With respect to Urethane molding, Urethane is very compressible and it is possible to trap air in the Urethane resin melt back in the injection machine barrel. Through we cannot guarantee that the following suggestions will solve your molding issue, but you might consider not using suck-back during Urethane molding as this can introduce air into the Urethane melt. Another option is to adjust the back pressure to try to push any encapsulated air back-up through the hopper as you auger the pellets. Again we repeat, we make no guarantee that the above suggestions will solve your molding issue.
Air that is encapsulated in the Urethane melt can be compressed by means of the high pressure used to inject the melt into the part cavity, effectively making the air bubbles almost disappear. Once the part begins to cool and the pressure is relieved, the air bubbles can re-appear.
Sintered vents are used to eliminate gassing/burn marks as a result of trapped air at the flow front. This could occur at a weld line. However trapped air mid-stream in a Urethane melt would not be removed by the Sintered vent. If you are attempting to remove trapped gas at the end of the flow front, we recommend you consider machining a custom vent into your mold at that location.
Please note that DME Air Poppets are not designed to vent gas from a part cavity. DME Air Poppets apply a small amount of air in the part ejection cycle to assist in part ejection from the core or cavity. Therefore DME Air Poppets are not recommended for removing air bubbles found mid-stream in a molded part.