Plastic Molding Manufacturing and Process
Plastic Molding Manufacturing process is a critical manufacturing technique in many industries today. Any method that can be used to reduce cost associated with injection molding parts made from thermoplastic, silicone rubber or standard Buna N rubber and thermo set plastics can utilize the process of Plastic Molding.
For years since around the early 1940’s until the late 1960’s molding a plastic part using the injection molding process was fairly simple and easy given the materials available during these years. After the birth of the “screw” which replaced the “plunger” new materials became more prevalent. The old tried and true polystyrene and polyethylene were easy materials to “shoot” into a mold and parts were never that complicated that the molder had much of a problem getting acceptable results. During most of time molding was fairly easy and straight forward. Because of the birth of the “screw” type molding machinery new and exotic materials became easier to process. All of a sudden the polymer chemist went berserk and starting coming up with what we now call engineered grade resins. Up until this time injection molding was thought of as more of an art than a science. These new resins and new product designers began to push the state of the art until many problems began to show up in the manufacturing processing world that needed to be resolved. Materials cost were climbing as were other cost in manufacturing. In the last 1960’s time frame several bright minds began to work on these issues and finally developed what is known today as Scientific Injection Molding or SIM for short.
Plastic Molding Technology gave birth to the molding of much tighter tolerances and thin wall molded parts. Many new guidelines were being called for from agencies such as the FDA. Guidelines for Process Validation. Scientific injection molding is a highly technical, scientific approach to developing and optimizing an injection molding process. SIM also works equally as well in processing thermoplastic materials as well as liquid silicone rubber and other thermo set resins. SIM follows a standardized data driven methodology to plan and collect important exact process data from the molding process.
Past practices would lead a process engineer to just shoot plastic into a plastic mold, look at the visual quality, measure a few key dimensions, and “tweak” the molding process until product specifications would be met not knowing anything about all the interdependent process parameters that would get “tweaked” in the process of his/her knob twisting adventures. This approach to arriving at a process did not insure that the process was stable enough to consider the “tweaking” to have been enough to call the process complete and repeatable.
The development of a stable process would now require the understanding of several process parameters that were otherwise unavailable prior to the invention of the pressure transducer usually installed under an ejection pin. This scientific approach has lead us to understand that there a few basic parameters that are absolutely necessary to capture and bring to every process. Melt temperature of the plastic as it enters the mold, fill time of plastic from the beginning of the flow of the resin to the completion of the flow as the gate freezes. The parameter of knowing the cooling rate of the mold to extraction rate of the temperature from the plastic as well as the cavity pressure are the important parameters. You can go so far as to say that the collection of these parameters will allow you to transfer any mold from one machine to another and with these same parameters you can virtually guarantee the exact same parts dimensions and containing the same physical properties. Now we have something we can call scientifically repeatable and can now assure the customer that we can give them the exact same part year after year. This assurance gives the customer and or final product user the confidence that his product will last through the design life expectancy. With the use of parameters being recorded we can now use computer programs to check on the viscosity variations that are thrown at us by the materials manufacturers that can cause us to change some of the parameters in order to process the resins to obtain the proper processing to the retention of the materials properties expected in the finish molding part. This also allows for the possible conclusion that the materials are not capable of attaining the proper expected properties and thus calling for the use of physical properties lab testing to verify.
Plastic molding technology has taken the next steps to the level of adding the ability to monitor important process parameters by adding in-mold cavity temperature or cavity pressure sensing, including closed-loop feedback from these signals. For example, the transfer of control from velocity to pressure can be controlled from the cavity pressure signal. Controlling the molding process using cavity pressure sensing allows identical parts to be produced consistently from machine to machine, part to part and set-up to set-up. Real-time process monitoring and closed-loop process control can be achieved with state-of-the-art injection molding machines as well as a host of auxiliary devices that can be added to any of the older injection molding machines.
In conclusion plastic injection molding process is a data-driven, scientific approach. Cycle times are minimized, machine efficiency are improved, and productivity is increased. plastic molding manufacturing process is a strong tool supporting the objectives and practices of a lean manufacturing culture. SIM reduces and in some cases eliminates many forms of waste by providing a predictable molding process.