Plastic Extrusion Materials
The variety of plastic extrusion materials is virtually limitless, and their usage is rapidly increasing with the development of advanced plastics that can replace rubber, metal, and wood in a growing number of applications.
Choosing the best plastic material for your custom extrusion can be a daunting task, but you can quickly narrow down your options if you know where to start. This article can help you do just that. It provides a basic breakdown of the different types of plastic polymers and compounds used for extrusion and enhancements that can be made to achieve the desired performance.
Types of Plastic Extrusion Materials
You can quickly identify which plastic extrusion materials will work for your part if you know where to look. Thermoplastics are categorized by their commercial availability and their molecular structure. All you need is a general understanding of what material properties are required for your application and you can begin narrowing your search by category.
Commercially, plastic extrusion materials are grouped by their cost, market share, and material properties into three categories: commodity, engineering grade, and high performance.
Commodity plastics are suitable for most applications and account for 90% of all thermoplastic use. They are readily available, easy to process, and the least expensive plastic for extruded products. Examples of commodity plastics include polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC).
Engineering Grade Plastics
Engineering grade plastics are designed with unique combinations of properties for improved performance in specific applications. Examples of engineering grade plastics include thermoplastic elastomers (TPE’s), polycarbonate (PC), polyamide (PA or Nylon), and acrylonitrile butadiene styrene (ABS).
High Performance Plastics
High performance plastics are engineered to have exceptional mechanical and/or thermal properties in order to meet high performance requirements in harsh environments. They’re some of the most expensive plastics, so they are typically reserved for low volume, specialty applications.
Unless your application is especially demanding, limit your search to commodity plastics. The other grades can be quite impressive, but they’re also more expensive. Commodity plastics can be compounded or enhanced to suit the majority of plastic extrusion applications.
Virtually all plastic polymers have a blend of two molecular structures, amorphous and crystalline. The proportions will change slightly during processing, but one structure typically remains predominant.
The molecular chains in amorphous polymers are rather jumbled and move freely when the polymer is pushed or pulled. This gives the polymer excellent flexibility, elasticity, and impact resistance. Amorphous polymers tend to have low shrinkage and can accommodate tighter tolerances. They also exhibit good impact resistance and moderate heat resistance, but generally have low strength and poor chemical resistance.
The molecular chains in crystalline polymers are very ordered and locked in place. This gives the polymer excellent strength and rigidity. Crystalline polymers tend to have a high degree of shrinkage so it can be difficult to hold tight tolerances. They exhibit high strength and rigidity with excellent chemical and heat resistance, but generally have poor impact resistance and exhibit a high degree of shrinkage.
Common Plastic Extrusion Materials
The table below outlines the distinctions between the most common plastic extrusion materials. It’s not exhaustive but is meant to serve as a starting point to understand what’s available. When making a final decision, each material attribute such as physical properties, dimensional stability, electrical performance, and thermal properties, should be taken into consideration.
|Commodity Plastics for Extrusion|
|Molecular Structure||Material||Properties||Uses and Applications||Other Grades||Notes|
|Crystalline||PE||Rigid or flexible
Excellent impact strength
High chemical resistance
High corrosion resistance
Poor heat stability
Poor dimensional stability
(low-density polyethylene) is a low-density grade PE.
It is lightweight and highly flexible, but has low tensile strength.
(high-density polyethylene) is a high-density grade PE.
It is rigid and has a high strength-to-weight ratio, but has poor mechanical properties with the exception of impact strength.
Excellent electrical resistance
High tensile strength
High heat stability
|PP - filled||PP is often filled with glass or carbon fibers to increase its tensile strength and heat-deflection temperature (HDT).|
|Amorphous||PVC||Rigid or flexible
Excellent chemical resistance
Excellent electrical insulation
|Rigid PVC||Rigid PVC (RPVC) is the unplasticized version of PVC. This type is heavily used in the construction industry.|
|Flexible PVC||Flexible PVC (FPVC) is the plasticized version of PVC. This type is used in the electrical, automotive, and medical industries.|
|Engineering Grade Plastics for Extrusion|
|Molecular Structure||Material||Material Properties||Uses and Applications||Other Grades||Notes|
|Crystalline-amorphous alloy||TPE||Choice of durometers
Good elastic properties
High tear strength
Oil and gas resistant
|TPV||Thermoplastic vulcanizate (TPV) is a low-cost alternative to rubber and is used in applications that require excellent elasticity and flex fatigue resistance.|
|TPO||Thermoplastic polyolefin (TPO) is a lightweight alternative to PVC and is used in applications that require increased toughness and durability over other TPE's.|
|TPU||Thermoplastic polyurethane (TPU) is the best wearing TPE material and has the highest tensile strength, but is heavier and more expensive.|
|TPS||Thermoplastic styrene (TPS) is very versatile in compounding with other materials and has good adhesion to most other plastics.|
High strength-to-weight ratio
Excellent electrical resistance
|PA6, PA6/6||PA6 and PA66 are the most commonly used forms of PA. PA6 typically has lower production costs and higher impact resistance, while PA66 has lower moisture absorption and better stability.|
|PA - filled||PA is usually filled up to 50% with glass or carbon fiber to improve mechanical properties and operating temperatures.|
Good dimensional stability
Moderate chemical resistance
|PC is the only engineering plastic that is transparent. It is also available in colored grades.|
Excellent electrical properties
Good abrasion resistance
|ABS can be modified by changing it's monomer ratios to obtain a wider range of properties, and is commonly combined with other materials for improved properties.|
We strongly encourage you to work with your extruder and material supplier to discuss which material would be best for your specific application. They can also provide strategies to enhance the material and design for optimal part performance and cost.
Reinforcements, Fillers, and Additives
Virtually all extrusion plastics can be enhanced with reinforcements, fillers, and additives to achieve the desired material properties and/or reduce cost.
Polymers can be reinforced internally by adding certain fibers to the polymer matrix, or externally by coextruding it with another material.
Reinforcing fibers such as glass fiber and carbon fiber can be added to the polymer matrix to significantly improve the mechanical and thermal properties of the extruded plastic material. This is the most common method of reinforcement and has been used to make plastic extrusions that can outperform metal in many applications.
When orienting the fibers in the same direction, as extrusion does with a continuous flow in the one direction, the tensile strength, compression strength, flexural modulus, and rigidity of the polymer composite significantly increases. Additionally, reinforcing fibers also improves thermal stability and decreases shrinkage and warpage.
Reinforcing materials can be extruded simultaneously (coextruded) with the plastic polymer to improve strength and provide additional performance benefits.
The most common coextruded reinforcement materials are other polymers with high durometer (hardness). During the coextrusion process, the two materials form a strong molecular bond to improve the strength of the finished part.
Alternatively, pre-formed metal can be coextruded with the polymer to create a plastic-metal hybrid extrusion. This method greatly improves strength and can provide additional benefits such as magnetic attraction and the ability to hold a bend without external support.
Plastic polymers can be filled with minerals, metals, and/or other polymers to reduce costs and/or enhance performance.
Extender fillers are less expensive than plastic and quite literally “extend” the use of the polymer to which they’re added, thus reducing the overall cost of the compound. Examples of extender fillers include calcium carbonate, silica, and clay.
Functional fillers go a step beyond extender fillers and provide performance enhancements in addition to cost savings. For example, conductive fillers such as carbon, graphite, and aluminum can improve electrical and thermal conductivity while reducing cost
Any material not deemed a reinforcement or filler would be considered an “additive” material. These are special ingredients that enhance or introduce a specific property without adding too much bulk. Common additives to plastic extrusion materials include:
- UV inhibitors and stabilizers to prevent UV degradation
- Adhesion promoters to improve bonding to other materials
- Internal lubricants to reduce abrasion and prevent sticking
- Impact modifiers to improve shock and crack resistance
- Flame retardants to resist combustion
- Colorants to color and protect against color fade
- Plasticizers to increase flexibility
- Foaming and blowing agents to decrease weight
Selecting a Plastic Extrusion Material
The first step in selecting a material is to determine which grade of plastic to explore. Unless your application is in an exceptionally harsh environment, start with the commodity grade plastics. Next, decide which molecular structure, amorphous or crystalline, will provide your desired properties. Within that category, choose the plastic that best fits your application. Remember, most plastics can be reinforced, filled, or enhanced with additives.
For the best match, ask your extruder or resin supplier about your selection. They can offer suggestions to optimize the material and design of your part for additional cost savings and performance benefits.
Resources for Engineers
If you’d like an expert opinion on the material or design of your plastic extrusion, call the extrusion team at Gemini Group. We’ve spent over 40 years extruding and coextruding dozens of commodity and specialty resins and we’re eager to help you succeed.
To learn more about plastic extrusion material selection and design, download your free copy of the Plastic Extrusion Design Guide. Simply fill out the form below to get access today!
Gemini Group’s extrusion team is comprised of Gemini Plastics, Sierra Plastics, and Gemini Plastics de Mexico. Our sales engineering team would love to hear from you. Feel free to contact them at 248-435-7271 or firstname.lastname@example.org.