Guide to Low-Volume Injection Molding

While injection molding is traditionally considered a manufacturing process only for mass production due to its high tooling costs, leveraging 3D printing to fabricate injection molds can empower you to use this process to produce high-quality and repeatable parts for prototyping and low-volume production.

In this comprehensive guide, learn how you can use 3D printed injection molds with both benchtop and industrial machines to efficiently and affordably produce hundreds of functional prototypes and parts that accelerate product development, reduce costs and lead times, and bring better products to market.

Injection molding is one of the leading processes for manufacturing plastics. It is a cost-effective and extremely repeatable technology that yields high-quality parts for large series production. As a result, it is widely used for mass-producing identical parts with tight tolerances.

Injection molding is a fast, intensive process where high heat and pressure are involved to inject molten material inside a mold. The molten material depends on the scope of the manufacturing project. The most popular materials are various thermoplastics, such as ABS, PS, PE, PC, PP, or TPU, but metals and ceramics can be injection molded as well. The mold consists of a cavity that accommodates the injected molten material and is designed to closely mirror the final features of a part. 

The molds are traditionally made out of metal by CNC machining or electric discharge machining (EDM). These are expensive industrial methods that require specialized equipment, high-end software, and skilled labor. As a result, the production of a metal mold typically takes four to eight weeks and costs anywhere from $2,000 to $100,000+ depending on the shape and the complexity of the part. For smaller part quantities, the cost, time, specialized equipment, and skilled labor required to fabricate the mold out of common tooling metals and manufacturing methods often makes injection molding at this scale unobtainable. However, there are alternatives to machining molds out of metal. Leveraging in-house 3D printing to fabricate injection molds for prototyping and low-volume production significantly reduces cost and time compared to metal molds, while still producing high-quality and repeatable parts.

In this video, we've partnered with injection molding service provider Multiplus to walk you through the steps of the injection molding process using 3D printed molds.

Desktop 3D printing is a powerful solution to fabricate injection molds rapidly and at a low cost. It requires very limited equipment, saving CNC time and skilled operators for other high-value tasks in the meantime. Manufacturers can benefit from the speed and flexibility of in-house 3D printing to create the mold and couple it with the production force of injection molding to deliver a series of units from common thermoplastics in a matter of days. They can even achieve complicated mold shapes that would be difficult to manufacture traditionally and can be used on both desktop and industrial molding machines, enabling development teams to be more innovative. Furthermore, product development benefits from the ability to iterate on the design and test the end-use material before investing in hard tooling. 

Even though 3D printing molds can offer these advantages when used appropriately, there are still some limitations to be aware of. We should not expect the same performance from a 3D printing polymer mold as from a machined metallic one. Critical dimensions are harder to meet, cooling time is longer because the thermal transfer occurs slower in plastic, and printed molds can more easily break under heat and pressure. However, companies across the industry are continuing to implement 3D printed molds into their short-run injection molding workflows, enabling them to quickly produce hundreds to thousands of parts. From designing functional prototypes with end-use materials, fabricating parts during pilot production, or manufacturing low-volume or custom end-use parts, 3D printing injection molds is a cost-effective and quick way to produce parts in limited quantities.

Stereolithography (SLA) 3D printing technology is a great choice for molding. It is characterized by a smooth surface finish and high precision that the mold will transfer to the final part and that also facilitates demolding. 3D prints produced by stereolithography are chemically bonded such that they are fully dense and isotropic, producing functional molds at a quality not possible with fused deposition modeling (FDM) 3D printing. Desktop SLA printers, like those offered by Formlabs, can seamlessly be integrated into any injection molding workflow as they are easy to implement, operate, and maintain.

As an alternative for mid-volume production of about 500 to 10,000 parts, machining molds out of aluminum can also reduce the fixed costs associated with manufacturing molds. Machining aluminum is five to ten times faster than steel and causes less wear on the tooling, which means shorter lead times and lower costs. Aluminum also conducts heat faster than steel, resulting in less need for cooling channels and allowing manufacturers to simplify mold designs while maintaining short cycle times.

To sum up, here’s an overview of the different injection molding methods and the mold types that result in the most efficient process and the lowest cost per part depending on the production volume:

The type of injection press does not have a significant influence on the process for low-volume injection molding; traditional large industrial injection molding machines can also be used with 3D printed injection molds. However, these machines are expensive, come with stringent facility requirements, and require skilled labor, and as a result, most enterprises outsource mid- and high-volume production to service providers and contract manufacturers.

If you are new to injection molding and are looking into testing it with limited investment, using a benchtop manual injection molding machine such as the Holipress or the Galomb Model-B100 could be a good option. Automated small-scale injection molding equipment such as the desktop machine Micromolder or the hydraulic machine Babyplast 10/12 are good alternatives for medium-series production of small parts.

Curious to learn about the different factors that make up the total injection molding cost? Read our comprehensive guide.

The workflow for low-volume injection molding includes the following seven steps:

The three major applications for low-volume injection molding are rapid prototyping, short-run injection molding, and on-demand or custom injection molding. 

Rapid prototyping helps companies turn ideas into realistic proofs of concept, advances these concepts to high-fidelity prototypes that look and work like final products, and guides products through a series of validation stages toward mass production.

In general, 3D printing is the most common way to produce rapid prototypes. However, in the later stages of the development process, there is often a need to produce slightly larger volumes of identical prototypes using the same materials and production processes as for the final parts. These prototypes can then be used for applications like beta and field testing. Combining 3D printed molds with injection molding allows manufacturers to rapidly and efficiently develop functional prototypes and speed up the product development process. 

For example, French startup Holimaker develops a manual injection molding machine that enables engineers and product designers to process plastic parts on their desktop in low quantities for prototypes, pilot production, or even a limited series of end-use parts.

The company offers feasibility studies for their customers, using 3D printed molds for a fast and affordable turnaround. This allows their clients to quickly and affordably prototype designs and validate final manufacturing conditions during the pilot production phase of new product introduction. 

By using the same manufacturing method including mold design and materials, these parts can be tested in the field and ensure the designs are ready to be produced at scale. The 3D printed mold designs can then be easily adapted for tool-grade steel during mass production.

Using 3D printed molds, Holimaker managed to shorten the lead time for producing molds for the injection molding process to 24 hours and they use 3D printed injection molds in 80% to 90% of their projects today.

Short-run injection molding provides manufacturers with a means to produce smaller series of end-use parts for products that are only produced in limited quantities or manufacture a pilot series of a product for testing the market before sinking too much capital into the venture. 

Using low-volume injection molding provides the opportunity to manufacture accurate and repeatable end-use parts without the high fixed costs associated with traditional injection molding. 

Multiplus is an injection molding solutions provider based in Shenzhen that covers the entire production cycle from design to manufacturing of plastic products and provides services to over 250 clients per year, including some Fortune 500 companies. Some of these clients require small batch production, which is traditionally expensive and time-consuming with injection molding due to the complexity of manufacturing hard tooling.

As the demand for small batch production started to grow, Multiplus turned to 3D printing to explore different materials in an attempt to find a cost-effective way to produce cheaper plastic molds for smaller orders and pilot production runs. Fabricating low-volume injection molds with Formlabs professional 3D printers reduced costs, labor, and time compared to machining aluminum molds, and could seamlessly be used with their Babyplast industrial injection molding machines.

Custom or rapid on-demand injection molding can be required to manufacture customized end-use parts for specific purposes, such as human factors, application, or occasion, often on an expedited timeline. The limited volume and/or short lead time means traditional injection molding with hard tooling is neither efficient nor feasible. In these cases, low volume injection molding with 3D printed molds is an ideal solution to speed up the process and deliver custom parts.

The example of Braskem, one of the world’s leading petrochemical companies, highlights the case for using 3D printed molding to fulfill rapid orders on-demand. During the first wave of the COVID-19 pandemic, the company needed to produce thousands of mask straps to protect its global workforce. Braskem identified injection molding as the ideal method to produce the parts, but without access to 3D printing, they would have had to outsource an expensive metal mold, costing the team money and valuable time.

Braskem’s team turned to injection molding using a Formlabs Form 3 3D printer to print the mold for its straps and the all-electric Cincinnati Milacron 110 Ton Roboshot injection molding machine to develop the straps. 

By leveraging 3D printing, the team was producing thousands of straps within a week of receiving the VP’s email and preparing them for shipping to offices around the world. 

Accelerate product development, reduce costs and lead times, and bring better products to market by incorporating 3D printing and low-volume injection molding into your development process.