5 Replies Latest reply on Jun 14, 2011 3:28 PM by Tom Strohscher

    Force to open molds

    Tom Strohscher

      I have a blow mold and mold hanger setup.

      Can SW help me determine the pressure it will take to open/crack the mold hangers open?

      I  know the closing force of the hanger perpendicular to the split line.

      I can not use the blow pressure because of exhaust vents and leaks.

      I'm triing to determine the maximum pressure we can apply to the product during it's forming process.

      demo mold hangers.JPG

        • Re: Force to open molds
          Jerry Steiger

          Tom,

           

          If I'm understanding you correctly, this is a tough problem to solve. You apply a certain pressure to the inside of your part, but you don't actually know what the real pressure applied to the part is because of vents and leaks in the system. I suppose you could fix that part of the problem by adding a pressure sensor in or very near the mold in the pressure line. But then you need to figure out how much force actually gets applied to your mold halves. To do that you need to accurately model the behavior of the plastic in your part. You are going to have large displacements and high temperatures, and most likely a fairly good variation in temperature across the part and during the expansion of your part. Just finding a good material model at the molding temperatures is going to be hard.

           

          I would think that there exists a program for blow molding that is equivalent to Moldflow for injection molding. Probably not inexpensive, but trying to work this out on your own will involve a massive investment in time and testing.

           

          Jerry Steiger

            • Re: Force to open molds
              Tom Strohscher

              Even if we forget about the material being blown can SW determine the pressure needed in the insife surface to overcome the force applied to the the out side?

               

              I have charted the close force reletive to closing cylinder pressure.  Then I have charted blow force and recorded the cylinder pressure at which my molds crack.  It's not great but it gets me in the ball park.  Each item shape produced gives a different trend.  And like you mentioned there are many other process variables to take into account that can affect it.  I don't need this to be exact but get me closer as we estimate larger products.  I would like to understand what the mold closing mechanism will support.  If I can use SW data in relation to my sample data in the current situation I should be able to use SW to estimate the future larger products.

               

              I don't want to design a mechanism for 1000 pounds when I may only need 100.

                • Re: Force to open molds
                  Jerry Steiger

                  Tom,

                   

                  If you are looking for order of magnitude results, then Simulation will be able to give you a good enough answer. Just apply the blowing pressure to the inside faces of the mold and see what happens. Apply a pressure that you expect to be more than enough to blow the mold open, and then run the analysis with a large number of substeps. Look at the rotation or deflection at the parting line. At some substep the mold should start to blow open and the analysis will fail to converge.

                   

                  I would try it out on the shapes you have data on and see how well it correlates before trying it on something new. And always be aware that a nice tight correlation on one or more sets of data doesn't mean that you will have a good correlation on any new design.

                   

                  Jerry Steiger

                    • Re: Force to open molds
                      Tom Strohscher

                      I think at this point I have more of an issue triing to get a simulation setup in SW then understanding correlation between data.

                       

                      I either get no movement or a large displacement and a failed calculation.

                       

                      Should I mate parts in the assembly to allow them move as it does in real life?

                    • Re: Force to open molds
                      Wayne Matus

                      I am not familiar with the blow mold process, but it sould to me like you should apply the estimated pressures, constrain the model where the cylinders prevent the mold from opening. Run a linear static analysis and look at the reaction forces on the constrained faces. This will tell you the force the cylinder has to exert to keep the mold close.