With so many varieties of plastic tubing to choose from, a design engineer has many difficult choices to make when prototyping a new medical device. Of all the materials used for such applications, polyethylene most often leads the way.
Introduced to the world of manufacturing at the time of FDR, polyethylene has since made many applications easier to manage, safer for consumers(compared to earlier metal counterparts), and cost-effective enough to mass-produce.
When choosing a type of polyethylene, mechanical factors always come first, because they are the basic requirements needed for a design to function. Fortunately, polyethylene is extremely versatile and most mechanical requirements can be met with many possible formulas. Cost must also factor into the decision, as all consumer products have a price point which limits their allowable manufacturing cost. Understanding the properties of the different grades can assist a design engineer in the selection of thermoplastic materials for products that use custom plastic tubing.
LDPE(Low Density Polyethylene) – The first invented grade of polyethylene, LDPE remains the most commonly used density. In addition to being useful for plastic tubing, LDPE is also used for plastic bags, food storage, computer/car components, general purpose containers, and many other things. While it has a lower tensile strength than the higher density grades, it has a higher resilience(maximum energy per unit volume that can be elastically stored) which makes it very flexible.
HDPE(High Density Polyethylene) – While it has many of the same applications as LDPE, it is harder, more opaque, and somewhat more resistant to heat and chemicals. It is often used for outdoor scenarios where there is a large temperature range as well as containment scenarios where chemicals need to be isolated from the environment over a wide area.
LLDPE(Linear Low Density Polyethylene) – Slightly harder to process than normal LDPE, LLDPE has higher tensile strength, impact resistance and puncture resistance. Basically this means that a thinner layer of plastic can remain intact under flexibility testing. Its primary use is in flexible tubing, but it is also used for plastic wrap, toys, lids, cable coverings and more.
UHMWPE(Ultra High Molecular Weight Polyethylene) – More expensive than most other grades of polyethylene, UHMWPE has the highest impact strength of any thermoplastic presently made. It is often referred to as high performance polyethylene and is typically reserved for “unbreakable” scenarios like artificial bone replacements, bulletproof vests, etc.
VLDPE(Very Low Density Polyethylene) – Because VLDPE is characterized by even lower heat resistance than LDPE, it is often used in packaging for frozen food and ice. Some tubing and stretch wrap is also made from VLDPE and it is commonly blended with other polymers as an impact modifier.
PEX(Cross-linked High Density Polyethylene) – PEX is almost exclusively used for long-term tubing scenarios. Many thermal properties of the plastic are improved by the cross-linking process. It maintains strength at a higher temperature and reduces flow. Under low temperatures, impact resistance, tensile strength and scratch resistance are improved. Cross-linking also improves the chemical resistance.