3D printing injection mold

(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.

How to use 3D printing for injection molding

Today, most plastic products in the world are made using injection molding. However, creating molds can be time consuming and costly. Fortunately, molds can not only be obtained on a metal milling machine, but also printed on a 3D printer.

Stereolithographic (SLA) 3D printing is an affordable alternative to milling aluminum molds on a machine. The 3D printed models are hard and isotropic, and the materials used have a thermal distortion temperature of 238°C at 0.45 MPa. This means that they can withstand the temperature and pressure of injection molding. nine0003

Download our free white paper to learn how to 3D print injection molds.

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3D printed aluminum framed molds and finished die-cast model.

With affordable desktop 3D printers, heat-resistant 3D printing materials and injection molding machines, you can create molds yourself to produce functional prototypes and small models from industrial plastics. In the case of small-scale production (approximately 10-1000 models), injection molds created using 3D printing save time and money by eliminating expensive metal molds. They also provide a more flexible approach to manufacturing, allowing engineers and design professionals to prototype injection molds, test mold configurations, or easily modify molds while continuing to iterate, thanks to short order lead times and turnaround times. nine0003

Stereolithography (SLA) technology is a great solution for injection molding. It is distinguished by the fact that it allows you to create molds with a smooth surface and high precision, giving these qualities to the finished model, as well as simplifying the process of removing from the mold. SLA 3D printing provides chemical bonding, density and isotropy of the manufactured models, which makes it possible to produce functional molds of a quality that cannot be achieved with Fused Deposition Modeling (FDM) printers. Desktop stereolithography printers, such as those offered by Formlabs, simplify your workflow because they are easy to implement, use, and maintain. nine0003

Formlabs created High Temp Resin for small production runs of injection molded models. It has the highest heat distortion temperature on the market and one of the highest among Formlabs resins: 238°C at 0.45 MPa. High Temp Resin can withstand high casting temperature and shorten the cool down time. Our white paper provides a case study from Braskem. She completed 1,500 casting cycles using a single 3D printed High Temp Resin profiling insert to make the face mask neck straps. The company printed the insert and placed it in a conventional metal mold integrated with the injection molding system. This is an effective solution for the production of medium batches of models. The printed insert can be replaced as the project parameters change or if it breaks. This solution allows molds to be created as needed with complex geometries that are difficult to create with traditional methods, and also provides the possibility of multi-stage casting. nine0003

However, High Temp Resin is quite brittle. In the case of more intricate shapes, it is easily deformed and cracked. For some models, it is difficult to complete more than a dozen cycles. To solve this problem, the young French company Holimaker used Gray Pro Resin. This polymer has a lower thermal conductivity than High Temp Resin, which increases the cooling time. However, it is softer and able to withstand hundreds of cycles.

Formlabs recently released Rigid 10K Resin, an industrial grade material with a high fiberglass content. It can convey a variety of geometric features and withstand injection molding processes. Rigid 10K Resin has a thermal distortion temperature of 218°C at 0.45 MPa and an elastic modulus of 10,000 MPa, making it strong, incredibly stiff and heat resistant. Novus Applications has cast hundreds of structurally complex threaded caps with just one Rigid 10K Resin mold. As more companies start using Rigid 10K Resin, we believe this resin will be a great help in 3D printing complex injection molds. nine0003

Download our free white paper to learn about practical use cases and how to make your own injection molds using 3D printing.

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Based on our customers' internal testing and case studies, we recommend selecting a 3D printing resin based on the criteria shown in the table below. Three stars mean that the polymer is very effective, one star means that it is not very effective. nine0049 ★ ★★ ★★★

The more complex the model and mold design, the more difficult the injection molding process. Using 3D printed molds, models can be molded from various materials such as polypropylene, polyethylene, thermoplastic elastomer, TPU or polyamide. Low viscosity material can reduce pressure and increase mold life. Polypropylene and thermoplastic elastomers are easy to process. Using them, you can perform a large number of cycles. In contrast, more technical plastics such as polyamide allow for fewer cycles. A release agent makes it easier to separate the model from the mold, especially for flexible materials such as TPU or thermoplastic elastomers. nine0003

The type of injection molding machine does not greatly affect the process. If you have little experience with injection molding and would like to try this method at no extra cost, we recommend using a desktop injection molding machine such as the Holipress or Galomb Model-B100.

White paper

Download our white paper to learn how to use 3D printed molds for injection molding to help you cut costs and order lead times, and see real world application examples 3D printed by Braskem, Holimaker, and Novus Applications. nine0003

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We recommend that you follow the design rules for additive manufacturing as well as the general design rules for injection molding, such as 2 or 3 degree taper, uniform model wall thickness, and rounded edges. Here are some helpful tips from users and professionals regarding resin printed molds:

To optimize dimensional accuracy:

• Plan the mold allowance for post-processing and dimensional changes. nine0113
• Print one batch of a mold to learn about dimensional deviations and account for them in a CAD mold model.

To increase mold life:

1. Open the sprue to release pressure in the cavity.

2. If possible, design one side of the floor flat and the other as designed. This will reduce the chance of blocks shifting and splashing.

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Create large air ducts from the edge of the cavity to the edge of the mold. This will improve the flow of material into the mold, minimize pressure, and reduce the chance of splashing in the sprue for faster cycle times.

3. Avoid thin cross-sections: under the influence of temperature, a surface with a thickness of less than 1-2 mm may be deformed.

For print optimization:

5. Modify the back of the mold to minimize the amount of material used: reduce cross-cuts in areas that do not support the cavity. This will save resin and also reduce the chance of printing errors or warping. nine0003

6. Add a beveled edge to remove the product from the work platform.

7. Add centering holes at the corners to align both models.

If you have any questions about the workflow, please read our article in the FAQ section titled "Injection Molding in 3D Printed Molds". For full details of the workflow and recommended practices, download our technical white paper. nine0008