3D printing molds for injection molding

(image courtesy of Formlabs)

Materials

A 3D printing material is suitable for creating injection mold if it has:

• High temperature resistance - A high heat deflection temperature is required to withstand the mechanical and thermal loads applied to the mold during material injection. Note though that the temperature decreases rapidly during solidification.
  
• High stiffness/toughness - Repetitively removing parts can cause wear to the mold, so materials with high stiffness are required to maintain mold accuracy over time.
  
• High level of detail - One of the main requirements of an injection mold is high dimensional accuracy and a smooth surface. Highly accurate molds will produce highly accurate parts.

The 3D printing technologies that cover best these requirements are SLA and Material Jetting. These technologies can produce parts with high dimensional accuracy and are ideal for prints intricate details and very fine features. Speciality materials that are available in these technologies, like Formlabs High Temperature resin or Stratasys Digital ABS, are ideal for molding and tooling applications. An outline of the properties that are most relevant to injection molding for these two materials is shown below.

For industrial applications, desktop SLA is not suitable. An in-depth article comparing the two most commonly used industrial 3D printing materials for mold manufacturing (Digtal ABS and Somos PerFORM) can be found here.

Property	Formlabs High Temp resin*	Stratasys Digital ABS**
Heat deflection temperature	289 ℃ @ 0.45 MPa	92 - 95 ℃ @ 0.45 MPa
Flexural modulus	3. 3 GPa	1.7 - 2.2 GPa
Impact strength (Notched IZOD)	14 J/m	65 - 80 J/m
Lowest layer height (resolution)	25 - 50 microns	16 - 30 microns
Minimum detail size	0.2 mm	0.2 mm

Mold Design

Describing specific technical design of mold features (such as gates, runners, air vents etc) is out of the scope of this article. An internet search will reveal a large amount of information on the subject. This post by Seattle Robotics is a good starting point for those new to injection mold design.

Here is a list of some good practices that should be followed when designing a 3D printed mold:

• When designing the mold for SLA printing, ensure that the inner faces of the mold are orientated so that no support is in contact with them. This will improve their surface quallity, as no support marks will present on these surfaces, minimizing the required post processing.
  
• Including shallow air vents (0.05 mm deep) from the edge of the cavity to the edge of the mold will help expel trapped air during the molding process.
  
• If the 3D printed mold is to be used for more than 20 runs, consider including channels in the design for embedding metal rods or tubes. These can help reinforce the mold, reduce warpage and improve cooling times.
  
• 3D printing the mold at a lower layer height can help produce smoother molded parts as the molds will have less visible print lines.
  
• Embossed and engraved details should be offset from the surface by at least 1 mm.

Specific restrictions on design will depend upon the injection molding machine that is used. However Stratasys suggest that molds made via their Material Jetting printers should be used to produce parts with maximum volume of 165 cm³ in 50 to 80 ton molding machines or manual hand presses.

Designing parts for injection molding

As with conventional injection molds, a designer should consider:

• Adding a draft angle of minimum 2^o degrees to aid in the removal of the part from the mold.
  
• Maintaining a uniform wall thickness across the entire part.
  
• Keeping all walls and features as thin as possible.
  
• Including radii on all edges and corners.
  
• Including thin ribs and gussets to add strength to a part rather than increasing wall thickness.

Learn more about the importance of draft angles in this article →

Reducing Flash

Flash is the name given to the material that comes out between the two halves of the mold during the injection process. This generally occurs when the two mold halves do not mate perfectly together, are not perfectly flush and flat or the mold is overfilled. Runners are used in the mold design to help reduce the likelihood of flash.

If designing for an aluminium frame, add 0.125mm of extra thickness to the back of the mold plates to account for the compression forces and to ensure a complete seal. Increasing the clamping force in the vise can also help mitigate flash, as can polishing the molds' split plane to give it as flat a surface as possible.

(image courtesy of Formlabs)

Release compound

Due to the relatively fragile nature of the materials used in 3D printed injection molds compared to traditional molds, applying excessive force to remove a part from the mold can lead to rapid mold deterioration. Including a release compound on the surfaces of the mold cavity before the injection stage can assist greatly with part removal.

Case study: a plastic motor fitting

This case study will compare manufacturing a custom plastic fitting for a motor housing. The requirements of the design are:

• The total number parts is 25.
  
• A high level of dimensional accuracy in needed to ensure a tight fit.
  
• The weight of the part is 0.005 kg (5 grams).
  
• The part must be made from ABS.
  
• The part must be black in colour.
  
• The overall diameter of the part is 40 mm.

Here are the available alternative manufacturing options:

Industrial FDM ABS 3D printing: Industrial FDM 3D printing has high repeatability and can produces parts with high dimensional accuracy and is able to print parts in small to medium batches. The cost of the ABS filament used in industrial FDM systems is typically around $90 - $110 per kg. The main restriction for any part produced via FDM 3D printing is its anisotropic mechanical performance: parts are significantly weaker in one direction. This means that a designer must have a good grasp on the loads the part will be subjected to and the orientation of the model in the print platform.

Injection molding ABS parts with SLA 3D printed molds: High Temperature SLA resins are able to produce functional injection molds with a high level of accuracy that are best suited for low level production runs. SLA resins retail at around $150 - $170 per litre. A benchtop injection molding machine has been used for the calculations in this example with the 3D printed molds inserted into aluminium support frames. ABS pellets are used for molding the part that cost approximately $2 - $3

Traditional injection molded ABS part: Traditional injection molded parts have a very high level of accuracy, excellent surface finish and a very high level of repeatability. The main downsides of traditional injection molding is the high initial setup cost and the number of design restrictions enforced in the designing of the molded part (draft angles, constant wall thickness etc). The cost of the ABS pellets is the same as above.

A summary of the cost (based on online quotes) of manufacturing the ABS fitting using the technologies discussed above is summarised in the table below. All prices do not include shipping.

	Industrial FDM*	3DP IM**	Traditional IM***
Cost of mold	N/A	$70. 85	$1660.72
Cost per part	$3.69	$0.05	$1.89
Total cost	$92.25	$72.10	$1711.48
Lead time	4 days	2 days	8 days

Next steps: How to produce parts with injection molding

Is low-run injection molding with 3D printed molds the best solution for your application? Then there are two ways for you to move forwards with you project:

If you have access to an injection molding machine and the know-how to design the mold, then 3D printing the mold in a heat resistant material is an option. An article discussing the advatages and disadvatages of the two most commonly used 3D printing materials for manufacturing low-run injection molds can be found here.

Otherwise, you can outsource the production to a professional injection molding manufacturer. Hubs offers access to a global network of Injection Molding service providers, who are able design a mold for your part and produce volumes from 100 to 10,000+ units.

Need advice on your injection molding project?

Speak with an expert

Rules of Thumb

• 3D printing the injection molds is the most cost effective way for low-run injection molding.
  
• Material Jetting and SLA are the most suited technologies for 3D printing injection molds.
  
• Use wide draft angles (2^o degrees or more) and a release compound to increase the lifetime of the molds.
  
• Keep the part volume lower than 165 cm³
  
• Each 3D printed mold can be used for approximately 30 - 100 runs (depending on the material being injection molded).
  

Ready to transform your CAD file into a custom part? Upload your designs for a free, instant quote.

Injection molding from 3D printed molds

Study of low-volume production of small plastic parts

BRIEF INFORMATION

Form 2 printer and injection using a Galcom Model-B100 injection molding machine. Two transparent resin molds were tested: one large butterfly mold and one mold with four small butterflies in one block. A third mold for the USB stick case was tested in high temperature polymer. These molds were 3D printed by Formlabs and the castings were made by Galomb Inc and Formlabs in a variety of materials.

Formlabs and Galomb, Inc.

LOW-VOLUME 3D PRINTED PARTS PRODUCTION

Most of the plastic products in the world today are made by injection molding. Using inexpensive desktop 3D printers and injection molding machines, molds can be created to produce small functional parts from manufacturing plastics.

For low volume production (approximately 10-100 parts), 3D printed molds save time and money. They also allow for a more flexible approach to manufacturing, giving engineers and designers the ability to easily modify molds and continue iterating their designs at low time and cost.

The Form 2 Stereolithographic 3D Printer (SLA) produces completely solid, smooth parts that can withstand the temperature and pressure of desktop injection molding. 3D prints produced using SLA are chemically bonded so that they are completely dense and isotropic, producing functional shapes with a quality not possible with FDM.

Formlabs is partnering with Galomb Inc., a manufacturer of low cost injection molding machines, to test and demonstrate the viability of SLA injection mold printing.

Fig. 2 : 3D printed molds in aluminum frames.

METHOD

Both clear and high temperature resins can be used to print small functional forms, with high temperature resins offering compatibility with a wider temperature range of

thermoplastic melts. Formlabs Clear Resin was chosen for its strength, high detail and smooth surface Clear Resin is preferred for its transparency as you can easily see when forms are being filled, but any of the standard Formlabs Resins (clear, white, black and grey) can also be used as they have similar mechanical properties. The plates were printed with a layer height of 100 µm and took approximately 5 hours per plate. Depending on the geometry, multiple shapes can be printed at once on the build platform to improve printing efficiency.

Fig. 3 : Printing setup in PreForm with cavities up.

Two forms were printed from transparent resin. Parts and subsequent molds were designed to match Galomb machine vise sizes, 1 in3 injection cylinder capacity, and Form 2 build volume. supports were polished.

The parts were then cured for one hour under a 405 nm UV lamp to achieve full mechanical strength and rigidity. For a better understanding of the impact of post-curing on parts, see the Formlabs UV post-curing booklet.

Fig. 4 : 3D printed molds in aluminum frames and die-cast parts.

The first was a large Formlabs butterfly logo and the second was four small Formlabs butterfly logos. Both molds had a cavity, narrow inlets, and sprue at the point of injection and were designed in Solidworks. The molds were inserted into aluminum frames to provide protection from the downward pressure and heating of the injection nozzle. Aluminum frames can also prevent the shape from deforming after repeated use. The frames shown in figures 2 and 4 were custom made by Whittaker Engineering in Scotland, but standard aluminum frames are readily available on request from injection molding machine manufacturers.

Plastic pellets can be purchased online or from suppliers such as IASCO-TESCO. To create different colors, the molten plastic was pre-mixed with powdered dyes prior to injection.

Using a Model-B100 benchtop injection molding machine, Galomb tested printing plates with 25 shots of LDPE. LDPE melts at approximately 325°F (163°C) and was chosen for its low melting point. Of note, Formlabs Clear Resin has an HDT at 0.45 MPa 73.1 ºC after post-cure (see Material Data Sheet). HDT is a measure of the thermal properties of a material, but does not rule it out for use, although LDPE has a higher melt temperature. Whether your 3D printed mold will withstand the injection molding process depends on the melt temperature of the injection material, part geometry, and cooling and cycle times.

RESULTS FOR CLEAR POLYMER

After 25 injections of LDPE, there was no noticeable deterioration in the surface of the molds (chips, cracks or scratches). LDPE is not prone to sticking to polymer molds when tested, but other plastics may require the use of a mold remover to assist in part removal. Adhesion of the part to the mold can cause mold wear during ejection. Release agent is widely available, and silicone release agent is compatible with Formlabs standard and high temperature resins.

The cycle time for each injection was approximately three minutes. This process was accelerated by using compressed air to cool the mold. Cyclic injection into forms printed on the Form 2 causes the form to heat up. To counteract this, the cooling time between open mold cycles must be increased. While the mold cannot deform, too much residual heat will reduce molding success if the mold is opened too early. Galomb also improved molding success by etching shallow (0.05 mm deep) vent holes (not shown) leading from cavity edge to mold edge so that air does not enter the cavity during injection.

Some of the injections showed a leak in the parting line due to deformation of the polymer mold during the cooling phase after several injections. Increasing the clamping force in the vise can help mitigate leakage, as can polishing the parting plane of the mold to make it as flat as possible. Galomb proposed incorporating channels into the mold design to include metal tubes and fill them with aluminum epoxy, as a strategy to strengthen the mold, reduce warping, and improve cooling time.

Fig. 5 : A range of injection molding parts made using 3D printed molds.

FURTHER TESTING WITH HIGH TEMPERATURE POLYMER

Clear resin molds have been successfully tested using LDPE, which has a relatively low melt temperature. Higher melt temperatures can cause thermal shock in clear resin printed parts, which manifests itself as a deformed mold surface.

Transparent resin molds experience thermal shock when exposed to higher temperature molten plastic.

Formlabs printed the shape of a USB device case in High Temp Resin to test

High temperature polymer molds showed no temperature degradation at the mold surface for any of the tested plastics.

*DURABLE RESIN polymer is under development and final specifications are subject to change.

High Temp and Standard are the polymers best suited for molding. Of the Formlabs resins, High Temp has the highest HDT at 0.45 MPa and low thermal expansion. It is also the stiffest material with the highest stretch factor.

The relatively high stiffness of High Temp Resin means that the mold will not deform when the part is removed. This makes the use of a release agent especially important for removing parts molded from rigid plastics such as polystyrene.

GENERAL TROUBLESHOOTING

Form overflow leak.

Leakage occurs when the injected plastic is forced out between the two mold halves. This can happen when the shape is overflowing, or if the split plane is not completely flat. Adding thin exit ports to the mold can help mitigate leakage from overpressure within the mold, facilitate part removal, and eliminate entrapped air that can cause bubbles in the molded part. Although not shown, the plates were tested without the aluminum frame. The downside to this approach is that these parts use more material, which increases printing cost and time, and the forms can be more prone to warping. With this method, a steel washer placed between the printing plate and the nozzle of the injection molding machine protects the print from direct contact and helps distribute forces. In addition, pre-sealing the injection molding cylinder against a metal block helps ensure there are no air pockets to disrupt the plastic flow.

Printed lines visible on some parts; this can be reduced by printing the form with a lower layer height. The plates used in this study were printed at 100 µm, but 50 or 25 µm could also be used. This will improve the cleanliness of the plate surface, but increase print time and reduce tank life.

Final USB device case molded from high temperature resin.

DESIGN INSTRUCTIONS.

When designing a form, consider what will print successfully and what will form successfully.

• Adding one to three degrees of recess on surfaces perpendicular to the direction of the recess will allow easier part removal and minimize mold degradation. Fillets should be applied to the inside edges to reduce buckling from internal plastic stress and facilitate removal of the part.
• Embossed and engraved parts must be offset from the surface by at least 1 mm.
• If you plan on using an aluminum frame, The surfaces of the split planes can be polished with fine sandpaper to reduce flare. add an extra 12mm thick plate to the back of the mold plate to account for compressive forces and ensure complete sealing.
• Be sure to orient the mold halves in the PreForm so that the cavity is facing up. This will prevent reference marks inside the cavity and make post-print processing easier.

90° angle

2° recess

Optimum condition 2° recess and rounding.

PROCESS SUMMARY

STEP 1:

Part design in CAD.

STEP 2

Mold design in CAD.

STEP 3

3D print the forms on Form 2.

STEP 4

Remove support material from molds.

STEP 5

Insert the plastic into the mould.

STEP 6

Remove the part from the mold.

technical study from a leader in the plastics industry

The French Industrial and Technical Center for Plastics and Composites - IPC, conducted a study evaluating the performance of injection molding using 3D printed molds. This article summarizes some of the findings, particularly those related to Formlabs 3D printers.

Injection molding is an economical and highly reproducible technology mass production of plastic parts with tight geometric tolerances. However, the high cost of traditional steel equipment makes it difficult to injection mold small batches of parts and can often be a barrier to new product introduction. By using 3D printed injection molds, engineers, fabricators and product designers can lower their costs, reduce lead times and bring better products to market. 3D printed injection molds are a great option for those who want to develop functional prototypes from end-use materials, produce a series of identical pre-production samples, or even custom or limited-edition end-use models.

Checkpoints

The IPC study was divided into three phases:

1 . Comparison of 3D printing technologies: The first classification was based on the technical data of several manufacturers. The thermal and mechanical characteristics were evaluated, respectively, in terms of deformation heat resistance (HDT) and tensile modulus. To select the three most promising materials for each of the identified technologies, four types of control points were developed to highlight critical properties. Due to the high resolution and smooth surface of the 3D printable models, resin-based solutions have been recognized as the best material choice for injection molding molds.

In general, a similar range of dimensional variations is measured for all considered 3D printing technologies: from ± 0.02 mm to ± 0.05 mm for small parts and from ± 0.05 mm to ± 0.2 mm for large parts .

The standard accuracy of machined metal tools should be ± 0.02 mm. This accuracy is desirable for a good fit along the parting line and to prevent reflow. IPC offers two methods for optimizing the parting line of a 3D printed resin tool.

2 . Guidelines for material characterization, design, and 3D printing.

3 . Injection Molding Tests: IPC performed two tests with different mold sets. In the first test, an "extreme test" scheme was used. Nearly a hundred polypropylene models were cast using a mono-material mold printed in High Temp Resin. The second test used a more complex "extreme test" scheme. Thousands of polypropylene models were molded using a mold of different materials: the core and mold inserts were printed from Rigid 10K Resin, and the frame was printed from Nylon 12 (Polyamide 12) using selective laser sintering technology.