Injection molding cycle: After the solid particles are heated and melted to fill the mold cavity, the plastic is cooled and formed. Molding shrinkage: The volume of the product cooled to room temperature is always smaller than the volume of the cavity of the molding die at normal temperature. Commonly used shrinkage rate. At present, mold designers generally use the average shrinkage or extreme value method to calculate the shrinkage value of injection molded parts.

1 Effect of injection molding properties on shrinkage

(1) The effect of plastic types on shrinkage

Different resin materials have different shrinkage rates. Even if the same kind of resin materials are produced by different manufacturers or the same material produced by the same manufacturer, the shrinkage rate is different. Moreover, due to the inherent characteristics of the resin itself, the shrinkage range is wide and narrow.

(2) Effect of glass fiber content on shrinkage

The plastic shrinkage of the same variety varies depending on the glass fiber content. When the glass fiber content increases, the shrinkage rate decreases.

Generally, a glass fiber having a mass fraction of 20% to 40% is added to the thermoplastic resin, and the shrinkage ratio can be reduced by 1/4 to 1/2.

However, it is concluded from the injection molding practice that in the flow direction of the flow, this situation is hardly affected by the wall thickness of the plastic part. In the vertical direction with the flow, the shrinkage rate decreases with the increase of the glass fiber content in the case of constant wall thickness; in the case of thin wall, 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 parting surface, gate form and size of the mold directly affect the flow direction, density distribution, pressure-preserving and shrinking action and molding time.

The use of direct gates or large-section gates can reduce shrinkage, but the anisotropy is large, shrinking in the direction of the flow direction is small, and shrinking along the vertical flow direction; conversely, when the gate thickness is small, the gate portion will prematurely condense. After hardening, the plastic in the cavity is not replenished in time, and the shrinkage is large.

The point gate is fast-sealed, and when the condition of the workpiece is allowed, a multi-point gate can be provided, which can effectively extend the dwell time and increase the cavity pressure, so that the shrinkage rate is reduced.

(2) Plastic 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. In general, the shape of the plastic part is complicated, the size is small, the wall is thin, there are inserts, the number of inserts is large and symmetric, and the shrinkage rate is small.

(3) Insert design

Although the metal insert in the injection molded product can meet the local functional requirements, it has a hindrance to the shrinkage of the injection molded product, so that the product is in a non-free contraction state before demolding, and there is an in-mold limiting effect, around the insert, not only It hinders the flow direction, density distribution and shrinkage of the stream, and the temperature of the insert itself is also low.

Therefore, in the injection molding process, the product having the insert has a smaller shrinkage rate than the general plastic part;

Moreover, if the design is too complex or oversized inserts, it will cause fluctuations in the shrinkage between the different structures of the entire plastic part. Due to the mutual defining function of the respective structures, the plastic parts with complicated structures generally have a smaller shrinkage rate than the plastic parts with a simple structure.

(4) Cooling system

The distribution of the mold cooling circuit affects the temperature of the cavity surface, which affects the cooling rate and shrinkage process of the injection molded product at various points.

The surface of the cavity is close to the mold cooling circuit, which is strongly influenced by the cooling medium, so that the plastic melt here is cooled quickly. On the one hand, the action time of the temperature change is shortened, and the actual specific volume value and balance of the plastic are made. The difference between the specific volume values ​​in the state is increased; on the other hand, when entering the in-mold shrinking stage, the surface temperature of the injection molded article here is already low, so the degree of shrinkage that can occur is small.

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

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

3 Effect of injection molding process conditions on yield

(1) Pressure

The pressure during injection molding includes injection pressure, holding pressure, and cavity pressure. These factors have a significant impact on the shrinkage behavior of the plastic parts.

Increasing the injection pressure can reduce the shrinkage of the product. This is because the pressure is increased, the injection speed is increased, and after the filling process is accelerated, on the one hand, the melt temperature is increased due to the shear heat of the plastic melt, and the flow resistance is reduced; on the other hand, the melt temperature can also be obtained. In the state of high temperature and low flow resistance, it enters the pressure-preserving feeding stage earlier. Especially for thin-walled plastic parts and small gate plastic parts, due to the fast cooling rate, the filling process should be shortened as much as possible.

The higher holding pressure and cavity pressure make the products in the cavity dense and shrinkage, especially the pressure in the pressure-holding stage has a greater influence on the shrinkage rate of the product. This can be explained by the fact that the molten resin is compressed under the molding pressure. The higher the pressure, the greater the amount of compression that occurs, and the greater the elastic recovery after the pressure is released, so that the size of the plastic part is closer to the size of the cavity, so the amount of shrinkage The smaller.

However, even for the same product, the pressure of the resin in the cavity is not uniform in each part; the injection pressure is different in the portion where the injection pressure is difficult to act and the portion where it is easy to act. In addition, the pressure of each cavity of the multi-cavity mold should be designed uniformly, otherwise the shrinkage rate of the products of each cavity will be inconsistent.

(2) Temperature

Material temperature: Temperature has a major influence on the melt viscosity of the polymer.

Above the viscous flow temperature, the viscosity of the polymer has a relationship with the temperature as with the low molecular liquid. As the temperature increases, the free volume of the melt increases, and the interaction between the molecules weakens, making the polymer fluid. Increasingly, the viscosity of the melt decreases exponentially with increasing temperature, because in the high polymer injection molding process, the mold filling ability of the melt is increased, and temperature is the primary means of viscosity adjustment.

Whether from the mechanism of crystallization and orientation of the polymer or the principle of thermal expansion and contraction, it is easy to think that the shrinkage of the product during the holding and cooling setting stages should increase with increasing temperature, but some experiments are just the opposite. The conclusion.

This situation can be explained as: the melt viscosity will decrease after the material temperature rises. If the injection pressure and the holding pressure remain unchanged at this time, the gate freezing speed will be slowed down, indicating that the dwell time is prolonged. The shrinkage is increased, and the density is also increased, so the shrinkage rate is lowered.

It can be seen from the above analysis that the influence of the material temperature on the molding shrinkage is the result of the combined effects of heat shrinkage, crystallization shrinkage, orientation shrinkage and pressure shrinkage. If the effects of the first three shrinkages are large, the final shrinkage of the product will follow. The temperature of the melt increases as the temperature rises; conversely, when the pressure-preserving effect is large, the shrinkage rate decreases as the temperature increases.

Mold temperature

After the thermoplastic melt is injected into the cavity, a large amount of heat is released and solidified. Different plastic varieties require the cavity to be maintained at an appropriate temperature. At this temperature, it will be most beneficial for the molding of plastic parts, with the highest molding efficiency and the minimum internal stress and warpage.

The mold temperature is the main factor controlling the cooling and setting of the product. The influence of the mold temperature on the molding shrinkage is mainly reflected in the process before the product is demolded after the gate is frozen. However, before the gate freezes, although the mold temperature rises, there is a tendency to increase the heat shrinkage, but it is also the higher mold temperature that causes the gate freezing time to prolong, resulting in an increase in the injection pressure and the pressure-preserving effect, and the feeding effect and The amount of negative contraction will increase.

Therefore, total shrinkage is the result of a combination of two reverse shrinkages.

The value does not necessarily increase as the mold temperature increases. If the gate freezes, the effects of injection pressure and holding pressure will disappear. As the mold temperature increases, the cooling set-up time will also prolong, so the shrinkage of the product will generally increase after demolding.

(3) Time

Molding time: refers to the period during which the screw compresses the melt material during the advancement, fills the cavity, and maintains the compression time of the compression. Injection time: refers to the time during which the injection screw continuously advances and continuously compresses the melt.

Before the gate is closed, the shorter the injection time, the greater the shrinkage rate, and the greater the extent of the shrinkage; when the injection time reaches or exceeds the setting time of the gate, even if the injection time is extended, the product quality and shrinkage rate There is no longer a change, the injection time control is closely related to the designed gate thickness, and the gate thickness can largely control the gate closure time.

When the gate is sealed, the extension of the injection time will not only be no longer effective, but will reduce the production efficiency, and may even cause cracks and the like near the gate. For the same plastic part, the injection time is determined by the injection speed.

The longer the holding time is, the better the compaction of the melt is. The higher the density of the plastic part, the smaller the shrinkage rate.

When the gate is sealed, the pressure reduction does not work for the shrinkage of the product. Excessive holding time will prolong the molding cycle.

In thermoplastic injection molding, the influence of the cooling time of the molded article in the cavity on the molding shrinkage of the product differs depending on the kind of the resin, the thickness of the product, the melt temperature, the temperature in the mold, and the crystallization method.

A long cooling time in the mold can reduce the shrinkage rate. In the case of the amorphous resin, the cooling time has little effect on the shrinkage of the product; however, in the case of the crystalline resin, if the cooling time is too long, the crystallization proceeds sufficiently, the crystallinity is high, and the molding shrinkage increases. However, in general, the cooling time is too long, the cooling can be performed uniformly, the material in the mold is sufficiently solidified, and the size of the product taken out from the mold is close to the size of the cavity, so that the molding shrinkage rate is small.

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