What Are The Influencing Factors Of Injection Molding Shrinkage?

1. The influence of the characteristics of the injection molding material on the shrinkage rate

(1) The impact of plastic types on shrinkage

Different resin materials have different shrinkage rates. Even if the same type of resin material is produced by different manufacturers or the same material with different batch numbers produced by the same manufacturer, the shrinkage rate is different. Moreover, due to the inherent characteristics of the resin, the shrinkage range is wide or narrow.

(2) The effect of glass fiber content on shrinkage

The shrinkage of the same type of plastic varies with the content of glass fiber. When the glass fiber content increases, the shrinkage rate decreases. Generally, when the glass fiber with a mass fraction of 20% to 40% is added to the thermoplastic resin, the shrinkage rate can be reduced by 1/4 to 1/2.

But from the practice of injection molding, in the direction of material flow, this situation is hardly affected by the wall thickness of the plastic part. In the direction perpendicular to the material flow, the shrinkage rate decreases with the increase of the glass fiber content when the wall thickness is constant; in the case of thin walls, the shrinkage rate of the plastic part is hardly affected by the glass fiber content. Impact.

2. The influence of mold structure characteristics on shrinkage

(1) Parting surface and gate

Factors such as mold parting surface, gate form and size directly affect the material flow direction, density distribution, pressure maintaining and shrinking effect and molding time.

The use of direct gates or large cross-section gates can reduce shrinkage, but the anisotropy is large, the shrinkage in the direction of the material flow is small, and the shrinkage in the direction of the vertical material flow is large; on the contrary, when the thickness of the gate is small, the gate part will condense prematurely After hardening, the plastic in the cavity cannot be replenished in time after shrinking, and the shrinkage is relatively large.

The point gate is quick to congeal and seal. If the part conditions permit, multiple point gates can be set, which can effectively extend the pressure holding time and increase the cavity pressure, so that the shrinkage rate is reduced.

(2) Plastic part structure

The shape, size, wall thickness, presence or absence of inserts, the number of inserts and their distribution have a great influence on the shrinkage rate. Generally speaking, plastic parts are complicated in shape, small in size, thin in wall, with inserts, and the number of inserts is large and symmetrically distributed, and the shrinkage rate is small.

(3) Insert design

Although the metal inserts in the injection molded products can meet the local functional requirements, they have a hindering effect on the shrinkage of the injection molded products, so that the products are always in a non-free shrinking state before demolding, and there is an in-mold limiting effect. Around the inserts, not only It hinders the flow direction, density distribution and shrinkage of the material flow, and the temperature of the insert itself is also low.

Therefore, in the injection molding process, products with inserts have a smaller shrinkage rate than general plastic parts;

Moreover, if the design shape is too complex or the size of the insert is too large, it will also cause the fluctuation of the shrinkage rate between the different structures of the entire plastic part. Due to the mutual restriction of various structures, plastic parts with complex structures generally have a smaller shrinkage rate than plastic parts with simple structures.

(4) Cooling system

The distribution of the mold cooling circuit affects the temperature of the cavity surface, thereby affecting the cooling rate and shrinkage process of the injection molded products.

The surface of the mold cavity is closer to the mold cooling circuit, and it is strongly affected by the cooling medium, so that the plastic melt here cools quickly. On the one hand, the time of the temperature change is shortened, and the actual specific volume value of the plastic is balanced. The gap between the specific volume values in the state increases; on the other hand, when the in-mold shrinkage phase is entered, the surface temperature of the injection molded product here is already very low, so the degree of shrinkage that can occur is small.

The layout and size design of the mold cooling channel directly affects the mold temperature distribution and the cooling process of the plastic parts, and its improper design will also affect the fluctuation of the molding shrinkage rate. The shrinkage rate increases where the cooling is fast. Due to the complex shape of the plastic parts, the inconsistent wall thickness, and the different filling sequence, uneven cooling often occurs, resulting in large shrinkage fluctuations.

In order to improve this situation, the cooling water can be passed through the higher temperature place first; even the warm water is passed through the place where the cooling is fast, and the cold water is passed through the place where the cooling is slow. This can reduce the fluctuation of shrinkage rate and avoid deformation and cracking of plastic parts.