When Plastics Need Memory

 

When we speak of memory in plastic materials, we are talking about a plastic parts ability to return to its original shape after it is deformed in some way. More specifically, we are talking about a part being able to return to its original shape after being held in a flexed position for an extended period of time.

The question is, what makes a material have good memory and what materials are best used for applications that require this property?

There are three properties to consider when looking for a material that has good memory properties. They are high tensile elongation at yield, high tensile strength at yield and good creep resistance. Lets break these down a little further.

1. Tensile elongation (or strain) at yield

When a part is flexed or bent, the outside surface of the part undergoes a tensile load. In other words, it is being stretched. When a material is put under a tensile load, it begins to stretch or elongate. At first the material experiences what is called elastic deformation meaning that if the load is released, the part will bounce back to its original size. At a certain point, the deformation will transition to what is called plastic deformation meaning that it is permanently deformed. The part will not return to its original shape after plastic deformation has begun. The plastic deformation occurs before fracture though.

The point at which the material transitions from elastic deformation to plastic deformation is called the yield point. It can be measured on traditional tensile strength testing equipment.

In order for a material to have good memory, it must have a relatively high tensile elongation (sometimes called strain) at yield. In layman’s terms, we need the material to be able to be stretched quite a bit before it will not bounce back anymore. This property has the most influence on the memory of the material.

2. Tensile strength at yield

It is also important that the material have a relatively high tensile strength at the yield point. Tensile strength is a measurement of the stress that is required to stretch the part. Tensile strength is commonly measured at the yield point (the point at which the material transitions from elastic to plastic deformation) and at break which is the stress required to fracture the specimen.  It is the stress required to stretch the specimen to its yield point that is important for memory. The material needs enough strength to avoid being pushed past the elastic range too easily.

3. Creep

According to Wikipedia, the definition of creep is:

“The tendency of a solid material to move slowly or deform permanently under the influence of mechanical stresses.”

Plastics are considered visceolastic materials in that they exhibit both the properties of a viscous liquid and an elastic material at the same time. Viscous liquids flow over time, like maple syrup, while elastic materials bounce back like a rubber band when a load is removed.

It would seem that a stress that is below the tensile strength of a material would not deform the plastic part and it won’t in the short term. But over time, even a small stress, will permanently deform the plastic part. This is the effect of creep in a nutshell.

On a molecular level, what is happening is that the long, chain like molecules that make up plastic are slowly sliding by one another. The bonds between the molecular chains in most plastic materials are weak compared to the bonds within the chains and under stress this will allow the chains to move in relation to one another.

In order for a plastic material to exhibit good memory properties, we need the material to avoid permanent deformations over time as much as possible. Only materials with excellent creep properties can have good memory properties.

Crystalline vs amorphous materials

Some plastic materials are considered to be crystalline and some are considered to be amorphous. Crystalline materials tend to have more rigid molecular structures and amorphous materials tend to have more random molecular structures. You could compare the molecular structure of an amorphous material to a plate of spaghetti. Crystalline materials tend to have better creep properties because of their more rigid molecular structures. Common crystalline materials are PP, PE, Nylon, Acetal and PBT. Common amorphous materials are PC, Acrylic, ABS and polystyrene. You might notice that amorphous materials are more likely to be clear. The amorphous molecular structure is more likely to allow light to pass through the material.

Materials with good memory

If you focus on tensile properties, we can clearly see that materials like nylon, acetal, polycarbonate and PBT exhibit properties that are most conducive to having good memory. They have excellent tensile strength and elongation properties. Also, notice how much better the tensile properties are for acetal homopolymer than copolymer. It should be noted that the properties of nylon are significantly different at 50% relative humidity than when tested dry as molded. In extremely, dry environments, the elongation of nylon is significantly lower.

Here are some common materials that their approximate tensile strengths and elongations at yield.

Material                                   Tensile Strength at Yield (PSI)       Elongation (strain) at Yield (%)

Nylon 6/6  (50% RH)                8000                                                         25

Nylon 6/6 unfilled (dry)         11900                                                        4.5

Acetal Copolymer                    8800                                                           10

Acetal Homopolymer             10300                                                          17

Acrylic                                         10200                                                         4.0

Polycarbonate                           9000                                                          7.0

Polypropylene Copolymer    3900                                                           5.2

PBT unfilled                               7500                                                           10

ABS                                                5400                                                         3.0

PC/PBT                                          7200                                                        4.0

PC/ABS                                         7830                                                         4.7

PPO                                                7200                                                         3.0

Testing for creep properties is a little more difficult but I will just say that Acetal, nylon and PBT are crystalline materials and have better creep resistance than the other materials listed. Polycarbonate is an amorphous material and does not have great creep properties but does have excellent tensile strength and elongation.

Conclusion

Ideally, for any application that requires a high amount of memory, the best materials are going to be crystalline materials which have better creep properties along with high tensile elongation and tensile strength.

It should also be noted that reinforcements or fillers such as glass fiber or mineral will tend to reduce the tensile elongation dramatically and thus are going to harm the memory properties.

There are really four materials that should be considered for applications requiring memory.

1. Acetal Homopolymer: Acetal homopolymers such as Delrin exhibit the best tensile strength and elongation and because it is a crystalline material, exhibits the best creep properties as well. Acetal also has excellent coefficient of friction, meaning that it is slippery, which is great for clasps and buckles. Acetal also has good stiffness. This should be the first choice if you need really good memory.
2. Acetal Copolymer: Acetal copolymers such as Celcon have excellent properties as well even though they are not as good as homopolymer. Copolymers tend to be less expensive because of the competition in the copolymer market though so you might consider it for that reason.
3. PBT: PBT similar to nylon in properties but is not affected by moisture like nylon is. Also, PBT is an excellent electrical insulator and is thus a really good choice for electrical connectors that might have snap fit features requiring memory.
4. PC: Although polycarbonate is amorphous, it still has pretty good memory properties. If you need a clear part with memory, PC is the only choice here. Also PC has extremely good impact strength.
5. Nylon: Nylon has excellent memory in wet conditions, but not such great memory in dry conditions. It can be used in some applications but caution must be taken. I will typically recommend that people consider PBT instead.
6. PPO: PPO is typically alloyed with either polystyrene or nylon. It has decent memory but not as good as other engineering grades. If you have cut a tool for ABS and need something with more memory, PPO might be a good option.

Memory in plastic materials is an important property to consider when choosing materials. For snap-fit features, memory is more important if the part will need to be repeatedly unsnapped and snapped. One time assembly type snap fits might make memory less important but you will want to make sure that the tab on the feature is not under constant stress to avoid the snap fit weakening over time. Plastic clasps and buckles which are commonly used need excellent memory properties in order to last for the lifetime of the product that they are installed on. Just remember good tensile strength and elongation, creep resistance, and crystalline.