Everyday we encounter thousands of plastic injection molded products that we don’t give a “high value” too, but we expect these products to work flawlessly when we need them. Examples could be the living hinge on a plastic lid, a cap on a soda bottle, or a zip tie. When using these everyday products do you ever consider all that makes them work? All of these injection molded parts perform simple functions and appear to be simple in design, but work because of decades of development in plastics material science and product design. Odds are you probably haven’t put much thought into their engineering unless you have been asked to design one.
Consider the injection molded zip tie example and think about all the factors that make the simple zip tie work. First you have the physical properties of the plastic material. The interface of the molded pawl (lever) and ratchet must be in perfect alignment to ensure they make contact. The angles and tolerances of the molded pawl and ratchet must be considered to ensure a positive lock. When you start to break down how the simple zip tie works you find it is not as simple as it seems. Now take understanding of the zip tie and place it with a first responder or hospital ER and envision a doctor who must quickly secure the device while wearing gloves and working under pressure. Recently I have been working on an injection molded medical device that serves as an endotracheal tube holder and provides a safe way to insert an orogastric tube into the esophagus of a patient. This device design solves many problems for hospital and first responder medical technicians. The device secures the ET tube assembly to the patient, eliminates the discomfort and risks of feeding tubes through the nose to access the stomach, and provides a bite block to prevent a patient from closing the air way. The zip tie is a critical functioning component and many factors have to be perfect for a zip tie to work in this environment. We will discuss some of the challenges encountered with this design and the steps Blackwell Plastics took to solve the problems.
The illustrations show how the design planned for the assembly of the final product and how it would function. The device includes two components that would be bonded together. To assemble the final part, the tube from the lower piece is inserted inside the cylinder on the top piece and solvent bonded in place. The material originally chosen for this application was flexible PVC, with a higher durometer for the top section and a lower durometer (softer/ more flexible) part for the bottom section.
We recognized the critical variables that would impact the mechanical performance of these performing areas. Before the tools were made we anticipated there might be adjustments to certain features so we stayed “steel safe” and inserted certain parts of the mold. This allowed us the flexibility to make quick and easy changes if needed. We also built the tools in stainless steel to allow broader choices of plastic materials including PVC. Following first shots the customer found that the “zip tie” would not hold the ET tube without it slipping. The pawl (lever) wouldn’t engage the ratchet (strap). Under close observation we saw that the strap was deflecting away from the lever because the amount of clearance or “play” behind the strap. We also noticed that the plastic lever would fatigue after a couple of uses and wouldn’t hold enough pressure on the strap to secure the lock. After the problems where identified the mold was easily modified to take up the clearance and thicken the pawl to restrict flex movement. The improvements made the lever stronger, but the 90 durometer PVC material still allowed the lever to bend out of position. The material needed to be flexible enough for the strap to bend, but rigid enough for the lever to engage the teeth on the strap. We were asking the one material to perform as both a rigid and flexible material. We elected to go with a LOWER DUROMETER MATERIAL to give flexibility we wanted in the strap and add material to the lever to pin it down and force it to make positive contact with the strap; this would allow a lower durometer material to make a positive lock. The final adjustments made to the lever were enough to give it the strength needed. Finding the right balance between strength and flexibility was difficult but crucial to the part’s performance. This small example illustrates how difficult it can be to predict exactly how a design will perform when you take into consideration all the different factors that can affect performance. In early stages of the project when planning for tooling it is important to anticipate design challenges and plan flexibility in the tool design so that modifications can be made to critical components without significant rework and cost.
These challenges where only a few of the issues we encountered while bringing this plastic injection molded product to life. The parts have gone through several transformations since the first samples were molded. Early on we understood the design may need to be adjusted and we where able to plan accordingly. Taking the proper precautions helped to save time and costly tooling changes. None of these changes happened overnight, and it took team work and the dedication of several people to make the product a success. That persistence will now benefit those in the medical community and those who receive medical care. If we can help you bring your product to life or for more information on product design and injection molded parts go to www.blackwellplastics.com You can also reach us at 713.643.6577 or email at Engineer@blackwellplastics.com.