Study on the bright lines on the frosted surface of injection molding products

1. Concept of product gloss
Gloss, as a surface feature of an object, depends on the specular reflection ability of the surface to light. The so-called specular reflection refers to the reflection phenomenon that the reflection angle is equal to the incident angle. Glossiness is a physical quantity that evaluates the ability of a material surface to reflect light under a set of geometric conditions. Therefore, it represents the reflection property with directional selection. Diffuse reflection refers to the phenomenon that light is reflected irregularly in all directions by a rough surface. Many objects, such as plants, walls, clothes, etc., appear to be smooth on the rough surface, but when you carefully observe them with a magnifying glass, you can see that the surface is uneven, so after the light that is originally parallel is reflected by these surfaces, it will diffuse to different directions, forming a matt surface.
2. Generation of bright lines
A mold is a tool used to copy and shape products. The high-gloss appearance products reproduce the mold surface perfectly by polishing the mold surface, and the products present high gloss. The frosted surface appearance products reproduce the rough surface of the mold through rough treatment of the mold surface (sand blasting, sun shading, laser carving and other processes), and the product surface will produce diffuse reflection and matt texture effect, without dazzling and dazzling feeling. In addition to large pits and protrusions on the surface of the mold frosted cavity, there are also many small bumps and pits at the micro level, so that the product surface can produce a wide range of diffuse reflection, absorb more light, and the product presents a matte texture effect. In the process of product production, due to mold, equipment, parameters and other reasons, the product replication mold surface is affected, resulting in problems such as light spots, shiny spots or bright lines on the product grain surface, and the product appearance quality is affected. This is a typical appearance problem encountered in the production of frosted surface products.
3. Cause analysis and improvement measures
(1) Abrasion of mold abrasive grain
During the long-term production of the mold, the high-pressure and high-speed injection material flow continuously scoured the mold cavity, especially near the gate or at the position where the local shear force is large, resulting in the wear of the local micro-convex of the mold grain surface, resulting in the increase of light reflection in the specific direction, the reduction of diffuse reflection, the shiny of the grain surface worn position, and the bright spot of the product. This wear process is related to the material fluidity, and the wear time is proportional to the material fluidity.
Improvement measures: the mold wear position shall be re-drawn, and the micro-convex of the wear surface shall be restored. At the same time, we should pay attention to the stability of parameters in production, and avoid using high pressure and high speed parameters for mold cycle production.
(2) Influence of injection molding process parameters and mold temperature
In the actual production process, the mold temperature and process parameters have a great impact on the concave and convex surface of the mold cavity of product replication.
① The influence of improper process parameters: low packing pressure, injection pressure and low filling speed make the product unable to tightly press and compact with the mold surface, the injection density is small or local shrinkage occurs, and the mold micro-convex structure cannot be copied, resulting in local brightness of the product surface.
Improvement measures: adjust the process parameters, appropriately increase the injection pressure, speed and pressure maintaining pressure, and avoid commissioning shrinkage. The improvement effect is obvious.
② Effect of mold temperature: too low mold temperature will make the molten injection molding material cool too fast when contacting the mold surface, and can not completely copy the mold micro-surface, which will make the part of the product shiny.
Improvement measures: improve the overall temperature of the mold. Generally, the dynamic and fixed mold is connected with a 50 ° - 70 ° water temperature machine, and the product's brightening effect can be improved.
(3) Mold cooling
The layout of the water channel is seriously unbalanced or the water channel is blocked in some areas, which causes local parts of the mold to overheat, resulting in local overheating of the product and bright lines.
Improvement measures: the cooling water path shall be evenly distributed, and the mold water path shall be regularly inspected and cleaned to avoid local overheating of the mold caused by water path blockage.
(4) Poor exhaust
Poor die exhaust and trapped air are another major cause of bright lines on the product surface.
The parting surface is the main channel for exhaust during the injection molding process. When the exhaust is poor, the edge of the parting line is prone to shine. The product is also easy to appear shiny at the joint of multi-strand material flow, that is, near the fusion line. The gas will rise rapidly during rapid extrusion. If it cannot be discharged quickly and effectively, the plastic material will be hot or even burnt, and the product will be shiny.
Improvement measures: fully exhaust the trapped gas position, mainly use the parting surface to increase the exhaust slot, and add the splicing exhaust for the welding line accessories to ensure the smooth exhaust. After the gas is discharged, the flow speed at the trapped gas can be increased. At the same time, with the adjustment of process parameters, the material flow compacts the cavity under the pressure, perfectly replicates the mold surface, and eliminates bright lines.
(5) Product materials
Material problems in actual production will also cause partial brightening of the frosted surface of the product.
① For example, for materials containing abrasive additives (such as glass fiber, calcium carbonate, flame retardant additives), with the passage of time, in the mold cavity containing texture, the texture details will be eroded by abrasive additives, which will also lead to the increase of product gloss.
Improvement measures: using hardened die steel or hard coating can improve the wear progress of the die.
② Whether the material is oily, dry and mixed with recycled materials or impurities.
These substances will decompose and gasify or scorch under high temperature, and the scorched substances or gas molecules will adhere to and deposit into the microgrooves of the mold grain, resulting in bright marks on the product.
Improvement measures: In actual production, wiping the mold grain surface with kerosene at the shiny position can eliminate the attachment on the mold frosted surface and eliminate the product bright grain.
(6) Product wall thickness change
① Wall thickness influence: the uneven thickness of the product is the biggest factor at the location where the wall thickness changes.
When the wall thickness changes, due to the difference of flow mode, the main reason is that the material flow speed is slow in the area with thick product, and the product surface is prone to blackening; Relatively speaking, the thinner wall section will not be under too much plastic pressure, resulting in a higher gloss of the product transition area.
Improvement measures: lengthen the transition area and add fillets at the transition position to make the material flow as uniform as possible, which can weaken the product vision and improve the brightness of the transition area.
② The corners of the product structure and the surface of the product with more ribs are also prone to gloss problems.
It is mainly caused by the material flow velocity and pressure at the corners or ribs.
Improvement measures: the thickness at the corner is optimized and uniform.
③ Rib effect: bright lines appear on the top surface of the rib. During the material forming process, the material flow will sink and float when the material flows through the rib, and there will be trapped air on the top of the rib, causing the product to shine.
Improvement measures: cancel or reduce the rib.
(7) Mold part deformation
The deflection and deformation of mold parts can also cause bright lines. This problem is most likely to occur when the mold ejects the parts. When ejecting, the product is affected by the unbalanced ejecting stress, and the frosted surface of the product is deformed and shiny.
Improvement measures: repair the mold, eliminate the deformation position, and the bright lines of the product disappear.
4. Summary
The problem of bright lines on the frosted surface is a common production problem in the injection molding process. The formation of the problem involves many aspects and the causes are relatively complex. This paper summarizes common problems and solutions based on actual production experience, provides production guidance for actual production, improves product exception handling efficiency and production efficiency.