Plastic Injection Molding Shrinkage

Plastic Injection Molding Shrinkage

Injection molding shrinkage is one of the properties when the material temperature drips. The rate of injection molding shrinkage is needed in determining the final workpiece dimensions. The value indicates the amount of contraction that a workpiece exhibits after it has been removed from the mold and then is cooled at 23℃ for a period of 48 hours.

Shrinkage is determined by the following equation:

Injection molding shrinkage rate is a range, as shown in the Tab 1. Injection molding shrinkage rate is not only related to plastic type and  temperature changes, but also by the influence of injection molding processing conditions and product. The material temperature, mold temperature, injection pressure and holding pressure, cooling strength, workpiece wall thickness, the complexity of the shapes, filling characteristics and mold structure will affect molding shrinkage. Therefore, the dimension precision of plastic products is lower than the metal products. The change of process conditions, plastics batches all can cause Injection molding shrinkage rate of fluctuations.

The Injection molding shrinkage rate wave error equation is:

Tab 1 Shrinkage for  thermoplastic / %

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Cold Slug Well of plastic injection molding

Cold slug well of plastic injection molding is divided into the sprue cold slug well and the runner cold slug well. The cold slug well location is generally designed in the end of the sprue or the runner.

Cold slug well are mainly in the form of the bottom with push rod and sprue puller. With a push rod at the bottom of the slag hole is composed of a push rod and push rod mounted on the rod on the fixed panel, so it is often used with a push rod or tube release mechanism, as shown in the figure a, b, c. Another is the bottom of cold slug well formed by a sprue puller, which pull lever core fixing plate, so it's not exercise with the ejection mechanism. Its main bodies are spherical and bacteria sprue puller, as shown in the figure d.

Cold slug well is to gather cold slug to prevent them into the cavity and affect the quality of plastic parts. It can be pushed out from the plastic injection mold with the sprue when demoulding. Cold slug well is to store material flow forwards cold slug. The shape of the sprue cold slug well are the Z-pin (Fig a), the reverse taper (Fig b), the annular ring （Fig c）and the spheral (Fig d). while in the mold opening process to use puller rod or inverted cone etc will be the main runner material pulled from the sprue bushing.

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The Shape and Position of Parting Line

For setting out plastic injection parts,material and putting insert, mold cavity consist of at least two or more parts, remove the product or material separation of part of the contact surface is called a parting line(PL). Parting line position, shape, and quantity depend on the plastic structure, applications and selection of gate type and other factors. From the perspective of simplified structure and guarantee the quality of products, the parting surface is perpendicular to the closing direction, inclined to clamp or parallel to the direction of movement and direction.

Parting surface can be classified as flat, angled, stepped, profiled parting surfaces as Fig 1. The flat parting surface is most commonly used. In order to meet the ejection, mold structure or application, angled, stepped and profiled parting surfaces may be used. But it may make the structure of mold more complex and mold processing more trouble. So the parting line position choice is important, which will affect the mould structure, plastic quality, mould manufacturing and maintenance operations.

The determination of parting surface position need to consider knockouting the workpiece off the mold, coaxial requirement, appearance requirement and mold parts manufactory as Fig 2,3,4,5.

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Gates desing of Plastic Injection Molding

Gate design of Plastic Injection Molding includes gate section shape, gate section size and gate location.

The factors influence the gate section shape and size, in terms of products, including the product's shape, size, wall thickness, dimensional accuracy, surface quality and mechanical analysis, etc. The factors from raw material used in the plastic properties of the influence gate design are the plastic molding temperature, viscosity, contractility, and presence of fillers, etc. In addition, at the time of the gate design, gate should also be considered for processing, mold release and removal of the difficulty level of the gate.

As usual, the section size of the gate should be smaller rather than larger. When determining the gate size, make sure the smaller, and then try to test mould, if necessary we can revise according to the cavity filling conditions, especially to a multi-cavity mold, which can be made simultaneously to the cavity filling evenly through the amendment.

The small gate can increase molten plastic material flow rate and molten plastic material through the small gate will have much friction to raise the material temperatures. Which apparent viscosity decreased and advantageous to the filling. In addition, due to the small gate of fast curing, which does not produce excessive feeding and reduce the internal stress of products. At the same time it can reduce injection molding cycle and improve the gate removal.

Gate should not be too small for some products, such as thick-walled products, must be more than two times in the process of injection pressure to meet the requirements of products.  Too small gate will cause the gate place too early curing, the feeding difficulties will cause the product defects.

The Gate location should locate in the thickest area, so that the plastic flow from thick to thin-walled area and keep the process identical in all parts of the gate to the cavity,prevent jetting or worming during the filling process.

The gate location should  locate in the main stress direction, because the plastic flow direction on the highest tensile stress and compressive stress. When choosing gate location, we should consider the size requirements of the product. Because the plastic gate mold filling, the plastic flow direction and perpendicular to the flow direction of shrinkage varies is not the same. So we need consider  the direction of the deformation and shrinkage.

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