TECOS has been offering support to Slovene and foreign partners in tooling, plastics and sheet metal processing with simulations and elastomeric analysis for 23 years. For some partners, we also provide comprehensive services, such as the development of new products.
Our customer L-TEK turned to us for help in the development of a portable ECG system. This system would allow an individual to monitor their own heart and other life functions. Through an application on their mobile phone, it can detect the correct or incorrect heart function, and then can seek help from their doctor if needed. The project is called SAVVY www.savvy.si.
Our input was the outer surface of the product. The basic shape of the product is two drop shaped elements connected by a silicon cable. The larger drop contains the electronics and battery, while only electronics can be found in the smaller drop. Human life functions are monitored through two electrodes that a person places on their chest. These electrodes are connected to electronics in both droplets via special buttons.
From the function of the product, the design requirements are as follows:
- The product must be waterproof, including both buttons and at both cable outlets.
- The product must be tough enough so as not to shatter on falling on the ground.
- The product should be as rounded as possible, because it is worn by the user under clothing, both in daily life and in activities such as running or cycling.
- The product is in contact with the skin, therefore medical material must be used which does not induce any reactions to the skin or transfer the substance from the product to the human body.
- The temperature of the product is from -20°C to +50°C.
- The weight of the product is very important, as the user wears the product all the time. The target weight was 21g, which we successfully reached in the end.
Figure 1: The drop shaped elements connected by cable
The construction of plastic products always starts with the known functions of the product, while at the same time the technological and technical constraints of processes and materials must be taken into account at the design stage. Processes include processing of tools, plastic processing, as well as integrating technology. The material characteristics are derived from steels used for tools, thermoplastic materials, dyeing agents for thermoplastics and adhesives used for bonding.
The polypropylene PP Bormed RF 830MO was selected as an injection molding material. The material has all the required certificates to allow it to be used for pharmaceutical and medical purposes. Bormed was a transparent material, so it had to be coloured with a green dye. The toughness and temperature stability of the material is satisfactory.
The largest disadvantage was the weight limit of the whole product at 21g. The basic thickness of the product had to be small enough so that the weight would not rise beyond the desired limit. At the same time, we had to be careful that the product had a satisfactory thickness for filling the mould cavities of the tools.
Additionally, the elements of fastening and joining must be mentioned. We had to pay attention to the appropriate thickness ratio on all the elements that were attached to the basic thickness of the product. An inadequate ratio leads to sink marks and deformation of products. Using MOLDFLOW analysis, we checked the filling, the injection pressure and the differences in volumetric shrinkages over the surface of the products. The regular model corrections and the implementation of simulations enabled us to quickly and efficiently optimise products to the desired shapes and specifications.
We had issues with the design of the ribs and other connecting elements. The ribs should be as thin as possible so that the base surface does not show sink marks, but at the same time it must be strong enough to perform its positioning function of the battery and the circuit. Very thin ribs can also not be filled or they do not have enough strength and break even during the removal phase of the product from the tool.
Figure 2: The drop shaped elements and charging station
The integration of technology into the final product was amongst the most demanding. The drop-shaped product appears very simple for manual installation, but automatic assembly is more complicated. Possible assembly operations were ultrasonic bonding or gluing. Ultrasonic bonding operates on the principle that mechanical energy at the weld site is converted to heat. The problem is that the connection energy is to be introduced over the weld, which in the case of a drop is not possible. With special construction solutions, we made the shape of ultrasonic welds in the form of a U profile with special energy routers, which enable the welding process to start at a precise location and control the flow of the molten material into the side walls of the weld.
Ultrasonic bonding is ideal for larger product lines, so L-TEK decided to change the method of bonding during the project start-up. Smaller product corrections were needed, since the bonding technology with adhesive on the joint should leave adequate clearance for the adhesive. However, in ultrasonic bonding, the product to be connected is spaced by the height of the “collapse”. This collapse means how much material is poured and the weld is filled. The bonding of PP products is problematic because the surface tension needs to be changed accordingly, if we want a waterproof compound. This can be obtained by appropriately activating the surface with various processes such as flame activation, plasma, or corona procedure. In the case of SAVVY, plasma surface activation was selected to ensure proper surface preparation and high-quality bonding.
Work on the SAVVY project did not always run smoothly, but all of our partners from SAVVY, L-TEK and TECOS did their best to reach the desired design and quality. The product is now on the market and users will now be able to comment on the usability and quality of the product.