A Pinhole is a small hole or cavity that penetrates the thickness of a blow molded balloon, as a result of a defect in the balloon. A defect could be a gel, debris, mold imperfection, or stress concentration from processing. Defects need to be properly inspected for during balloon inspection, in order to minimize pinholes. Therefore, it is advised to establish an inspection criterion regarding these defects, which sets the acceptance criteria. Typically, specifications are stated as the area of the defect. These defects can be inspected with a Tappi chart, which has calibrated marks corresponding to different areas. Tappi charts have areas between .02mm^2 to 5mm^2. The most requested callout is .2mm^2. These areas are shown as both lengths and diameter. Magnified inspection of balloons might be required depending on inspection criteria. The use of polarized light can be used to help visualize these defects as well as residual stresses through photo elasticity.
Pinholes occur in areas with extremely thin walls or high residual stress in the material. These typical failure points are then amplified by defects in the material. The preferred and standard failure mode is a longitudinal tear across the length of the balloon. The highest stress during loading should be the hoop stress. In this case the failure plane is normal to the hoop stress direction resulting in a longitudinal failure.
A pinhole failure occurs at a pressure lower than a longitudinal failure. This premature failure results in an increase in the standard deviation of the balloon burst pressure. The rated burst pressure (RBP) of a device is dependent on the average burst pressure and standard deviation of the balloon. An increase in standard deviation can significantly lower the RBP of the balloon and consequently the whole device.
Balloons used in medical applications are inspected for both dimensional and functional specifications. A typical functional test consists of a burst and a leak test. Burst and leak testing employs a series of increasing pressure steps, until balloon rupture. This test is critical to the validation and verification of both device design and the manufacturing processes and techniques.
Control over all raw materials is required. Any moisture absorbed during processing either through extrusion or blow molding could result in variation in material properties. Also, the storage of extrusion and balloons is critical. Both extrusion and balloons storage conditions should be controlled. An easy way to manage moisture is to keep the materials is bags with dry air. Desiccation of raw material is a common safety measure to decrease moisture during storage. It is also critical to control the extrusion processing. Extrusion should be dried prior to being extruded and then stored properly. There are other parameters during the extrusion process that can increase tendency to pinhole. If the extrusion die is too hot, run speed is too fast, or excess stresses are imparted during the extrusions process, additional pinhole failures may be observed.
In addition to controlling the temperature and moisture, the cleanliness of the material should be taken into account. Gel is a broad term to describe a small defect that creates a disruption in the polymers structure. Under this description is contaminant, unmelted polymer, highly entangled unmixed polymer, highly crosslinked polymer, or highly oxidized polymer. These defects create weak points in the material. The walls of the balloon thin out during the blow forming process while the defects stay the same size. As the wall thickness approaches the thickness of the defect the likelihood of premature failure increased.
There are several critical parameters to consider in order to get a high-performance balloon. Some critical design elements include material, wall thickness, and geometry. One common balloon material is polyethylene terephthalate (PET). PET is capable of thin walls
Other materials such as polyether block amide (PEBA) and polyamide (nylon) 11 or 12 are alternative options to PET. PEBA is offered in a wide range of durometers which allows catheter engineers the ability to optimize burst and flexibility for a given clinical application. Nylon does not have the same range of flexibility as PEBA, but nylon does offer higher burst pressures in comparable sizes and wall thickness.
PET is classified as a non-compliant material. Non-compliant balloons grow less than 5% between nominal pressure and RBP. In order to get compliance as low as 5% from a balloon the starting extrusion needs to properly sized. Ideally the sizing will take the material to its ultimate elongation during the blow forming process. PET has high ultimate elongation properties. As a result, the starting extrusion for a PET balloon is smaller than Nylon or PEBAs materials. The high strain and stresses amplify the defects in PET, resulting in an increased tendency of pinholing.
Although both nylon and PEBA offer improved performance over PET, they can still pinhole. If pinholing still occurs even after material changes, additional considerations should be made. Can the balloon wall thickness be increased? If not, what is the transition shape/angle between the major diameter and tail/neck/cuff diameter? A more gradual taper angle added fillets between transitions, and lower extrusion expansion ratios should all be considered to address pinholes. Finally, multilayer extrusions can increase balloon performance. By adding additional layers to the extrusion either of the same material or different materials the stress concentrations are constrained to an individual layer not the entire thickness.