Release time：2021-11-19
3C electronic shell materials need flame retardancy, balance of strength and toughness, thin-wall molding and dimensional stability, so halogen-free flame retardant pc/abs is the best choice.
However, halogen-free flame retardant pc/abs is not omnipotent. When it is used as a 3C electronic shell product, the headache is that the plastic screw hole is easy to crack.
So why did the screw hole crack? How to solve it?
reason:
① The material formulation design is defective, and its toughness is insufficient
② Low weld strength
③ Material degradation during processing
④ Influence of antirust oil and other chemicals
⑤ Difference of metal plastic shrinkage coefficient
⑥ Unreasonable screw hole design
1、Materials with flame retardant precipitation and insufficient toughness
Let's start with the influence of the material itself.
Due to the need of flame retardant, halogen-free flame retardant pc/abs must be added with flame retardant. The commonly used halogen-free flame retardant is polymer small molecules, such as organophosphorus flame retardant (MW is dozens to hundreds). The addition of these small molecules will affect the toughness of pc/abs.
More importantly, these flame retardants contain monomolecular or small molecular residues (such as TPP) that have not yet been fully polymerized. Because of their small molecular weight, they are easy to migrate from pc/abs material to the surface of the material, thus forming micro cracks on the surface of the material, leading to cracking.
In addition, in halogen-free flame retardant pc/abs, PC is notch sensitive material. When a small crack in PC does not stop in time, it will expand into a large crack.
Adding ABS to PC can significantly reduce the cracks caused by notch sensitivity of PC, because the rubber phase butadiene (b) in ABS is dispersed in pc/san matrix, forming a "Sea Island" structure, which can effectively prevent the generation and development of cracks.
This is because the toughness of the material is also related to the particle size, content and dispersion of the selected ABS rubber. Generally speaking, the smaller the ABS particle size is, the higher the rubber content is, the more uniform the rubber dispersion is, and the higher the screw hole strength is.
2、Low weld line strength and gas trapping during injection molding
The low strength of the welding line at the screw hole is also an important reason. When the threaded hole is injection molded, the two strands of material will converge to form a fusion line. If the notch, rubber turnover and uneven rubber Distribution occur at the position where the cohesive force of the converging material flow is weak and the edge is located, the strength of the relevant area will be reduced and the risk of cracking will be increased.
In addition, in some factories, nuts are pre placed in the mold cavity and then injected with plastic melt. Because the edge of the nut is generally serrated, gas may be left in the serration during the injection molding process, resulting in the loose combination of plastic and nut, which is often referred to as "trapped gas". Such defects may lead to the generation and growth of cracks, and it is necessary to strengthen the exhaust design in the mold.
3、Material degradation
In halogen-free flame retardant pc/abs materials, PC is prone to chain breaking reaction under the stimulation of heat, oxygen, shear, especially water and alkaline groups, and ABS is also prone to degradation under high temperature, resulting in "internal injury" to pc/abs, and the performance of impact resistance, heat resistance and other products will be reduced.
However, the injection molding method of placing nuts in advance requires a long injection cycle, and the material can stay in the barrel for more than 3 minutes, which also increases the risk of material degradation.
A simple method to evaluate the thermal stability and degradability of materials is to keep the materials at a certain temperature in the injection molding machine for a period of time, and then inject them into the air to see the smoke. The more smoke, the more serious degradation.
4、Influence of chemicals such as antirust oil and thread tightening agent
In order to prevent iron screws from rusting, screw manufacturers will coat the screw surface with anti rust oil. Antirust oil generally contains gasoline, kerosene, fatty acids, alkaline earth metal salts and other acid-base substances.
In order to make the combination of studs and screws closer, some manufacturers will drop thread tightening agent at the joint of studs and nuts, which contains acrylate, maleic acid, acetylphenylhydrazine and other substances.
The above chemicals may cause corrosion and cracking of halogen-free flame retardant pc/abs.
After magnifying the pc/abs spline surface before and after coating with threaded fasteners, we found that after coating, the surface corrosion was serious, and a large number of cracks appeared on the spline surface. The cracks would grow with time, resulting in the final cracking.
5、Metal plastic shrinkage coefficient difference
Due to the different thermal expansion coefficients of plastic and metal, the stress cracking caused by different shrinkage during thermal expansion and cold shrinkage is also one of the reasons for the cracking of screw holes.
The linear expansion coefficient of iron is 11 × ten-6/℃, the linear expansion coefficient of halogen-free flame retardant pc/abs is 75 × ten-6/℃, the latter is more than 7 times the former.
When the self tapping screw enters the screw hole, it is usually in an interference state, that is, the plastic screw hole is in an expansion state. When the temperature becomes cold, the shrinkage of metal screws is small, while the shrinkage of plastic screw holes is large, and the screw holes may crack due to shrinkage stress. This is also the reason why screw holes are easier to crack in winter.
6、Unreasonable screw hole design
Unreasonable screw hole design is mainly reflected in the guide hole design, screw hole depth, screw hole inner diameter, screw interference, screw hole thickness and so on.
When we screw in the screw, the screw often enters obliquely, which leads to the breakage of the screw hole. In order to prevent the screw from cracking due to the direction deviation when it is introduced, a guide hole must be designed to guide the screw to lock.
In addition, when the self tapping screw enters, the plastic fragments will be cut off, and the extrusion of these fragments may cause the screw hole to burst. Therefore, the length of the screw hole must be longer than the length of the screw to accommodate the plastic fragments cut by the self tapping screw.
And in order to prevent the back shrinkage, the thickness of the screw column is generally designed to be relatively thin, which is also one of the reasons why the screw column is easy to crack.
There are many reasons for broken screw holes, so how to avoid this situation?
1. Reasonable material selection
Choose halogen-free flame retardant pc/abs with reliable toughness, especially the formula design of screw hole strength. For example, the halogen-free flame retardant pc/abs an9000, which is specially formulated for screw hole cracking, has strengthened the design in PC selection, flame retardant selection, rubber phase and distribution, thermal stability and other aspects to ensure a higher product yield.
2. Prevent degradation
Design a reasonable process flow, reduce the residence time of materials in the screw, and prohibit the use of anti rust oil, threaded fasteners and other chemical reagents to corrode halogen-free flame-retardant pc/abs to prevent material degradation.
3. Improved structural design
Appropriately thicken the stud thickness and widen the stud inner diameter to prevent interference during tightening and increase the screw hole depth.
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