How to prevent shrinkage fluctuation of plastic parts for precision injection molding products?

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

The shrinkage rate is governed by the suitability of the mold design and also varies depending on the resin batch, and if the pigment is changed, the shrinkage rate also varies. The control of shrinkage is difficult because the setting and reproducibility of molding conditions and the action of each molding cycle fluctuate depending on the molding machine, which produces fluctuations in the actual shrinkage rate, etc.

1. Main factors affecting shrinkage rate

The mold size can be obtained by adding the shrinkage rate to the product size, so the main factors of shrinkage rate need to be considered in the mold design.

Resin pressure

Resin temperature

Mold temperature

Gate cross-sectional area

Injection time

Cooling time

Wall thickness of the product

Reinforcement material

Orientability

Injection speed

Resin pressure

Resin pressure has a great influence on the shrinkage rate. If the resin pressure is high, the shrinkage rate becomes smaller and the size of the product is larger. Even within the same cavity, the resin pressure varies depending on the shape of the product, resulting in a difference in shrinkage rate. In the case of multi-cavity molds, the resin pressure in each cavity is likely to be different, and as a result, the shrinkage rate of each cavity is also different.

Mold temperature

Whether it is a non-crystalline resin or a crystalline resin, the shrinkage rate will increase if the mold temperature is high. The precision molding requires that the mold temperature be maintained at a specific temperature. In-mold design, attention must be paid to the cooling circuit design.

Gate cross-sectional area

Generally speaking, the shrinkage rate changes when the gate cross-sectional area is changed. Shrinkage decreases with a larger gate size, which is related to the flow of the resin.

Wall thickness

The wall thickness also affects the yield. For non-crystalline resins, the tendency of the resin to affect the shrinkage rate of the wall thickness is different, as the wall thickness is large, the shrinkage rate is also large, and vice versa, the shrinkage rate becomes small. For crystalline resins, it is important to avoid particularly large variations in wall thickness. In the case of multi-cavity molds, if there is a difference in the wall thickness of the mold cavity, the shrinkage rate will also vary.

Reinforcement content

When using glass fiber reinforced resin, the more glass fiber is added, the smaller the shrinkage rate is. The shrinkage rate in the flow direction is smaller than the transverse shrinkage rate, and the difference is larger according to the resin.

Orientation

Although there are large differences in orientation, orientation exists for all resins. The orientation of crystalline resins is particularly large and varies due to wall thickness and molding conditions.

In addition, there is the generation of post-molding shrinkage, the main factors of which are

Internal stress relief

Crystallization

Temperature

Humidity

2. Measures that can be taken

Flow channel, gate balance

As mentioned above, the shrinkage rate varies depending on the resin pressure. In the case of single-cavity molds with multiple gates and the case of multi-cavity molds

To fill the mold equally, gate balancing is required. The resin flow is related to the resistance to flow in the runner, so it is better to balance the runner before taking the gate balance.

Mould cavity arrangement

To make the molding conditions easy to set, it is necessary to pay attention to the cavity arrangement. Since the molten resin will be heated into the mold, the mold temperature distribution will be concentric circles centered on the gate in a general cavity arrangement. Therefore, when choosing the cavity arrangement of multi-cavity mold, it is easy to take the flow channel balance and the concentric circle arrangement with the gate as the center.

3. Prevent molding deformation

The reason for deformation is that there is internal stress due to uneven shrinkage, so it is necessary to prevent uneven shrinkage. In the case of a round product with a hole in the center of the gear, a sprue must be placed in the center. However, when there is a big difference in shrinkage between the flow direction of resin and the vertical direction, there is a disadvantage of oval shape, so when high precision roundness is needed, 3-point or 6-point gating is required.

However, it is necessary to pay sufficient attention to the balance of each gate. When using side gates, 3-point gates will increase the inner diameter of cylindrical products. In cases where gate marks are not allowed on the exterior and end surfaces, using fewer inner multi-point uniform gates will give good results.