Plastic molding properties (continuation 2)

2024-03-13 14:42:57

Plastic molding properties

(continuation 2)

プラスチック成形特性 (続き2).jpg

2. Calculation of shrinkage rate The shrinkage of plastic parts during molding can be expressed by the shrinkage rate, as shown in formulas (1-1) and (1-2).

Actual value=(a-b)/b×100 (1-1)
Q = (c-b)/b×100 (1-2)
In the formula: Qactual——actual shrinkage rate (%);
Q-meter - Calculates shrinkage (%).
a——The size of the plastic part in one direction at the molding temperature (mm).
b——The size of the plastic part in a certain direction at room temperature (mm).
c——The size of the mold in one direction at room temperature (mm);

Since the actual shrinkage represents the actual shrinkage of the plastic part, and its value is significantly different from the calculated shrinkage, Q is used as the design parameter when designing the mold and calculating the size of the cavity and core.

3. Factors affecting the change of shrinkage rate In actual molding, not only the shrinkage rates of different types of plastics are different, but also the shrinkage values of different batches of the same type of plastic or different parts of the same plastic part are also different, and they also have something in common. The main factors affecting changes in shrinkage include:

(1) Types of plastics Each plastic has its own shrinkage range, and even the same type of plastic has different shrinkage rates and anisotropy due to differences in fillers, molecular weights, and ratios.

(2) Characteristics of plastic parts: The shape, size, wall thickness, presence or absence of inserts, and the number and arrangement of inserts also greatly affect the shrinkage rate of plastic parts.

(3) Mold structure Especially in extrusion molding and injection molding, the parting surface of the mold, the pressure direction, and the shape, arrangement and size of the gate system also greatly affect the shrinkage rate and directionality.

(4) Molding process Extrusion and injection molding processes generally have larger shrinkage rates and obvious directionality. Preheat conditions, molding temperature, molding pressure, dwell time, filler geometry and cure uniformity all affect shrinkage and directionality.

As mentioned above, mold design is based on the shrinkage range specified in the instruction manual of each plastic, and needs to be based on various shapes, sizes, wall thicknesses, presence or absence of inserts, parting surfaces, pressure forming direction, mold structure, etc. Design molding conditions. The selection of shrinkage rate should comprehensively consider the shape, size, location, molding process, etc. of the supply port. In extrusion and injection molding, it is often necessary to select different shrinkage rates based on the shape, size, wall thickness and other characteristics of each part of the plastic part.

In addition, the molding shrinkage rate is also affected by various molding factors, but it is mainly determined by the type of plastic and the shape and size of the plastic part. Therefore, by adjusting various molding conditions during molding, the shrinkage rate of the plastic part can be appropriately changed. Chapter 3 discusses in detail the rules for selecting shrinkage during mold design.

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