Plastic Extrusion Process and Design Crash Course

Plastic extrusion is a manufacturing method used to make products by forcing material through a die that forms a continuous profile matching the die as it is extruded. The process shares similarities to metal extrusion; however, instead of producing a metal product, it is designed for creating products made from plastic polymers. There are plastic extruded parts all around us in our everyday lives; for example:

• Plastic tubes and piping (e.g., PVC plumbing)
• Deck railing
• Plastic gutters
• Plastic films and sheets
• Wire insulation

Plastic extrusion involves several stages, from melting and extrusion to cooling and processing stages. It begins with loading raw material, typically polymer resin pellets, into a hopper. Additives such as fillers and coloring agents are mixed in with the resin pellets to create a homogenous mixture for melting and extrusion. Similar to plastic injection molding, a hopper of plastic pellet material feeds a barrel containing a rotating screw that works to push the material forward. As the material passes between the screw and the barrel walls, it undergoes shearing forces and friction, which heats and begins melting. Heaters on the barrel apply additional heat, assisting the melting process. Drag forces generated by the screw force the melted polymer through a filter and breaker plate to remove contaminants and create an even distribution of polymer through the die assembly at the barrel’s end.

A well-thought-out design can reduce costs, mitigate defects, and ultimately lead to a higher-quality product. In the sections below, we will go over tips and considerations when designing a part for the plastic extrusion process.

Pro Tip: Design with uniform wall thicknesses. Wall thickness should not be less than 0.025"-0.050", depending on part size.

With extrusion processes, it is crucial to maintain relatively even cooling rates across the profile as it is extruded. If cooling is uneven in areas, it can lead to defects such as warping and twisting and cause features to fall out of specification. One of the best ways to create a balanced extrusion with even cooling is to design using uniform wall thicknesses. Uneven walls can affect the flow of the material and cause uneven cooling, so the more uniform the walls are, the better.

Another factor to keep in mind with wall thickness is the minimum wall thickness. Extremely thin-walled parts can be very challenging or impossible to produce efficiently, leading to issues. Minimum wall thickness scales with overall part size, so the larger the part, the thicker the walls need to be to support the profile. For smaller parts, a wall thickness between 0.025"-0.030" is typically suitable, while for larger parts, you will want to increase the minimum thickness to around 0.040"-0.050".

Pro Tip: Never leave corners completely sharp; use a radius of 0.016” or larger.

Radii help promote good material flow as it is extruded through the exit die. If corners are left completely or excessively sharp, it can create a concentrated area of stress. These stress concentrations lead to issues such as breaking and warping in the extrudate. That said, designers should always add radii to sharp corners to mitigate this issue. We recommend using at least 0.016” or larger radii and maintaining uniform wall thickness by using outside and inside corner radii that share the same center point.

Pro Tip: Avoid hollows when possible, and do not design hollows within each other. Protrusions within hollows should be minimal and extend no more than the wall thickness.

Hollow sections within extrusion profiles are called lumen. Lumen are typically more challenging to produce and regulate specifications. As such, hollow features have looser tolerance bands than external features. Hollows may also require additional steps and tools, such as vacuum sizing and air pressure to keep the shape of the part during cooling, which can cause the price to increase. Generally, it is best to avoid hollow sections whenever possible.

If your design requires hollows or multi-lumen features, then keep the following in mind to make them more manageable:

• Do not design a hollow inside of another hollow. The position of a nested cavity cannot be easily managed and can impact the surrounding features before the part has wholly cooled, causing an out-of-specification part.
• Details and protrusions, such as legs within hollows, should be avoided as they are also difficult to control. They should be minimal and protrude no deeper than the wall thickness if required.

Pro Tip: Avoid excessively tight tolerances and only apply tighter tolerances to critical features.

To hold tighter feature tolerances, specialized tooling and setups may be required, increasing costs and lead times. Try to design for standard manufacturing tolerances and apply tighter tolerances to features critical to form, fit, and function. Extruded thermoplastics react strongly to temperature, expanding and contracting with it. For this reason, maintaining tight length specifications can be particularly difficult. As extrusion length increases, so should the allowable tolerance. For more information on standard plastic extrusion tolerances, visit our manufacturing standards page.

Choosing a suitable material for your project is crucial to your extruded part’s overall functionality and longevity. In addition, the material can be a primary driver for manufacturability and cost. The first step to picking a plastic resin is determining what characteristics your part will require for its intended use and environment, as not all plastics perform equally. These characteristics may include the following:

• Environmental resistance (temperature, UV rays, weather, etc.)
• Rigidity / Flexibility
• Impact resistance and strength
• Chemical resistance (e.g., cleaning agents, gasoline, oil, etc.)
• Physical appearance
  

You can keep costs low by avoiding more exotic, less common materials and picking one that is more common and meets your part’s end-use requirements. To help you understand the differences between the different plastic resins and make the right choice for your project, we’ve compiled the table below, which compares commonly available materials for plastic extrusion. Xometry’s experts can also help guide you to the most suitable material and filler selection for your project and answer any questions.