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ABS Injection Molding

ABS plastic is an engineering plastic widely used in injection molding, and the products produced combine the characteristics of acrylonitrile, butadiene, and styrene monomers, with good comprehensive performance. ‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌

Injection Molding Manufacturers

Injection molding manufacturers can provide high-level plastic injection molding technology, thereby bringing high-quality injection molding services to customers. Injection molding can achieve automation, so it can be applied to various products that require large-scale production, such as daily necessities packaging, etc; The high precision of ABS injection molding also enables it to be applied in various high-demand fields such as medical, agricultural, industrial, automotive, and electronic equipment.

Parameter Settings for Injection Molding

Before starting production, injection molding manufacturers need to debug and set various process parameters to achieve the best production results.

Setting of Back Pressure Value

In practice, the manufacturer recommends a back pressure of 5-10MPa. If lower back pressure is used, it will increase the residence time of the material in the barrel, resulting in inconsistent shapes between the formed parts. Increasing back pressure will also increase the heat generated by friction and reduce the time required for plasticization.

Final Injection Position Setting of the Screw

When switching from the filling stage to the pressure-holding stage, the position where the screw is located is the final injection position of the screw. Insufficient padding may cause shrinkage marks on the surface of the product. In daily operations, the length of the padding is generally set to 5-10mm. To prevent damage to the mold and injection molding machine, manufacturers generally set the final position of the screw to fill two-thirds of the mold cavity.

Setting the Injection Speed of the Injection Molding Machine

When the injection speed is high, the flow resistance is smaller, the flow length is longer, and the strength of the fusion line is stronger. However, accelerating the injection speed means that more exhaust holes need to be set up, otherwise it will lead to poor exhaust, trapped air, and an increase in temperature and pressure inside the mold cavity, resulting in defects such as burn marks, material degradation, short shots, and fusion marks in the molded parts. So a reasonable exhaust system should be designed to avoid or reduce defects caused by trapped air. In addition, it is necessary to regularly clean the surface of the mold and exhaust facilities to prevent blockages.

Setting of Injection Pressure

Setting the injection pressure as high as possible by the manufacturer can better utilize the injection speed of the injection molding machine, accelerate the injection process, and improve production efficiency. Before the mold is filled, the pressure will switch to the holding pressure stage, which greatly reduces the risk of mold damage.

Setting of Holding Time and Cooling Time

The ideal holding time should be the minimum value between the solidification time of the gate and the solidification time of the part (which can be calculated or estimated). For the first trial mold, CAE software can be used to predict the filling time and then set the holding time to 10 times the filling time. The cooling time can be estimated or calculated as 10 times the injection time, which includes pressure holding time and continuous cooling time. For example, if the predicted injection time is 0.85s, the pressure holding time is 8.5s, and the additional cooling time is 8.5s. This ensures that the parts and flow system are fully cured, and subsequent demolding is relatively simple.

Gradual Increase in Injection Volume

The weight of plastic parts and sprue runners can be measured using CAE software. By adding this information to the known screw diameter or inner diameter of the material barrel, the injection volume and starting point position for each injection can be estimated. Therefore, if only 2/3 of the mold needs to be filled, the holding pressure needs to be set to 0 MPa. This way, when the screw reaches the injection endpoint position, the mold filling will stop in time to protect the mold from damage. Next, increase the filling amount by 5% to 10% in each step until 95% of the cavity volume is filled. At this point, using a compression safety valve can prevent plastic from flowing from the nozzle. After the screw rotation is completed, it is necessary to immediately retreat a few millimeters to release the back pressure established during the plasticization stage.

The Method of Coordinating Injection and Pressure Holding during Injection Molding Process

In the process of manufacturing products by plastic injection molding manufacturers, it is usually necessary to understand injection and pressure holding. There are similarities between the two, but more importantly, they are different. Therefore, injection molding manufacturers need to coordinate the two.

During injection and pressure holding, the injection direction valve is always open, and unless an injection servo valve is used, it will not close or turn during the process.
On the display screen of a typical injection molding machine, both injection and pressure holding have speed and pressure control.
The segmentation of injection can be distinguished by time (called time injection), but the most accurate differentiation is based on the screw position, which is the same as the segmentation of pressure holding.

Based on the above situation, some engineering and technical personnel do not use the pressure holding section when setting injection molding process parameters, but instead, use the post-injection section or several post-injection sections for pressure holding. It can be used in some simple injection molding situations such as two-stage injection and two-stage pressure holding. However, this method is generally not used in the injection molding of complex plastic parts.

Solution for Cracks Generated by Manufacturers During Injection Molding Process

During the injection molding process, surface cracking or cracks may occur in the final molded part, which will greatly affect the performance and aesthetics of the molded part. At this time, injection molding manufacturers need to find the cause of this problem and corresponding solutions.

When designing, the plastic structure is unreasonable. The sharp corners and notches in the shape and structure of plastic parts are most prone to stress concentration, leading to cracks and fractures on the surface of the plastic parts. Therefore, the sharp corners of these inner and outer corners should be replaced with arcs or rounded corners as much as possible. The experiment shows that the radius of the arc and the wall thickness at the corner should be 1.7 times the optimal transition radius of the arc.
If the residual stress during the molding process is too high, a direct gate with minimal pressure loss and high injection pressure can be used in mold design and manufacturing, or the gate diameter can be reduced by changing the forward gate to multiple needle-shaped gates or side gates. When designing the side gate, a convex gate can be used, which means that the cracked part can be removed after forming. In terms of process operation, the simplest method is to reduce residual stress by reducing injection pressure, as injection pressure is directly proportional to residual stress. It is also possible to increase the temperature of the material and mold appropriately, reduce the temperature between the melt and the mold, control the cooling time and speed of the billet in the mold, and give the oriented molecules a longer recovery time.
Improper selection of raw materials or contamination of raw materials. Long-term practice has shown that using low-viscosity plastics for processing is less likely to produce cracks. Therefore, in the production process, suitable molding materials should be selected based on the processing situation in reality. During the operation, special attention should be paid not to mixing plastics such as polyethylene (PE) and polypropylene (PP), as this can easily cause cracks. During the molding process, excessive or improper use of release agents can also cause cracks in the product, so the use of release agents should be minimized or avoided as much as possible.
The mold cracks and replicates onto the surface of the product. During the injection molding process, due to the repeated injection pressure on the mold, the sharp edges in the cavity will be repeatedly rubbed, and cracks will occur to a certain extent, especially around the cooling holes. When cracks on the surface of the mold cavity replicate onto the surface of the plastic part, it can cause the same cracks to appear at the same position of the product. When such cracks occur, the injection molding manufacturersshould immediately inspect the corresponding cavity surface for the same cracks. If cracks are caused by replication, the maintenance personnel should be notified to repair the mold.
Under the action of external forces, residual stress is concentrated. In general, such defects are most likely to occur near the top rod. After such defects occur, the ejection device should be carefully inspected and readjusted. The parts with the highest demolding resistance, such as protrusions and reinforcing ribs, are most suitable for setting top rods. If the number of top rods set cannot be expanded due to the limitations of the pushing area, the method of small area multiple top rods can be used for setting. If the demolding angle of the mold cavity is not enough, scratches will also appear on the surface of the plastic part, forming wrinkled patterns.