Polymers Help Reduce Vibration in Appliances
Designers can use vibration-damping materials to improve performance and enhance the user’s experience.
Vibration in appliances can cause a range of problems, from excessive noise to wear and tear on components. Unwarranted vibration may give consumers the perception of poor quality or poor design. The vibration of large appliances (such as a washing machine) can be transferred to the floor. In some cases this vibration can be a minor annoyance, but in others it could cause physical damage to the structure around the appliance. For handheld appliances, vibration can cause discomfort or, in the extreme case, possibly even injury. Fortunately, vibration can be reduced through a combination of product design and materials that reduce vibration.
What causes vibration?
Vibration is essentially oscillatory motion that results from a physical system being pushed out of equilibrium. Sound is a type of vibration. Although the vibration of a violin’s strings is generally a desirable sound, sound waves from an appliance vibrating is typically unwanted noise.
Sometimes the cause of a system being pushed out of equilibrium is a discrete force, such as the opening of a valve, and sometimes an external force, such as the turning of an electric motor, continuously drives the system. In an appliance, the rotation of the motor or other imbalances can cause vibration of the housing, which can be heard and/or felt. Vibrations that have a frequency at or near a material’s natural frequency are of particular concern because they can cause mechanical resonance, in which the system can oscillate with increasing reinforced amplitude.
How can vibration be reduced?
The first line of defense is to fix the source of vibration, such as improving the balance of an oscillating mechanism. Another tool is to isolate the vibrating part with a sub-frame or springs to minimize the energy of vibration from being transferred to the rest of the system. If mechanical resonance is a concern, it can be cancelled using a tuned mass damper.
Insulating barriers can be used to prevent the transmission of vibration energy. Mass-loaded, flexible polymer sheets, for example, can be used for sound deadening. Similarly, materials can be used for absorbing vibration energy and sound waves. Polymer foams with open cells and polymer composites with fibrous structures (such as fiberglass) can absorb vibrations.
Another solution is that appliance housings or other components made from polymers can dampen vibrations by absorbing the vibration energy into the polymer molecule. A new polymer technology available to appliance designers was created to perform this function even better than existing polymers.
Polymers for vibration damping
Because different polymer types have different molecular structures, they have different vibration damping capabilities, which can be measured with a laboratory test called dynamic mechanical analysis (DMA). Under stress, a polymer has both elastic behavior (like an elastic solid) and viscous behavior (like a Newtonian fluid). DMA is a spectroscopy technique that measures the ratio between the viscous and elastic responses of a viscoelastic material such as a polymer that exhibits both behaviors.
This ratio is reported as the tangent of the phase angle (delta) or “tan delta,” and it corresponds to a material’s vibration damping capability. A higher tan delta peak indicates that a material has a greater ability to absorb and dissipate the energy of vibration at a given temperature.
Thermoplastic elastomers (TPEs) are inherently one of the most effective polymers for vibration damping, because they have both hard and soft segments in the polymer chain.
Where can TPEs be used in appliances?
Thermoplastics are well known to appliance designers for benefits such as design freedom and being lightweight, strong, and colorable. A key benefit of thermoplastics over other materials is that they can be easily melted (and remelted) and formed into diverse final shapes using melt processing, such as injection molding or thermoforming. Complex and compact shapes or unique contours can be readily created. Multiple components can be integrated into one piece to eliminate the need for screws, welds, and bonding materials.
Vibration-damping TPEs are a special kind of thermoplastic that have all these benefits as well as the ability to absorb significantly more vibration. In contrast, thermoset polymers, such as urethane foams used for vibration damping, have more limited design freedom, because they cannot be formed using melt processing techniques.
A special melt processing technique called “overmolding” can integrate different types of polymers into one piece during molding to form the final shape. The stiffness and heat deflection temperature properties of polycarbonate (PC), for example, can be combined with the soft and elastic touch of a TPE. One type is molded on top of another with complete bonding created during the forming process. The overmolding elastic polymer can completely cover the more rigid substrate, or it can partially cover the substrate. A common example of overmolding is a toothbrush with a soft grip of TPE on the hard plastic substrate. Designers can take advantage of this freedom to create systems that reduce and absorb vibration.
For example, a TPE can be used to overmold a gasket on a housing. Because the gasket is an integrated part, the extra assembly step of attaching the gasket is no longer needed, which reduces manufacturing cost. The TPE gasket will act as a vibration isolator and will reduce vibrations where the enclosure parts meet by absorbing the vibration energy. TPE gasket materials have good compression set properties that result in long-lasting seals, even at elevated temperatures.
TPEs can also be overmolded on the outside of a housing to add impact protection and abrasion resistance, as well as absorb vibration.
Overmolding grades can be formulated to bond to various types of polymers or even to metal. For example, a TPE can be designed to overmold onto PC, ABS, and PC/ABS blends, or to nylon (i.e., polyamide), polypropylene (PP), polystyrene (PS), or copolyester (COPE).
Overmolding polymers and part design
Polymers used for overmolding must be able to flow easily into thin-walled sections. Overmolding TPEs are shear-thinning—at a higher shear rate, they have a lower viscosity (i.e., flow more easily). The following design principles are important to consider when planning an overmolded part:
- Use uniform wall thicknesses for the substrate and overmold; any transitions should be gradual
- Thicknesses from 0.060” to 0.120” are typical for the overmolded material
- Use radii (0.020” minimum) in sharp corners to help reduce localized stress
- Long draws should have a 3° to 5° draft.
Shrinkage of an overmolding TPE can be greater than that of the substrate in some cases, which can result in warpage or cupping. These problems can be minimized by using the following techniques:
- Use a higher modulus substrate
- Add stiffening ribs to the substrate part
- Minimize the TPE thickness
- Use a TPE with a lower Shore hardness
- Relocate the gate to minimize the flow length to thickness ratio.
The versatility of thermoplastic elastomers can be harnessed to create new and better designs, including those that reduce the negative effects of vibration. Polymer compound suppliers can answer questions about choosing the best hard-and-soft polymer combinations and can work with designers to minimize unwanted vibration in an appliance design.