Indirect Manufacturing Thermoforming with MJP Parts

It is often advantageous to use rapid prototyping in the fabrication process of another part.  The 3D printed part is typically destroyed or discarded after being used and can make one or more of the final desired part.  This type of process is termed “indirect manufacturing” or “indirect prototyping” and the 3D printed parts are commonly referred to as “indirect tooling.”  Many traditional manufacturing processes can leverage rapid prototyping in this way, the most common uses being plastic injection molding, thermoforming, sheet metal forming, sand/investment/flask casting, indirect and direct printed silicone/urethane molding and wax molding for investment casting. 

Injection Molding

Thermoforming

Sheet Metal Forming

Investment Casting

Direct Printed Eggshell Mold

2-Half Direct Printed Mold

 

Direct Printed 3D MJP Molding Capability

Indirect Manufactured MJP Mold                                                          (Using MJP as a Master Pattern)

Wax Molding                                                                  (for casting)

                                                                             Rapid Tooling / Indirect Manufacturing Examples using MultiJet Printing Technology

 

Thermoforming is a manufacturing process that covers all processes which involve heat to shape polymers.  The process consists of heating a thermoplastic material to a pliable forming temperature, forming a specific shape in a mold, and trimming to create a usable product.  The sheet is heated with infra-red elements or with convective heating in an oven and the material is often stretched into or onto the mold.  Vacuum forming is a simplified version of thermoforming and is very common.  In this process, the sheet is stretched onto a single surface mold and forced against the mold by a vacuum.  Trapped air is evacuated with the assistance of a vacuum system.

Vacuum thermoforming

Top Left:  Vacuum forming machine clamped plastic sheet Top right:  Heated plastic forming around dies with vacuum pressure Bottom:  Four final formed parts after forming, cooling, and removal from molds

 

The same air system can be reversed after the part is cool to facilitate cooling and release of the part from the tool.  Draft angles and a release agent are both typically needed in the design of the mold to allow release of the plastic after forming.  Deeper parts can be formed if the sheet is mechanically or pneumatically stretched prior to bringing it into contact with the mold surface and applying the vacuum.  The “Draw Ratio” is a measure of the extent of sheet stretching; a measure of the area of the sheet after being formed to that before forming Drawing:

Draw Ratio = (Surface Area of the Part) / (Footprint of the part)

The most common material used is high impact polystyrene (HIPS).  Many other opaque and clear thermoplastic materials can be used including acrylic and polycarbonate, ABS, Polypropylene, PVC, etc. 

Thermoforming requires plastic in sheet form which is more expensive compared to the powder or pellets used in processes like plastic injection molding.  Also, recycling any trimmed area is typically not practical as part of the manufacturing process, as can be done with injection molding.  However, there are many other advantages which drive its widespread use in industry.  For example, fairly low forming pressures are needed which enables comparatively low cost tooling with lower complexity to be utilized.  Relatively large size molds can be economically fabricated which may otherwise be cost prohibitive with other processes.  Since the molds experience low forces, molds can be made of relatively inexpensive materials and mold fabrication time is reasonably short.  These attributes result in comparatively shorter lead times and also help make this process more compatible with rapid prototyping.  Finally, controlling to very tight functional engineering tolerances can be difficult with this method; however, often this shortcoming can be designed around.  Therefore, thermoforming is a very good solution for prototyping and low quantities of parts as well as medium/large size manufacturing runs (often utilizing multiple molds).  Thermoforming is very common in all product categories and is used extensively to manufacture a wide range of products the most common being food and product packaging. 

The processes of thermoforming typically involve seven steps. 

1. Clamping - The edges of the film are clamped to control the movement of the sheet and to make air tight seal
2. Heating - Heating is accomplished with Infra-red heaters (similar to oven elements) that are often mounted within an aluminum reflector plate.  Heat uniformity is important and is controlled by the machine design.  Sometimes this requires a series of controlled heat zones, but this is typically controlled by a simple timer.
3. Sheet Leveling - (controlled sag of the heated film) - The sheet material will sag under its own weight.  This must be controlled by optimizing the process time, visually, or with a laser system that can feedback pressure to limit the sag amount.
4. Pre-stretch (bubble formation) - After the plastic has reached its forming temperature, it can be pre-stretched.  Pre-stretch is important for deep parts with minimum draft and high mold surface detail.  This can be done visually or with a laser measurement and pneumatic feedback system.
5. Vacuum Forming (including plug assist) – Vacuum pressure is applied to draw the air trapped between the sheet and the mold.  Plug assist is a term used to describe the use of a male tool positioned over the forming area of the machine and is used to force the material into the female cavities.
6. Cooling and Release - The formed plastic must be allowed to cool on the mold before release.  If released too soon the part can be damaged as it is still at an elevated temperature and soft.  Adhesion of the sheet to the mold also typically decreases when the sheet is cooled.  High cooling cycles can use air or water mist.
7. Trimming - The clamping and forming process requires extra material.  The process is also not exact and so for these reasons the sheet must be oversized compared to the final part needed.  This extra material is cut with a metal stamping die and the extra material can collected be recycled (outside of the manufacturing process).    

 

 

(1)Sheet leveling sag during heating, (2) Pre-stretch, (3) Vacuum formed shape  (4/5) Part removal

 

Using Rapid Prototyping In the Design of a Machined Metal Tool

A thermoformed tool is created using Computer Aided Design (CAD).  The heating, stretching, and forming process of thermoforming using a sheet with constant thickness has led to number of design rules and best practices that are well documented.  However, there is no method to determine the exact final geometry or quality of the final thermoformed part based on 3D geometry.  In fact, thermoforming is commonly known as “dark art” for this reason and iterative testing is typically used in the tool design process.  However, a good designer can create a wide variety of functional geometries for different needs. 

Thermoformed part design from SolidWorks CAD

 

The alternative to a 3D printed part is a complex CNC machining operation, likely with numerous part and machine setups.  This requires an expert machinist and can take substantial time even after the job makes it out of the queue and to the shop floor.  This need for complex machining is particularly cumbersome with respect to the iterative design process required for a thermoforming tool.  Therefore, even if a machined steel tool will ultimately be needed, rapid prototyping can be used in the tool design process to quickly test and iterate on the final design.  Therefore, quick tool design is one key value that rapid prototyping brings to the thermoforming process.

 

Thermoforming Tooling with Rapid Prototyping In Design and Manufacturing

The time and cost in a traditional machined tool is relatively more expensive and time consuming when the designers only require a few parts to be made.   Therefore, using rapid prototyping for thermoforming avoids the main machining time shortcoming and allows designers and engineers to consider thermoforming for more things in their workflow like jigs and fixtures and low volume packaging, etc…  Also, many shops have a thermoforming machine as part of their overall capability, but that typically sits unused much of the time.  While thermoforming utilizing 3D printed rapid tooling is very powerful and commonly used in prototyping, it also has numerous advantages and can be cost effective even for the manufacture of a final part in low to medium volume.  Functional updates are simple and geometric complexity of the tool is easily created.  Rapid prototyped plastic dies are faster to generate and cheaper compared than traditionally manufactured dies made out of metals or plastics and yet they can be used to form tens to hundreds or even thousands of parts depending on the specific design.  The high cost of a metal machined tool allows numerous rapid prototyped tools to be 3D printed and used for the same total cost.  With the design freedom gained from rapid tooling a more complex shape can be used for functional purposes or aesthetics needs.  Finally, often a thermoforming operation requires the formed part to be marked with identifications such as time/date, batch, etc.  For this need, rapid prototyping can be used directly to make multiple unique dies, or the 3D printed part can be used as an insert in a standard machined metal tool.  For this use, the insert can be very competitive to print vs. using a traditional process as each individual insert is typically unique and that type of character-based marking can be complex to machine.  Also, inserts are smaller compared to the full tool size, and will only have to undergo fractions of cycles compared to the main tool usage.  Therefore, 3D-printed tooling has many advantages over traditional machining for many different possible uses and should be considered for all applications including aerospace, automotive, consumer products, electronics, energy, industrial and robotics among others.  Military uses are also possible as well as they will often produce one-off replacements for the repair of damaged vehicles and aircraft using metal forming processes.

 

Designing and Leveraging the Properties of MJP Technology in Thermoformed Tooling

There are numerous basic design principles that are documented and well-know which help direct tooling design to create highly functional and defect free products using thermoforming.  However, as previously discussed, the process of preheating the plastic sheet and the resulting sag with different materials and thicknesses make it difficult to predict design based on 3D CAD.  However, the use of MJP rapid tooling does not substantially change the complexity or basic design rules.  Therefore, any literature, experience or testing a person may have with thermoforming most likely transfers to 3D printed tooling. 

There are a number of important printer and material requirements that enable a safe and functional use of rapid tooling with the thermoforming process.  Of course, the printer needs to reproduce geometry sufficiently to create the basic features and the build size is important to allow the most ease-of-use and flexibility.  However, the three main capabilities of MJP technology that bring substantial value to thermoforming are

1. The materials’ ability to handle a high temperature.
2. The technologies smooth surface quality.
3. The ability to form extremely small features for things like text marking, complex surface texture and small vent holes or porous vent structures.

 

In terms of the high temperature requirements, the material itself is most important.  One of the great strengths of 3D Systems MultiJet Printing (MJP) technology is in its broad and individually optimized material set focused on different customer applications.  For example, all the Rigid and Engineering materials can be used for drilling/taping/machining/pressing and achieve the good surface finish, sharp corners, fine features, and create high fidelity true-to-CAD parts that is well known for MJP technology.   The rigid white, clear, and gray materials (M2R-CL/WT/GRY) are optimized for general-purpose properties.  They are fairly rigid with good heat deflection temperatures, and yet maintain reasonable elongation.  They are good for concept modeling and light end-use prototype part needs.  The engineering materials M2G-CL (Armor) and M2G-DUR (ProFlex) were designed for the most demanding applications as they can be substantially twisted, flexed and deformed without cracking or breaking.  They are excellent for things like prototyping snap fits and for room temperature forming like sheet metal forming.   However, the specialty material M2S-HT90 was designed and optimized for high temperature applications and is the material of choice for thermoforming.  The operational melting temperatures of the thermoforming plastics are the range of 140 to 200C.

Material	Glass Transition Temperature (tg)	Rec. Mould Temperature	Rec.Forming  Temperature	Drying Temperature
PS	94°C	82°C	150-175°C	70°C
ABS	88-120°C	82°C	150-180°C	70/80°C
PP	5°C	90°C	150-180°C	65°C
Acrylic / PVC	105°C	N/A	140-190°C	75°C
PC	150°C	127°C	170-205°C	90°C

Typical thermoforming plastics with respective forming temperatures

 

M2S-HT90’s heat deflection temperature of 90C provides very good resistance to these hot plastic contact temperatures.  This high heat deflection temperature also enables rapid cycle times without overheating and protects small features from distortion during the forming process.  In addition, M2S-HT90 also has an elongation before break of 6-7%.  This elastic property of the material allows sufficient flexibility that is required to remove numerous final formed parts without damaging the tool or destroying small features.  This level of elongation is also synergistic with some tooling design requirements like assembling multi-cavity thermoforming tools using standard screw hardware (Refer to Example 3 for more details).  While the M2S-HT90 material address the elevated temperature and elongation requirements, the MJP technology itself address the remaining surface, feature, and fidelity needs. 

3D Systems MultiJet Printing (MJP) uses a high-resolution inkjet technology combined with a melt-away support material system.  The MJP printing process generates a part with over 1.1 billion drops per cubic inch of part volume allowing the creation of even the smallest details.  The support wax technology instantly solidifies in place during part formation and greatly contributes to sharp features and smooth surfaces.  Thermoformed plastics get very thin due to the sag and the thin plastic is able to reproduce even the smallest of surface imperfections.  MJP technology creates surfaces that are smooth enough that it does not impact the surface shape of the film whatsoever.   Also, because of its inherent smooth surface quality and its high fidelity capability, MJP is able to create very small and highly detailed surface textures which are often a desirable attribute of a thermoforming tool.

Left:  M2R-GRY part showing numerous detailed surface textures Right:  HIPS vacuum formed part showing surface textures

 

Finally, porous structures or small hole features are often synergistic with the air venting needed by a vacuum forming tool. This is particularly true for relatively deep cavities and for the formation of very fine features like in a graphic for text marking at the bottom of a cavity. It can also be advantageous to make a porous lattice structure that is smaller than the feature size to allow uniform flow of air for a more uniform feature formation.

Left: Extremely small holes used to vent air in lettering Right:  MJP shells and infill capability making a lattice venting pattern

 

The wax support also allows for simple and hands-free post processing removal of support from even the most complex geometry and entrapped cavity without scarring of the part surfaces.  There is nothing that typically needs to be done to accurately match your part design dimensions with an MJP printer.  For most geometries, the part should come out true-to-CAD the very first time printed and every time printed.  If needed, the printer was designed with a scale factors for each material that is built into the 3D Sprint software.  Also, part accuracy is independent of orientation for even the smallest and sharpest of features so you don’t have to worry about numerous special part design rules, part orientation and placement issues or special printer setups.  The high-resolution capability of MJP also allows for things like the formation of printed threads for assembly and fixture needs.  A few thermoforming examples will be presented next to facilitate such knowledge; however, ultimately each design is unique and one must iteratively test to determine the appropriate load/pressure requirements for their specific needs (material, thickness, and geometry).

 

Example 1:  Vacuum Formed Multi Cavity Part

A vacuum forming tool was designed and 3D printed with MJP 2500 acrylate in M2S-HT90 material as an example to help facilitate design rules, special capability, and best practices.The design intent was to create the packaging to hold five small parts: 3x 3D printed plastic parts, 1x metal dowel pin, and 1x screw.The part was created in SolidWorks CAD and measures 67mm (2.6”) on the sides and 11mm (0.43”) thick.6-10 deg drafts were used on all the feature walls including the outside edge of the part and rounds were used on all top and bottom corner cavities to facilitate part release.

Sheet metal part to be formed with 3D printed MJP rapid tool.

 

Numerous 1mm holes were used in the two top round features and the bottom larger square feature to form the bulk cavities. A positive logo feature was placed on the bottom of the larger 30mm (1.18”) diameter feature.  Holes were cut around the logo (0.5mm, 0.197”) to allow air passage during the forming process. The 0.7mm (0.028”) logo thickness and surrounding vent hole pattern were determined through iteration.

Round Vacuum Forming Tooling Feature with Drafts, Rounds, and Venting

 

Iterative process used in sizing and placing air venting holes

 

Company name and product identification markings were added to the vacuum formed part. The text measured about 6.3mm (0.25”) in height and the cuts were 1mm (0.04”) deep. Two different venting schemes were used to create the vacuum formed text (CAD and 3D Sprint tips/workflows are shown later as part of Example #3). In the first scheme 0.4mm (0.016”) holes were cut through the part at locations determined iteratively.

Small holes printed to achieve air venting

 

In the second scheme, a shells and infill lattice pattern was used behind the text for air venting. 

Shells and Infill pattern created behind 3D SYSTEMS text to create uniform air flow

The features for the dowel pin and machine screw as well as the 0.6mm (0.024”) vent size and hole pattern were iteratively determined.  Drafts and rounds were again used to assist in part removal.

Feature and hole size/pattern for dowel pin and screw features

Curved air venting cuts were added to the sides of the smaller square features that held the dowel pin and the screw.  This was done to show the melt away support capabilities of the MJP technology and could be useful for improved feature formation, adding shallow sidewall marking features, or for part release purposes in a system where the air is reversed. 

Left:  CAD geometry for straight holes and curved sidewall air flow Right:  Actual features shown in the 3D printed part

A 1mm (0.04”) relief was cut into the entire bottom of the part to allow for air venting robustness to all the formed features (i.e., so none of the vent holes could be obstructed in the machine). Threaded features were added at the corners and in the center of the part to allow attachment of numerous forming dies to a backup alignment plate that allowed four of the parts to be manufactured at once.

1mm relief along the part edge and threaded features for screw mounting

 

The following are pictures of the front and back of the final design and a highlight of the key design elements.

Left:  Top of mold die, Right:  Bottom of mold die

 

MJP Thermoforming Tool: 1. Smooth surface, 2. Small/sharp feature detail in 3DS Logo, 3. Small air vent holes around 3DS logo, 4. Porous lattice air vent structure,  5. Printed threads for multi-tool assembly, 6. Angled internal pipe structure for side wall air flow

 

Flatness of final printed part

 

Final MJP Rapid Tool and final thermoformed part

 

Example 2:  Complex Formed Feature Testing

            In conjunction with the testing above, numerous other different diagnostic parts were created and tested with different forming properties (feature depth, feature size, venting hole size, etc…).  The idea was to stress the material and look for useful trends and capabilities.  The following is a list of key findings. 

1. MJP technology prints highly true-to-CAD and can make extremely small holes. Holes 0.5mm (0.02”) are easily possible. Smaller holes sizes are can be created but can be more difficult to post process. 

Left:  Hole test diagnostic with variable heights Right:  MJP is able to print deep holes down 0.2mm (0.0075”), but deep and small holes are more difficult to post process

 

1. Even the smallest 0.5mm (0.02”) holes would transfer a mark onto standard thickness film.

 

2. The maximum recommended draw ratio is 0.75. Any draw ratio that’s bigger than 0.75 will be either be drastically thin or rupture the plastic sheet.   

 

3. High quality vacuum form machines have photoelectric sensors that senses if a plastic sheet is sagging too much and changes the pressure of the chambers accordingly to keep the plastic sheet from sagging. Cheaper and simpler vacuum form machines rely on a technician and sag level to create an optimal part. 

 

4. Higher quality vacuum forming machines are equipped with a pre-stretch feature where plastic sheets are stretched via positive pressure (sheet is blown up) after they are heated to proper forming temperature to ensure uniform part thickness and feature reproduction.

 

5. Webbing is a major issue in vacuum forming as it can arise from sharp features that are 90° or less.  The more the plastic sheet sags before forming, the more likely webbing will occur on more obtuse angled features.

 

6. While HT90 material is optimal for thermoforming, all Rigid, enGineering and Specialty MJP materials can be used for thermoforming, especially for single trials or very low cycle rates which allow for sufficient cooling.

 

Example 3:  SolidWorks and 3D Sprint Workflow and 4-Part Tooling Plate Design

Most the parts and features of the thermoform design from Example 1 were simply created, feature-by-feature in SolidWorks.  Basic initial dimensions were taken from the individual parts to be packaged.  Final feature sizes and the performance of the vacuum forming tool was optimized through iterative testing of numerous 3D printed parts.  However, the use of 3D Sprint slicing software to leverage the Shells and Infill capability to create lattice venting patterns will be expanded on for clarity. 

            In order to define different areas of a part with unique properties in 3D Sprint the parts need to be separate bodies in the CAD design.  For example, to create the shells and infill lattice pattern behind the “3D SYSTEMS” text in Example 1, a second part was created and assembled behind the text.  This allowed that part to be selected and defined separately in the 3D Sprint software with a lattice pattern.

A part needs to be created as separate body in CAD in order for 3D Sprint to define its Shells and Infill properties

 

A slightly more difficult but more robust design that creates a stronger link between the lattice pattern and the thermoforming text features is also possible.  The process is similar, but the backup part is made to have an inverse of the die features.  In this way, the protruded text of the backup part can be inserted into the cut text of the main tooling die.  This allows the lattice pattern to connect to the text features on the sides as well as on the bottom.  This type of design dramatically increases the cycle robustness of the features.

Use the cavity tool or a Boolean subtraction to form the functional features in the top and bottom mold halves

 

Two easy methods of creating this backup part is to use a Boolean subtraction or the cavity tool in assembly mode.  Begin by creating a simple square backup part, but one that is thick enough for both the backup lattice and the text thickness.  This part is assembled to the tooling die such that it overlaps the main part in the SolidWorks assembly.  Finally, use the cavity tool or a Boolean operation to cut the geometry of the backup part using the geometry form the main tooling die.   Finally, a third method to create a separate lattice part is to just create it feature-by-feature in the part file, but with the “merge part” checkbox unchecked.  No assembly file is required.  When saved as an .stl file in SolidWorks all three of these methods will import the backup part as individual separate parts into 3D Sprint.  There are many tutorials available for these types of feature creation in SolidWorks.  Other methods involving offsetting surfaces are also possible for those more skilled in the art.  Once the parts are imported into 3D Sprint, simply use the Shells and Infill functionality and define the backup part to be a lattice.

Left:  Infill lattice selection in 3D Sprint Right:  Infill lattice cell size 0.6mm, thickness 0.2mm

The lattice pattern shown here used the Jacks pattern with a 0.6mm cell size and a 0.2 mm thickness (rod size).  Air can be easily carried through this type of lattice even when multiple mm thick.  Anything thinner than about 2 mm can lack sufficient strength and is not recommended.

The goal was to make hundreds of parts with this tool.  Therefore, a second part was designed (with four mounting locations for four of the vacuum forming dies) and printed with the M2S-HT90 material in a MJP ProJet 2500 printer.

Left:  Tooling plate to hold four molds Right:  Tooling plate with four molds attached (all printed with MJP M2S-HT90)

This entire part was created in a single part file in SolidWorks.  Cuts in the main part followed by filling with protrusions using the “merged part” unchecked created different areas that could be individually set with different lattice structures in 3D Sprint.

Left:  Different sections created in a single part file for lattice venting using 3D Sprint Right:  Actual MJP part printed in M2S-HT90 material with Shells and Infill lattice structures

 

Thru holes with chamfers were used to allow assembly at the corners and center.  Large lattice patterns were used to create the air venting as a fine mesh was not required and would be less robust.  The center cyan colored area venting was used directly under the part to allow air flow to all the individual cavities and text features.  A second venting (shown in yellow above) was used around the edges of the die to pull the heated plastic sheet down against the tool.  A flat section (shown in magenta above) created a flat area directly outside the forming die where the individual parts could be cut to create the final part.  Finally, two different methods were tested and worked similarly to reduce the cost of this part and improve the flatness.  In the first method, shells and infill was used with a 1mm shell thickness and an internal lattice pattern and drain holes for the support material.  In the second method, a shell feature in CAD was used on the main body of the part to hollow out the part.  Both of these operations reduced the amount of build material substantially without causing other issues.  These type of features are often synergetic with better flatness as well.

Left:  Shells and Infill with drain holes for part cost reduction Right:  Shell feature added in CAD for part cost reduction

The following is a picture of the positioned in the vacuum forming machine.  Approximately 400 parts were made with this tool without any noticeable degradation in formed part quality.  This equates to about 100 formed parts per each die.

Complete fixture with 4x of the MJP 3D printed mold attached to the 3D printed backup plate that allows 4 parts to be made in a single step

 

400 Parts were generated without noticeable degradation, 100 from each 3D Printed MJP Tool.

 

Tool showing no tooling degradation after making approximately 100 parts in each die