5. Mold Designing
LAPEROS® LCP has anisotropic characteristics which, along with the basic rules of design, must be taken into consideration when designing a mold.
Physical properties of molded parts are influenced by the direction and the extent of orientation of LAPEROS itself and fillerss. The orientation results from the shear force of the flowing material. For this reason, the flow pattern of the material in the mold cavity must be considered in relation to the properties required of molded parts. Generally, the thinner the wall of molded parts, the stronger the orientation will be.
Molded parts designers and mold designers should cooperate closely with each other so that the molded parts will satisfy properties requirements.
In addition, at weld lines, the inherent strength of LAPEROS is sometimes not fully obtained; therefore, efforts must be made during the mold designing stage to avoid weld lines.
As LAPEROS is not corrosive to molds, all standard mold steels are usable. Some fillers are corrosive to molds and in such a case, selection of a suitable steel or hardening is necessary, as in the case of all other molding materials filled with similar additives.
Table 5-1 Abrasion Resistance of Mold against A130
D2 is the plastic mold steel of Hitachi Metals Tool Steel, Ltd., and 420 is the plastic mold steel of Uddeholm K.K.
When ease of machining is compared, runner shapes are ranked in the order of semi-round, trapezoid and round, but considering cross-sectional area and pressure loss, the order is round, trapezoid and semi-round. Accordingly, round or trapezoidal runners are recommended. The optimum runner size depends on the required length and product size when cost advantages are taken into consideration, and diameters of 2 to 5 mm are generally adopted. Runner length should be as short as possible. In multi-cavity molds, the distance between cavities must be made equidistant wherever possible in order to decrease irregular filling between the cavities.
If the sprue size is too large relative to the nozzle hole diameter of the injection molding machine, jetting will occur in the sprue and may result in blisters due to air entrapment. Therefore, it is recommended that the sprue hole diameter on the nozzle touch side be approximately 0.5 mm larger than the nozzle hole diameter of the injection molding machine, and the taper of the sprue be 0.5° to 1°.
LAPEROS by nature is a material of easy mold releasing, and at the same time, it is a high flow material; therefore, if there is any defect on the mold surface, it will be transferred to the molded parts and mold releasing is also sometimes affected. Sprues and runners must be polished carefully. At the end of sprues and runners, a slug well should be provided to prevent cold slug from running into molded parts.
Gate design is particularly important when designing molds for LAPEROS. Because LAPEROS has anisotropic characteristics, gate location must be decided in consideration of the mold filling pattern. In general, the mechanical strength of injection molded products is high in the direction in which the molten resin flows in the mold; therefore, gate location in flow direction is recommended when possible.
When a product shape is complicated and flow direction is random during mold filling, it is not necessary to consider the influence of gate location. For the gate system, the popularly adopted side gate, pin-point gate and submarine gates are all adaptable.
LAPEROS generates almost no gas during molding. Even so, it is good to provide air vents to remove air inside mold cavities for easy filling. Air vents for LAPEROS are approximately 1/100 to 2/100 mm deep. The depth of this range has a limited cross-sectional area of air passage, so it is advised to provide air vents not only at air trapping spots but also over a wide area