What are methods for testing plastic stress?

Time:2023-04-04 10:59:04 / Popularity: / Source:

1. Solvency

⑴ acetic acid immersion
Acetic acid (CH3COOH) used must be more than 95% acetic acid and repeated use shall not exceed 10 tests.
①Surface stress test: Pour acetic acid (glacial acetic acid) into glassware, and completely immerse product in acetic acid for 30 seconds. After 30 seconds, sample was taken out with a clip and immediately rinsed with clean water (tap water) to check whether surface of sample was whitish and cracked. Conclusion: There should be no cracking, and surface is allowed to be slightly whitish.
②Internal stress test: Dry samples that pass surface stress test and soak them in acetic acid for 2 minutes. After 2 minutes, sample was taken out and immediately rinsed with clean water (tap water), sample was inspected for whitening and cracks. Judgment: There should be no fracture, there should be slight cracks and surface whitening at inserts.
(2) Methyl ethyl ketone + acetone addiction method: completely immerse the whole machine in a 1:1 mixed solution of methyl ethyl ketone + acetone at 21 degrees Celsius, take it out and dry it immediately, review it according to above method.
Principle: According to phenomenon of medium stress rupture, that is, after solvent molecules penetrate between macromolecules of resin, mutual force between molecules is reduced. Where internal stress is large, force between molecules is weakened before immersion. After immersion in solution, these weakened places are further weakened, causing cracks. Places with small internal stress will not crack in a short time. Therefore, size and location of internal stress of plated part can be determined from time and degree of cracking on the surface of part to be plated. So as to determine whether plastic parts are electroplated.

2. Instrument method

Use polarized light to illuminate plastic parts, depending on number of colored light bands, analyze strength of internal stress, it is only suitable for transparent parts. Equipment required by polarized light method is expensive, complicated to operate, and accuracy is not high, because changes before and after processing of part are not significant, light bands emerging on spectral band are not necessarily effects of internal stress, such as ripples on the surface of workpiece, which will also affect inspection results. However, this method has no effect on performance of parts. It is a non-destructive inspection. Inspected parts can be electroplated and used.
3. temperature sudden change
This method is to repeatedly cool and heat plastic parts to be plated, and evaluate internal stress according to time when cracks appear. It is suitable for all kinds of plastic molded parts. Equipment required for temperature sudden change method is simple, but test time is longer. Repaired plastic parts have been damaged and cannot be used continuously.

How much does wall thickness of plastic parts affect quality?

plastic stress, acetic acid immersion 
Wall thickness of plastic parts processing has a great influence on quality. When wall thickness is too small, flow resistance is large, it is difficult for large and complex plastic parts to fill cavity. Minimum size of plastic parts processing wall thickness should meet following requirements:
Have sufficient strength and stiffness;
Can withstand impact and vibration of demolding mechanism when demoulding;
Can withstand the tightening force during assembly.
Injection molding factory stipulates a minimum wall thickness value, which varies with variety, grade and product size of plastic parts. Excessive wall thickness in processing of plastic parts not only wastes raw materials, but also increases molding pressure for thermosetting plastic molding processing. For thermoplastics, it increases cooling time. In addition, it also affects quality of plastic parts processing. Wall thickness of same injection molded part should be as uniform as possible, otherwise additional stress will be generated due to uneven cooling and solidification speed.
It is very important to reasonably determine wall thickness of plastic parts. Wall thickness of plastic part is first determined by use requirements of plastic part: including strength of part, quality cost, electrical performance, dimensional stability and assembly requirements.
Generally, wall thickness has an empirical value, which can be determined by reference (such as an iron with a general wall thickness of 2mm, and a vacuum cleaner with a general wall thickness of 2.5mm). Points to note are as follows:
Wall thickness of plastic parts should be as uniform as possible to avoid too thin, too thick and sudden changes in wall thickness. If plastic part requires a change in wall thickness, gradient or arc transition should be adopted, otherwise plastic part will be deformed and affected due to uneven shrinkage. Molding process problems such as strength of plastic parts and affecting fluidity during injection molding.
Wall thickness of plastic parts is generally in range of 1-5mm. The most commonly used value is 2-3mm.
Try not to design reinforcing ribs and screw columns too thick. Generally, it is recommended to take half of wall thickness of body to be safe, otherwise it will easily cause appearance problems such as miniatures.
Try not to design the part as a separate plate, size is very small, otherwise deformation will cause part to be uneven.

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