When designing plastic molds, after determining the mold design, the overall design of each part of the mold can be carried out, that is, the specifications of each template and part, the concave mold and core specifications, etc., are determined. At this time, relevant material shrinkage rates and other key design parameters will be involved. Therefore, only by accurately grasping the shrinkage rate of the molding plastic can the specifications of each part of the concave mold be determined. Even if the mold design is properly selected, but the parameters used are not good, it is unlikely to produce qualified plastic parts.

Plastic Shrinkage Rate and Influencing Factors

Thermosetting plastics are characterized by expansion after heating and shrinkage after cooling. Naturally, the volume will also decrease after pressurization. In the injection molding process, the molten plastic is first injected into the mold cavity, and after filling, the material cools and solidifies. When the plastic part is removed from the mold, shrinkage occurs, which is called molding shrinkage. During the period from the removal of the plastic part from the mold to its stabilization, the size will still undergo subtle changes. One change is further shrinkage, which is called post-shrinkage.

Another change is that some water-absorbing plastics expand due to moisture absorption. For example, when the moisture content of nylon 610 is 3%, the size increase rate is 2%; when the moisture content of glass fiber reinforced nylon 66 is 40%, the size increase rate is 0.3%. However, the main effect is molding shrinkage. Currently, the method for determining the shrinkage rate (molding shrinkage + post-shrinkage) of various plastics generally recommends the requirements of DIN16901 in the national standard. That is, at 23℃±0.1℃, the difference between the mold cavity size and the relative plastic part size measured after molding and placing for 24 hours under the standard of 23℃ temperature and 50±5% humidity is calculated. The shrinkage rate S is expressed by the above formula: S={(D－M)/D}×100%(1)

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

If the mold cavity is calculated according to the known plastic part size and material shrinkage rate, then D=M/(1-S). In stamping dies, in order to simplify the calculation, the following formula is generally used to find the mold size:

D=M MS(2)

If a more accurate calculation is required, use the following formula:

D=M MS MS2(3)

However, when determining the shrinkage rate, because the actual shrinkage rate is affected by many factors, only natural numbers can be used, so the calculation of the concave mold size using formula (2) is mostly satisfactory. When manufacturing the mold, the concave mold is processed according to the lower error, and the core is processed according to the upper error, so that appropriate adjustments can be made if necessary.

The main reason why it is difficult to accurately determine the shrinkage rate is that the shrinkage rate of various plastics is not a constant, but a range. Because the shrinkage rate of the same material produced by different factories is different, even the shrinkage rate of the same material with different production batches produced by the same factory is different. Therefore, each manufacturer can only provide the customer with the shrinkage rate range of the plastic produced by the factory. Secondly, the actual shrinkage rate in the molding process is also affected by factors such as the shape of the plastic part, mold design, and molding conditions. The following is a detailed introduction to the influence of these factors.

Plastic Part Shape

For the wall thickness of the molded part, generally because the cooling time of the thick-walled tube is longer, the shrinkage rate is also large, as shown in Figure 1. For general plastic parts, when the difference between the size L in the direction of material flow and the size W perpendicular to the material flow direction is large, the shrinkage rate difference is also large. From the perspective of material flow distance, the pressure loss at the part away from the gate is large, so the shrinkage rate at this point is also larger than that near the gate. Because ribs, holes, convex molds, and carvings have shrinkage resistance, the shrinkage rate at these positions is smaller.

Mold Design

The gating method also affects the shrinkage rate. When using a small gate, the shrinkage rate of the plastic part increases because the gate solidifies before the pressure holding is completed. The cooling circuit structure in the injection mold is also an important part of the stamping die. If the cooling circuit design is inappropriate, the temperature imbalance of each part of the plastic part will cause shrinkage difference, resulting in dimensional deviation or deformation of the plastic part. In the thick-walled part, the effect of mold temperature distribution on the shrinkage rate is more significant.

Molding Conditions

Barrel temperature: When the barrel temperature (plastic temperature) is high, the pressure transmission is good, and the shrinkage force is reduced. However, when a small gate is used, the shrinkage rate is still large because the gate solidifies early. For thick-walled plastic parts, even if the barrel temperature is high, the shrinkage is still large.

Supplementary material: In the molding conditions, try to avoid supplementary material to keep the plastic part size stable for a long time. However, insufficient supplementary material cannot maintain pressure, which will also increase the shrinkage rate.

Injection pressure: Injection pressure is a factor that greatly affects the shrinkage rate, especially the holding pressure after filling. Under normal circumstances, when the pressure is high, the density of the material is high, and the shrinkage rate is smaller.

Injection speed: The injection speed has a small effect on the shrinkage rate. However, for thick-walled plastic parts or very small gates, and when using reinforced materials, the shrinkage rate is smaller when the injection speed is increased.

Mold temperature: Generally, when the mold temperature is high, the shrinkage rate is also large. However, for thick-walled plastic parts, the mold temperature is high and the flow resistance of the material is small, and the shrinkage rate is smaller.

Molding cycle: The molding cycle has no direct relationship with the shrinkage rate. However, it should be noted that when the molding cycle is accelerated, the mold temperature, material temperature, etc. will inevitably change, which will also affect the change of the shrinkage rate. When conducting material experiments, molding should be carried out according to the molding cycle determined by the required production capacity, and the size of the plastic parts should be tested. The following is an example of a plastic shrinkage rate experiment using this mold.

Mold size and manufacturing dimensional tolerances

The production and processing specifications of mold cavities and cores, in addition to the basic specifications calculated according to the formula D=M(1 S), also involve the issue of dimensional tolerances in processing. According to international conventions, the processing dimensional tolerance of a mold is 1/3 of the dimensional tolerance of the plastic part. However, due to differences in the range and reliability of plastic shrinkage, it is first necessary to reasonably determine the standard tolerance of molded parts made of different plastics. That is, the standard tolerance of molded parts made from plastics with a large shrinkage range or weak shrinkage stability should be larger. Otherwise, a large number of substandard waste products may result.

Therefore, various countries have formulated national industry standards or national standards for the standard tolerances of plastic parts. China has also formulated ministerial-level technical standards. However, most lack corresponding standard tolerances for mold cavities. The French national industry standard specifically stipulates the DIN16901 standard for plastic part standard tolerances and the corresponding DIN16749 standard for mold cavity standard tolerances. This standard has a significant impact worldwide and can be used as a reference for the plastics processing industry.

Regarding the standard tolerances and allowable errors of plastic parts