Plastic is a material used worldwide in a wide range of applications and products from medical to consumer products to aerospace. All these products must be manufactured in an efficient way to bring value to the end user. The most widely used manufacturing methods for plastic products are injection molding and additive manufacturing, otherwise known as 3D printing. Each of these manufacturing methods have advantages and disadvantages which should be carefully considered when deciding on a plastic product manufacturing method.
Injection Molding Manufacturing Process
Injection molding is the process by which molten plastic is forced through a channel into a mold to form a part. There are a few different inputs to this process that are relevant to the advantages and disadvantages of the injection molding manufacturing process. The first step of injection molding is to design a part and a corresponding mold to make the part. The special mold tooling will then need to be CNC machined, typically out of aluminum or steel. Each mold can be used for many units but can only be used to produce one single product design. Once the mold has been CNC machined, the tooling is setup on the machine and the injection molding manufacturing process is carried out over the course of many thousands of units. Molten plastic is continuously injected into the mold, the plastic is then cooled in the shape of the part, released, and the process is started again.
Advantages of Injection Molding
The advantage of the injection molding manufacturing process is that it can create high volumes of identical units very cost effectively. As the volume increases, the price per unit decreases dramatically, enabling impressive economies of scale. Injection molded parts also have great mechanical properties and are isotropic, meaning they have the same mechanical properties, regardless of direction.
Disadvantages of Injection Molding
The disadvantages of the injection molding manufacturing process are mainly due to the constraints imposed by the tooling. Although high volumes of identical units can be produced cheaply, high volumes of unique units are uneconomical. This is because a unique mold tool would have to be designed and fabricated for every single product that is being manufactured. Likewise, injection molding is also not efficient at producing plastic products in low volumes. The reason being that the high initial cost of the tool must be distributed over a fewer number of units, increasing the cost per unit.
It is also not advisable to use injection molding during the new product development process because of the fluidity of the product design and the high likelihood that design changes will be required. This is because with every new design change the mold tool must be modified. The mold tools are often very difficult to modify and often are required to be completely scrapped and remade. The mold tool fabrication adds higher costs and longer lead times to the product development cycle.
Design complexity should also be a consideration with injection molding because there is a direct relationship between the geometric complexity of the product and the cost of the tooling. Each tool must be CNC machined and a design with high complexity will either require a longer run time, resulting in higher cost, or will be impossible to machine. As a result, the injection molding manufacturing process is most efficient when basic shapes, with a low amount of complexity are required in high volumes.
Additive Manufacturing/3D Printing
3D printing is a process by which thousands of layers are printed consecutively and fused together, creating a three-dimensional object. The 3D printing plastic product manufacturing process can either be done via extrusion of molten thermoplastic material, with a laser over a polymer powder bed or with a laser over a bed of photo-curable resin. The 3D printing setup process is less intensive than injection molding as it does not require any tooling. The operator need only ensure that the printer is properly calibrated, with the correct parameters, and build orientation before sending the digital file to the printer to print.
Advantages of 3D printing
There are several advantages of 3D printing that are a result of its layer by layer manufacturing method. As material is being selectively added layer-by-layer, there is no need for a mold tool to create the parts. This lowers the fixed costs and shortens the time to market as the lead time is only needed to make the product, not the product and the tooling. For this reason, 3D printing is commonly used for rapid prototyping applications where multiple product iterations are desired with both speed and cost being critical performance metrics. 3D printing can produce low volumes of multiple product designs, quickly and cost effectively.
In contrast to the injection molding manufacturing process, products with high-complexity do not increase the cost. 3D printing can create products with complex geometries and fine detail features with no increase in cost. Likewise, mass customization is also possible with 3D printing, whereby each product produced can be customized and differentiated from all the other products. To do this with injection molding, it would require a special mold tool for each individual product, dramatically increasing the cost.
Disadvantages of 3D Printing
Although 3D printing can produce plastic products more efficiently in low volumes and with complex geometries, 3D printing still cannot compete with injection molding at higher volumes of identical products. Likewise, injection molded parts are isotropic, while 3D printed parts are anisotropic, meaning that 3D printed parts are stronger in the XY direction than they are in the Z direction. Some of these anisotropic tendencies can be mitigated through the optimization of printing parameters and improving the layer adhesion, but Z strength will always be slightly different than the XY strength.
Get Started Manufacturing Plastic Products
In summary, if the products will be produced in low volumes, have highly complex designs, undergo multiple iterations or need to be customized, then 3D printing will be the best manufacturing method. But if identical products must be created in large volumes, then utilizing an injection molding manufacturing process will likely be the best option.