Today's KNOWLEDGE Share : 𝗡𝘂𝗰𝗹𝗲𝗮𝘁𝗶𝗼𝗻, 𝗰𝗿𝘆𝘀𝘁𝗮𝗹𝗹𝗶𝘇𝗮𝘁𝗶𝗼𝗻, 𝗮𝗻𝗱 𝘀𝗵𝗿𝗶𝗻𝗸𝗮𝗴𝗲 𝗰𝗼𝗻𝘁𝗿𝗼𝗹 𝗱𝘂𝗿𝗶𝗻𝗴 𝗶𝗻𝗷𝗲𝗰𝘁𝗶𝗼𝗻 𝗺𝗼𝗹𝗱𝗶𝗻𝗴

Today's KNOWLEDGE Share

𝗡𝘂𝗰𝗹𝗲𝗮𝘁𝗶𝗼𝗻, 𝗰𝗿𝘆𝘀𝘁𝗮𝗹𝗹𝗶𝘇𝗮𝘁𝗶𝗼𝗻, 𝗮𝗻𝗱 𝘀𝗵𝗿𝗶𝗻𝗸𝗮𝗴𝗲 𝗰𝗼𝗻𝘁𝗿𝗼𝗹 𝗱𝘂𝗿𝗶𝗻𝗴 𝗶𝗻𝗷𝗲𝗰𝘁𝗶𝗼𝗻 𝗺𝗼𝗹𝗱𝗶𝗻𝗴

In injection molding of semi-crystalline polymers, 𝗰𝗿𝘆𝘀𝘁𝗮𝗹𝗹𝗶𝘇𝗮𝘁𝗶𝗼𝗻 𝗯𝗲𝗴𝗶𝗻𝘀 𝗮𝘁 𝗻𝘂𝗰𝗹𝗲𝗮𝘁𝗶𝗼𝗻 𝘀𝗶𝘁𝗲𝘀 — small points where polymer chains start to arrange into ordered structures.

The way nucleation and crystallization happen directly affects shrinkage, part dimensions, and mechanical properties.

How holding pressure and cooling influence crystallization:

𝗗𝘂𝗿𝗶𝗻𝗴 𝗵𝗼𝗹𝗱𝗶𝗻𝗴 𝗽𝗿𝗲𝘀𝘀𝘂𝗿𝗲, the polymer is still soft and partly molten.

Applying pressure for a longer time keeps the melt dense and delays the formation of nucleation sites, because it reduces the space available for crystals to start growing.

However, once the temperature drops close to the crystallization point, nucleation starts anyway, and the effect of pressure becomes smaller.

The influence of holding pressure on crystallization depends on how much pressure is applied, how long it is maintained, and how much heat is transferred from the melt to the mold surface.

𝗗𝘂𝗿𝗶𝗻𝗴 𝗰𝗼𝗼𝗹𝗶𝗻𝗴, the polymer changes structure quickly.

As the material cools below the crystallization temperature, nucleation speeds up — especially near the surface of the part, where the hot polymer is in direct contact with the colder mold steel, causing rapid heat transfer and faster solidification.

𝗧𝗵𝗲 𝘁𝗵𝗲𝗿𝗺𝗮𝗹 𝗴𝗿𝗮𝗱𝗶𝗲𝗻𝘁 — the temperature difference between the surface and the center of the part plays a big role:

Fast cooling at the surface creates many small nucleation points, leading to a fine crystalline structure. This usually gives higher stiffness but lower impact resistance.

Slower cooling in the center allows larger crystals to grow, which improves impact resistance but slightly reduces stiffness.

In the end, everything comes back to temperature control.

Even the influence of holding pressure is tied to temperature — it affects how quickly nucleation sites form, depending on the pressure level, the time of application, and the speed of heat transfer between the material and the mold.

For good part quality, it is most important to optimize cooling, manage thermal gradients, and apply holding pressure correctly to balance shrinkage, internal stresses, and mechanical properties.

Note: image source — https://lnkd.in/dwPBmpmg.

source: Krstina Jankovic