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What is the relationship between mould size and plastic shrinkage rate?

May 12, 2017

The shrinkage rate of plastics and its influencing factors

The properties of thermoplastics are inflated after heating, shrinking after cooling, and of course the volume will shrink after pressurization. In the process of injection molding, the molten plastics are first injected into the mould cavity, the filling is cooled and solidified after the melt is finished, and the shrinkage occurs when the plastic parts are removed from the mould, which is called forming shrinkage. Plastic parts from the mold to stabilize this period of time, the size will still appear small changes, a change is to continue to shrink, this contraction is called after contraction. Another change is that some hygroscopic plastics swell with moisture absorption.

The contraction rate s is expressed by the following: S={(D-M)/D}x100% (1)

Among them: s-shrinkage rate; D-mould size; Dimensions of M-plastic parts.

If the mould cavity is computed according to the known plastic parts size and the material shrinkage rate, the D=M/(1-S) is used in the mould design to simplify the calculation, and the mould size is generally evaluated by the following type:

D=M+MS (2).

If more precise calculations are required, the following type is applied: D=M+MS+MS2 (3)

However, when the shrinkage rate is determined, the approximate value can be used only by the effect of the actual shrinkage rate, so the size of the computed cavity is basically satisfied with the type (2). When making molds, the cavity is machined according to the deviation, and the core is machined according to the deviation, and it can be appropriately trimmed when necessary.

2. Shape of plastic parts

For the wall thickness of the forming part, the cooling time of thick wall is usually longer, thus the shrinkage rate is also larger, as shown in Figure 1. For the general plastic parts, when the melt flow direction l size and the perpendicular to the melt flow direction w size difference is larger, the shrinkage rate is also larger. From the melt flow distance, far away from the gate part of the pressure loss, so that the shrinkage rate of the place is larger than near the gate. Because of reinforcement, holes, convex sets and engraving and other shapes with shrinkage resistance, so the shrinkage of these parts is small.

III. MOULD structure

The gate form also affects the shrinkage rate. With the small gate, the shrinkage rate of the plastic parts is increased by the cure of the gate before the end of the holding pressure. The structure of cooling loop in injection mould is also a key of mould design. Cooling circuit design is inappropriate, because the plastic parts of the temperature imbalance, resulting in shrinkage difference, the result is to make the size of the plastic parts or distortion. In the thin-walled part, the effect of temperature distribution on shrinkage is more evident.

The use of direct gate or large cross-section gate can reduce shrinkage, however, the anisotropy of large, along the direction of the flow of material narrowing, along the direction of the vertical flow contraction large; Conversely, when the gate thickness is small, the gate part will be premature condensation hardening, the plastic contraction in the cavity can not be supplemented by a timely contraction.

Point gate coagulation quickly, in the workpiece conditions permitting, can set a multi-point gate, can effectively prolong the retention time and increase the pressure of large cavity to reduce the shrinkage rate.

IV. Forming conditions

Barrel temperature: The barrel temperature (plastic temperature) high, the pressure transfer better and reduce shrinkage force. However, when the gate is cured, the shrinkage rate is still larger because of the early gate. For thick-walled plastic parts, even if the barrel temperature is higher, its shrinkage is still larger.

Supplement: In the forming conditions, minimize the material to make the size of the plastic parts remain stable. However, insufficient replenishment can keep the pressure and increase the shrinkage rate.

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