Do you understand key points of plastic precision injection molding?

Time:2020-01-06 09:00:33 / Popularity: / Source:

With rapid development of electronics, telecommunications, medical, automotive and other industries, requirements for high precision and performance of plastic products are increasing day by day. Precision injection molding requires not only high dimensional accuracy, low warpage, but also excellent optical properties. Injection moulding is one of the most important plastic molding methods. How to improve level of injection moulding technology, produce high-precision plastic products, and create high value-added products. Injection mold design is an important link.
In injection mold design, in addition to general molding design, special consideration should be given to following:
1) In order to obtain desired dimensional tolerance, consider appropriate mold dimensional tolerances.
2) Consider preventing occurrence of mold shrinkage fluctuations.
3) Consider preventing deformation of forming.
4) Consider preventing release deformation.
5) Minimize error in plastic injection mold making.
6) Consider preventing mold accuracy fluctuations.

1. Appropriate mold size and tolerances

1.1 Relationship between dimensional accuracy of product and precision injection molding

Draw a product map, considering molding design, plastic injection mold making and molding process.
First, mold surface size can be obtained from product drawing size. Mold is made according to size of mold drawing to obtain actual size of mold. Molded article can be obtained by using precision injection molding to obtain actual size of product. Question is how this actual size fits within required dimensional tolerances of drawing.

1.2 Appropriate shrinkage rate

As described above, even in same resin using same pigment, shrinkage ratio varies depending on molding conditions. In precision injection molding, degree of shrinkage change is small, and it is expected that shrinkage rate and actual shrinkage are as small as possible. Mainly to use actual shrinkage rate of similar products in the past to estimate shrinkage rate, but also to use experimental mold to find actual shrinkage rate, and then modify and design production model.
However, it is almost impossible to accurately presume shrinkage rate, and it is inevitable to correct mold after mold trial. As a result of correction, recess will increase in size and projection will be downsized. Therefore, shrinkage ratio is set to a small value for size of concave portion, and a large value for size of convex portion. When outer diameter of gear becomes larger, it cannot be engaged. When gear is small, only backlash becomes large, so shrinkage ratio should be set to a small value.

2. Prevent occurrence of molding shrinkage fluctuations

Precision injection molding must be based on assumption that precision injection molding can be made to required size. However, even if mold size is constant, actual size of product will vary depending on actual shrinkage. Therefore, in precision injection molding, control of shrinkage is very important. Suitability of molding design governs shrinkage rate, and also because of resin batch. However, if pigment is changed, shrinkage rate also varies. Due to different molding machines, setting and reproducibility of molding conditions, movement of each molding cycle fluctuate, actual shrinkage rate fluctuates, so control of shrinkage is difficult.

2.1 Main factors affecting shrinkage rate

Size of precision injection molding can be determined by size of product plus shrinkage rate, so in injection mold design, main cause of shrinkage needs to be considered.
Main factors affecting molding shrinkage rate are
(1) resin pressure, (2) resin temperature, (3) mold temperature, (4) gate cross-sectional area, (5) injection time, (6) cooling time, (7) product wall thickness, (8) reinforcing material, (9) orientation, (10) injection speed. These effects vary depending on changes in items such as resin and molding conditions.
(1) Resin pressure
Resin pressure has a great influence on shrinkage rate. If resin pressure is large, shrinkage ratio becomes small, and product size is large. Even in same cavity, resin pressure varies depending on shape of product, so that a difference in shrinkage rate occurs. In case of a multi-cavity mold, resin pressure in each cavity is likely to be different, as a result, shrinkage ratio of each cavity is also different.
(2) Mold temperature
If mold temperature is high, shrinkage ratio becomes large regardless of amorphous resin or crystalline resin. Precision injection molding maintains mold temperature at a specific temperature. At molding design, attention must be paid to design of cooling circuit.
(3) Gate cross-sectional
Generally speaking, when gate cross-sectional area is changed, shrinkage rate also changes. Shrinkage ratio becomes smaller as gate size becomes larger, which is related to fluidity of resin.
(4) Product wall thickness
Thickness of product wall also affects yield. For amorphous resin, resin tends to have a different influence on shrinkage rate of wall thickness. Wall thickness is large, shrinkage ratio is also large. On the contrary, shrinkage ratio is small. For crystalline resins, it is necessary to avoid a particularly large change in wall thickness. In case of multi-cavity modes, if wall thickness of cavity is different, shrinkage will also vary.
(5) Reinforcing material
When glass fiber reinforced resin is used, the more glass fiber is added, the smaller shrinkage rate is, shrinkage rate in flow direction is smaller than transverse shrinkage rate. According to difference between resin, shape and number of gates must be considered to prevent twisting and warping.
(6) Directivity
Although orientation is largely different, there is directionality for all resins. Orientation of crystalline resin is particularly large and varies depending on wall thickness and molding conditions.
In addition, there is also shrinkage after molding. Main factors affecting shrinkage after forming are 1 internal stress relaxation, 2 crystallization, 3 temperature, 4 humidity.
precision injection molding 

2.2 Possible measures

(1) runner, gate balance
As described above, shrinkage rate changes due to resin pressure. In case of a single-cavity multi-point gate and a multi-cavity mold, it is necessary to perform gate balancing in order to perform same filling. Resin flow is related to flow resistance in flow path, so it is preferable to take flow path balance before taking gate balance.
(2) cavity arrangement
In order to make setting of molding conditions easy, it is necessary to pay attention to cavity arrangement. Since molten resin puts tropics into mold, in case of general cavity arrangement, mold temperature distribution is concentric with gate as center. Therefore, when selecting cavity arrangement of multi-cavity mode, it is necessary to take flow path balance easily, and take concentric arrangement centered on gate.

3. Prevent deformation of molding

Reason for deformation of molding is that there is internal stress under uneven shrinkage, so it is necessary to prevent uneven shrinkage. In case of a circular product having a hole in the center of gear, a gate must be provided at center at injection mold design. However, when flow direction of resin differs greatly from vertical direction, there is a disadvantage that an ellipse is generated. When a roundness of higher precision is required, it is necessary to set a gate with 3/6 drops.
However, it is necessary to pay full attention to balance of each gate. When side gate is used, 3-point gate will increase inner diameter of cylindrical product, inner multi-point uniform gate is less used in the case where outer surface and end surface do not allow gate mark, and good results can be obtained.

4. Prevent demoulding deformation

Precision products are generally small, product wall thickness is thin, and some have many thin ribs. Chess design must consider that product is not deformed, and can be properly removed. For resins with a small shrinkage rate, when molding pressure is high, it is necessary to pay attention to fact that product is easily left in cavity. When gear is molded by a resin having a small shrinkage ratio, gear portion cavity is preferably designed on ejector plate of ejection side.
When using top pin, pay attention to number of top pins and top pressure position without deformation. A perforated gear requires a core pin, it is necessary to be placed on ejecting side template to facilitate parallel ejection when ejecting.
For horn-like articles, a punching template can be used for ejection, and ejection of template can prevent deformation. Generally, precision products have a small draft angle. In order to reduce mold release force, mirror processing is required, grinding direction must be draft direction. It is necessary to set a block core that is easy to grind in direction of draft.

5. Smallest plastic injection mold making error

5.1 Appropriate mold construction according to desired processing method

In order to obtain desired size of product, there must be a corresponding mold size, and mold needs to be processed with extremely high precision, and is limited by finishing machinery.
In order to maintain precision of precision injection molding, high wear resistance and quenching is required for this purpose. Precision of quenching mold by grinding machine and electric discharge machine can reach 0.01 mm or less.
When machining with a EDM machine, it is necessary to pay attention to wear of electrode end. When machining cavity for gear of Fig. 3 with a wire electric discharge machining machine, it is necessary to design a structure that can be processed through as much as possible at injection mold design.
From viewpoint of preventing grinding deformation and shortening processing time, it is necessary to select a steel material having less quenching deformation, design a shape having less quenching deformation. When shape is complicated, bonfire cooling is not uniform and is prone to quenching deformation.
precision injection molding 

5.2 Total split mold

In order to machine quenched parts to a higher precision, a grinding machine is used. Therefore, it is necessary to use an insert to form a split mold.
This mold has following characteristics:
(1) Since a suitable material be selected, a suitable hardness mold material can be used.
(2) It is possible to use mold materials with high corrosion resistance and wear resistance.
(3) Since heat treatment is possible separately, it is easy to set heat treatment conditions.
(4) It is possible to use a mold material having good mirror workability, mirror surface processing operation is also easy, so that mirror surface can be improved.
(5) Since it is easy to grind in drawing direction, it is advantageous to use a mold having a small draft.
(6) Due to hardening, mold precision holding time can be extended, and mold life is long.
(7) It is easy to set exhaust at any position, so filling mold is easy.
(8) Easy grinding and processing.
(9) Accuracy of mold parts can be improved, so that accuracy of product may be improved.
(10) Cavity and core can be made with small tolerances, so parts are interchangeable and easy to repair.
(11) Since grinding process is main part, processing efficiency is high.
(12) Number of parts is large, and it is necessary to greatly improve machining accuracy of each part.
(13) Limited to specific processing methods
(14) Fully quenched

6, Prevent errors in mold accuracy

To ensure positioning of sliders in each cycle, it is necessary to prevent fluctuations in accuracy of mold. In order to maintain accuracy of sliding member, sliding member should be honed and ground. Matching of sliding portion of side core should have a positioning and retracting portion.

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