Since the 90s, membrane keypads, keyboards & switches were extensively used in the industrial sector particularly in household appliances like washing machines, because of their attractive appearance, low weight, compact size, and lengthy lifespan. The procedure of using pressure-sensitive adhesives to join the keyboard’s membrane switch onto the washing machine housing is easy and efficient and the bond strength is sufficient. It’s one of the most popular techniques for assembly. As the standard of living continues to rise and improve, the occasional keyboard lift-up issue will not be a problem for the individual consumer however, in extreme cases it can also impact the image and reputation of the appliances manufacturers. This is a concern which major appliance manufacturers are devoted to and are keen to resolve.
This article brings together years of research results of Linepro in the bonding field and in the home appliance industry and offers possible solutions to this membrane keypad and switch issue.
Analysis of the reason for the keypad lifting up
The keypad of the membrane switch in the washing machine is made up of mechanical micro switches with distinct components as well as the membrane panel. When in use, the buttons on the panel are controlled to perform functions by extremely flexible film. The design of the bonding system consists of disk film, pressure sensitive adhesive, and chassis.
Membrane keyboards typically make use of transparent and flexible polyester film with buttons displayed on their back. The casing is molded with a mold. It is typically employed materials include Engineering plastics (ABS) or polypropylene (PP) and the the panel is flat. Pressure-sensitive adhesives typically make use of adhesive films that are not substrate-free as well as PET-based double-sided tapes. The disk film as well as the chassis are linked through adhesive force.
If there is external or internal stress and the force exceeds the actual force of adhesive from this pressure-sensitive adhesive it will break off from the side that has lower force of adhesive and the adhesion breaks down. Based on feedback from customers, the disk film lift up occur most frequently after both low and high temperature environmental shifts, and it is primarily on the side in which there are buttons.
In the illustration, from the point of the view of defective products that are on sites, it is evident that the tape remains on the other side of the film disk after the disk film has been arched, and it is then separated from the casing’s surface, the reason why the failure of adhesive in the area between tape and the casing takes place.
Membrane keyboards and casings which have sufficient matching level in ambient temperature outstrip the casing by 2 to 3mm after being exposed to an environment with a temperature below 25 degrees during 24 hours of experiment. The results of experiments show that the casing’s deformation is higher than the deformation for the keyboard during change in temperature of the environment. The difference in both causes internal stress, which is in opposition to the force of adhesive on the tape and causes the failure of bonding.
Methods to enhance the lift up of membrane keypad
Based on our prior research, we have discovered that in order to overcome the issue of failure in bonding it is necessary to start by reducing internal stress and increase the force of bonding.
Methods to decrease internal stress
On the other side, you should consider reducing the amount of deformation that is shared between materials that come from the same source. It is suggested to choose materials that have similar or identical thermal expansion coefficients to the greatest extent feasible for the disk membrane as well as the casing to decrease the deformation of the two materials. However, it is possible when designing the look of the casing that it leave a space for deformation of the membrane, so that there is no interference between the membrane as well as the casing following deformation, which will reduce the stress inside. If the material’s structure and the ambient temperature aren’t easily changed and the adhesive strength of the tape needs to be increased to the greatest extent feasible.
Methods to increase adhesion
Select a glue method which is more suitable to bond plastic materials
The surface of a material is characterized by a certain energy. Materials with different surface energies have a significant impact on the extent in which tapes wet. If the energies of the substance are greater than that of the tape it can be easily joined by the tape and a stronger bonding strength can be easily established. If the energy of the surface of the product is less than the power of the tape, the material will not be joined by the tape, and it is hard to determine the strength of the bond.
Materials like ABS and PET are both materials that have higher surface energy in plastic products. However, some materials, like PP are not high in surface energy materials. Printing inks’ surface energies can differ due to the varied ink types. However, in the majority of cases the surface energy of the printed surface is lower.
To bond different materials, pressure-sensitive adhesives that have distinct performance characteristics must be chosen. For materials with higher surface energy, it is advised to select adhesives that offer superior adhesion and resistance to aging high surface energy materials. If you are looking for materials with lower surface energy, it’s suggested to select products with greater wetting capacity and are able to quickly increase bonds.
Use a standardized and optimized bonding processes
Increase the effectiveness of bonding area
The strength of the adhesive tape is proportional with the effective area of bonding. In other words, the greater the area of bonding is, the greater the strength of bonding. The case is designed with holes for buttons. In this location, the area of the adhesive of the tape will be reduced and it is inevitable that the strength of the local adhesive decreases accordingly. When the number of key holes is too concentrated, the bonding area shrinks and will become the weakest point within the whole bonding system. It is suggested to design the layout for the button, take into account the area that can be bonded simultaneously and the most sensible layout will maximize the local bonding area to it.
Appropriate surface treatment
The surface cleanliness of the material directly affects the effect of bonding tape. Any foreign objects like dirt, dust and moisture can hinder bonding between the tape as well as the bonding material which can affect the strength of the build-up. Particularly, plastics are more likely to leave release agents on the surface when they are being molded. The presence of release agent can greatly affect the ability to wet the tape as well as affect its bonding capacity. In general, prior to bonding, the surface of the tape should be dry, clean and smooth, devoid of moisture, dust, and oil.
The bonding tension should be increased.
Once the keyboard’s membrane is secured to the case’s surface, appropriate pressure must be applied to the position of bonding to ensure that the tape will fully adhere to the surface to be bonded in order to establish the bonding force. In order to avoid damaging the structure of the product applying more pressure and making sure that the force is uniform across the bonding area will increase the adhesion initial to the surface.
Make sure you are able to guarantee the curing time and temperature for the bond.
The temperature of the bonding is crucial in the formation of the bonding strength. It is influenced by the environment and the temperature of the surface of the tape while the process of bonding. When temperatures are low, the movement of the glue molecules slows and it becomes difficult to rapidly penetrate the surface that is to be bonded and this is evident in the low adhesion at first and the slower establishment of bonding strength. To achieve more efficient performance, it is suggested to complete the design of the tape in an environment that is not less than 15 degrees Celsius.
At Linepro, we understand the problems and effects of membrane keypads, keyboards and switches for consumer electronics or home appliances. For getting consultations on membrane keypads for any application, share your project ideas and we will start guiding you the best available solutions.