There are three forms:
1. Quench stress.
2. Construct volume strain.
3. Freezing of molecular orientation, difference between linear expansion coefficients of metal inserts and plastics is too large, chemical reaction fission caused by chemical solvents.
Type 1
This kind of stress sometimes disappears by itself when product produces bubbles or dents, can be eliminated by annealing.
Type 2
Configuration volume strain. It is caused by shrinkage in different directions formed by geometric dimensions of part. Thickness of part becomes more obvious. It can only be eliminated by annealing.
Type 3
Freezing of molecular orientation. This is the most important kind of residual stress. Residual stress remaining in the part due to molecular orientation and freezing is called frozen molecular orientation. This force will cause parts to crack, dimensional instability and deformation when heated.
Appearance of residual stress is closely related to injection conditions.
Mainly consider from five aspects: mold temperature, part thickness, injection pressure, injection time, and material temperature.
We mainly slow down cooling rate of molten material during molding, so that stress is fully relaxed, so as to minimize orientation of frozen molecules, that is, to minimize residual stress.

 

⑴Acetic acid immersion
Acetic acid (CH3COOH) used must be more than 95% acetic acid and number of repeated use shall not exceed 10 tests.
①Surface stress test: Pour acetic acid (glacial acetic acid) into glassware, immerse product in acetic acid for 30 seconds. After 30 seconds, take out sample with a clamp, immediately rinse it with clean water (tap water) to check for whitening and cracks on the surface of sample. Judgment: There must be no cracking, and a slight whitening of surface is allowed.
②Internal stress test: Wipe dry samples that pass surface stress test and immerse them in acetic acid for 2 minutes. After 2 minutes, take out sample and immediately rinse it with clean water (tap water is sufficient), inspect sample for whitening and cracks. Judgment: There must be no cracks, slight cracks and whitening of surface are permitted at inserts.
⑵Methyl ethyl ketone + acetone addiction method: completely immerse whole machine in a 1:1 mixed liquid of methyl ethyl ketone + acetone at 21 degrees Celsius, take it out and dry it immediately, and review according to above method.
Principle: According to phenomenon of medium stress cracking, after solvent molecules penetrate between macromolecules of resin, mutual force between molecules is reduced. Force between molecules before immersion is weakened in the place where internal stress is large, weakened place is further weakened after being immersed in solution, causing cracking, and place with low internal stress will not crack in a short time. Therefore, size and location of internal stress of plated object can be determined from time and extent of surface cracking of plated object, so as to determine whether plastic parts are electroplated.

Using 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. Instruments required by polarized light method are expensive, complex in operation, and not very accurate, because parts before and after processing do not change significantly, light bands emerging on spectral band are not necessarily affected by internal stress, such as ripples on the surface of parts will also affect results of inspection. However, this method does not have any effect on function of parts. It is a non-destructive inspection. Parts that have been inspected can be electroplated and used.

This method is to repeatedly heat and cold plastic parts to be plated, evaluate internal stress according to time when cracks appear. It is applicable to all kinds of plastic molded parts. Equipment required for sudden temperature change method is simple, but test time is longer. Plastic parts after inspection have been damaged and cannot be used continuously.

Supramolecular structure of polymer has a very obvious influence on injection molding conditions and product performance. Polymers can be divided into crystalline and amorphous types according to their supramolecular structure. Molecular chains of crystalline polymers are arranged regularly, while molecular chains of amorphous polymers are arranged irregularly and amorphously. Different forms show different technological properties.
Generally, crystalline polymers have heat resistance and higher mechanical strength, while amorphous polymers are opposite. Polymers with simple molecular structure and high symmetry can generate crystals, such as PE. Although molecular chain is large, interaction between molecules can also generate crystals, such as POM, PA, etc. Polymers with high molecular chain rigidity are not easy to crystallize, such as PC, PSU, PPO, etc.
Criteria for evaluating crystal morphology of polymers are crystal shape, size and crystallinity.

 

High density and crystallinity means that most of molecular chains have been arranged in an orderly and compact structure, intermolecular force is strong, so density increases with increase in crystallinity. For example, density of PP with 70% crystallinity is 0.896 kg/m3. When crystallinity increases to 95%, density increases to 0.903 kg/m3.
Tensile strength: High crystallinity and high tensile strength. For example, polypropylene with a crystallinity of 70% has a tensile strength of 27.5 MPa. When crystallinity is increased to 95%, tensile strength can be increased to 42 MPa.
Impact strength: Impact strength decreases with increase of crystallinity. For example, notched impact strength of polypropylene with 70% crystallinity is 15.2 kgf.cm/cm. When crystallinity is 95%, impact strength decreases to 4.86 kgf.cm/cm. .
Thermal performance: Increase in crystallinity helps to increase softening temperature and heat distortion temperature. For example, polypropylene with a crystallinity of 70% has a heat distortion temperature of 125℃ under load, while it is 151℃ with a crystallinity of 95%. Rigidity is one of demolding conditions for injection molded products, higher crystallinity will reduce cooling cycle of product in mold. Crystallinity will bring brittleness to low temperature. For example, embrittlement temperature of isotactic polypropylene with crystallinity of 55%, 85% and 95% is 0℃, 10℃ and 20℃ respectively.
Warpage: Increase in crystallinity will reduce volume and increase shrinkage. Crystalline material is more likely to warp than amorphous material. This is because when product is cooled in mold, difference in temperature causes difference in crystallinity, which causes uneven density and unequal shrinkage, which leads to higher internal stress and warpage, reduces stress crack resistance.
Gloss: Increase in crystallinity will increase compactness of product and increase surface gloss of product, but presence of spherulites will cause scattering of light waves and reduce transparency.