What is Packing pressure?

Packing pressure is a key parameter in the plastic injection molding process, directly impacting the quality, dimensional stability, and appearance of the final part. It refers to the pressure applied during the post-filling stage of the injection cycle, once the mold cavity is full, to compensate for the material’s shrinkage as it cools.

This phase—also known as the packing or holding phase—is essential to ensure the cavity is completely filled and that no voids, bubbles, or sink marks remain. Applying the correct packing pressure helps produce denser parts with better mechanical properties and a more uniform surface finish.

The optimal packing pressure depends on several factors, including the type of plastic material, the geometry of the part, the design of the gating and runner system, and the mold temperature. Too much pressure can cause flash, internal stress, or warpage, while insufficient pressure can result in incomplete parts or visible surface defects.

During this stage, the molten material inside the cavity begins to solidify from the mold walls inward. Packing pressure pushes additional molten plastic into the cavity to compensate for the volumetric reduction caused by cooling. This is particularly important in thick or complex areas of the part, where shrinkage is more likely to occur.

In high-precision molds, such as those used in automotive, electronics, or medical applications, precise control of packing pressure is critical. Any deviation can affect dimensional tolerances or lead to rejections due to functional or cosmetic defects. For this reason, it’s common to program multi-stage holding profiles with gradually decreasing pressure values, all adjusted via the machine’s controller to match the material’s behavior during cooling.

At Plásticos Lezo, we place great emphasis on optimizing the injection process and precisely controlling all parameters, including packing pressure. Thanks to our machines equipped with advanced control systems and integrated sensors, we can fine-tune this pressure in real time, ensuring a stable and high-quality production process.

Our technical team analyzes each project to define the most suitable pressure profile, taking into account the product’s requirements, the selected thermoplastic, and the client’s specifications. We also conduct fill and pack simulations using specialized software, allowing us to anticipate potential defects before mold manufacturing and adjust the design to improve process efficiency.

Correct packing pressure also enhances process repeatability by reducing cycle-to-cycle variation and ensuring that every part has consistent characteristics. This is crucial for large production runs or parts that must meet strict dimensional or functional certifications.

In short, packing pressure is much more than just a technical value—it is a key control variable for ensuring the quality and reliability of the final product. When properly set, it enables the production of stable, durable, and visually appealing plastic parts, meeting the high standards demanded by sectors where precision is non-negotiable.