With the rapid development of the automotive industry, the application of plastic parts in automobiles is becoming more and more widespread. The application of automobile plastic parts has significant advantages in reducing automobile quality, saving fuel, promoting environmental protection and recyclability. Automotive plastic parts are mostly injection-molded. Tiger skin, poor surface finishing, sink marks, welding lines, warping and deformation are common defects in automotive injection parts. These defects are not only related to materials, but also to structural design and mold design. It has a lot to do with the molding process. Today Asia Billion will share with you a few common problems and solutions for our automobile bumper plastic injection mould and moulding production experience.
First, the pressure line
As shown in the figure, there are obvious pressure lines around the bumper fog lamp area, which affects the appearance and surface quality of the product. Since the bumper is a part of the outer surface of the car, it has stricter requirements on the apparent quality and have a serious impact for the production quality.
Cause analysis and improvement measures:
Mould: In our case, a gate is set near the hole around the fog lamp. When this gate is opened, the pressure on both sides of the hole reaches a pressure line formed by the balance due to the influence of the hole.
The pressure line described in the case is actually a material flow line, which often appears in the area where the welding mark is located. The solution is to reduce the pressure difference around the welding mark as much as possible, or make the pressure difference is not enough to move the material which is solidifying.
Second: Tiger skin pattern:
The tiger skin pattern often appears on large-sized automotive injection parts such as bumpers, dashboards, door panels, and pillars. It is a wavy stripe surface defect that is approximately perpendicular to the direction of melt flow and forms different gloss on the surface of the part. The different surface finishing looks like patterns on tiger skin, commonly known as tiger skin.
1. Where do tiger skin patterns generally appear?
Tiger skin is easy to produce on parts with thin wall thickness and large flow range area. It has the following characteristics:
(1) The surface of the injection molded part has periodic alternating light and dark stripes;
(2) The stripes are approximately perpendicular to the flow direction of the melt;
(3) The stripes are divided into bright and dark areas, the gloss of the bright area is high, and the gloss of the dark area is poor;
(4) If the front side of the part is light, the back side is dark, and the two appear alternately.
2. Why is tiger skin easy to appear?
The more toughening systems in the material, the easier the tiger skin pattern will appear. The tiger skin is rarely seen in poorly tough materials, such as reinforcing materials, non-toughened nylon, and PBT. PP materials used in automotive injection parts require higher impact properties. Toughening components such as EPDM and POE are added, which are very prone to tiger skin defects.
How to improve tiger skin texture?
1) Increasing the runner diameter, increasing the thickness and width of the gate in mold design are effective solution to eliminate tiger skin. The thickness of the gate should preferably be 0.7 to 0.8 times the wall thickness. The purpose of this action is to reduce the pressure loss of the melting in the runner and the gate, and reduce the expansion effect of the mold when entering the cavity.
2) In the injection molding process, increasing the melting temperature and the mold temperature, adjusting the injection speed are conducive to eliminating tiger skin marks. In generally, using a lower rate injection speed is beneficial to eliminate tiger skin texture, because the melting flow is more stable at low speeds.
Three: material flowers
The flower-like cold material near the car bumper not only affects the apparent quality of the part, but also affects its mechanical properties. A car bumper is a safety device that absorbs and mitigates external impact, protects the front and rear of the vehicle body. Generally PP with better mechanical strength is selected.
Cause analysis and improvement measures:
Mold: The mold used in the product of this case includes hot runner and cold runner, but there is no cold material well on the cold runner, which is directly connected to the product.
Molding process: For the injection molding process, it is found that some needle valve gates are not tightly sealed, there is residual cold material on the gates, and the pressure of the closed needle valve is very low, which can’t serve as the sealant. In this case, the pressure of the sealing needle is increased by using a pressure pump, and the cold material disappears after the hot gate no longer leaks.
Like the structure of a typical hot runner nozzle, the front end of the nozzle is in contact with the mould plate. Although the contact area is limited, the heat at the front end of the nozzle will be conducted to the colder plate. In addition, there is sometimes a cooling circuit around the nozzle due to the cooling needs. It further increases the possibility that the temperature at the tip of the nozzle is too low. The temperature at the front end of the nozzle is low, causing a small amount of cold material at the front end. Especially when the needle seal is not tight, the melt overflows and the cold material is more likely to be formed. Therefore when the hot runner nozzle turns into a cold runner, it is recommended to Cold wells are also set up.
Automobile bumper parts have large surface, low defect rate, and it is difficult to find peeling defects, but this defect seriously affects the appearance quality of the parts.
Cause analysis and improvement measures:
In terms of molding process: the judgment of the product in this case may be caused by cold materials. Through inspection, it was found that the material temperature and the hot runner temperature were normal, and the open hot runner used by the mold was also normal. Because manual pick-up is used during production, when the pick-up time is longer, each hot nozzle will have different degrees of drooling. After shortening the pickup time, it was found that the hot nozzle drooling phenomenon was effectively improved. Therefore, it is possible that the cold material produced by drooling was flushed into the cavity and caused delamination, and it was found on the scene that the length of the worker's picking time determines whether the next mold product will delaminate. In view of this, the solution taken by the product in this case is to change the ejection and retreat distance of the injection molding machine from 16mm to 20mm, no more drooling of the hot nozzle, and no delamination on the part.
Cold material flushing into the cavity may cause delamination too. The source of cold material may be too low material temperature, too low nozzle temperature, no cold material well or cold material well length in the runner is not enough, too low hot runner temperature, too low mold temperature, etc.
The bumper of a car has a bubble on the surface and the position is roughly fixed. It is located at the turning position corresponding to the trunk cover. The probability is high that the bubble will adversely affect its apparent quality.
Cause analysis and improvement measures
1) The product structure: the bubbles are located on the two sides of the trunk cover at the corresponding arc positions. The bubbles appear on the left and right arcs, and the chances are different; the bubbles sometimes is very obviously, sometimes they are only slightly uneven, which is difficult to find. After spraying painting, it is very obvious; using an air heater to bake at 600℃, a slight bubble quickly come. Take the product with slight unevenness on the surface, and there are holes in the section viewing.
2) The molds: there is no vent groove in the mold part corresponding to the bulging position, and it is difficult to vent gas during the injection process. Therefore, an venting groove is added to the mold.
3) The molding process: find the position of the screw corresponding to the bulging part, and this section uses low-speed injection. Finally solve the bubble problem.
Now Asia Billion are very expereinced for the automobile bumper plastic injection mould making, no matter for the front bumper or rear bumper , we can help you to develop them with enough expereince and engineering capacity , if you have troubles for your automotive mould and molding project, or have any RFQ, you can let us know freely .