1. Gate Design
The injection mold gate is a crucial part of the entire gating system. Its location, type, and number directly affect the flow state of the molten material within the mold cavity, thus leading to changes in plastic solidification, shrinkage, and internal stress. Commonly used gate types include side gates, point gates, submarine gates, direct gates, fan gates, and thin-film gates.
Therefore, the gate location should be chosen to minimize the plastic flow distance. A longer flow distance increases the flow difference between the inner flow layer and the outer frozen layer, resulting in greater internal stress caused by flow and shrinkage between the frozen layer and the central flow layer, leading to increased part deformation. Conversely, a shorter flow distance reduces the flow time from the gate to the end of the part, resulting in a thinner frozen layer during mold filling, lower internal stress, and reduced warpage.
For example, on large, thin-walled precision plastic parts, using a single center gate or side gate will result in significant warping deformation after molding because the radial shrinkage rate is greater than the circumferential shrinkage rate. Using multiple point gates or film-type gates can effectively prevent warping deformation; therefore, flow ratio calculations must be performed during the design phase.
When using point gate molding, the location and number of gates also significantly affect the degree of deformation due to the anisotropic shrinkage of the plastic.
For the experiment on the distribution of different gate numbers for flat, box-shaped plastic parts: using 15% glass fiber reinforced PA66, the part weighing 1450g had many reinforcing ribs along the flow direction of the four walls. The same process parameters were used. Gate methods: (a) direct gate, (b) 5-4 point gates, (c) 9-8 point gates. The experimental results showed that setting the gate according to method b yielded the best results and met the design requirements. The gate design based on 'c' is worse than a direct gate, with warpage exceeding design requirements by 3.6~5.2mm. Multiple gates shorten the flow ratio (L/t) of the plastic, resulting in more uniform melt density and shrinkage within the mold. Simultaneously, the molded part can fill the cavity with lower injection pressure, reducing molecular orientation tendencies, lowering internal stress, and minimizing part deformation.
2. Cooling System Design
Uneven cooling rates during injection molding can lead to uneven shrinkage, causing bending moments and warping.
For example, in a precision, flat, large plastic shell mold, a large temperature difference between the cavity and core causes the melt on the cold mold cavity surface to cool quickly, while the layer near the hot mold cavity surface continues to shrink. This uneven shrinkage leads to warping. Therefore, the cooling system design of injection molds requires strict control of the temperature balance between the core and cavity. Therefore, for precision flat plastic shell parts, materials with high molding shrinkage are prone to deformation. Production tests show that temperature differences should not exceed 5°C to 8°C.
Secondly, it is necessary to consider the uniformity of temperature across the plastic part, that is, to maintain a uniform temperature throughout the core and cavity, ensuring even cooling rates and uniform shrinkage, effectively preventing deformation. The design of the cooling system should be determined through rigorous process trials based on theoretical calculations. Therefore, the placement of cooling water holes on the mold is crucial.
After determining the distance from the pipe wall to the cavity surface, the distance between cooling water holes should be minimized as much as possible. If necessary, a non-uniform arrangement should be used, with more densely spaced cooling water holes where the material temperature is high and more sparsely spaced where the material temperature is low, to maintain a relatively uniform cooling rate. Simultaneously, since the temperature of the cooling medium increases with the length of the cooling channel, the length of the cooling circuit should not be too long.
3. Ejection Mechanism Design
The design of the ejection mechanism also directly affects the deformation of the plastic part. If the ejection mechanism is unbalanced, it will cause uneven ejection forces, leading to deformation of the plastic part. Therefore, the ejection mechanism should be designed to balance with the demolding resistance. The cross-sectional area of the ejector pins should not be too small to prevent excessive force per unit area on the plastic part, which could lead to deformation.
The ejector pins should be placed as close as possible to areas with high demolding resistance. For precision flat plastic shell parts, as many ejector pins as possible should be used to reduce deformation, and a combined demolding mechanism combining ejector pins and push-plates should be employed.
When producing large, deep-cavity, thin-walled plastic parts using soft plastics, the demolding resistance is relatively high, and the material is relatively soft. If only mechanical ejection is used, the plastic part will deform. Using a multi-component combination or a combination of pneumatic (hydraulic) and mechanical ejection will yield better results.
