Warp deformation refers to the shape of the injection molded products distorted and uneven, deviating from the requirements of the shape accuracy of the parts, which is one of the common and difficult to solve product defects in injection mold design and injection production.
With the development of the plastics industry, especially the development of the electronic information industry, the appearance and performance requirements of plastic products are increasingly high, such as the deformation of the plastic shell parts, such as notebooks and palmtops, thin and thin mobile phones, etc., One of the most important indexes to evaluate product quality is more and more concerned by die designers.It is hoped to predict the possible causes of warpage in the design stage so as to optimize the design and reduce the warpage of the product to reach the product Design accuracy requirements.
1, the cause of warping deformation
Warpage is the product during the injection process, resulting in uneven stress and shrinkage.Demolition poor, insufficient cooling, the shape and strength of the workpiece is not appropriate, the mold design and process parameters are also poor so that the plastic parts of the music change.
Uneven mold temperature, plastic parts of the internal temperature is not uniform.
Differences in plastic wall thickness and cooling uneven, leading to differences in shrinkage.
Thick plastic parts to the pressure differential pressure and cooling speed difference.
Plastic parts when the temperature is high or the top of the uneven force.
Plastic parts improper shape, with curved or asymmetrical shape.
Poor mold accuracy, positioning is not reliable, resulting in plastic parts easily warp deformation.
Improper feeding position, poor injection process parameters, so that the direction of contraction significantly, uneven contraction.
Flow direction and perpendicular to the direction of the molecular chain orientation differences, resulting in different shrinkage rate.
Bump wall thickness to asymmetric cooling, lack of cooling time, after cooling off improperly.
2, the impact of mold structure on the warpage of injection molded parts
In the mold design, the main factors that affect the warp deformation of plastic parts are three major systems, namely pouring system, cooling system and ejection system.
Gate design
Injection molding gate is the key part of the whole casting system, its location, form and the number of gates directly affect the filling of the melt in the mold cavity flow state, resulting in plastic curing, shrinkage and internal stress variation. Type of gate with side gate, point gate, submarine gate, sprue, fan gate and film gate.
The location of the gate should be chosen to minimize the plastic flow distance.The longer the flow distance, the worse the flow between the inner and outer layers, so that the internal stress caused by the flow and shrinkage between the frozen layer and the central flow layer Large, plastic deformation also will increase;
Conversely, the shorter the flow distance, the shorter the flow time from the sprue to the end of the flow of the part. The thickness of the frozen layer decreases during filling and the internal stress decreases and the warpage decreases.
Such as precision thin wall larger plastic parts, the use of a center gate or a side gate, because the radial shrinkage rate is greater than the circumferential shrinkage, molded plastic parts will have a greater distortion; if the switch to more points Gate or film-type gate, you can effectively prevent warpage, so the design must be calculated when the flow check.
When using point gate molding, the same due to shrinkage of plastic anisotropy, the gate location, the number of plastic parts have a great extent the degree of deformation.
The number of flat box-shaped plastic parts in the distribution of the number of different tests: The use of 15% glass fiber reinforced PA66, weighing 1450g plastic parts along the wall along the direction of flow along the wall with a lot of ribs using the same basic parameters. (A) sprue, (b) 5 to 4 sprue points, (c) 9 to 8 sprue points The results of the test, according to b to set the gate with the best results to meet the design requirements. c design of the gate is worse than the sprue, warpage deformation beyond the design requirements of 3.6 ~ 5.2mm.
Multi-gate can make the plastic flow ratio (L / t) shortened, so that the in-mold melt density and shrinkage more uniform at the same time, plastic parts can be filled with less injection pressure mold cavity, reducing the plastic molecular orientation Propensity to reduce internal stress and reduce the deformation of plastic parts.
Cooling system design
In the injection process, the plastic parts cooling rate will also form a non-uniform shrinkage of plastic parts, shrinkage difference that led to the generation of bending moments so that plastic parts warpage.
Such as precision flat large plastic shell mold cavity, core temperature difference is too large, cold cavity surface melt quickly cooled down, and close to the hot mold cavity material layer will continue to shrink, contraction is not Uniform plastic parts will warp.Therefore, the injection mold cooling system design should be strictly controlled core, cavity temperature balance.
The precision flat plastic shell pieces, forming shrinkage and deformation of the larger materials, production tests show that the temperature difference should not exceed 5 ° ~ 8 °.
Second, you should also consider the temperature of the plastic parts on both sides of the same, that is, to maintain the core, the cavity uniform temperature throughout the cooling rate of plastic parts around the equilibrium shrinkage, effectively prevent the deformation of the cooling system design Based on the theoretical calculation, the process should be rigorously determined by trial and error, therefore, the mold cooling water hole setting is essential.
After the pipe wall to the cavity surface distance is determined, the distance between the cooling water holes should be as small as possible, using uneven arrangement of the density, that is, the material temperature high cooling water hole density, the material temperature is low Cooling water hole row sparse some to maintain the cooling rate is basically the same.At the same time, as the cooling medium temperature increases with the cooling water channel length increases, the length of the cooling circuit waterway should not be too long.
Top of the body design
The design of the ejector mechanism also directly affects the deformation of the plastic parts.If the unbalanced ejector mechanism causes the plastic parts to be deformed due to unbalanced ejecting force, the ejection mechanism should be designed to balance the mold release resistance. Ram's cross-sectional area should not be too small, in case plastic parts per unit area is too large and produce deformation.
Push rod arrangement should be as close as possible to the mold release resistance large parts.For precision flat plastic molded case, as much as possible to set the top bar to reduce the deformation of plastic parts, The compound stripping mechanism.
The use of soft plastic to produce large deep cavity thin-walled plastic parts, due to mold release resistance larger, and the material is softer, if the full use of mechanical ejection, the plastic parts will be deformed, if the switch to multi-element Combined or gas (liquid) pressure and the mechanical ejection of the combination will be better.
3, filling cooling and warpage deformation
Under the action of injection pressure, the molten plastic is filled into the mold cavity and cooled and solidified in the cavity, in which the temperature, pressure and speed are coupled with each other, which has a great influence on the quality of the plastic part .
High pressure and flow rate will produce high shear stress, causing parallel to the flow direction and perpendicular to the flow direction of the molecular orientation difference, the formation of larger plastic parts stress.Temperature on the warpage deformation is mainly reflected in the following Aspects:
Plastic parts inside and outside surface temperature difference will cause thermal stress and thermal deformation;
Plastic parts of the temperature difference between different regions cause uneven shrinkage;
Different temperature conditions will affect the shrinkage of plastic parts.
Therefore, strict control of the appropriate injection process parameters is to reduce the warping deformation of the important means.
4, parts shrinkage and warpage
The direct cause of the warpage of an injection molded part is the non-uniform shrinkage of the molded part, and the shrinkage itself is not important for the warpage analysis, but the difference is important in the shrinkage. In the injection molding process, As the polymer molecules along the flow direction of the plastic in the flow direction of the shrinkage than the vertical shrinkage rate, leaving the injection molded parts have warping deformation.
Generally uniform shrinkage only caused by changes in the volume of plastic parts, only uneven shrinkage will cause warping deformation. Crystalline plastic shrinkage in the direction of the flow and vertical shrinkage than non-crystalline plastic, and its shrinkage is also Compared with non-crystalline plastic, crystalline plastic parts tend to be warped and deformed much more than non-crystalline plastics.
5, Residual thermal stress and warpage
In the injection molding process, the residual thermal stress is an important factor in causing warp deformation. Since residual thermal stress has a very complicated effect on the warpage of the part, the mold design is usually analyzed and predicted by the injection molding CAE software.
6, concluded
There are many factors that affect the warping deformation of precision flat and thin plastic parts. The structure of the mold, the thermophysical properties of the plastic material and the process parameters of the injection molding process all affect the warpage of the parts to varying degrees. The experimental study of the warp deformation mechanism of the workpiece must focus on a number of factors that should be considered comprehensively.