Plastics, Starting from Beginning

The filling stage is an important stage in injection molding cycle. Starting from the pellet as a material, in the hopper, conveyed by a screw in the chamber, until the melt thermoplastics injecting into the cavity, filling stage is one of the major factors in the plastic production process. In the industry nowadays, the high production rate is important. To be a good vendor of custom molding, you should have the ability to fulfill the demand from the customer and the plastic market.

In my earliest days of troubleshooting a mold in the press we had to do it by hand with dykem blue. Grinding away here and there to force a mold into production. Now you can cut times by analyzing the filling stage; the determinations are a necessity to improve the production rate without neglecting the quality performance of the product produced. This element can give the benefit to the industry to achieve profitability and survive in this challenging plastic industry. SOLIDWORKS Plastics can help you do this analysis.

Many factors should be considered, such as the mold cost, tool room time, and lead times. The material is chosen and the cycle time influences the production and quality rate of the product. The parameters that should considered include the temperature that was used to heat the pallet, the pressure to force the melt and the cycle time to guide the process in this stage. As the first stage of the injection molding process, the filling stage should give the lead into the next stage, packing. The importance of this stage is to control the good running process with consideration in cooling stage and ejection stage down the line. The review of cooling stage and ejection stage becomes important for this analysis. Finally, the analysis of filling stage in injection molding should be accurate to achieve the good quality rate of production without increasing the cost.

Simulating unwanted part defects such as short shots, weld lines, and vent locations are the key to what we are looking for with the filling stage. SOLIDWORKS Plastics will help at this stage to find the part defects and help eliminate them before they become a production problem.

A short shot is a molded part that is incomplete because the insufficient material injected into the mold. Entrapped air can cause it, insufficient machine injection pressure (resulting from high melt resistance and a restricted flow path), pre-mature solidification of the polymer melt, and mechanical defects. SOLIDWORKS Plastics will help with viewing short shots, vents, injection pressures along with all the heat options you will need.

Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. They occur when melt flow fronts collide in a mold cavity. A poor knit line can cause only cosmetic blemishes, or it can significantly weaken the structural integrity of a part. Strength at the knit line can be as little as 20 percent of the nominal strength of the part—or it can be 100 percent as strong, depending on a host of variables. Weak weld lines have their origins in material choice, part design, tooling, and processing. Some materials are more or less “forgiving” where it comes to weld lines.

Part design matters because non-uniform wall thickness can vary the shear and flow rate of the melt front, resulting in a split flow path. Tooling effects include multiple gates into a cavity and projections in the mold like bosses and ribs, as well as holes or depressions, all of which can interrupt and split melt flow into separate fronts. A temperature variation in one section of the mold surface also can create a non-uniform flow front.

Air trap appears in rib part, for example, in some products, the rib is very deep, and then the air cannot be pushed out when plastic flows in that area. And in some products, the thickness varies so much, air traps easily. Also, air traps also in the place where the plastic meets at the end of the fill. The phenomenon of the air trap: the product cannot fill with the full material; parts become white, worse still, get black. And in some transparent plastic, there is a small bubble.

Luckily, there are solutions for injection molding air trap: Make air venting in the part where the air traps. Make the inserts or pins, then the air will go through the gap, but it is very important to keep the accurate dimension. Otherwise, there will be flash. Molding parameter adjustment: we can slow down the injection speed. Thus the air can be pushed out by the material flow. And for transparent material, we can ease or solve the air tap problems by increasing the machine back pressure.

With all of the information in front of you during an analysis of the fill process designers, engineers, companies, and customers can rely on SOLIDWORKS Plastics to provide valuable information. Know that finding short shots, weld lines, and air traps alone give you a head start on production-ready parts. Filling might be one of the shortest sections of total cycle time but can be the most important.

Find the areas that cause you the most problems in production and use SOLIDWORKS Plastics to help solve that problem. Look at your venting, weld lines, and filling patterns to go back to help solve issues in a production mold that has been running for years to shave a couple of seconds off for more production.






Jeff Osman

Jeff Osman

Jeff Osman has more than 23 years of experience in the mechanical CAD industry. As Senior Technical Sales Specialist Plastics NA, he is responsible for all technical Sales of SolidWorks products, focusing on SolidWorks Plastics, for North America and has been with SolidWorks for 19 years. Prior to joining SolidWorks, Jeff was a senior technical manager with Microcadam, a division of IBM. In addition, he has held several manufacturing positions with companies Processed Plastics, Plano Molding and Furnas/Siemens Electric.
Jeff Osman

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