven process work well for M2S-HT90.   Avoid leaving thin features in the steam process for more than about 4 hrs.  The steam process can cause cracking for M2S-HT250.  To reduce the chance of cracking, only use the oven process with M2S-HT250.  Also, always keep M2S-HT250 parts in the oven for at least 8 hrs.  A full 8 hr soak in the oven enables the M2S-HT250 material to achieve the full Heat Deflection Temperature of >250C.    

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  1. Fine Wax Removal*:  Mineral Oil or EZ Rinse-C.
  2. Soapy Wash and Rinse*:  Warm soapy wash and warm rinse.  The use of extreme temperature differences (hot fine wax removal to cold soapy wash, or hot soapy wash to cold rinse) can cause part cracking.  Keep all the bath temperatures approximately the same during processing.  Specialty materials are particularly sensitive to cracking if exposed to bath temperature differences.

* M2S-HT250 parts can be easily broken during post processing.  The material has a high heat resistance, but is more brittle than other MJP materials.  Careful handling and precaution can improve the success in post processing.   This is true for M2S-HT90 as well, but to a lesser extent.

 

Engineering Materials:  M2G-CL, M2G-DUR

  1. Part Removal from Plate:  Freezer or heat gun / hot plate.
  2. Bulk Wax Removal**:  The mechanical properties of engineering materials M2G-CL (Armor) and M2G-DUR (ProFlex) are more sensitive to both high temperature and to steam.  Therefore, the steam process is not recommended.  To achieve mechanical properties as published in the 3D Systems technical specifications for engineering materials, the standard oven process is the best recommendation.  Even when using the oven process, very thin features may experience mechanical property changes if held for over 4 hrs. 

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  1. Fine Wax Removal:  The engineering materials are also very sensitive to exposure to the EZ Rinse-C fine wax removal solvent.  The materials will soften substantially even for very short exposure time to EZ Rinse-C (<5 min).  The material properties of the engineering family are robust to extended contact with Minerial oil.  Using mineral oil is highly recommended for M2G-DUR and M2G-CL.  
  2. Soapy Wash and Rinse:  Warm soapy wash and warm rinse.  The use of extreme temperature differences (hot fine wax removal to cold soapy wash, or hot soapy wash to cold rinse) can cause part cracking.  Keep all the bath temperatures approximately the same during processing.

** Under some circumstances, it is possible for a user to use the EasyClean Steam Mode for engineering materials depending on their part geometry and requirements.   It highly recommended that the user try this process for their specific application and part geometry before purchasing the equipment or making a permanent change.  It is a faster process and does not affect the properties as much if done quickly (<30min).  This can work well for many types of general concept model and aesthetic parts of medium to smaller size, but without any small/thin parts or features.  EZ Rinse-C is never recommended for Engineering materials unless the goal is to create nonstandard mechanical properties for some special customer application.

 

Elastomeric Materials:  M2-ENT, M2-ENT, M2-BK70

  1. Part Removal from Plate:  Freezer or Heat Gun / hot plate.  Most all elastomeric parts will work fine processed in the freezer.  However, some very thick parts, or parts with thin features extended out from a thick area that are connected to the build plate may experience cracking when using the freezer.  If cracking appears in an elastomeric part, the user should avoid using the freezer with that part and use the heat gun or hot plate process instead.
  2. Bulk Wax Removal:  The mechanical properties of elastomeric materials M2-EBK, M2-
    ENT and M2E-BK70 are more sensitive to both high temperature and to steam.  Therefore, the steam process is not recommended.  To achieve mechanical properties as published in the 3D Systems technical specifications for elastomeric materials, the standard oven process is the best recommendation.   Even when using the oven process, very thin features may experience mechanical property changes if held for over 4 hr.  Also, for geometries with small features that extend out from the part, it may be necessary to hand remove the support wax part wax as it melts to avoid the weight of the support from damaging the part. 
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  1. Fine Wax Removal:  The elastomeric materials are also very sensitive to exposure to the EZ Rinse-C fine wax removal solvent.  The materials will soften substantially even for very short exposure time to EZ Rinse-C (<5 min).  The material properties of the elastomeric family are robust to extended contact with Mineral oil.  Using mineral oil is highly recommended for M2-ENT, M2-EBK and M2E-BK70.  
  2. Soapy Wash and Rinse:  Warm soapy wash and warm rinse.  The use of extreme temperature differences (hot fine wax removal to cold soapy wash, or hot soapy wash to cold rinse) can cause part cracking.  Keep all the bath temperatures approximately the same during processing.

 

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Summary of post processing recommendations for all materials

 

Post Processing Special Conditions

Problems with Warp:

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(top) Thin wall of part is warped compared to (bottom) true-to-CAD

 

  • For Elastomeric and Engineering materials, always use the EasyClean system in the “Oven Mode” or use a convection oven.  Also, use mineral oil for the fine wax removal.  Use of EZ Rinse-C can cause a lot of distortion, especially in parts with thin walls.
  • During the bulk wax removal, DO NOT let the part set in the melted support wax.  Also, DO NOT let the melted support wax pool in cavities of the part.  Some part geometries naturally shed all the support wax, others may need to be rotated after the support wax melts to drain out the support.
  • Use the minimum time necessary in the steamer/oven (bulk wax removal) and mineral oil (fine wax removal).
  • Avoid uneven heating.  Applying heat and cooling slowly allows for the part to come to temperature evenly.
    1. When putting parts in the oven, don’t let the part touch bare metal.  Metal transfers heat into the part more quickly and can cause the area touching the heat to expand more quickly than the surrounding part, causing upward warp.
    •  Use a convection oven, or the EasyClean System in “Oven Mode” and keep each stage of cleaning as close to 65°C as possible.
    • The parts being cleaned should go directly from 65°C oven to 65°C oil to 65°C water to a slow cooling environment.  If the part cycles from hot to cold to hot multiple times, the warp will be more pronounced and unpredictable.
    • For parts with very thin features, allow them to cool slowly.
    • Consider printing in M2S-HT90 as this material is stiffer at elevated temperatures

 

Particularly for parts with very long and thin walls, it is good practice to cover the parts with some insulation to stop convective cooling from pulling away heat unevenly. This can be as simple as using paper towels to wrap the part up while it cools.  The most difficult parts can be thin and tall features that are connected to a relatively thicker piece.  Slow cooling is key to reducing warp for these type of parts. 

EASY FIX:  If a part is deformed after post processing, the best practices is for parts to be placed back into a 65°C oven for ~5-10 minutes while resting on a flat surface and supporting long overhangs.  The printed parts retain memory and will naturally return to their as-printed geometry when reheated.  Additionally, for thin features one should cover the part after removal from the oven, or simply turn the oven off and let the part cool slowly as the oven cools.

 

Batch Processing:  The MJP melt away support offers the customer the ability to batch process high volumes of parts with relatively little effort.  A large convection oven can be used for the bulk wax removal.  Place the parts on a grill and let the wax drip into a pan.  Do not let the parts set in the support wax and do not let the support wax pool in the parts.  Most parts are able to drain all their wax without user intervention if placed in the correct orientation with respect to gravity.  One or more large ultrasonic baths can be used for the fine wax removal.  Avoid stacking parts in a pile that may be damaged.  A large plastic tub can be used for the wash and rinse and requires some user interaction.

 

Eliminating Part Yellowing:  Parts can be placed into an oven at 65-70C overnight to bleach out yellow areas.  If the parts are left in the oven for extended periods of time, they will then turn even more yellow. 

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Left:  MJP Part with slight yellowing after normal post processing

Right:  MJP Part with reduced yellowing after a 4hr dwell in an oven

 

Nonstandard Elastomeric Material Properties for Medical Modeling:  For medical modeling applications and possibly other needs, the customer requires a slightly softer elastomer material with better rebound.   This can be achieved for the M2-ENT and M2-EBK materials by using the EZ Rinse-C fluid in the fine wax removal step.  The parts will come out substantially softer and will “rebound back” when compressed.  The materials will also experience a reduction in tear strength. 

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Left:  Standard post processing with mineral oil

Right:  Post processing with EZ Rinse-C provides a softer and more flexible part with better rebound

 

Overnight processing: The Rigid and Specialty materials can be left in the oven at 65°C overnight for post processing.  This can be an efficient way to process any part, especially for large parts with a lot of support.  The parts can be put into the bulk wax removal step in the EasyClean or a convective oven before the user goes home at night.  They will then be ready for fine wax removal the next morning.  This is only recommended for the oven process, and not the steam process.

Small internal passageways:  The best method of processing parts with small and/or long internal passageways is to leave the parts in the mineral oil bath overnight.  This can be done with all families of materials without changing their material properties. 

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ProJet M2R-CL rigid clear material part with complex internal passageways

 

After the overnight soak, the mineral oil needs to be removed from the internal cavities with gravity, air pressure or water pressure.  For extremely complex geometry, this process might need to be done more than once.  Also, make sure that the mineral oil bath is clean.  If there is substantial support wax dissolved in the mineral oil it will become solid upon cooling and will be stuck within the passageways of the part.

Shells and Infill:  Shells and lattice infill is a feature of 3D Sprint that allows parts to be shelled out and filled with a lattice structure.  This makes for a less expensive part and a part with high strength-to-weight.  In terms of post processing it is similar to internal passageways because the support needs to be removed from within complex features of the part.  For these parts, remove as much of the support wax first.  Rotate the part at different angles to help drain the wax.  Always cut your drain holes with this in mind.  As a final step, the shells and infill part can be submerged in the mineral oil bath overnight.  This will remove any remaining support wax.

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Left:  Part cut away showing internal 3D Sprint feature “Shell and Lattice Infill”

Right:  Shell and Lattice Infill with drain holes visible