Generally, when designing a plastic mold, after determining the mold structure, the detailed design of each part of the mold can be carried out, i.e., determining the dimensions of each template and part, cavity and core dimensions, etc. This will involve the main design parameters such as material shrinkage. This will involve the main design parameters such as the shrinkage of the material. 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 will not be possible to produce quality plastic parts.
Plastic shrinkage and its influencing factors
The characteristic of thermoplastic is that it expands after heating and shrinks after cooling, and of course the volume will be reduced after pressurization. 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 part is removed from the mold. 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 forming shrinkage. At present, to determine the shrinkage of various plastics (forming shrinkage + shrinkage) method, generally recommended the German national standard in DIN16901 provisions. That is, 23 ℃ ± 0.1 ℃ when the mold cavity size and after molding placed 24 hours, at a temperature of 23 ℃, relative humidity of 50 ± 5% of the difference between the corresponding plastic part size measured.
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 in order 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. Secondly, the actual shrinkage rate during the molding process is also influenced by the shape of the plastic part, the mold structure and the molding conditions. The following is a description of the effects of these factors.
Part shape
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. In general, when the difference between the melt flow direction L dimension and the dimension perpendicular to the melt flow direction W is large, the shrinkage rate 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 of these parts is smaller because of the shrinkage resistance of the reinforcement, holes, tabs and engraved shapes.
Mold structure
The form of gate also has an effect on the shrinkage rate. When a small gate is used, the shrinkage of the molded part increases because the gate cures before the end of the holding pressure. The structure of the cooling circuit 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 the mold temperature distribution on the shrinkage rate is more obvious.
Forming conditions
Barrel temperature: When the barrel temperature (plastic temperature) is higher, the pressure transfer is better and the shrinkage force is reduced. However, when using small gates, the shrinkage is still higher due to early curing of the gates. For thick-walled plastic parts, even if the barrel temperature is higher, the shrinkage is still 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.
Injection speed: The effect of injection speed on shrinkage is small. However, for thin-walled plastic parts or very small gates, as well as the use of reinforced materials, injection speed is accelerated, then the shrinkage rate is small.
Mold temperature: Usually the shrinkage rate is higher when the mold temperature is higher. However, for thin-walled plastic parts, high mold temperature results in small flow impedance of the melt, *] while shrinkage is smaller.
Forming cycle: The forming cycle is not directly related to the shrinkage rate. However, it should be noted that when the molding cycle is accelerated, the mold temperature and melt temperature are bound to change, which also affects the change of 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)°C 240(235-240-230-220)°C 250(245-250-240-230)°C 260(225-260-250-240)°C Injection speed 1425 240)℃Injection speed 1425px3/s Injection time 0.44~0.52s Holding time 6.0s Cooling time 15.0s
Mold size and manufacturing tolerance
In addition to the machining dimensions of mold cavities and cores, the basic dimensions are calculated by the formula D=M(1+S), and there is a problem of machining tolerances. According to the usual practice, the processing tolerance of the mold is 1/3 of the tolerance of the plastic part, but because of the range of shrinkage and stability of plastics vary, the first must be rationalized to determine the size tolerance of different plastics 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.
