This shrinkage phenomenon will continue until the finished product cools down to room temperature, and then the volume shrinkage change does not reach an equilibrium state. However, some materials even under room temperature, because of moisture absorption, light, heat, and other conditions, and it will start the stress release of the product, so it will cause continuous shrinkage and change in product volume. After shrinking, it is generally necessary to check whether the size of the product has stabilized and there is no shrinkage and deformation. The product will be required to stand for 48 hours before measuring the size.
The so-called product warping refers to the deformation of the surface appearance of the molded product, which makes the appearance of the product unable to meet the design size and shape. If the overall shrinkage of the product is fairly uniform, the product will not deform or warp. In other words, the main cause of the deformation or warping of the product is the uneven shrinkage of the product at the local location. However, there are many reasons for the uneven shrinkage and deformation of the product, for example, the internal stress (flow stress or shrinkage stress) of the product generated during the molding process, or because the product has a more severe molecular chain orientation effect at certain positions or The orientation of the reinforcing fibers is either the overall temperature distribution of the product during the molding is very different or the temperature changes are inconsistent, or the product pressure during the molding process is very different (for example, oversaturation occurs near the gate, The pressure at the end of the flow far from the gate is insufficient due to saturated pressure, etc.), or at different positions in the thickness direction of the product, during the molding cooling and solidification stage, the degree of pressure felt is different, because there are many reasons for uneven shrinkage, And will affect each other; so to make the product has a more uniform shrinkage after molding, and the degree of deformation and warpage of the product can be accepted or can be adjusted and controlled by the molding conditions, the shrinkage of the plastic product needs to be first And warpage characteristics.
Effects of material types on product shrinkage
Non-crystalline plastic has a lower shrinkage value, so non-crystalline plastic has better uniformity than crystalline plastic. For injection products, the thickness distribution of the product or the design of the gate location or gate type is often different, so that during injection filling, a high molecular orientation effect in the flow direction will be produced due to the high shear effect of the flow field. This molecular orientation effect will also cause the overall shrinkage of the product to have different directions or asymmetry, which means that the shrinkage of the plastic in the flow direction and the vertical flow direction will be different. Generally, the injection products will flow in the direction of flow and have a larger shrinkage due to a more serious molecular orientation effect. Besides, the thickness of the product will also cause different degrees of shrinkage. Generally speaking, thicker products or thicker areas in the finished product will have a slower cooling rate, and thinner products or thinner thickness areas will produce more shrinkage.
The crystalline plastic is produced along with the crystallization during the cooling process, and the molecular chains will interact closely to form a crystalline region, causing more volume reduction, so the crystalline plastic will have a larger value of shrinkage. The degree of crystallinity produced during the injection process of plastics will also affect the shrinkage of the product. Therefore, general crystalline plastics will show anisotropic shrinkage values, especially for products with varying product thicknesses. In the thicker area of the product, the plastic has a slower cooling rate here, and the molecular chain has enough time to be discharged into the crystallization position, which can produce higher crystallinity. Therefore, the shrinkage value in this thicker area is also It will be larger.
Effects of additives or reinforcements in material formulations on shrinkage and warpage
For non-crystalline plastics or crystalline plastics, if a granular or flake filler (such as inorganic fillers: Talc, CaCO3, Mica, glass bead) is added to the plastic formula, the plastic shrinkage value will be reduced and Can produce a more uniform overall shrinkage, and these plastics formulated with inorganic additives generally have a higher rigidity value (Flexural Modulus), but will reduce the Impact Strength of the plastic. If the plastic is formulated with reinforcing materials, such as GlassFiber, CarbonFiber, fiber reinforcement materials with relatively large diameters, because of the high shear effect of the flow field, the fiber orientation of the fiber reinforcement material will be caused. Therefore, generally, fiber-reinforced plastic injection parts, The shrinkage in the cross-flow direction will be higher than that in the flow direction, so the issue of the amount of deformation and warpage of the injection parts of fiber-reinforced plastics, and the effect of fiber orientation will be an important determinant.
For pure plastics without added reinforcing fibers, the warpage of injection molded products is mainly affected by the thickness distribution of the product and the mold temperature distribution. The product thickness difference is too big or the male and the female mold temperature difference is too large or the local mold temperature difference is too large will cause the product to produce the more serious warping phenomenon. For fiber-reinforced plastics, the reinforcing fibers (GlassFiber, CarbonFiber, organic fibers, etc.) in the plastic composition have high rigidity and extremely low heat shrinkage characteristics, so the reinforcing fibers will inhibit the overall shrinkage of the plastic. And because the reinforcing fiber has a large aspect ratio, there will be a flow direction effect during injection filling, which causes the shrinkage value of the reinforcing fiber plastic injection product to have non-uniform directionality. Generally, the reinforcing fiber plastic injection part is along The alignment direction of the fibers (generally the flow direction) will have a smaller amount of shrinkage relative to its vertical direction. For the same reason, for a granular filler with a length-to-diameter ratio close to one, although the shrinkage value of the injected product will be smaller than that of the pure material, the non-uniform directionality is not as good as that of fiber-reinforced plastic products.