RobS
Super Member
OK, this is interesting enough to make a deal. I'll share some molding information if someone else can come through with a simple (impossible?) explaination of how the resins are made.
Some years back I did a stint as a Supplier Quality Engineer focusing on our plastic suppliers. These were all molders, not raw material suppliers. I learned enough to be dangerous but not enough to start my own plastics company so here goes.
There are two primary kinds of plastics: Thermosets and Thermoplastics. Thermoplastics are your milk jugs and anything that can be remelted. Thermosets are your phenolic pot handles and anything that will not remelt. I'm sure there are many others but most fit into these two categories. Teflon may be a slightly different beast but I don't know much about it other than the interesting story I'll share later.
Thermoplastics start as a resin usually in bead form. It can be mixed with re-grind, usually mold sprues and such right at the press. Most thermoplastic molding is done in injection molding machines (presses) where the resin is heated to melt and injected under pressure into the mold. If you've put together a plastic model kit you've seen exactly what comes out of the press. You'll have your parts and all the runners and sprues to carry the material into the part mold. Other than model kits, this stuff is trimmed away and you never see it.
The molds are usually water cooled to help the cycle time. The faster you can cool the resin, the faster you can open the press and get your next shot started. Typical cycle times are well under a minute per shot. Most operations run completely automated with parts falling to a conveyor for further processing.
Interestingly enough, one of the most sophisticated molding operations in the world is supposed to be the Lego company where they make the kids toy blocks. Those simple little blocks are held to incredible tolerances. Tolerances are one of the skills of a mold maker. The mold must be made to account for material shrinkage. Note now uniform and square a lego block is. Then compare to your cheapie, county fair prize toy. You'll see parting line flash, sprue marks and various indentations where the part hasn't shrunk evenly. This is the mark of quality in injection molding.
Thermosets on the other hand, are molded several different ways. There is compression, transfer and injection. Injection is very similar but the cycle times are a little longer and the materials tend to be much "messier". Compression and transfer both start by premolding a "puck" of material. This puck is then placed directly into the mold cavity (compression) or into a transfer cavity (transfer). The mold closes and the material is squeezed into the cavity (compression) or is transferred in with a seperate ram. In either case, the internal mold pressure is generally higher with thermosets than thermoplastics and heat is now added to the mold to cure the material. Once cured the mold is opened. Thermoset parts are a little like concrete, lots of compressive strength but not as much tensile. And very heat resistant so good for pot handles.
Many plastic parts are then post-processed with machining, welding (sonic, spin etc.) or assembled.
The one experience I had with teflon was pretty cool. I visited a supplier who made small teflon rings for us. They were lathe cut from bar stock much like machining metal. Their bar stock was purchased as a raw material so I don't know much about it. This same supplier though, made teflon rings for the turret of the M1 tanks. Very cool to see. They laid up teflon blocks in ring shape on a huge sheet of plywood (the ring was eight feet in diameter or so). Again, I'm not sure how they did the bonding to create the ring from the segments. Once laid up and cured/dried they put the whole thing on a huge lathe including the plywood. Spun the whole thing slowly and cut the ring. It was neat to see and that is one thing I enjoyed most about that job... seeing products other than my own being made.
Another supplier of ours also made CD "Jewel Boxes". This was early in the life of CDs but just to be competitive it was fully automatic and would mold and assemble a CD box every few seconds. Cycle time is important in high volume!
Hope this has been useful to someone. It's fun to talk about. Now who knows how that resin is made?
/forums/images/graemlins/smile.gif
Some years back I did a stint as a Supplier Quality Engineer focusing on our plastic suppliers. These were all molders, not raw material suppliers. I learned enough to be dangerous but not enough to start my own plastics company so here goes.
There are two primary kinds of plastics: Thermosets and Thermoplastics. Thermoplastics are your milk jugs and anything that can be remelted. Thermosets are your phenolic pot handles and anything that will not remelt. I'm sure there are many others but most fit into these two categories. Teflon may be a slightly different beast but I don't know much about it other than the interesting story I'll share later.
Thermoplastics start as a resin usually in bead form. It can be mixed with re-grind, usually mold sprues and such right at the press. Most thermoplastic molding is done in injection molding machines (presses) where the resin is heated to melt and injected under pressure into the mold. If you've put together a plastic model kit you've seen exactly what comes out of the press. You'll have your parts and all the runners and sprues to carry the material into the part mold. Other than model kits, this stuff is trimmed away and you never see it.
The molds are usually water cooled to help the cycle time. The faster you can cool the resin, the faster you can open the press and get your next shot started. Typical cycle times are well under a minute per shot. Most operations run completely automated with parts falling to a conveyor for further processing.
Interestingly enough, one of the most sophisticated molding operations in the world is supposed to be the Lego company where they make the kids toy blocks. Those simple little blocks are held to incredible tolerances. Tolerances are one of the skills of a mold maker. The mold must be made to account for material shrinkage. Note now uniform and square a lego block is. Then compare to your cheapie, county fair prize toy. You'll see parting line flash, sprue marks and various indentations where the part hasn't shrunk evenly. This is the mark of quality in injection molding.
Thermosets on the other hand, are molded several different ways. There is compression, transfer and injection. Injection is very similar but the cycle times are a little longer and the materials tend to be much "messier". Compression and transfer both start by premolding a "puck" of material. This puck is then placed directly into the mold cavity (compression) or into a transfer cavity (transfer). The mold closes and the material is squeezed into the cavity (compression) or is transferred in with a seperate ram. In either case, the internal mold pressure is generally higher with thermosets than thermoplastics and heat is now added to the mold to cure the material. Once cured the mold is opened. Thermoset parts are a little like concrete, lots of compressive strength but not as much tensile. And very heat resistant so good for pot handles.
Many plastic parts are then post-processed with machining, welding (sonic, spin etc.) or assembled.
The one experience I had with teflon was pretty cool. I visited a supplier who made small teflon rings for us. They were lathe cut from bar stock much like machining metal. Their bar stock was purchased as a raw material so I don't know much about it. This same supplier though, made teflon rings for the turret of the M1 tanks. Very cool to see. They laid up teflon blocks in ring shape on a huge sheet of plywood (the ring was eight feet in diameter or so). Again, I'm not sure how they did the bonding to create the ring from the segments. Once laid up and cured/dried they put the whole thing on a huge lathe including the plywood. Spun the whole thing slowly and cut the ring. It was neat to see and that is one thing I enjoyed most about that job... seeing products other than my own being made.
Another supplier of ours also made CD "Jewel Boxes". This was early in the life of CDs but just to be competitive it was fully automatic and would mold and assemble a CD box every few seconds. Cycle time is important in high volume!
Hope this has been useful to someone. It's fun to talk about. Now who knows how that resin is made?
/forums/images/graemlins/smile.gif
