Injection molding and 3D printing: what to choose?

Injection molding and additive manufacturing: what is the most advantageous process?

New technologies have been joining forces with the traditional methods for manufacturing polymers. Based on the need, like the number of parts and custom or complex geometries to manufacture, one can evaluate what is the most advantageous process. This process has indeed a direct influence on delivery time, investment costs, and tooling needed.

Two of the main technologies to manufacture polymer parts are injection molding and additive manufacturing. The main differences between injection molding and 3D printing are in the delivery time of the parts, the investment costs, and the overall look of the parts. Injection molding requires high-priced molds and custom tools that require some testing and long lead time to be set up. The results of this waiting are paid off by high surface finish and an enormous number of possible production volumes.

On the other hand, 3D printing allows for exceptionally fast production of the most complex shapes with no onerous tool required. These differences make injection molding the preferred production method for mass production, while additive manufacturing is mostly suitable for small to medium batches of highly complex parts.

What is injection molding?

Injection molding is a manufacturing process that is excellent for the massive production of identical parts with tight tolerances. It allows to produce parts made of plastic, glass, rubber, or even metal.

This production method has been discovered in the nineteenth century, straight after the invention of the first plastic materials, because it exploits one of the main properties of thermoplastic polymers: moldability. Since then, techniques and technologies have become more and more optimized up to the point that today it has been estimated that about 80% of the durable plastic items we find every day are manufactured by plastic injection molding.

The process is very easy: melted material is injected into the mold, filling the cavities. Then, the mold is cooled, and the parts are ejected by pins. The engineering of this simple solution is anything but simple: tools and injection pressures are accurately engineered to achieve smooth surface finish and a high repeatability of the process, making it ideal for high batches.

However, due to the very high pressure, the resolidified polymer might shrink and warp. To avoid this from occurring, rib details might be designed on the final part and a flow analysis can be run to validate the process.

Among the typical surface defects of injection molded parts there are sink marks, weld lines and steaks of pigment. While the first appear on the surface opposite to a rib detail, the second occur where the material is forced to flow around obstacles like recesses and holes.

Injection molding requires tools that are generally machined from aluminum or steel. These tools are usually made up of water-cooling channels for temperature control, an injection point (gate), runner systems (connecting parts), and electronic measuring equipment which continuously monitors temperature. The least expensive injection molding tooling consists of 2 halves, the male tool, and the female tool. But the boundaries of the process are constantly being pushed by engineers and toolmakers with more complex tooling, retractable cores, and multiple gates.

Advantages and disadvantages of injection molding

Injection molding is one of the preferred manufacturing methods for polymer parts. However, the process must be analyzed in all its pros and cons to have a better understanding of its benefits and limitations. Advantages of injection molding are:

• speed: high number of parts per hour. Speed depends on the complexity and size of the mold;
• repeatability: process repeatability brings convenience for mass production and high batches;
• fine tolerances and very high surface quality achievable;
• low labor costs, automated process;
• large material choice (also in colors) and multiple plastic materials could be used together;
• customizable: possibility to include inserts to the molded parts.

Here are the disadvantages of injection molding:

• limited design flexibility: shapes are limited in the complexity; design for extraction rules must be followed:
  ○ limited to thin-walled parts: use uniform wall thicknesses to prevent inconsistencies in the cooling process resulting in defects like sink marks. Furthermore, thick parts result more expensive since the cooling process takes longer;
  ○ avoid sharp edges and undercuts; 
  ○ draft angles are recommended for better de-molding.
• difficult to modify molds and, consequently, molded parts;
• post processing required to eliminate bridges, sprue, and runners for some shapes;
• high investment costs for expensive molds and tooling. These costs must be split by the high number of parts to manufacture to achieve convenience. Usually, the larger the part, the more difficult and expensive the mold will be. These points determine that it’s too expensive for small runs of parts;
• long set up lead times due to extensive testing during designing of mold tool and prototyping;
• long lead time for production and delivery of molds;
• need to stock molds in warehouse;
• no orientation of fibers according to a selected direction, when molding fiber-reinforced composites.

Some materials, like PEEK are more complex to manufacture, since they require a higher melting temperature and might be subjected to shrinkage. This material can be injection molded and this technology is suitable for the mass production of PEEK parts.

Advantages of 3D printing

3D printing adds polymeric or composite material, layer by layer, to manufacture net shapes. It is considered the quickest technology to produce very complex parts. FFF technology is especially known to be very easy to implement in a company’s environment. Currently, it is the most used technology for rapid manufacturing of functional prototypes.

The main advantages of additive manufacturing, especially FFF technology, are:

• Very short lead time: print on demand and just in time;
• No necessity of tools;
• complex and customized geometries;
• create digital warehouses eliminating warehouse and obsolescence costs;
• Reduce logistic costs approaching the production point to the final point of use;
• Possibility to create integrated assemblies with reduced part count;
• Material waste only due to support structures and raft;
• Low labor cost: the printer doesn’t need operators while in function;
• High part strength ;
• Shorten supply chain with printer capability inhouse ;
• Option to reduce weight thanks to infill flexibilities.

One of the most difficult high-performance polymers to process is PEEK and it can be 3D printed with excellent quality with Roboze FFF printers. For the mass customization of PEEK products, Roboze 3D printing is the perfect solution, as it ensures a repeatable process and high crystallinity of the PEEK parts.

Difference between injection molding and 3D printing: how to choose the most suitable solution?

Choosing between 3D printing and injection molding is easy because they have many differences. If there is the necessity to reduce lead time, eliminate tooling and have custom shapes, traditional methods show big limitations. Companies having to decide on the preferred production method must consider all these aspects to analyze the convenience.

To choose an option between 3D printing and injection molding, one must analyze:

• Production volumes required: injection molding is famous for being convenient for mass production. The cost per part strictly depends on the production volumes. Furthermore, the more complex the part, the higher the cost to injection mold it, due to the higher costs to machine the metal molds. Usually, the larger the part, the higher the costs. On the other hand, 3D printing costs do not depend on the production volume but stay almost steady. Depending on the shape and the size of the parts to manufacture, a break-even point can be calculated to evaluate what is the most convenient technology between 3D printing and injection molding. The break-even point might vary over a wide range between some thousand to tens of thousands of parts. In very general terms, injection molding is to be considered convenient for more than ten thousand parts, while 3D printing is the technology to choose for less than five thousand parts.

• Immobilization of capital and goods: costs are paid upfront with injection molding: tools and material must be bought in advance and the high batches of parts needed are all available as soon as they are ready. It results convenient in economic terms, but it might not be in financial terms: in many cases, the monthly consumption of these parts is low but the total costs have been paid upfront. Since this capital has been immobilized, it cannot be used by companies for different expenses. Furthermore, it generates stock in the warehouse that represents an additional cost and might become obsolete if unsold in a short time. Thanks to the possibility to print just in time and on demand, 3D printing doesn’t require immobilization of either capital or goods. This flexible production method can allow smaller batches to be ready periodically, only when needed;
• Lead time required: injection molding is a quick process only after all the tools have been manufactured and tested. The design and testing phase, however, might take some time. The manufacturing of the mold as well. Having a .stl file is all that is needed for 3D printing: the lead time is very low;
• Investment costs: molds and tools can get very expensive, depending on size and complexity. The range cost of a single injection mold varies from 1k to 80k USD. 3D printing does not require any tool for production, so the investment is only on the machine;
• Production flexibility: every shape to manufacture with injection molding requires at least one brand-new mold, since modifications on molds are mostly not possible. Imagine how high the tooling costs can be if many different shapes must be manufactured. The 3D printing process does not depend on the part shape, all shapes can be manufactured with the same machine and without tools, guaranteeing a high flexibility;
• Surface roughness and dimensional tolerances: injection molding ensures very high-quality finish and extremely low tolerances, depending on the quality of the mold. 3D printing ensures medium tolerances and usually a surface that can be used as is or polished for applications that need so;
• Complexity of the part: injection molding can manufacture mid-complex parts, but it’s very often limited by the design for injection molding: no undercuts, thin parts and draft angles are some of the characteristics. Furthermore, the higher the complexity, the higher the tooling costs. 3D printing is perfect for manufacturing very complex parts
• Customizability of the part: customizing parts means customizing injection molds: this is almost impossible. There is the possibility to eliminate material from the mold with machining operations, however the operations that can be performed are limited and this process isn’t reversible. Since 3D printing is not bound to any tools, changing the design of the parts is easy and quick, so it’s always convenient to customize parts at no additional cost.

  Process	Injection molding	3D Printing
  Geometry	Limitations	No limitations, flexible and complex parts, infill options
  Tooling cost	Very High	None
  Lead time	High	Very low
  Stocks needed	Yes, molds and material	Filament only
  Cost	Increases with complexity	Depends on material quantity only
  Profitability	Based on large batches	Independent of number of units
  Goal	Mass production	Mass customization

Will 3D printing replace injection molding?

3D printing will substitute injection molding for the production of small to medium batches, since it can ensure shorter lead time and economic convenience with no need for expensive tools, but it will probably not be the preferred production technology for the mass production of polymer parts.

The industry demands are becoming more and more difficult to meet in terms of design complexity, since markets are switching from mass production to mass customization. Therefore, technologies must adapt in order to satisfy the industry needs. While costs increase with complexity with injection molding, this is not the case for 3D printing. This advantage, together with the flexibility of additive manufacturing, will be key for a wider implementation of this production process in the future. 

Today injection molding is the most used technology but as soon as engineers and designers stop thinking about design for injection molding and start thinking about design for additive manufacturing, more and more doors will open to mass customization. With a repeatable process, as the one that is guaranteed with Roboze ARGO printers, 3d printing gets aligned with the repeatability of injection molding to guarantee functional parts. 

Our team of experts at Roboze will help you evaluate if our technology is the best solution for your production needs.