Skimming, also known as flash, flash, flash, etc., mostly occurs at the parting position of the mold, such as the parting surface of the mold, the sliding part of the slider, the gap of the insert, the aperture of the ejector pin, etc. If the flash is not solved in time, it will be further enlarged, so that the stamping die will collapse locally and cause permanent damage. The edge of insert gap and ejector pin hole will also make the product stuck on the die, which will affect the demoulding.
In essence, the burr is the surplus left on the product after the gap between theplastic material and the matching part of the mold is cooled. It is very simple to solve the problem of sticking front, that is, to control the melt from entering into the mold fit gap. There are generally two situations when the plastic melt enters the mold fit gap: one is that the mold fit gap is originally large and the colloid can easily enter it; In another case, the mold fit gap was not big, but the molten colloid forced into it under pressure.
On the surface, it seems that it can be completely solved by strengthening the manufacturing precision and strength of the die. It is absolutely necessary to improve the manufacturing precision of the mold, reduce the fit gap of the mold and prevent the molten colloid from entering. However, in many cases, the strength of the mold cannot be strengthened indefinitely, and the colloid cannot penetrate into it under any pressure.
There are both mold reasons and process reasons for the appearance of Pifeng. Check the process reasons, first check whether the clamping force is sufficient, and only check the mold reasons when the edge is still generated when the clamping force is sufficient.
Method for checking whether clamping force is sufficient:
1) Increasing the injection pressure step by step, with the increase of injection pressure, the front edge increases correspondingly, and the front edge is mainly produced on the parting surface of the mold, which shows that the clamping force is not enough.
2) Gradually increase the clamping force of the injection molding machine. When the clamping force reaches a certain value, the flap on the parting surface disappears, or when the injection pressure is increased, the flap on the parting surface will no longer increase. This clamping force value is considered to be sufficient.
The method of checking whether the mold manufacturing precision causes the edge-sticking;
With lower material temperature, lower filling speed and lower injection pressure, the product is just filled (the product shrinks slightly). At this time, it can be considered that the ability of the melt to protrude into the fit gap of the mold is very weak. At this time, if the tipping edge occurs, it can be judged that it is a problem of mold manufacturing accuracy, which needs to be solved by mold repair. It can be considered to give up using technological methods to solve the formation of the front. It should be noted that the above "three low" conditions are indispensable. High material temperature, fast filling speed and high injection pressure will lead to an increase in the local pressure of the mold cavity, enhance the ability of the melt to penetrate into the mold fit gap, and expand the mold to produce a front, although the product is not full of glue at this time.
The analysis of the causes of whipping is based on the premise that the clamping force is sufficient, and it is difficult to analyze the causes of whipping when the clamping force is insufficient. The following analysis is based on sufficient clamping force. According to several situations of wearing front, there may be the following reasons for wearing front:
The first case: as mentioned above, under the conditions of low temperature, low speed and low pressure, when the product is not full of glue, the tipping edge has already occurred. The main possible reasons are: insufficient precision of mold manufacturing and too large fit gap;
The second case: when the product is just full of glue, there is local shrinkage and no front; When the injection pressure is increased to improve the local shrinkage of the product, the front will be produced. Possible reasons are:
1) the material temperature is too high. If the material temperature is too high, the viscosity of the melt is low and the fluidity is good, and the stronger the ability of the melt to penetrate into the fit gap of the mold, it will lead to the formation of the front.
2) The injection speed is too fast and the injection pressure is too high (resulting in supersaturation of filling). Too fast speed, too high injection pressure, especially too high injection pressure, will enhance the melt's ability to penetrate into the mold fit gap, and lead to the occurrence of flash.
3) The fluidity of plastics is too high. The better the fluidity of plastic, the lower the viscosity of melt, and the stronger the ability of melt to drill into the fit gap of mold, it is easy to produce a front. When the mold manufacturing has been completed, the depth of the exhaust groove of the mold, and the fit gap of the mold have been finalized, another kind of plastic with good fluidity will be used for production, which will produce a front.
4) The strength of the mold is insufficient. When the design strength of the mold is insufficient, when the mold cavity bears the pressure of the plastic melt, it will deform and expand, and the colloid will protrude into the gap of the mold, resulting in a front.
5) Unreasonable product design. The local glue position of the product is too thick, and the shrinkage during injection molding is too much, which will lead to local shrinkage. In order to adjust the local shrinkage of products, it is often necessary to use higher injection pressure and longer injection time to fill and maintain the pressure, which leads to insufficient strength and deformation of the mold.
6) The mold temperature is too high. High mold temperature can not only keep good fluidity of plastics and reduce pressure loss, but also reduce the strength of the mold, which will also lead to the occurrence of frogging.
The second case is the most frequently encountered problem in injection molding production, which can't be solved by all the technological means, and is the most troublesome for injection molding technicians. In this case, the most important means is to repair the mold. The solutions are:
1) local glue reduction of products. The shrinkage problem of the product can be improved, the injection pressure will be reduced, the deformation of the mold will be small, and the front can be restrained after the glue position is thinned. This is the most effective and commonly used method.
2) Increase the glue feeding point. By increasing the pouring point, the injection process and pressure can be reduced, and the pressure on the mold cavity will be reduced, which can effectively solve the problem of falling front. Increasing the pouring point, especially at the shrinking position of the product, can have an immediate effect on reducing the injection pressure of the mold cavity. It is also one of the commonly used methods.
3) Strengthen the local part of the mold. Sometimes the deformation of the formwork can be strengthened by adding a brace between the movable formwork and the ejector plate.
