Plastics Technology - Dedicated to improving Plastics Processing.
Issue link: http://pty.epubxp.com/i/827817
Many factors must be considered to determine if an injection molding machine has the capability to mold parts on a consistent basis. The injection unit must have the correct shot volume to avoid degradation and unmelt. It must also have pressure in abundance and a flow rate high enough to allow for proper part filling. Plastic variables also must be consid- ered, focusing on how important it is to process from "the plastic's point of view." It's important to dry the material to appro- priate moisture content, process within the recommended melt temperature, select the correct flow rate for injection, apply adequate hold time and pressure, and cool long enough to maintain dimensional stability. However, these variables only focus on the injection aspect of the process. The clamp end of the machine is responsible for providing force to counter the pressure that is applied from the injection unit. How much tonnage is applied, location of that force, type of clamping mechanism, and size of the mold base all affect the success of the molding process. Determining the correct tonnage is not a simple task. It provides a foundation that must be rock- solid to avoid flash and mold damage. Is it possible to apply too much clamp force, narrowing the process window before molten material is ever injected? Clamp Tonnage: More Is Better…Right? In the following examples, we will focus on using a Decoupled Molding II processing strategy. During the process, the filling of the cavity is separate from the pack/hold phase. The filling stage should yield a part that is 95% to 98% visually full, resulting in zero pressure at the end of the mold cavity. As the machine transfers from velocity to pressure control for the pack/hold phase, the cavity becomes visually full. Additional material is packed into the cavity to minimize sinks and decrease dimensional variation. Inside the cavity, the pressure increases, so the clamp must provide enough force to overcome the pressure applied in the cavity. DETERMINING CLAMP TONNAGE The starting point for determining appropriate clamp tonnage is to calculate the projected area of a single part. Then any cored-out surface area shall be removed from the overall surface area of the single part. Once the total surface area of the part is determined, we must multiply it by the number of cavities in the mold. If the mold has a cold runner, its entire surface area must be taken into consideration as well. After obtaining the total projected area of all parts plus the cold runner, we must then multiply that area by a tonnage factor. Tonnage factors used to be found on the material specification sheets, but nowadays that information rarely finds its way onto those data sheets. Typical tonnage factors can range from 2 to 10 tons/in. 2 , but for a starting point we recommend 3 tons/in. 2 . As an example, let's propose molding a lid for a square leftover dish (Fig. 1) with a hot-runner mold and eight cavities: The starting point for determining appropriate clamp tonnage is to calcu- late the projected area of a single part. Determining the correct tonnage provides a foundation that must be rock-solid to avoid flash and damage. But applying excessive force can create problems with the part, mold, and machine. Get more insights on Injection Molding from our expert authors: short.ptonline.com/moldingKH Learn more at PTonline.com KNOW HOW INJECTION MOLDING By Jeremy Williams Material selected for this lid was a low-viscosity LDPE, indicating that the pressure required from the injection unit to pack the part would be fairly low. FIG 1 34 JUNE 2017 Plastics Technology PTonline.com K now How INJECTION MOLDING