Enterprise name: Zongsheng Precision Mould Co., Ltd.
Contact: Mr. Nie 13923298099 (Wechat same number).
Miss Jia 15820657628 (Wechat same number).
Tel: 0757 2226 9630.
0757 color 22269276.
Fax: 0757 2226 9631 22269276.
Web site: en.fszongsheng.com.
Add: no. 1, Rongye 6 road, Fengxiang industrial zone, Daliang, Shunde district, Foshan city, Guangdong province
When planning the plastic mold, after determining the structure of the mold, that is, the scale of the mold and parts, the scale of the cavity and core, etc., each part of the mold can be planned in detail, and the main planning parameters such as data shortening will be touched at this time.
Therefore, as long as the shortening rate of plastic forming is mastered in detail, the scale of each part of the die cavity can be determined.
Even if the structure of the selected mold is correct, it is impossible to produce qualified plastic parts if the parameters are not suitable.
Thermoplastics are characterized by swelling after heating, shortening after cooling and, of course, shortening after pressure.
In the process of injection molding, the molten plastic is first injected into the mold cavity.
After filling, the melt cools and condenses, and when the plastic part is removed from the mold, it will be shortened.
This shortening is called forming shortening.
In the process from mold removal to stability, the scale of plastic parts will still change slightly.
One change is continuous shortening, which is called post-shortening.
Another change is that some hygroscopic plastics swell because of their hygroscopicity.
For example, when the water content of nylon 610 is 3%, the scale is added by 2%; when the water content of glass fiber reinforced nylon 66 is 40%, the scale is added by 0.3%.
But forming shortening plays a major role.
At present, the German standard DIN16901 is generally used to determine the shortening rate of all kinds of plastics (shortening after molding + shortening after molding).
That is to say, at 23 (?
) the difference between the dimensions of the mold cavity measured under the condition and the corresponding dimensions of the plastic parts.
C and 50 (?
When the cavity scale of the mold is placed 24 hours after forming, the relative humidity of 5% is calculated.
The shortening rate s is expressed by the following formula: s = (dmurm) / dumb100% (1).
In the meantime: s-type shortening; D-type mold scale; M-type plastic part scale.
In order to simplify the calculation of mold planning, according to the known scale of plastic parts and the shortening rate of data, when the mold cavity is d=m/ (1murs), the following formula is generally selected to calculate the mold scale.
If more accurate accounting is required, choose the following formula: d=m+ms+ms2 (3).
However, when determining the shortening rate, because the practical shortening rate is affected by many factors, we can only choose the approximate value, so the cavity scale calculated by formula (2) basically meets the requirements.
In the mold manufacturing process, the mold cavity is processed according to the lower deviation, and the core is processed according to the upper deviation, and appropriate repair can be carried out if necessary.
The shortening rate of all kinds of plastics is not a fixed value, but a scale, which is the main reason why it is difficult to determine the shortening rate accurately.
Because the shortening rate of the same kind of data produced by different manufacturers is different, and even the shortening rate of the same kind of data produced by different batches of the same factory is also different.
As a result, each factory can only supply users with a reduction in the size of the plastic produced by the factory.
Secondly, the practical shortening rate in the forming process of PET crystal bottle cap die is also affected by the shape of plastic parts, die structure and forming conditions.
The impact of these elements is described below.
Generally speaking, for the wall thickness of the formed parts, the shrinkage is also larger because the cooling time of the thick wall is longer.
For general plastic parts, when there is a great difference between the L size of the melt flow direction and the W size perpendicular to the melt flow direction, the shrinkage is also quite different.
From the point of view of the melt flow distance, the pressure loss away from the gate is large, so the shrinkage there is also larger than that near the gate.
Because the shapes of ribs, holes, bumps and carvings have shrinkage resistance, the shrinkage of these parts is relatively small.
The form of gate also has an effect on shrinkage.
When using a small gate, the shrinkage of the plastic parts is increased because the gate is solidified before the end of packing.
The structure of the cooling loop in the injection mold is also a key in the mold design.
If the cooling circuit is not designed properly, the shrinkage difference will occur due to the uneven temperature of the plastic parts, and the result is that the size of the plastic parts is too poor or deformed.
In the thin-walled part, the effect of die temperature distribution on shrinkage is more obvious.
The machining dimensions of mold cavity and core are not only the basic dimensions calculated by Dumm (1cm S) formula, but also a problem of machining tolerance.
According to the usual practice, the machining tolerance of the mold is 1x3 of the tolerance of plastic parts.
However, due to the differences in the range of shrinkage and stability of plastics, the dimensional tolerances of plastic parts formed by different plastics must first be rationally determined.
In other words, the dimensional tolerance of plastic parts formed by plastic parts should be larger because of the large range of shrinkage or poor stability of shrinkage.
Otherwise, there may be a large number of over-sized waste products.
To this end, countries have specially formulated standards or industry standards for dimensional tolerances of plastic parts.
China has also formulated ministerial-level professional standards.
However, most of them do not have the corresponding dimensional tolerances of crystal easy-to-pull bottle cap moulds.
The German standard specially formulates the DIN16901 standard for the dimensional tolerance of plastic parts and the corresponding DIN16749 standard for the dimensional tolerance of the mold cavity.
This standard has a great influence in the world, so it can be used as a reference for the plastic mold industry.
In order to reasonably determine the dimensional tolerances of plastic parts formed by materials with different shrinkage characteristics, the concept of forming shrinkage difference VS is introduced into the standard.
In the formula: VS- forming shrinkage difference VSR- forming shrinkage in the direction of melt flow VST- and the forming shrinkage in the vertical direction of melt flow.
According to the vs value of plastics, the shrinkage properties of all kinds of plastics were divided into four groups.
Groups with low VS values are high precision groups, and so on, groups with high VS values are low precision groups.
The precision technology, 110, 120, 130, 140, 150 and 160 tolerance groups are compiled according to the basic dimensions.
It is also stipulated that 110, 120 and 130 groups of dimensional tolerances can be selected for plastic parts formed with stable shrinkage characteristics.
The dimensional tolerances of plastic parts formed with moderately stable shrinkage characteristics are 120, 130 and 140.
If 110 groups of dimensional tolerances are selected for plastic parts formed with this kind of plastics, a large number of over-sized plastic parts may be produced.
The dimensional tolerances of plastic parts formed with poor shrinkage characteristics are 130, 140 and 150 groups.
The dimensional tolerances of plastic parts formed with poor shrinkage characteristics are 140, 150 and 160 groups.
When using this tolerance table, you should also pay attention to the following points.
The general tolerances in the table are used for dimensional tolerances that do not indicate tolerances.
The tolerance of direct marking deviation is used to mark the tolerance zone of plastic parts.
The upper and lower deviation can be determined by the designer.
For example, if the tolerance zone is 0.8mm, you can choose the following upper and lower deviations.
0. 0, 0. 8, 0. 4, 0. 4, 0. 2, 0. 5, etc.
There are two groups of tolerances An and B in each tolerance group.
Among them, An is the size formed by the combination of die parts, which increases the error caused by the misfit of die parts.
This added value is 0.2mm.
Where B is the size directly determined by the mold parts.
Precision technology is a set of tolerance values specially established for plastic parts with high precision requirements.
Before using this plastic tolerance, you must first know which tolerance groups are applicable to the plastic used.
Foshan Shunde Zongsheng Precision Mould Co., Ltd.
Tel: 0757 Tel: 22269630 / 13923298099 (Mr. Nie).
Web site: www.fszongsheng.com.
Add: no. 1, Rongye 6 road, Fengxiang industrial zone, Daliang, Shunde district, Foshan, Guangdong
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