The Influence of the Gating System's Design on Warpage of Plastic Parts

As one of the main indicators to measure the quality of plastic parts, the degree of warpage has attracted more and more attention from designers and end users of plastic parts. Reducing the warpage of products has become one of the key issues that need to be paid attention to in the optimization of the injection molding process and mold structure.
 
It is generally believed that the main factors affecting the warpage of plastic parts are raw materials, design of plastic parts, mold design and molding process conditions. The orientation ability and crystallization properties of plastics will significantly affect the warpage of plastic parts, and oriented materials are more likely to warp than non-oriented materials. The warpage tendency of crystalline polymers is greater than that of amorphous polymers, and additives will increase the warpage degree of injection molded products. Also, shapes, wall thicknesses, reinforcing ribs and surface decorative relief of products can affect the filling performance of plastic materials and the cooling effect of molding, resulting in uneven distribution of product orientation, internal stress and shrinkage and warping being inevitable. In addition, the setting of casting systems and cooling systems of molds, the exhaust performance, and the design of the ejection mechanism of the mold can all affect the orientation and shrinkage of the product, thereby significantly affecting the warpage of the product after the product is released from the mold. Many process parameters such as plasticizing quality of injection molding melt, melt temperature, injection molding pressure, dwell pressure, dwell time, and mold temperature all affect the warpage of the product.
 
The influence of the gating system's design on warpage of plastic parts
The location, type and number of gates in the injection molding will affect the filling state of the plastic in the mold cavity. Therefore, the improper number and location of the gates, and too small or too long gates and runners will cause large flow and great flow resistance; the corresponding injection pressure must also be increased.
 
The polymer is easy to be stretched and squeezed; the residual stress is great, and it is easy to warp. In order to reduce residual stress, an appropriate flow length and thickness ratio should be adopted, and the filling balance of the melt should be ensured. The location of the gate has a great influence on the filling method, and also affects the flow's length distribution in the filling process. The longer the flow distance is, the greater the internal stress caused by the flow and feeding between the frozen layer and the central flow layer becomes; on the contrary, the shorter the flow distance is, the shorter the flow time from the gate to the flow end of the part becomes. The thickness of the frozen layer thins in the filling process. The internal stress is reduced, and the warpage will be greatly reduced. 
 
Selecting the type and number of the gate is also important to control warpage. For large flat molded parts, if multiple point gates or film gates are used instead, warpage can be effectively prevented. In addition, the use of multiple gates can also shorten the flow ratio of the plastic, so that the density of the material in the mold cavity is more uniform and the shrinkage is more uniform. Meanwhile, the entire plastic part can be filled with less injection pressure. The small injection pressure can reduce the molecular orientation tendency of the plastic, reduce its internal stress, thus, reducing the deformation of the plastic part.
 
For the size of the gate, usually, when the size of the gate section is large, it is conducive to the transmission of pressure, increasing the cavity pressure, delaying the freezing time of the gate, and increasing the amount of feeding, thus, reducing the shrinkage rate of the injection molded product. However, sometimes too big gates will cause obvious traces on the surface of the injection molded product. Generally, a small size gate is used and arranged in a hidden position, which can not only increase the good appearance of the product, but also reduce the subsequent machining. Moreover,  filling and pressure maintaining can be improved by increasing the number of gates.
 
The location and number of gates should generally be determined with the goal of volumetric shrinkage at the end of the flow approaching the design value at the optimum feeding pressure. The locations of gates are chosen based on a uniform desirable shrinkage value at the end of the flow. In practice, the gate design criteria for warpage are similar to the fill balance design criteria, but there are slight differences in areas with a thick and thin wall where gates are set. In order to achieve the purpose of equalization of pressure maintaining, it is preferable to set the gate in the thicker area when the flow is completed. It is worth noting that changing the gate position is an important factor in reducing the effect of orientation, and sometimes the only way to control the orientation is to change the position of the gate.
 
The relationship between locations and quantity of gates and quality of plastic parts
The gate is the melt channel connecting the runner and the cavity in the gating system of the injection molding mold. The design of locations and quantity of gates is closely related to the quality of plastic parts.
 
The relationship between the gate position and jet flow
If the gate can be arranged as an impact gate, the plastic melt will immediately impact a certain barrier such as cavity walls and core pins after pouring, so that the plastic flow can be stabilized, and the jet flow can be reduced.
 
The relationship between the position and number of gates and joint lines
A joint line is a line formed when the wavefronts of two strands of plastic melt meet. Joint lines are negative in terms of the appearance or strength of plastic parts. For each additional gate, at least one joint line will be provided, so under the premise that the cavity can be filled as scheduled, the number of gates should be as less as possible. In order to reduce the number of gates, the pouring position that can cover the largest plastic part area should be found for each gate within the flow ratio of the plastic flow. 
 
The relationship between the location and number of gates and air pockets
Air pockets are defects in which the air in the cavity and the gas released from the melt are surrounded by the melt. The existence of air pockets can not only affect appearance and strength, but also lead to short shots or scorch marks in severe cases. Each additional gate increases the chance of air pockets. When the thickness difference of plastic parts is great, if the gate position is not set properly, air pockets will occur due to the gas getting into another gate.
 
The relationship between the locations of gates and hysteresis effect
The hysteresis effect is the effect that when the plastic melt flows to the connecting part between thick and thin parts, the flow resistance at the thin part is great, and the melt is stagnant. This will produce stagnant marks or short shots in severe cases. Gates should be placed furthest away from where hysteresis can occur to eliminate or mitigate hysteresis.
 
The relationship among positions of gates and shrinkage marks and shrinkage holes
The gate should be placed at the thick wall to ensure that the plastic flow of the feeding can last for the longest time, so that shrinkage marks and shrinkage holes will not easily occur for the thick wall due to much shrinkage.
 
The relationship between locations of gates and overflow
The layout of the cavity and the position of the gate should be symmetrical to prevent the mold from being subjected to eccentric load and causing an overflow.
 
The relationship between locations of gates and the balanced flow
For a mold with a single cavity, the plastic melt wave front arrives at each end of the cavity at the same time, which is called the balanced flow. The design of the balanced flow makes the distribution of pressure, temperature and volume shrinkage of plastic melt more uniform, and the quality of plastic parts is better. Therefore, the selection of the gate position is based on whether the balanced flow is obtained or not.
 
The relationship between positions of gates and planeness of plastic parts
If the setting of the gate can form a unidirectional flow, that is, after the plastic melt enters the cavity, its wave front can move with a straight form; then, the shrinkage of the plastic in the flow direction and the vertical flow direction will not restrain each other. Plastic parts with good planeness can be produced. If the setting of the gate can make the plastic melt flow through the straight part of the cavity first, and then to the curved part of the cavity, it can reduce the asymmetry of the residual stress on the center plane of the plastic part and the possibility of warping can be reduced.