Design Plastic Mold For Several Important Points

- Nov 09, 2017 -

Design plastic mold for several important points

When the plastic mould is designed, the mold structure can be determined in detail, and the parts of the mould can be designed in detail. The dimensions of each template and parts, the size of the cavity and the core size are determined.

At this time, the main design parameters such as shrinkage rate of materials are involved.

Therefore, only the shrinkage rate of plastic forming can be determined to determine the size of each part of the cavity.

Even if the selected mold structure is correct, but the used parameter is improper, it is impossible to produce qualified plastic parts.

Plastic shrinkage and its influencing factors

Thermoplastic properties expand after heating, then shrink after cooling, and of course the volume will shrink when pressed.

In the process of plastic injection forming, injecting molten plastic into the mold cavity, the first end of the filling liquid cooling solidification, removed from the mold plastic parts is a contraction, this is called forming contraction.

In the period of time when the plastic parts are removed from the molds to stability, the size will still appear small changes. One kind of change is to continue to contract, which is called the post-contraction.

Another change is that some of the hygroscopic plastics have been dilating due to moisture absorption.

For example, nylon 610 moisture content is 3%, and the size increase is 2%;

The moisture content of glass fiber reinforced nylon 66 is 0.3 %.

But the main effect is to form contraction.

At present, the methods of plastic shrinkage (forming shrinkage and contraction) are determined, and the provisions of DIN16901 are generally recommended in German national standards.

23 ℃ to + / - 0.1 ℃ when the mold size and shape after 24 hours, at a temperature of 23 ℃, relative humidity for 50-5% measured under the condition of the corresponding parts size to calculate the difference between the.

S= {(d-m)/D} by 100%(1)

Among them: s-systolic rate;

D-die size;

M- plastic parts size.

If the mold cavity is calculated according to the size of the known plastic parts and the shrinkage rate of the material, it is D= M/(1-s) in the mould design to simplify the calculation.

D = M + MS (2)

If more precise calculation is required, the following formula is applied: D=M+MS+MS2 (3).

However, in determining the shrinkage rate, due to the fact that the actual contraction rate has to be influenced by many factors, the approximate value can only be used. Therefore, the formula (2) calculation of the cavity size also basically meets the requirements.

In the manufacture of molds, the cavity is processed in accordance with the following deviation, the core is processed by deviation, which can be properly trimmed when necessary.

The main reason for the difficulty in determining the shrinkage rate is that the shrinkage rate of plastic is not a fixed value, but a range.

Since the shrinkage rate of the same materials produced by different factories is different, even the shrinkage rate of different batches of the same materials produced by a factory is different.

Therefore, the factories can only provide users with a range of shrinkage of the plastic produced by the factory.

Secondly, the actual shrinkage rate of the forming process is also influenced by the shape of plastic parts, mould structure and forming conditions.

The effects of these factors are introduced.

Shape of plastic parts

For the wall thickness of the forming part, the shrinkage rate is also larger due to the longer cooling time of the thick wall, as shown in FIG. 1.

For the general plastic parts, the shrinkage rate difference is also larger when the dimension of the melting flow direction of L is different from that of the W dimension perpendicular to the melting flow direction.

From the distance of molten material flow, the shrinkage rate of the gate is larger than that of the gate.

The shrinkage rate of these parts is small because of the strength of reinforcement, hole, punch and engraving.

The mould structure

The gate form also affects the shrinkage rate.

With the small gate, the shrinkage rate of the plastic parts increases as the water gate is solidified before the pressure is finished.

The cooling loop structure in injection mold is also a key in mould design.

The cooling circuit is not designed properly, but the temperature is unbalanced in the plastic parts, and the result is that the plastic parts are too poor or deformed.

In thin-wall part, the influence of mold temperature distribution on shrinkage rate is more obvious.

The forming conditions

Cylinder temperature: when the cylinder temperature (plastic temperature) is high, the pressure transmission is better and the shrinkage force decreases.

However, when the gate is solidified, the shrinkage rate is still higher.

For thick wall plastic parts, the shrinkage is still larger, even if the barrel temperature is higher.

Replenishing: in the forming condition, reduce the material as far as possible to keep the size of the plastic parts stable.

But the replenishing deficiency can not maintain the pressure, will also increase the shrinkage rate.

Injection pressure: the injection pressure is a factor that influences the shrinkage rate, especially the pressure of the pressure after filling.

In the general case, when the pressure is greater, the density of the material is large, and the shrinkage rate is smaller.

Injection rate: the effect of injection rate on the shrinkage rate is small.

However, it is very small for thin-walled plastic parts or sprue, and when the reinforcement material is used, the rate of injection is small.

Mold temperature: the shrinkage rate is also larger when the mold temperature is higher.

But for thin-walled plastic parts, the flow resistance of molten material is small, and the shrinkage rate is smaller.

Forming cycle: the forming cycle is not directly related to the shrinkage rate.

However, it is important to note that, when the forming cycle is accelerated, the mold temperature and the melting temperature must also change, which will also affect the change of shrinkage rate.

During the testing of materials, the forming cycle should be formed according to the forming cycle determined by the required yield, and the size of the plastic parts shall be inspected.

The example of plastic shrinkage test with this mold is as follows.

Injection machine, clamping force 70 t screw diameter of 35 mm screw rotation speed 80 RPM Φ forming conditions: high injection pressure 178 mpa cylinder temperature 230 ℃ (225-230-220-230) (235-240-230-240) of 240 ℃ to 250 ℃ (245-250-240-250) (225-260-250-260) 260 1425 ℃ injection speed px3 / s injection time is 0.44 ~ 0.52 s the holding time of 6.0 s cooling time of 15.0 s

Mould dimensions and manufacturing tolerances

The machining dimension of mold cavity and core is not only calculated by D=M(1+S) formula, but also a problem of machining tolerance.

According to practice, the machining tolerance of mould is 1/3 of the tolerances of plastic parts.

However, due to differences in the range and stability of plastic shrinkage, it is necessary to rationalize the size tolerance of plastic parts.

That is, the size tolerance of the plastic forming parts with a large shrinkage rate or a stable reduction rate should be larger.

Otherwise, there might be a lot of junk that is too bad.

To this end, countries have formulated national standards or industry standards for the size tolerance of plastic parts.

China has also developed ministerial professional standards.

However, the size tolerances of die cavity are not suitable.

In Germany, the standard of DIN16901, the standard of DIN16901 and the corresponding mold cavity size tolerances, is specially formulated in the German national standard.

This standard has a great influence in the world, so it can be used for reference in plastic mold industry.

Dimensions tolerance and allowable deviation of plastic parts

In order to reasonably determine the size tolerance of the molded plastic parts of different shrinkage characteristic materials, the standard introduced the concept of delta VS.

Delta v = VSR_VST (4)

In the formula: VSR- forming shrinkage of VSR- melt flow direction of VSR- molten material flow direction - forming shrinkage rate of vertical direction of molten material flow.

According to the value of plastic delta VS, the shrinkage characteristics of various plastics were divided into four groups.

The group with the lowest value of delta VS is the high-precision group, and so on, the largest group of delta VS is the low precision group.

The precision technology, 110, 120, 130, 140, 150 and 160 tolerances were prepared according to the basic dimensions.

It is also stipulated that the size tolerance of plastic forming parts with the most stable shrinkage characteristics can be used in groups 110, 120 and 130.

The size tolerance of plastic forming parts with moderate and stable shrinkage is 120, 130 and 140.

If the size of plastic parts of this type of plastic is used for the size of 110 groups, it is possible to produce a large number of oversized plastic parts.

The size tolerances of plastic molded parts with poor shrinkage were selected from groups 130, 140 and 150.

The size tolerances for plastic molded parts with the worst shrinkage are 140, 150 and 160.

The following points should also be noted when using this tolerance table.

The general tolerances in the table are used for tolerances that do not indicate tolerances.

The tolerance of direct label deviation is used to mark the tolerance zone of the dimension of plastic parts.

The upper and lower deviation can be determined by the designer.

For example, the tolerance band is 0.8mm, which can be composed of various upper and lower deviation.

0.0;

0.8;

+ / - 0.4;

0.2;

- 0.5, etc.

Each tolerance group has A and B tolerance values.

In this paper, A is formed by the combination of mould parts, which increases the error of the mould parts.

The added value is 0.2mm.

B is the size determined by the mould parts directly.

Precision technology is a set of tolerance values for high precision requirements for plastic parts.

Before you use the plastic parts, you must first know which of the tolerances you use.

Mold manufacturing tolerance

Standard DIN16749 of the corresponding mould manufacturing tolerance was developed by German national standard for the tolerance of plastic parts.

There are four kinds of tolerance in this table.

No matter what kind of material is molded, the tolerance of mould manufacturing tolerances that do not specify dimensional tolerance size are used in the tolerance of serial no. 1.

The specific tolerance value is determined by the basic size range.

No matter what material plastic parts of the medium precision dimensions of the mold manufacturing tolerance for the serial number 2 tolerance.

No matter what material plastic parts are more accurate size of the mold manufacturing tolerance for the serial number 3 tolerance.

Precision technology corresponding mold manufacturing tolerance for the serial number 4 tolerance.

It is reasonable to determine the reasonable tolerances of various materials and the corresponding mould manufacturing tolerances. This will not only bring convenience to the mould making, but also reduce the waste and improve the economic efficiency.