When designing plastic mold, after mold structure is determined, various parts of mold can be designed in detail, that is size of each template and part, cavity and core size are determined. This will involve major design parameters such as material shrinkage. Therefore, size of each part of cavity can be determined only by specifically grasping shrinkage rate of shaped plastic. Even if selected mold structure is correct, if parameters used are not correct, it is impossible to produce a qualified plastic part.

Characteristic of thermoplastics is that they expand after heating, shrink after cooling, and of course volume will shrink after pressurization. In injection moulding process, molten plastic is first injected into mold cavity, after filling is completed, melt is cooled and solidified, shrinkage occurs when plastic part is taken out from mold, and this kind of shrinkage is called forming shrinkage. Plastic part is taken out from mold for a period of time, and size still changes slightly. One change is to continue shrinking. This shrinkage is called back shrinkage. Another variation is that some hygroscopic plastics swell due to moisture absorption.
For example, when water content of nylon 610 is 3%, dimensional increase is 2%; and when water content of glass fiber reinforced nylon 66 is 40%, dimensional increase is 0.3%. But main role is forming shrinkage. At present, method for determining shrinkage ratio of various plastics (forming shrinkage) is generally recommended in German national standard DIN16901. That is, corresponding mold cavity size difference between 23℃ ± 0.1℃ and result measured after molding for 24 hours, at a temperature of 23℃ and a relative humidity of 50 ± 5%.
Shrinkage ratio S is represented by following formula: S = {(D - M) / D} × 100% (1) wherein: S - shrinkage ratio; D - mold size; M - plastic part size.
If mold cavity is calculated according to known plastic part size and material shrinkage rate, then D=M/(1-S). In order to simplify calculation in mold design, mold size is generally obtained by following formula: D=M MS(2)
If a more accurate calculation is required, following formula is applied: D=M MS MS2(3)
However, when determining shrinkage rate, since actual shrinkage rate is affected by many factors, only approximate value can be used. Therefore, calculation of cavity size by formula (2) basically satisfies requirement. When manufacturing mold, cavity is machined according to lower deviation, and core is processed according to deviation, so that it can be properly trimmed if necessary.
Main reason why it is difficult to accurately determine shrinkage rate is that shrinkage rate of various plastics is not a fixed value but a range. Because shrinkage rates of same materials produced by different factories are not same, even same batch of same material produced by a factory has different shrinkage rates. Therefore, each factory can only provide users with a range of shrinkage of plastics produced by plant. Secondly, actual shrinkage during forming process is also affected by factors such as shape of plastic part, mold structure and forming conditions. Following is an introduction to impact of these factors.

 

For wall thickness of formed part, shrinkage rate is also large due to long cooling time of thick wall, as shown in Fig. 1. For a general plastic part, when difference between flow direction L dimension of melt and dimension W perpendicular to flow direction of melt flow is large, difference in shrinkage ratio is also large. From flow distance of melt, pressure loss away from gate portion is large, and thus shrinkage rate at this point is also larger than that near gate portion. Since shape of reinforcing ribs, holes, bosses, and engravings has shrinkage resistance, shrinkage rate of these portions is small.

Gate form also has an effect on shrinkage. When a small gate is used, shrinkage rate of plastic part is increased because gate is solidified before pressure is maintained. Cooling circuit structure in injection mold is also a key in mold design. If cooling circuit is not properly designed, shrinkage is caused by uneven temperature of plastic parts, result is that size of plastic part is excessive or deformed. In thin-walled part, effect of mold temperature distribution on shrinkage is more pronounced.

 

Factors such as parting surface, gate form and size of mold directly affect flow direction, density distribution, pressure-preserving and shrinking action, molding time.
Use of direct gates or large-section gates can reduce shrinkage, but anisotropy is large, shrinking in flow direction is small, and shrinking along direction of vertical flow is large; on the contrary, when gate thickness is small, gate portion will coagulate and harden prematurely, plastic in cavity will not be replenished in time after shrinking, and shrinkage is large.
Point gate is fast-sealed. When condition of workpiece is allowed, a multi-point gate can be set, which can effectively extend dwell time and increase cavity pressure, so that shrinkage rate is reduced.

When barrel temperature(plastic temperature) is high, pressure transmission is better and contraction force is reduced. However, when a small gate is used, shrinkage rate is still large due to early curing of gate. For thick-walled plastic parts, even if barrel temperature is high, shrinkage is still large.

In forming conditions,feeding is minimized to keep size of plastic part stable. However, if feed is insufficient, pressure cannot be maintained, and shrinkage rate is also increased.

Injection pressure is a factor that has a large influence on shrinkage rate, especially pressure retention after filling. In general, when pressure is high, density of material is large and shrinkage rate is small.
Pressure in injection moulding includes injection pressure, holding pressure, and cavity pressure. These factors have a significant impact on shrinkage behavior of plastic parts.
Increasing injection pressure can reduce shrinkage of product. This is because pressure is increased, injection speed is increased, and filling process is accelerated. On the one hand, melt temperature is increased by shear heat of plastic melt, and flow resistance is reduced; on the other hand, it is also possible to enter pressure-preserving feeding stage earlier in a state where melt temperature is still high and flow resistance is small. Especially for thin-walled plastic parts and small gate plastic parts, due to fast cooling rate, filling process should be shortened as much as possible.
Higher holding pressure and cavity pressure make products in cavity dense and shrinkage, especially pressure in pressure-holding stage has a greater influence on shrinkage rate of product. This can be explained by fact that molten resin is compressed under molding pressure. The higher pressure, the greater amount of compression that occurs, the greater elastic recovery after pressure is released, the closer size of plastic part is to cavity size, and thus the smaller amount of shrinkage.
However, even for same product, pressure of resin in cavity is not uniform in each part; injection pressure is different in portion where injection pressure is difficult to act and portion where it is easy to act. In addition, pressure of each cavity of multi-cavity mold should be designed uniformly, otherwise shrinkage rate of products of each cavity will be inconsistent.

Injection speed has little effect on shrinkage rate. However, when thin-walled plastic part or gate is very small, and when reinforcing material is used, injection speed is increased and shrinkage rate is small.

After thermoplastic melt is injected into cavity, it releases a large amount of heat and solidifies. Different plastic varieties require cavity to be maintained at an appropriate temperature. At this temperature, it will be more favorable for molding of plastic parts, molding efficiency of plastic parts is higher, internal stress and warpage deformation are smaller.
Mold temperature is main factor controlling cooling forming of product. Influence of mold temperature on molding shrinkage is mainly reflected in process before product is demolded after gate is frozen. However, before gate freezes, increase of mold temperature has a tendency to increase heat shrinkage but it is also higher mold temperature that causes gate freezing time prolong, resulting in an increase in injection pressure and holding pressure, both feeding effect and negative shrinkage amount are increased.
Therefore, total shrinkage is result of a combination of two types of reverse shrinkage, and value does not necessarily increase as mold temperature increases.
If gate freezes, influence of injection pressure and holding pressure will disappear. As mold temperature increases, cooling set-up time will also prolong, so shrinkage of product will generally increase after demolding.

There is no direct relationship between forming cycle and shrinkage. However, it should be noted that when forming cycle is accelerated, mold temperature, melt temperature are also inevitably changed, thereby also affecting change in shrinkage rate. In material test, forming shall be carried out in accordance with forming cycle determined by required output, and dimensions of plastic part shall be inspected. An example of a plastic shrinkage test using this mold is as follows.

clamping force 70t screw diameter Φ35mm screw speed 80rpm forming conditions: higher 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 dwell time 6.0s cooling time 15.0s

 

Machining dimensions of mold cavity and core have a problem of machining tolerance in addition to calculation of basic dimensions by D=M(1 S) formula. By convention, machining tolerance of mold is 1/3 of plastic part tolerance. However, due to differences in the range and stability of plastic shrinkage, it is first necessary to rationalize dimensional tolerances of plastic parts formed by different plastics. That is, dimensional tolerance of plastic molded part with a large shrinkage ratio or a poor shrinkage ratio should be made larger. Otherwise, there may be a large number of waste products of exceptional size. To this end, countries have developed national or industry standards for dimensional tolerances of plastic parts. China has also developed ministerial professional standards. However, most of them do not have corresponding dimensional tolerances of mold cavity. In German national standard, DIN16901 standard for dimensional tolerances of plastic parts and corresponding DIN16749 standard for dimensional tolerances of mold cavities are specified. This standard has a large impact in the world and is therefore available for reference in plastic mold industry.

In order to reasonably determine dimensional tolerances of plastic parts formed by different shrinkage characteristics, introduces concept of forming shrinkage difference ΔVS. △VS=VSR_VST(4)
Where: VS-form shrinkage difference VSR-formation shrinkage rate of melt flow direction VST- form shrinkage in the direction perpendicular to melt flow.
According to plastic ΔVS value, shrinkage characteristics of various plastics were divided into four groups. Group with a small ΔVS value is a high-precision group, a group with a large ΔVS value is a low-precision group. Precision technology, 110, 120, 130, 140, 150 and 160 tolerance groups were developed according to basic dimensions. It is also stipulated that dimensional tolerances of plastic molded parts with stable shrinkage characteristics can be selected from 110, 120 and 130. Dimensional tolerances for plastic molded parts with moderately stable shrinkage characteristics are 120, 130 and 140. If 110 dimensional tolerances for plastic molded parts of this type are used, it is possible to produce a large number of oversized plastic parts. Dimensional tolerances of plastic molded parts with poor shrinkage characteristics are selected from 130, 140 and 150. Dimensional tolerances of plastic molded parts with poor shrinkage characteristics are selected from 140, 150 and 160. When using this tolerance table, you should also pay attention to following points. General tolerances in the table are for dimensional tolerances that do not indicate tolerances. Tolerance for direct labeling deviation is tolerance band used to dimension tolerances for plastic parts. Upper and lower deviations can be determined by designer himself. For example, if tolerance band is 0.8 mm, following various upper and lower deviations can be selected: 0.0; -0.8; ±0.4; -0.2; -0.5 and so on. There are tolerance values for groups A and B in each tolerance group. Where A is size formed by combination of mold parts, which increases error caused by incompatibility of mold parts. This increase is 0.2 mm. Where B is size directly determined by mold part. Precision technology is a set of tolerance values specifically set for use with high precision plastic parts. Before using plastic part tolerances, you must first know which tolerance groups are suitable for plastic used.

German national standard sets standard DIN16749 for tolerance of mould. There are 4 tolerances in the table. Regardless of plastic part of material, mold manufacturing tolerances in which dimensional tolerances are not specified are all using tolerance of serial number 1. Specific tolerance value is determined by basic size range. Regardless of material, mold manufacturing tolerances of medium-precision size is tolerance of No. 2. Regardless of material, mold manufacturing tolerances of higher precision size of plastic part is tolerance of No. 3. Corresponding mold manufacturing tolerances of precision technology is tolerance of No. 4.
Reasonable tolerances of various plastic parts and corresponding mold manufacturing tolerances can be reasonably determined, which not only brings convenience to mold manufacturing, but also reduces waste and improves economic efficiency.