What are the design criteria for preform?
The design and molding of mold billet is the key of injection blow molding.P generally speaking, if you can form a good blank, you can blow out a good container.Containers of different shapes or sizes require a blank design and die design.The design of the billet requires a comprehensive understanding of the injection blow molding process and polymer properties.
(1) the ratio of the height of the container to the diameter of the neck thread directly determines the shape of the blank, that is, the mandrel
The length and diameter ratio (L/D) of the mandrel /D shall not exceed 10 generally. This is because the mandrel is a cantilever beam in the mold of the mold, and the mandrel /D is larger due to the high injection pressure during mold filling, resulting in uneven distribution of wall thickness.If the filling speed of the melt is controlled by a program, or the tip of the mandrel is temporarily fixed by sliding thimble during the filling process to make the mandrel in the mandrel, the L/ D of the mandrel can be larger.
(2) the moulding billet was mainly subjected to circumferential stretching while the axial stretching was very small.The larger the swelling ratio (that is, the ratio of the diameter of the container body to the diameter of the blank), the greater the possibility of uneven distribution of the thickness of the container wall.The blow ratio is generally less than 3, and in some cases it can be locally up to 4.
(3) for a container with an ellipse cross section, if its ellipse ratio (that is, the length ratio of the long and short axes of the ellipse) is less than 1.5, a round cross section blank can be well formed.When the ellipse ratio is no more than 2, a round mandrel with a cross section and an oval mold cavity can be used to shape the blank.When the ellipse ratio is greater than 2, the mandrel and blank cavity are usually designed to be elliptical.With the increase of the elliptical ratio, the design difficulty and manufacturing cost of the mold are increased, so it should not exceed 3.
(4) except the neck, the thickness of blank wall is generally 2~5mm.When the wall thickness is greater than 6mm, it is difficult to adjust the temperature of the blank.The ratio of the maximum wall thickness to the minimum wall thickness on the cross section of the billet is less than 2, while the ratio on the longitudinal section should not be greater than 3.
(5) when designing the size of the blank (such as the size of the neck) and the size of the blow molding mold cavity, the post-shrinkage of the container should be taken into account.Shrinkage rate is related to the type of polymer and molding conditions. For soft plastics like PE and PP, shrinkage rate is 1.6-2.0%, while for hard plastics (such as PC, PS,PAN, etc.), shrinkage rate is about 0.5%, and the container wall thickness is larger (such as neck thread).