Glossary: What Is Creep in Molded Plastic Parts?

In plastic molded parts, “creep” refers to the phenomenon in which the part gradually deforms over time when subjected to a constant load for an extended period.

This phenomenon is particularly significant in plastics compared to metals.

Here is an example of this phenomenon:

Suppose you place a book on a plastic shelf bracket.

At first, it only sags slightly, but as the load continues to be applied,

Initially → Sags slightly; 1 day later → Sags further; 1 year later → Sags significantly.

In this way, the deformation progresses even if the load itself does not increase.

This is called creep.

Why does plastic creep?

Plastic is not a perfect solid like metal; it also possesses the properties of a viscous material.

Therefore, when subjected to a load,

instantaneous elastic deformation → deformation that progresses over time → does not fully return to its original state even after the load is removed

This is the behavior it exhibits. Temperature has a particularly significant influence.

Generally speaking,

The higher the temperature, the more rapidly creep progresses.

It is safe to assume this.

For example, even plastic parts that pose no problems at room temperature may experience changes in dimensions or clamping force due to creep in high-temperature environments, such as the engine compartment.

Where do these issues arise in injection-molded parts?

In actual products, there are cases such as the following:

Parts and Applications	Problems Caused by Creep
Plastic Bracket	Deflection Due to Load
Screw fastening section	Decrease in Clamping Force
Snap-Fit	Decreased holding power
Gear	Changes in Tooth Shape
Bearing Retainer	Changes in Hole Diameter
Connector	Decrease in Contact Pressure
Plastic Hook	Gradually stretching or deforming
Automotive Parts	Deformation at high temperatures

Screw fasteners, in particular, are a clear example.

When a plastic part is tightly fastened with a screw, it initially provides sufficient clamping force. However, over time, the plastic around the screw may gradually deform, causing the clamping force to decrease.

Difference from “Stress Relaxation”

Stress relaxation is often mentioned in the same context as creep.

Although these two phenomena are similar, they occur under different conditions.

・Creep

Keep the load and stress constant

→ Strain (deformation) increases over time

・Stress relaxation

Keep the amount of deformation constant

→ Stress and clamping force decrease over time

For example, if a plastic clip is held in a state where it has been spread apart by a certain amount, it will initially exert a strong spring force, but this force will diminish over time.

Varies Significantly by Material

Creep characteristics vary considerably depending on the plastic.

For example, as a general trend:

Crystalline resins

PP, PE, PA, POM, PBT, and others exhibit significant changes in creep behavior depending on temperature and load.

On the other hand, amorphous resins

ABS, PC, PMMA, PS, and others also exhibit creep.

Therefore, it cannot be generalized that “since it’s a resin, it will creep to this extent”; factors such as resin type, temperature, stress, time, and the presence or absence of fibers must be taken into account.

What happens when glass fibers are added?

This is a very important consideration in injection molding.

For example,

adding glass fibers—such as PP with 30% glass fiber (GF30%)—generally helps suppress creep deformation.

This is because glass fibers themselves have higher rigidity than the resin, which helps limit the resin’s deformation when subjected to prolonged loading.

However, this does not mean that “adding glass fibers completely eliminates creep.”

Especially in injection-molded parts, creep characteristics vary significantly depending on fiber orientation.

For example,

stiffness and creep characteristics change depending on the load direction and the orientation of the glass fibers—such as in the case where the fiber direction is greater than the direction perpendicular to the fibers.

This is also related to the “issue of different shrinkage rates depending on fiber orientation” that we discussed previously.

To summarize in a nutshell:

Creep is “the phenomenon in which a resin gradually deforms over time even when subjected to a constant load.”

In the design of injection-molded parts, this is a very important property because it is necessary to consider not only **“whether the strength is sufficient” but also “whether the part will remain free of deformation even 10 years later”**.