In recent years, as consumption has entered an era of upgrading, consumers have increasingly favored various high-end products with superior quality, fashion and beauty, energy saving and environmental protection. In the era of increasingly fierce competition, manufacturers have become more demanding on appearance of various products, which invisibly promotes development and application of hot runners in various industries. Hot runner has changed from an open large nozzle to a cold runner at first, to a needle valve to a cold runner, then to direct injection of needle valve, further development to high requirements of needle valve inversion and color change. Because appearance of product directly reflects quality of product, appearance of product is not only related to hot runner, but also greatly related to rationality of injection parameters and mold design.

●Glue mouth halo: cloud-like discoloration generated near gate, often in the form of sun and moon halo.
●Fog spots: Fog-like discoloration generated near gate can sometimes be found in obstructions in plastic flow channel.
●Spray flow mark: from a restricted area (such as a nozzle or gate) to a thicker and open area, flow mark formed by bending and folding like a snake.
●Ripple: Produce a circle and circle pattern near glue port. Formed like ripples on the surface of river.
●Sink marks: local collapse of surface of product (or dimpled or ravine-shaped)
Shrinkage holes: vacuum holes in product due to plastic shrinkage
●Welding line: When the two strands of glue meet, temperature of the two strands of glue is inconsistent and the air is not discharged in time when melting glue, forming a weld line.
●Tiger skin pattern: When melt flow reaches a certain flow length, flow resistance is so large that upstream pressure is too late to provide enough kinetic energy to allow front edge to continue to move forward at original speed, so front edge slows down or even stops. When pressure transmitted from upstream to leading edge continues to increase, accumulated energy of leading edge is sufficient to overcome its forward resistance, leading edge bounces close to next equilibrium point of resistance and thrust. Here, frontier slows down or stalls again. This is repeated. In belt-shaped area where leading edge is stagnant and bounces perpendicular to flow direction, because contact with mold wall is different, degree of duplication of mold surface is different, luster varies from one to another, forming flow marks like tiger stripes, called tiger skin patterns.

(1) High or low mold temperature will cause different appearance defects. Mold temperature is a key factor in solving poor appearance. When setting mold temperature, you can start from recommended value of material manufacturer. Decrement (or increment) for each adjustment can be 5℃, glue is injected 10 times. After molding condition is stable, decide whether to adjust further according to result. For example, the most common occurrence of glue mouth halo occurs on back cover of TV or large flat surface that hits surface of product, especially when hot runner is used. Main reason for this is that material temperature at glue port is very different from material temperature in other places, so design of mold water transport and mold temperature are very critical at this time. First, add separate water transport around glue port, water transport height value should be close to glue plane, cold water or room temperature water should be taken close to glue port for rapid cooling. Then temperature difference between front and rear molds should not be too large.
(2) Size and shape of glue mouth are unreasonable, resulting in poor appearance. Gate is too small or cavity is too thin at glue inlet, melt flow is large, cross-sectional area is small, shear rate is large, and shear stress tends to increase, so that melt is broken and gate halo is bad. When cross-sectional area suddenly increases from gate to cavity, plastic flow is unstable, and it is easy to produce jet flow. From gate to cavity, cross-sectional area should gradually change, such as a tab gate or a fan gate, plastic flow can be smoothly transitioned, and jet flow can be avoided. Impact gate can avoid jet flow. Impact gate guides poured melt to a fixed surface to relieve stress, stabilize plastic flow and avoid jet flow. Overlapping gates and latent gates can be designed as impact gates. (Figure 1)
(3) Improper gate location. Improper gate location makes flow length too long and flow resistance is too large. If injection pressure is insufficient, cavity cannot be filled, melt density is small, and probability of sink marks or shrinkage holes is high. Gate should be placed as thick as possible to ensure a sufficient supply of supplementary flow and reduce possibility of sink marks or shrinkage holes.

 

(1) Whether size of flow channel is reasonable, too large will make rubber flow less forceful, causing ripples, too small will cause flow channel shear force to be large, and speed of fire will increase, resulting in jet flow marks, etc. Problem of reasonable runner size reduction can be calculated through analysis and theory.
(2) Whether temperature is stable and uniform, and control is accurate. Temperature is core point of hot runner. If temperature in runner is too high, it will cause carbonization of rubber material, forming a surface pattern, or forming a bright circle due to temperature difference of several glues when melting glue, and many other appearance problems. It is possible to verify whether temperature is uniform and reduce occurrence of problems through thermal analysis in the early stage and infrared rays in the later stage.
(3) Optimize structure of hot runner and use materials with good heat insulation, as far as possible to minimize influence of temperature of hot runner on mold temperature.

Whether shooting speed is reasonable, when shooting speed is high, flow rate of melt from thin gate into cavity is high, shear rate is large, shear stress is large, fluid is difficult to stabilize, and it is easy to produce fog spots, silver streaks or jet flow marks, tiger skin patterns, etc. In order to avoid unstable plastic flow, reduce screw speed... In this way, after melt enters cavity, due to slow advancement of front, a large amount of melt heat is taken away by colder mold wall... (Figure 2). As shown, solidified layer is thickened and extends to the front. Fountain flow at the front is blocked by extended solidified layer, can only bypass front of solidified layer and roll to downstream mold wall, just like a jumping frog. Shape of such a solidified layer is like throwing a stone in the center (gate) of pool, forming a circle of concentric ripples. Solution is high mold temperature, use of an optimized curve of screw speed versus stroke, and slow injection of glue. For example: car lined with tiger skin texture, PP material, wall thickness 2.5MM, when high-speed injection (5 seconds injection time), tiger skin texture is more obvious, reduce shooting speed, and perform low-speed injection, but when low-speed injection (>5.6 seconds injection time), tiger skin lines are not obvious.

 

There are many factors that affect appearance of bad appearance. In addition to above mentioned, there are also exhaust design of mold, design of water transportation, selection of steel, design of size of glue port, structure of the product, whether wall thickness is uniform, physical parameters of rubber, and grade will have a great influence. In addition to injection speed of molding parameters, selection of injection machine model, size of pressure, temperature of melt, and temperature of mold are all factors that affect defects.