Injection Molding with Thermoplastic Resins
Thermoplastic molding refers to the process of injection molding thermoplastic resins. Thermoplastics is a type of polymer that melts and becomes pliable when reaching a certain temperature, allowing it to be molded into various forms. This type of polymer can also be ground up and reused after hardening, as opposed to thermoset polymers, which cannot be melted and used again. The third type of polymer is elastomer, which can also be thermoplastic, in which case this type of material is referred to as thermoplastic elastomers. Thermoplastic resins are the most common type of material used in the injection molding process. The Molding Process The equipment required for thermoplastic molding is the same: a press, a steel mold, and plastic resin. The resin is poured into a hopper on top of the press, and is then drawn into the press where it is heated in a barrel and injected into a steel mold where cavities carved out by journeyman mold makers form the molten plastic into the shape desired. The steel mold is usually equipped with water lines running through it to cool the parts before the mold opens and parts are ejected. Each time the mold opens, molds parts, and ejects them is called a cycle. The typical injection molding cycle is measured in seconds. Decreasing the cycle time can lead to improvements in efficiency and the bottom line, as long as it does not jeopardize the functionality of the part. The Mold The mold is the steel frame that contains the carved out cavities to actually form the part. Every injection molding job requires a steel mold to form the molten material into a usable functional part. There are different types of molds for different applications, but the most common is a class A hardened steel mold, capable of producing hundreds of thousands or millions of parts without wearing out. For the typical thermoplastic molding process, a steel mold will run in a horizontal molding press, meaning the mold will open and close on a horizontal plane. In this case, when the mold opens and ejects the parts, the runner is also ejected. The runner is the residual molten plastic that hardened in the channel getting the plastic to the cavity in the mold. This is true of a cold runner mold, which is the most common injection mold. A hot runner mold, in contrast, injects plastic directly into the cavity from a nozzle, and therefore has no runner. A hot runner mold can assist with automation, but it is more costly than the more common cold runner mold. Equipment Required for Automation While the necessary equipment is the same, a molder can invest in auxiliary equipment to automate the process and run parts without an operator, and sometimes even in a ‘lights out’ environment. A vacuum conveying system sucks the plastic resin while in pellet form from a drum or gaylord into the hopper on the press. As long as enough resin material is in the drum the conveying system will automatically supply a steady stream of thermoplastic resin into the press. There are ways to regulate the amount of pellets drawn into the hopper at a time. Some molders even have ways to measure the moisture content of material, and dry it accordingly before it enters the press. Some molders use a moisture analyzer when required. Sprue Pickers Assist with Automation When a cold runner mold is used in a horizontal press, the runner will eject with the parts and either drop to a bin below, or be grabbed by a robotic arm called a sprue picker. A sprue is another term for a runner. Sprue pickers assist in the automation process because it separates the parts from the runners, allowing the parts to collect in a tub by themselves and not need to be sorted or separated from the runners. Process Monitoring Adds Consistency and Reliability A Process Monitoring System should be utilized for automated molding as it will monitor, as the name implies, the process as it is happening. If something began to run out of spec, such as temperature or pressure, an alarm would sound to alert a technician. These monitoring systems also function as material resource planning software, allowing the user to input specific job information and schedule production. These are some of the ways a molder can automate the injection molding process. If precision parts manufacturing is required, automation could assist to provide to a consistent and reliable process. Contact Michiana Global Mold today to see how we can reduce costs on your plastic part requirements.
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As their name suggests, production molds are used in the final stages by the injection mold makers. Unlike prototype molds, production molds are meant to be used repeatedly for long periods of time, so they must be durable and able to handle harsher conditions. Since it is the final stage of the process, the end product is meant to meet all of the project requirements and specifications.
Custom mold makers will skip the prototype mold if customers request that a project only goes through to production instead of working with a prototype. While the prototype can save money, it can also require more time as the production of the first few parts is meant to be a test run. Custom injection molds that go straight into production molds are given a much longer duration for the design process to ensure the first product is nearly perfect. Production molds are not meant to be modified or changed. If the first product is too far off of the necessary requirements, you may need to start from the beginning. However, without the additional time for modifications, these injection molds can actually save time as long as the design process goes smoothly. If you want to avoid using prototype molds, you need to find expert injection mold makers. New companies do not have the knowledge or experience required to create a final product that is nearly perfect on the first round. A number of things can go wrong and many considerations can be missed in the design stages. These are problems that expert injection mold makers have already encountered and learned to fix before moving to the production mold. In the event that there is a problem with the production mold, the experts can make modifications to avoid starting over again. |
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