How to solve the crack problem in PVC injection molding?
PVC plastic parts are cracks, mainly due to the plastic stress is too large or the stress concentration caused by the specific analysis is as follows.
1) Injection mold
①If the position of the top rod is incorrect or improperly installed, the ejection resistance increases and the local stress concentration of the plastic part causes cracks in the plastic parts. Therefore, the location of the top and the quality of the installation should be checked
②If the plastic parts of the slope is too small, the release resistance increases, in the forced release, the plastic parts have been too large a top effect and cracks, which should be appropriate to increase the mold slope, in order to facilitate the release
③If the sectional area of the top rod of the ejector is too small, the stress on the top of the ejection piece will be uneven and the residual stress will be concentrated, so that the cross section of the ejector rod should be properly increased
④Because of the large temperature difference, the gate is prone to crack, so the ring stiffener can be designed around the gate to reduce the crack
⑤For the bad fluidity of plastics, such as PSF, PPO, PC etc., must improve the injection pressure to melt fills the cavity, the plastic forming cracks at the gate, in this regard, can use a special gate, such as tab gate and side gate, gate forming part separation the crack.
⑥If the cavity is sharp edge, sharp changes in the cracks, the crack cavity will be reflected to the plastic parts, in this regard, should repair the cavity, chrome polishing treatment.
⑦The push rod at the top of the plastic parts of the afterburner position, should be part of the biggest ejection resistance, such as the boss and the stiffener, so that is conducive to the plastic mould, so as not to appear stress concentration.
⑧If the gate or improper location, very easy to crack, which can give priority to the use of small pressure loss, sprue can withstand high pressure, or positive gate is multiple point gate and side gate, and the gate diameter decreases.
⑨Injection, spray nozzles should be in close contact with the gate, die stress is quite large, repeatedly die in this force will crack because of fatigue crack, this will also affect the plastic parts, in this regard, they must be repaired to the mold.
2) Injection molding process
①If the holding time is too long and the plastic parts are subjected to too much pressure, there will be a great residual stress, which will make the plastic parts crack. Therefore, the packing time can be shortened appropriately
②If the injection pressure is too large, the residual stress will increase too. This kind of stress easily causes cracks in the plastic parts, and the injection pressure should be reduced appropriately
③We must use higher injection pressure if the injection molding, in order to reduce the residual stress in the plastic parts, can reduce the melt and mold temperature, mold temperature and improve the barrel, extend the cooling time of plastic parts, the molecular chain orientation can have time to recover.
3) Plastic parts
①If the outline of rapidly changing place of plastic parts, but also the stress concentration, the most easy to crack, in this regard, in plastic design, the shape of the exterior angle and the interior should as far as possible with a large circular arc.
②For the plastic parts with embedded parts, metal and plastic due to thermal expansion coefficient differences, such as thermoplastic thermal expansion coefficient is 6 ~ 11 times of steel, aluminum 3 ~ 6 times, therefore, the overall contraction in the plastic block will hinder the plastic, the resulting stress will be lead to plastic cracks, in order to reduce or avoid crack due to insert appears, from the following aspects:
First of all, in the selection of materials, as far as possible the use of thermal expansion coefficient close to the resin metal, such as zinc, aluminum, copper and other non-ferrous metals
Secondly, the insert as far as possible in the high molecular weight resin made of plastic parts, this kind of resin crack resistance is stronger. Moreover, the plastic thickness around the insert should be thicker, and its minimum wall thickness will vary with the variety of plastic
4) raw materials
①Mold release agent and melt difficult to dissolve, such as excessive dosage, easy to crack, in this regard should be as little as possible or no release agent
②If the recycled material is added too much, the content of impurities and volatile matter of recycled material is more, the plastic parts produced are lower in strength and easy to crack. Therefore, the amount of recycled material should be appropriately controlled
③The plastic parts made of low viscosity and loose resin are not easy to crack, and they can be used preferentially when selecting materials
④If the resin contains more water, this kind of resin is easy to decompose and brittle when heated, and the smaller residual stress is easy to cause cracks in the plastic parts. This should be dried by the resin
⑤Crystalline resins are better than non crystalline resins in resisting the ability to produce cracks. Therefore, crystalline resins should be used more often
⑥In the varieties of raw materials must be replaced when the adhesion material injection molding machine the parts completely clean, so as to avoid the mixing performance of different raw materials, caused by plastic crack.