Splay mark often appear on large automotive injection molded parts such as bumpers, dashboards, door panels and columns, and are a kind of surface defect of wavy stripes, which are about perpendicular to the flow direction of the melt, forming different gloss marks on the surface of the workpiece, which looks like the pattern on tiger skin, commonly known as splay mark.

1. Where do splay mark generally appear?

Splay mark is easy to produce on parts with thin wall thickness and large processes, and it has the following characteristics:
(1) Alternating light and dark stripes appear on the surface of the injection molded parts;
(2) the fringes are roughly perpendicular to the flow direction of the melt;
(3) The stripes are divided into light area and dark area, the gloss of the bright area is high, and the gloss of the dark area is poor;
(4) If the front of the workpiece is a light area, the back side is a dark area, and the two appear alternately.

2. Why are splay mark so susceptible?

The more toughening systems there are in the material, the more likely it is that splay mark will appear. splay mark are rarely produced in materials with poor toughness, such as reinforced materials, non-toughened nylon, PBT and other materials in the molding process. PP materials used in automotive injection molded parts are very prone to splay mark defects due to the high impact performance and the addition of toughening ingredients such as EPDM and POE.

3. How to improve splay mark?

1) Increasing the diameter of the runner and expanding the thickness and width of the gate in the mold design are effective measures to eliminate splay mark. The gate thickness should reach 0.7~0.8 times of the wall thickness. The aim is to reduce the pressure loss of the melt in the runner and gate and to reduce the effect of mold expansion when entering the cavity.

2) In the injection molding process, it is beneficial to improve the melt temperature and mold temperature, and adjust the injection speed, which is conducive to eliminating splay mark. In general, a lower rate of fire is beneficial for the elimination of splay mark, as the melt flow is more stable at low velocities.