Precision injection molding, must be based on the premise that the mold can be made according to the desired size. However, even if the mold size is certain, the actual size of the product varies according to the actual shrinkage. Therefore, in precision injection molding, the control of shrinkage rate is very important.

The mold design is appropriate or not to control the shrinkage rate, but also vary with different resin batches, if you change the pigment, the shrinkage rate is also different. Due to the different forming machines, the setting of forming conditions, reproducibility and the action of each forming cycle have fluctuations, and the actual shrinkage rate fluctuates, so the control of shrinkage is difficult.

1. The main factors affecting the shrinkage rate

Mold size can be obtained by the product size plus the shrinkage rate, so in the mold design, need to consider the main factors of shrinkage rate: Resin pressure

resin temperature

mold temperature

gate area

injection time

cooling time

products wall thickness

reinforced material

stereospecificity

injection speed

resin pressure Force

Resin pressure has a great influence on the shrinkage rate, if the resin pressure is large, the shrinkage rate becomes smaller, and the product size is large. Even in the same mold cavity, the resin pressure also varies due to the shape of the product, so the shrinkage rate difference. In the case of multi-cavity mold, the resin pressure in each mold cavity is easy to produce differences, and the shrinkage rate of each mold cavity is not the same.

Mold temperature

Whether it is an amorphous resin or a crystalline resin, if the mold temperature is high, the shrinkage rate will be larger. Precision molding to maintain the mold temperature at a specific temperature. In the mold design, we must pay attention to the cooling circuit design.

Gate cross-sectional area

Generally speaking, when the gate cross-sectional area is changed, the shrinkage rate also changes. The shrinkage rate becomes smaller as the gate size becomes larger, which is related to the fluidity of the resin.

Product wall thickness

Product wall thickness also affects the yield. For the non-crystalline resin, because the resin has a different tendency to affect the shrinkage rate of the wall thickness, the wall thickness is large, the shrinkage rate is also large, and on the contrary, the shrinkage rate is smaller. And for crystalline resins, the wall thickness change must be avoided especially large. In the case of multi-cavity mold, if the wall thickness of the mold cavity is different, the shrinkage rate will also be different.

Reinforcement material content

With glass fiber reinforced resin, the more glass fiber is added, the smaller the shrinkage rate, the flow direction of the shrinkage rate is smaller than the transverse shrinkage rate, according to the resin its difference is larger, in order to prevent distortion and warping, It is necessary to consider the shape of the gate, the position of the head and the number of gates.

Orientation

Orientation is present for all resins, although it varies greatly. The orientation of crystalline resins is particularly large and varies due to wall thickness and forming conditions.

in addition, also the contraction after forming, the main factors are:

internal stress relaxation

crystal

temperature

humidity

2. Measures that can be taken

runner, gate balance

As mentioned above, the shrinkage rate changes due to resin pressure. In the case of single-cavity mold multi-point gate and multi-cavity mold,

Want to also fill the mold, it is necessary to carry out gate balance. The flow of resin is related to the flow resistance in the runner, so it is best to take the runner balance before taking the gate balance.

Mold cavity arrangement

In order to make the setting of molding conditions easy, it is necessary to pay attention to mold cavity arrangement. Because the molten resin will be tropical into the mold, in the case of general mold cavity arrangement, the mold temperature distribution is concentric circles centered on the gate. Therefore, when selecting the cavity arrangement of the multi-cavity mold, it is easy to take the flow channel balance, and take the concentric return arrangement

3. Prevent forming deformation

The reason for forming deformation is that there is internal stress under uneven contraction, so it is necessary to prevent uneven contraction. In the case of circular products with holes in the center of the gear, the gate must be set in the center. However, when the flow direction of the resin is different from the shrinkage rate in the vertical direction, it has the disadvantage of producing ellipses. When the roundness is needed with higher precision, it is necessary to set the gate to 3 or 6 points.

But it is necessary to pay full attention to the balance of each gate. When using the side gate, the 3 point gate will increase the inner diameter of the cylindroid product, and use the inner multi-point evenly divided gate less in the case that the appearance and end face do not allow the gate trace, and good results can be obtained.

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