Maximizing part quality is a main goal of any injection molding project. Not only does part quality impact the performance and success of the end product, but it also plays a role in production time and cost: Fewer part errors means less rework and a quicker time to the customer and thus, to market. One of the best ways to maintain a high level of part quality is to improve each step of the molding process, specifically the filling, packing and holding stages. Here, we will examine what each stage involves and how to optimize them.
Injection Molding Phases and How to Optimize Them
As a short refresher, the injection molding process includes three general phases:
- The filling stage: In this step, the bulk of the molten plastic material is injected under pressure into the mold.
- The packing stage: At this point, pressure is adjusted, and additional material is injected into the mold to account for material shrinkage and backflow.
- The holding stage: During this phase, the material is held in place at a pressure equilibrium until gate freeze occurs, at which point the cooling process begins.
The Filling Phase of Plastic Injection Molding
The injection mold filling procedure is the process by which the bulk of the material is inserted into the mold and cavities — up to 95% or even 98% of the necessary resin. During this phase, it is critical that the temperature is set to the correct specification for the material. If the temp is too high or low, you risk impurities or imperfections in the final product. Additionally, pressure needs to be at the recommended level to ensure a smooth, uniform, relatively fast fill — usually just a few seconds.
As with the temperature setting, most materials will have a recommended pressure setting which should be followed for best results. The next phases, injection mold pack and hold, are where testing and fine-tuning would be most effective.
The Pack and Hold Phases of Plastic Injection Molding
After the filling process, a shift must occur in pressure and timing in order to carry out the packing and holding steps effectively. As mentioned above, these stages can often be grouped together and accomplished through one change in pressure and a single time setting. We will first look at that process in more detail, and then list some situations where one change won’t necessarily work.
Injection molding hold times, packing times and pressure are critical to ensuring the proper formation and quality of the finished product. If these times and pressures are not optimized, too much material may be injected into the mold, resulting in flash and wasted material. Alternatively, too little material may remain in the mold, resulting in sink and other defects.
The most effective way to optimize pack time, hold time and pressure is through a gate seal study, which measures product weight as a function of time. Then, you can fine-tune the time and pressure until the exact product weight (and thus the correct amount of material in the mold) is achieved.
There are also scenarios where timing and pressure will differ from the packing stage to the holding stage, such as with:
- Softer plastic materials
- Especially large molds or large gates
- Hot runner molds
- Valve gated molds
In these instances, the solution is to conduct separate weight vs. time measurements for each phase. Then, make a second pressurization adjustment for the holding stage to ensure that the product weight remains uniform throughout the process.
For more information on plastic injection molding, review this complete guide for more resources on different processes, tool to calculate press tonnage size your project will require, how to evaluate injection molding costs, and more.
Injection Molding Experts
Crescent Industries offers a wide range of injection molding services and extensive expertise. We optimize every molding process that we perform, determining the ideal time and pressure for filling, packing, and holding to ensure consistent quality product.