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Comparing the Plastic Extrusion & Injection Molding Processes in Manufacturing

18 March 2016- 16 min read


For industrial manufacturers, the methods of injection molding and/or extrusion are specified to make products with different shapes and sizes.

The molten die-casting method is the basis of the injection molding process. The injection-molding unit consists of two elements: the clamping unit, and the injection unit. Unlike extrusion, injection molding forms three-dimensional shapes.

Joseph Brahman patented the first hydraulic press in 1795. However, the process was more  fully developed in 1820 after Thomas Burr developed the first hydraulic powered press for producing shapes. It was not until 1894 that the process was expanded to include brass and copper alloys for the non-continuous extrusion of finished parts.  Eventually, in the 1930’s, the injection molding process as we know it today was established.

In terms of plastic extrusion, this technology was pioneered by Thomas Hancock in 1820 and Edwin Chafee in 1836 for the processing of rubber. The first thermoplastic extrusion is credited to Paul and Ashley Troester much later, in 1935. This is a method by which molten plastic or varying alternative materials are  pushed continuously, driven by feed screws through a two-dimensional die opening. Following this, it passes through a series of templates or blocks  where the molten form retains the desired shape  as it cools. In the extrusion process, the finished product has a two-dimensional form which is continuous in length. The extrusion method produces linear shapes which can be cut to multiple  lengths and / or notched, punched or otherwise fabricated, often continuously in line during the process .

Both extrusion and injection molding have their own advantages. An advantage in using the extrusion process over other methods is its ability to create complex cross-sections. Additionally, both stiff or soft  materials can be formed into any shape and the finished materials have a smooth surface finish when compared to other processes.  With  both the injection molding and extrusion processes, there is minimal waste as the scrap can be recycled again.

The Take Away:

  1. Injection molding forms three-dimensional shapes.
  2. The injection molding process was first established in the 1930s.
  3. The thermoplastic extrusion process was fully developed in 1935.
  4. Extrusion is a method using molten plastic or many other materials that are pushed through a two-dimensional die opening.
  5. In both processes , the scrap can be recycled reducing waste.
  6. The injection molding process is based on the molten die-casting method.
  7. In the extrusion method, the finished products have a two-dimensional form which are continuous in length and can be cut to multiple lengths and  punched or notched.
  8. Complex cross sections are created using the extrusion method.
  9. In the extrusion process, both stiff   or soft  materials can be formed into any shape and the finished materials have a smooth surface finish.
  10. The extrusion process is ideal for  construction materials such as siding , decking, trim or drainage, cosmetic parts such as wall protection, window casements, office furniture or functional components like louvers, brackets, retainers or supports.
  11. The injection process is ideal for manufacturing toys, interior automotive components and many other 3 dimensional products.