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, the control of shrinkage is very important in precision injection molding.
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 injection molding manufacturing process conditions and the action of each plastic injection molding cycle fluctuate depending on the molding machine, and the actual shrinkage rate fluctuates.
1. The main factors affecting the shrinkage rate
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 during 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 large, the shrinkage rate becomes smaller and the size of the product is larger.
Even in the same mold cavity, the resin pressure varies depending on the shape of the product, so the shrinkage rate varies.
In the case of a multi-cavity injection mold, the resin pressure in each cavity is easy to produce differences, and as a result, the shrinkage rate of each cavity is not the same.
Mold temperature
Whether it is a non-crystalline resin or a crystalline resin, if the mold temperature is high, the shrinkage rate will be larger.
Precision injection molding to maintain the mold temperature at a specific temperature. In the mold design, must pay attention to the cooling circuit design.
Gate cross-sectional area
Generally speaking, the shrinkage rate changes when the gate cross-sectional area is changed. The shrinkage rate becomes smaller as the gate size becomes larger, which is related to the flowability of the resin.
Wall thickness of the product
The cavity wall thickness of the product wall also affects the yield. For non-crystalline resins, as 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 vice versa, the shrinkage rate becomes smaller.
For crystalline resins, it is important to avoid particularly large variations in wall thickness. In the case of multi-cavity mold, if there is a difference in the wall thickness of the mold cavity, the shrinkage rate will also be different.
Reinforcing material content
When using glass fiber reinforced resin, the more glass fiber is added, the smaller the shrinkage rate is, and the shrinkage rate in the flowing direction is smaller than the transverse shrinkage rate.
According to the resin, its difference is larger, to prevent distortion fly warpage, must consider the gate shape flying fluke mouth position and the number of gates.
Orientation
Although there are large differences in orientation, however, 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 tropicalized into the mold, the mold temperature distribution will be in concentric circles centered on the gate in a general cavity arrangement.
Therefore, when choosing the cavity arrangement for multi-cavity injection molds, 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 under 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 gate must be set 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, a 3-point or 6-point rating 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 the case where gate marks are not allowed on the exterior and end surfaces, good results can be obtained by using fewer internal multi-point uniform gates.
English
العربية
简体中文
Nederlands
Français
Deutsch
Italiano
Português
Русский
Español