The gate land should be as short as possible to achieve lower pressure for filling and to improve de-gating by minimising the height of gate vestige. The only reason for not using very short lands is the loss of strength of the steel at this area, which may cause the steel to bruise or fracture. In the experience, land lengths should be from 0, 25 to 10 mm for the average molding.
Too small a gate can be recognized by blemishes at the gate and by surface imperfections. However, even before it affects the resin it dramatically affects the available injection pressure so that the product will not be filled, and the gate will freeze off too soon. Too large a gate often results in an unsightly vestige. It will affect the mold-closed time, and requires an increase in cycle time. While too small a gate can easily be correctcd by increasing the gate size, too large a gate will require a new gate insert, or even a new cavity if there was no insert.
High shear rates can raise the local melt temperature at the small gate significantly, thus reducing the viscosity and allowing the material to flow easily within the cavity space. High shear rates can also significantly improve the surface finish of the product, especially its gloss. If the gate is too large, there is no temperature rise as a result of shear in the gate, which may result in premature gate freeze-off and sinks in the product.
As discussed, We know that the shear rate at the gate must be greater than 1000 s、In thin-wall molding, shear rates of 100,000-1, 000, 000 JT1, dependent on the material, may be needed for best results (i.e., to be able to use the lowest possible molding temperatures). This reduction of viscosity by shearing is often essential.
Reduction in viscosity achieved is through high velocities and sheer rates may be the most effective way to mold otherwise difficult-to-proctors high viscosity materials. However, there is a limit to the amount of shear a resin can take before it starts to degrade.
What happens at shear rates greater than 1, 000, 000 s is not fully known. There is a point at which the molecules can no longer slide against each other in response to an applied shear stress- In other ‘words, the material can only be stretched so far before it starts to be torn apart.
In a fully stretched condition, the material viscosity is as low as it can be, because the molecules have aligned as much as possible in the direction of flow. If this condition occurs at 100 000 s 1, there is no point in shearing the material any further, because it will not provide any more viscosity reduction. If we could determine the limit to viscous stretching, We would obtain a physical limit to the amount of shearing.
All materials have a maximum permissible shear rate at which they degrade. The more heat-sensitive they are, the lower the permissible shear rate is, however, that it is difficult to find the maximum shear rate; because degrading is also affected by the length of time the material is subjectcd to that shear rate.