Since the 1990s, membrane switches have been widely used in the industrial market, especially in household appliances such as washing machines, due to their good appearance, light weight, small size, and long life. The process of using pressure-sensitive adhesive to assemble the keyboard membrane switch to the washing machine housing is simple and convenient, and the connection strength is appropriate. It is currently one of the mainstream assembly methods. As people’s living standards continue to improve, the occasional keyboard membrane lift up problem will not only affect the satisfaction of individual consumers, but in severe cases, it may even affect the brand image of home appliance manufacturers. This is a problem that major appliance brands attach great importance to and are eager to solve.

In this article, JRPanel combines years of service experience and research results in the bonding application field and the home appliance industry, and puts forward some feasible ideas and solutions for this problem.

1.Analysis of the cause of the keypad lift up

The membrane switch/keypad of the washing machine is a combination of a mechanical micro switch with discrete components and a membrane panel. During operation, the buttons under the panel are driven to achieve functions through the highly elastic film panel. The structure of the entire bonding system includes disk film, pressure-sensitive adhesive and chassis.

Membrane keyboards generally use flexible and transparent polyester (PET) film, with button patterns printed on the back. The casing is injection molded by a mold, commonly used materials such as engineering plastics (ABS) or polypropylene (PP), and the surface of the panel area is relatively flat. Pressure-sensitive adhesives often use substrate-free adhesive films or PET-based double-sided tapes. The disk film and the chassis are connected by adhesive force, and holes are opened at the buttons to achieve functions.

When there is external force or internal stress and this force is greater than the actual adhesive force of the pressure-sensitive adhesive, the tape will first separate from the side with the smaller adhesive force, that is, the adhesion failure occurs. According to customer feedback, the disk film lift up mostly occurs after experiencing high and low temperature environmental changes, and is concentrated in the position where there are buttons.

As shown in figure, from the point of view of defective products on site, the tape stays on the side of the disk film after the disk film is arched, and is separated from the surface of the casing, that is, the adhesive failure between the tape and the casing occurs.

The laboratory simulation test further verified this phenomenon. Membrane keyboards and casings that have a suitable matching degree at room temperature will exceed the casing by 2～3mm after being placed in a low temperature of -25℃ for 24 hours. The experimental results show that the deformation of the casing is greater than that of the membrane keyboard during the environmental temperature change. The difference between the two produces internal stress, which opposes the adhesive force of the tape and leads to bonding failure.

2. Methods to improve membrane keypad lift up

Based on the previous research, we know that to solve the problem of bonding failure, we must start from two aspects: reduce internal stress and improve bonding force.

2.1 Methods to reduce internal stress

On the one hand, consider reducing the relative deformation between different materials from the source. It is recommended to use materials with the same or similar thermal expansion coefficients as much as possible for the disk membrane and the casing to reduce the relative deformation between the materials. On the other hand, it can be considered when designing the appearance of the casing to properly reserve a space for the deformation of the disk membrane to avoid interference between the disk membrane and the casing after deformation, thereby reducing internal stress. When the product structure and ambient temperature are not easy to change, the adhesive force of the tape should be improved as much as possible.

2.2 Methods to improve adhesion

2.2.1 Choose a glue system that is more suitable for bonding plastic materials

The surface of a substance has a certain surface energy. Adhesive materials with different surface energies have a great influence on the degree of tape wetting. When the surface energy of the material is higher than the surface energy of the tape, the material is easily bonded by the tape and a higher bonding strength is easily established. When the surface energy of the material is lower than the surface energy of the tape, the material is not easily bonded by the tape and it is difficult to establish the bonding strength.

Materials such as PET and ABS are materials with higher surface energy in plastic products. However, materials such as PP are low surface energy materials. The surface energy of printing inks will be different due to the variety of inks. But in most cases, the surface energy of the printed surface is low.

For bonding different materials, pressure-sensitive adhesives with different performance characteristics should be selected. For higher surface energy materials, it is recommended to choose products with better adhesion and aging resistance to high surface energy materials. For materials with low surface energy, it is recommended to choose products that have better wetting ability and can quickly build up bonding strength.

2.2.2 Adopt standardized and optimized bonding process

(1) Increase effective bonding area

The adhesive strength of the tape is proportional to the effective bonding area. That is to say, the larger the actual bonding area of the tape, the higher the bonding strength. The case will be designed with button holes. In this area, the adhesive area of the tape is reduced, and it is unavoidable that the local adhesive strength will decrease accordingly. When multiple key holes are too concentrated, the effective bonding area will become smaller and become the weakest place in the entire bonding system. It is recommended that when designing the button layout, consider the bondable area at the same time, and the reasonable layout maximizes the local bond area.

(2) Appropriate surface treatment

The cleanliness of the surface of the material will directly affect the bonding effect of the tape. The presence of foreign matter such as surface dust, dirt, grease and moisture will hinder the contact between the tape and the bonding surface, thereby affecting the build-up strength. Especially plastic products are prone to leave release agent on the surface during the molding process. The presence of the release agent will significantly affect the wetting ability of the tape and affect the bonding strength. Generally, before bonding, the surface of the material should be clean, dry, and smooth, free of dust, moisture, and oil.

(3) Increase the bonding pressure

After the membrane keyboard is attached to the surface of the case, a certain pressure should be applied to the bonding position so that the tape can fully contact the surface to be bonded to establish the bonding strength. On the basis of not damaging the product structure, increasing the applied pressure and ensuring uniform force on the bonding area can effectively improve the initial adhesion of the tape.

(4) Guarantee the bonding temperature and curing time

The bonding temperature is critical to the establishment of bonding strength. It includes the environment and the surface temperature of the tape during bonding. Under low temperature conditions, the movement of glue molecules slows down, and it is difficult to quickly infiltrate the surface to be adhered, which is manifested by low initial adhesion and slow establishment of bonding strength. In order to obtain better performance in practice, it is recommended to carry out the adhesive construction of the adhesive tape in an environment not lower than 15℃。