What exactly is the relationship between mold size and plastic shrinkage?

When designing a plastic mold, after the mold structure is determined, the detailed design of each part of the mold can be carried out, i.e., the dimensions of each template and part, cavity and core dimensions, etc. This will involve the main design parameters such as the material shrinkage rate. Therefore, the dimensions of each part of the cavity can only be determined if the shrinkage rate of the molded plastic is specifically known. Even if the selected mold structure is correct, but the parameters used are not appropriate, it is impossible to produce quality plastic parts.

 

I. Plastic shrinkage rate and its influencing factors

The characteristics of thermoplastic are expansion after heating and shrinkage after cooling, of course, the volume will also be reduced after pressure. In the injection molding process, the molten plastic is first injected into the mold cavity, and after filling, the molten material cools and cures, and the shrinkage occurs when the plastic part is removed from the mold, which is called forming shrinkage. This shrinkage is called forming shrinkage. During the time between the removal of the part from the mold and the stabilization of the part, there will still be small changes in size. Another change is the expansion of some hygroscopic plastics due to moisture absorption.

 

For example, nylon 610 moisture content of 3%, the size increase of 2%; glass fiber reinforced nylon 66 moisture content of 40% when the size increase of 0.3%. However, the main role is played by the molding shrinkage. The current method of determining the shrinkage of various plastics (forming shrinkage + post-shrinkage), generally recommended the German national standard DIN16901 provisions. That is, the difference between the size of the mold cavity at 23℃±0.1℃ and the size of the corresponding plastic part measured after molding for 24 hours at a temperature of 23℃ and relative humidity of 50±5%.

 

The shrinkage rate S is expressed by the following formula: S={(D-M)/D}×100%(1)

 

Where: S - shrinkage rate; D - mold size; M - part size.

 

If the mold cavity is calculated based on the known part size and material shrinkage, it is D=M/(1-S) In mold design, the following formula is generally used to find the mold size to simplify the calculation.

 

D=M+MS(2).

 

If a more precise calculation is to be implemented, the following formula is applied: D=M+MS+MS2(3)

 

However, in determining the shrinkage rate, since the actual shrinkage rate is affected by many factors and only approximate values can be used, the cavity size calculation using equation (2) also basically meets the requirements. In the manufacture of the mold, the cavity is processed according to the lower deviation, and the core is processed according to the upper deviation, so that it can be properly trimmed if necessary.

 

The main reason why it is difficult to determine the shrinkage rate accurately is, first of all, because the shrinkage rate of various plastics is not a fixed value, but a range. The shrinkage rate of the same material varies from factory to factory, and even the shrinkage rate of different batches of the same material from one factory is not the same. Therefore, each factory can only provide the user with a range of shrinkage rates for the plastics produced by that factory. Second, the actual shrinkage rate during the molding process is also affected by the shape of the plastic part, mold structure and molding conditions.

 

II. The shape of the plastic part

For the wall thickness of the molded part, the shrinkage rate is generally higher due to the longer cooling time for thick walls, as shown in Figure 1. For general plastic parts, when the difference between the melt flow direction L size and the perpendicular melt flow direction W size is large, the shrinkage rate difference is also large. In terms of the melt flow distance, the pressure loss is higher in the part away from the gate, so the shrinkage rate is also higher in that part than in the part near the gate. The shrinkage rate of these parts is smaller because of the shrinkage resistance of reinforcement, hole, tab and carving shapes.

 

III. Mold structure

The form of gate also influences the shrinkage rate. When using a small gate, the shrinkage of the molded part increases because the gate is cured before the end of holding pressure. The cooling circuit structure in the injection mold is also a key in the mold design. If the cooling circuit is not designed properly, the shrinkage difference will be caused by the unbalanced temperature in all parts of the mold, and the result is that the size of the part will be oversized or deformed. In thin-walled parts, the influence of mold temperature distribution on shrinkage rate is more obvious.

 

Parting surface and gate

 

Factors such as parting surface, gate form and size of mold directly affect the direction of material flow, density distribution, pressure-holding and shrinkage replenishment, and molding time.

 

The use of direct gate or large section gate can reduce the shrinkage, but the anisotropy is large, the shrinkage along the direction of material flow is small, along the vertical material flow direction shrinkage is large; on the contrary, when the thickness of the gate is small, the gate part will condense and harden too early, the plastic in the cavity shrinkage is not timely replenished, shrinkage is larger.

 

Point gate condensation sealing fast, in the case of the conditions of the parts, can be set up multi-point gate, can effectively extend the pressure-holding time and increase the cavity pressure, so that the shrinkage rate is reduced.

 

IV. Forming conditions

Barrel temperature: When the barrel temperature (plastic temperature) is higher, the pressure transfer is better and the shrinkage is reduced. However, when using small gates, the shrinkage is still larger because of early curing of the gates. For thick-walled plastic parts, the shrinkage is still higher even if the barrel temperature is higher.

 

Refill: In the molding condition, refill is minimized to keep the size of the part stable. However, insufficient replenishment will not maintain the pressure and will increase the shrinkage.

 

Injection pressure: Injection pressure is a factor that has a large impact on shrinkage, especially the pressure holding page number 335 after the end of filling. In general, the shrinkage rate is smaller when the pressure is higher because of the density of the material.

 

Pressure in injection molding includes injection pressure, holding pressure and mold cavity pressure. These factors all have a significant effect on the shrinkage behavior of the molded part.

 

Increasing the injection pressure can reduce the shrinkage of the product. This is because the pressure increases, so that the injection speed increases, the mold filling process is accelerated, on the one hand, due to the shear heat of the plastic melt and increase the melt temperature, reduce the flow resistance; on the other hand, the melt temperature is still high, the flow resistance is small state earlier into the pressure-holding replenishment stage. Especially for thin-walled plastic parts and small gates, due to the fast cooling rate, it is more important to shorten the mold filling process as much as possible.

 

The higher holding pressure and cavity pressure make the product in the cavity dense and shrinkage reduced, especially the pressure in the holding phase has more influence on the shrinkage of the product. This can be explained by the fact that the molten resin is compressed under the molding pressure, and the higher the pressure, the greater the compression that occurs, and the greater the elastic recovery after the pressure is lifted, making the molded part size closer to the cavity size, and therefore the smaller the shrinkage.

 

However, even for the same product, the pressure of the resin in the mold cavity is not uniform in all parts; the injection pressure is not the same in the parts where the injection pressure is difficult to act and the parts where it is easy to act. In addition, the pressure of each cavity of multi-cavity mold should be designed uniformly, otherwise it will produce inconsistent shrinkage rate of each cavity.

 

Injection speed: injection speed has a small effect on the shrinkage rate. However, for thin-walled plastic parts or very small gates, as well as the use of reinforced materials, the injection speed is accelerated, the shrinkage rate is small.

 

Mold temperature: After the thermoplastic melt is injected into the cavity, it releases a large amount of heat and solidifies. At this temperature, it will be the most favorable for the molding of plastic parts, the highest efficiency of plastic parts molding, the smallest internal stress and warpage deformation.

 

Mold temperature is the main factor to control the cooling and shaping of the product, and its influence on the molding shrinkage is mainly shown in the process after the gate freezes and before the product is demolded. Before the gate freezes, the mold temperature rises to increase the tendency of thermal shrinkage, but it is also the higher mold temperature that makes the freezing time of the gate longer, which leads to the enhanced effect of injection pressure and pressure retention, and the complementary shrinkage and negative shrinkage will increase.

 

Therefore, the total shrinkage is the result of the combined effect of the two types of reverse shrinkage, and its value does not necessarily increase with the increase of the mold temperature. If the gate freezes, the influence of injection pressure and holding pressure will disappear, and as the mold temperature increases, the cooling and setting time will be extended, so the shrinkage rate of the product after demolding will generally increase.

 

Forming cycle: The forming cycle is not directly related to the shrinkage rate. However, it should be noted that when accelerating the forming cycle, mold temperature, melt temperature, etc. are bound to change, which also affects the change in shrinkage rate. For material testing, the molding cycle determined by the required output should be followed and the dimensions of the molded part should be checked. An example of a plastic shrinkage test using this mold is as follows.

 

Injection machine: clamping force 70t, screw diameter Φ35mm, screw speed 80rpm.

 

Molding conditions: maximum injection pressure 178MPa, barrel temperature 230(225-230-220-210)℃, 240(235-240-230-220)℃, 250(245-250-240-230)℃, 260(225-260-250-240)℃, injection speed 57cm3/s, injection time 0.44 ~ 0.52s, holding time 6.0s, cooling time 15.0s.

 

V. Mold size and manufacturing tolerances

Mold cavity and core processing size in addition to the D = M (1 + S) formula to calculate the basic size, there is a problem of processing tolerances. According to practice, the processing tolerance of the mold for the tolerance of the plastic parts 1/3. but because of the plastic shrinkage range and stability differences, the first must be rationalized to determine the size tolerance of different plastic molded plastic parts. That is, by the shrinkage rate range is larger or shrinkage rate stability is poor plastic molding plastic size tolerance should be achieved some large. Otherwise, there may be a large number of size of the scrap. For this reason, the countries on the size tolerance of plastic parts specially developed national standards or industry standards. China has also developed a professional standard at the ministerial level. But most of them do not have the corresponding size tolerance of mold cavity. German national standards specifically developed the dimensional tolerances of plastic parts DIN16901 standard and the corresponding mold cavity dimensional tolerances DIN16749 standard. This standard has a large influence in the world, and thus can be used for plastic mold industry reference.

 

VI. About the dimensional tolerances and allowable deviations of plastic parts

To reasonably determine the shrinkage characteristics of different materials molded plastic size tolerance, let the standard introduced the concept of forming shrinkage difference △ VS. △VS=VSR_VST(4)

 

Where: VS-forming shrinkage difference VSR-forming shrinkage in the direction of melt flow VST-forming shrinkage in the direction perpendicular to the melt flow.

 

The shrinkage characteristics of various plastics are divided into four groups according to the △VS value of the plastic. The group with the smallest △VS value is the high-precision group, and so on, and the group with the largest △VS value is the low-precision group. And the precision technology, 110, 120, 130, 140, 150 and 160 tolerance groups are prepared according to the basic size. It is also stipulated that the dimensional tolerances of plastic parts formed with the most stable shrinkage characteristics can be selected from 110, 120 and 130 groups. If the dimensional tolerances of the plastic molded parts with medium stability of shrinkage characteristics are selected in the group of 110, it is possible to produce a large number of oversized plastic parts. The dimensional tolerances of molded parts made of plastics with poor shrinkage characteristics are 130, 140 and 150 groups. The dimensional tolerances of molded parts made of plastics with the worst shrinkage characteristics are chosen in groups 140, 150 and 160. When using this tolerance table, the following points should be noted. The general tolerances in the table are used for dimensional tolerances where no tolerance is specified. Tolerances with direct deviations are the tolerance zones used for dimensional tolerances of plastic parts. The upper and lower deviations can be determined by the designer. For example, if the tolerance zone is 0.8 mm, the following upper and lower deviations can be used: 0.0; -0.8; ±0.4; -0.2; -0.5, etc. In each tolerance group there are two sets of tolerance values, A and B. Where A is the size formed by the combination of mold parts, increasing the misalignment formed by the mold parts not fitting tightly at the counterpart. This increase is 0.2 mm. where B is the dimension determined directly by the mold part. Precision technology is a set of tolerance values established specifically for the use of plastic parts with high precision requirements. Before using this tolerance for plastic parts, you must first know which tolerance groups apply to the plastic used.

 

VII. The mold manufacturing tolerances

The German national standard for plastic parts tolerance has developed the corresponding mold manufacturing tolerance standard DIN16749. 4 kinds of tolerances are set in the table. No matter what kind of material plastic parts, which does not specify the size tolerance size of the mold manufacturing tolerances are used serial number 1 tolerance. The specific tolerance value is determined by the basic size range. The tolerance of the mold manufacturing for medium precision dimensions of plastic parts of any material is the tolerance of serial number 2. Tolerances for the manufacture of molds of higher precision dimensions of plastic parts of any material are the tolerances of serial number 3. Precision technology corresponding mold manufacturing tolerance for the tolerance of serial number 4.

 

It can reasonably determine the reasonable tolerance of various material plastic parts and the corresponding mold manufacturing tolerance, which not only brings convenience to the mold manufacturing, but also can reduce the scrap and improve the economic efficiency.