After filling you must pressurize the mold to get a good imprint of the cavity to minimize the effects of cooling. Short shots, sinks, voids and dimensional variations in parts are caused because of insufficient packing pressure in the mold cavities.

Many times a mold is intentionally under packed to keep the mold from flashing. It is an easy way to squeeze some pretty good-looking parts out of a mold by packing gently with molder’s finesse. However, in the long run it is going to cause problems.

It is important that any new mold be packed to a minimum pressure of 3000 psi or 200 mpa at the end of the cavity. Any mold that cannot be pressurized to this degree at the last area to fill will most likely be problematic. This does not always mean that every mold is run at this pressure. In some cases it might be run at a lower pressure to get dimensions or save material; however, the 3000 psi minimum is a really good number to ensure that there is sufficient pressure to keep the short shots out of the system. It is important to also know that the mold can be run at this pressure, or higher if necessary.

The higher the mold can be pressurized without adverse effects, the better the tool is built. In large molds it is sometimes difficult to achieve this result and mold builders are pretty upset when their mold fails. However, once you figure out the cause and fix it, you’re happy because not only did you learn something, but your customer likes the mold, which is one of the keys to success in this industry.