With people's pursuit of high-quality life, thin-walled plastic parts are increasingly used in people's daily life. Thin-wall injection molding is not much different from traditional injection molding of plastic parts, except that there are some unique features in design of plastic parts and mold design, which require special attention from practitioners.
Due to development trend of miniaturization and portability of consumer electronic devices such as mobile phones and wearable devices, design of related plastic parts has become thinner and thinner. Thin-walling has become goal pursued by plastics consumer industry because of its advantages such as reducing product weight and dimensions, facilitating integrated design and assembly, shortening production cycle, saving materials and reducing costs, and has become a new research hotspot in plastic molding industry.

 

Basic definition of thin-wall injection molding has following points:
Ratio of flow length to thickness L/T, that is, ratio of flow length L from melt entering mold to the furthest point of cavity where melt must be filled to corresponding average wall thickness T is 100 or 150 thin-walled injection molding；
Injection molding method where thickness of molded plastic part is less than 1 mm, projected area of plastic part is above 50c㎡;
Wall thickness of molded plastic part is less than 1 mm (or 1.5mm), or t/d (plastic part thickness t, plastic part diameter d, for disc-shaped plastic parts) injection molding below 0.05 is defined as thin-walled injection molding .

Relatively speaking, design ideas and methods of thin-walled products are more complex, are further affected by molding limitations and material selection. Thin-walled products should have high impact strength, good appearance quality and dimensional stability, can withstand large static loads, and fluidity of molding material should be good. In design process, rigidity, impact resistance and manufacturability of product should be considered.

Considering that wall thickness of thin-walled plastic parts is relatively thin, it is easy to cause excessive flow resistance of plastic during molding process, it cannot be filled with large-scale plastic parts. On the basis of guaranteeing rigidity and strength of plastic parts, it is recommended that wall thickness of plastic parts be selected between 1 and 5 mm, its consistency and uniformity are required. According to analysis of requirements, in design of the overall wall thickness of plastic part, it is also necessary to consider influence of screw column, reinforcing rib on appearance and strength of mechanism. Different materials have different wall thickness recommendations, as shown in figure below:

 

Wall thickness of thin-walled parts should be greater than minimum wall thickness of selected plastic.

There are three main functions of stiffeners, namely:
1. Without increasing wall thickness of product, enhance strength and rigidity of product to save amount of plastic, reduce weight, and reduce costs;
2. Overcome product deformation caused by uneven stress caused by product wall thickness differences;
3. Facilitate flow of plastic melt, and provide channels for smooth filling of melt at some places where wall thickness of plastic product body is too thin.

 

For reinforcement ribs of thin-walled parts, recommended thickness is 0.5~0.7 times wall thickness, height is less than or equal to 3 times wall thickness, and demolding slope is 2~5 degrees.

Generally speaking, for inside of plastic part, self-tapping screws are also needed to install remaining parts and set corresponding screw posts. For screw column, it mainly includes reinforced ribs and non-reinforced ribs, length of bottom is generally controlled within range of c=(0.2~0.5)×height of screw column. In addition, thickness also needs to take into account design of structures such as through holes and corners, a thin-walled structure needs to be used, and corresponding structure and size changes need to be made.

As thickness of thin-walled plastic parts continues to decrease, it is also necessary to consider use of materials with good fluidity. This type of material has higher thermal deformation temperature and impact strength, and has good dimensional stability.

For injection molding of thin-walled plastic parts, because resin itself has good fluidity, flow distance to thickness ratio (flow length ratio L/T) generally exceeds 150. For production of thin-walled plastic parts, plastic resins with medium to high fluidity and low viscosity are generally selected as raw materials. For resins with poor fluidity, modification methods are generally used to reduce their viscosity and increase MFR, to meet requirements of at least 30g/10min, in order to meet production needs.

For thin-walled plastic parts, corresponding material has higher requirements for impact resistance. Generally, it is required that at room temperature, impact strength needs to exceed 640J/m.

Generally, it is required to ensure that thin-walled plastic parts themselves will not be deformed, aging, or dents and other problems at 70~90℃. Considering impact performance, fluidity, heat resistance and other requirements of plastics, PP, PA6 and other materials are generally considered as raw materials for thin-walled plastic parts.

When forming thin-walled products, a specially designed special mold for thin-walled products is generally required. Compared with standardized molds of conventional products, molds of thin-walled products have undergone major changes from mold structure, gating system, cooling system, exhaust system and demolding system.
Figure 1 is a plastic part box and corresponding mold design. Material is PP, wall thickness of plastic part is 1.0mm, which is a typical thin-walled plastic part. Because wall thickness of thin-wall injection molding is thinner, resistance of plastic in filling stage is relatively large, so a higher injection pressure is required. This will put forward higher requirements on strength and rigidity of mold.
In design of mold support plate 8, movable mold plate 12, fixed mold plate 14, thickness of mold is generally 30% to 50% higher than that of ordinary plastic mold, and a support column 5 is also required. For movable mold plate and fixed mold plate, there should be a conical surface positioning setting, so as to ensure good side support and accurate positioning, avoid offset and bending. In addition, plastic filling speed and pressure of thin-walled plastic parts will be higher, which will increase wear of mold. Therefore, for mold material used, its own strength, hardness, rigidity, etc. are very important. Generally speaking, you can choose to use die steel for pre-hardening or heat treatment, hardness of its surface is required to be controlled at 48~52 HRC.

 

For forming thin-walled products, especially if thickness of product is very small, a large gate should be used, and gate should be thicker than wall thickness. For plastics with good fluidity, you can choose a point gate to design gating system; for plastics with medium fluidity, gates are generally designed at thicker parts of plastic part. In process of injection molding, transition is generally from thicker part to thinner part, so as to reduce warpage and dent of product. Use of sheet-type multi-gate or latent gate design can directly fill cavity with plastic melt, thereby reducing loss of plastic pressure. In addition, choice of hot runner technology can reduce viscosity of melt, which can also achieve requirements of fast filling.

Thin-walled products are not like traditional thick-walled parts that can withstand greater residual stress caused by uneven heat transfer. In order to ensure dimensional stability of product, control shrinkage and warpage within an acceptable range, it is necessary to strengthen cooling of mold to ensure a balanced cooling. Better cooling measures include use of unclosed cooling lines in core and cavity modules, increasing cooling length, can enhance cooling effect, and adding high-conductivity metal inserts where necessary to accelerate heat transfer.

Thin-wall injection molding molds generally need to have good venting properties, and it is best to perform vacuum operation. Due to short filling time and high injection speed, it is very important to fully vent mold, especially full vent of flow front gathering area, to prevent trapped gas from burning. Gas is usually discharged through core, ejector rod, stiffener, stud and parting surface, etc. End of runner should also be fully exhausted.

Considering that wall of plastic part is relatively thin, generally has structures such as bosses and ribs, plastic part may be deformed or damaged during demolding process. Along thickness direction, plastic part shrinks less, at the same time, holding pressure is higher, which will make it shrink less, position of ribs is also prone to adhesion. In order to avoid mold sticking and top-piercing problems, it is recommended to use more and larger thimbles to eject plastic parts.
Injection molding process for thin-walled parts

In production of thin-walled plastic parts, melt is directly filled into mold cavity through runner and gate, melt on mold wall will form a solidified layer, which will make flow channel of melt smaller. Because wall thickness of plastic part is very small, its flow channel will become narrower, which may cause problem of undershot. Therefore, thin-wall injection molding generally requires high speed and high pressure to successfully complete filling.

In production of thin-walled plastic parts, injection molding process parameters are very critical. A special thin-wall injection molding machine is generally required. Thin-walled injection molded parts generally have a relatively small amount of glue, so required barrel size is relatively small to reduce residence time of melt in barrel and prevent melt from degrading. Injection molding of plastic parts with different wall thicknesses has different corresponding parameters. For plastic parts with a wall thickness of less than 1mm, recommended injection pressure range is 138~241 MPa, and filling time is 0.1~0.5s.