Ratio of size change of injection molded product caused by its shrinkage after molding and cooling to size of mold cavity is called molding shrinkage rate (referred to as shrinkage rate). Molding shrinkage rate generally refers to degree of dimensional shrinkage of part after molding.

1. Plastic variety characteristics, such as glass fiber content, stress distribution, molecular orientation, etc. of plastic. Same plastic has different shrinkage rates and anisotropy due to different relative molecular weights, fillers and formulas of resin. For example, if relative molecular weight of resin is large, filler is organic, and resin content is high, shrinkage rate of plastic will be large. During molding process, thermoplastics have larger shrinkage, wider shrinkage range and obvious directionality compared with thermosetting plastics due to factors such as volume change caused by crystallization, large internal stress, large residual stress frozen in plastic part and strong molecular orientation. In addition, shrinkage of thermoplastics after molding, annealing or humidity treatment is generally larger than that of thermosetting plastics.

 

2. Shape and structure of plastic parts, such as wall thickness, presence or absence of inserts and position of inserts.
3. Mold structure and mold pouring system, such as gate form, size, quantity, position, etc.
4. Mold temperature control system, such as mold temperature, uniformity and balance of mold temperature distribution.
5. Molding process parameters, such as pressure, time, barrel temperature, injection speed, etc. Maintaining pressure and duration also have a great influence on shrinkage. The higher pressure and the longer time, the smaller shrinkage, but directionality is strong. Therefore, adjusting mold temperature, pressure, injection speed and cooling time during molding can also appropriately change shrinkage of plastic parts.
6. How to place and post-process plastic parts after demolding.

 

Dimensions of moving and fixed molds and molded parts must first know molding shrinkage rate of plastic before calculation. There are several ways to obtain molding shrinkage rate data.
1. Look up table to obtain molding shrinkage rate data. Due to different characteristics of plastics, their shrinkage rates are also different, shrinkage rate range is wide, and average value is generally taken.
2. Usually customer specifies empirical value and provides it to mold supplier.
3. For a small shrinkage rate range, average value is generally taken according to theoretical value.
4. In order to obtain an accurate shrinkage rate, before designing mold, a mold similar to plastic part can be selected for testing and shrinkage rate value can be actually measured as basis for mold design. Precision injection molds usually use this method to obtain an accurate shrinkage rate.
5. In the absence of certainty, manufacturing of spline molds is adopted to verify correctness of shrinkage rate. At present, method for determining shrinkage rate of various plastics (molding shrinkage + post-shrinkage) generally recommends provisions of DIN16901 in German national standard. That is, it is calculated by difference between mold cavity size at 23℃±0.1℃ and corresponding plastic part size measured at 23℃0 and 50%±5% humidity for 24 hours after molding.

For molding shrinkage rate of high-precision plastic parts, following aspects are usually considered to design and manufacture molds:
1. Shrinkage rate of thin-walled products is smaller, and shrinkage rate of thick-walled products is larger.
2. Shrinkage rate of "anisotropic" plastics (such as crystalline plastics) in direction of melt flow is larger, and shrinkage rate perpendicular to flow direction is smaller (but opposite is true for fiber-reinforced plastics).
3. Shrinkage rate of "anisotropic" plastics (such as PC, etc.) in melt flow direction and vertical direction is same.
4. Shrinkage rate near gate is smaller, and shrinkage rate away from gate is larger, and shrinkage rate from gate to the end is gradual.
5. Shrinkage rates of different brands of plastics are also different (even very different), so actual shrinkage rate of molded plastic parts should be used as standard.
6. Shrinkage rate of mold with a small gate or thin gate is larger, and shrinkage rate of mold with a large gate is smaller.
7. For products with similar wall thickness, shrinkage rate of parts with high mold temperature is larger, and shrinkage rate of parts with low mold temperature is smaller.
8. Shrinkage rate of plastic parts in length direction is larger, and shrinkage rate in width direction is smaller.
9. For long strips of plastic parts, if gate is in the middle, shrinkage rate is smaller, and if gate is at one end, shrinkage rate is larger.
10. For molds with simple structures (fewer tendons, columns, and bones), shrinkage rate is larger, otherwise, shrinkage rate is smaller.
11. Take a smaller shrinkage rate for outer shape of plastic parts and a larger shrinkage rate for inner shape to ensure that there is a margin for correction after mold trial.
12. When determining shrinkage rate, product shape, structure, pouring system form, dimensional tolerance, molding process and working environment should be comprehensively considered.
13. For plastic parts that need post-processing, shrinkage rate should be determined considering specific conditions of post-processing.
14. Modify mold according to actual shrinkage situation. Generally, cavity selects lower limit and dynamic mold takes upper limit to facilitate mold trimming.
15. Test mold and adjust process appropriately, and slightly modify shrinkage value to meet requirements of plastic part.
For later reading, please refer to "Talking about mold optimization from perspective of injection molding process 7, 8".