A mold is a tool that imparts a fixed geometry to finished product to repeat mass production of finished product. In addition to product shape and flesh thickness design, it is also necessary to consider gate position, flow path arrangement, cooling line configuration, slider, thimble, mechanism and so on.

Traditionally, in injection mold design, first consideration is mechanism factor, parting surface, axial position, slider thimble, gate position, flow path arrangement, etc., and finally how plastic should be filled. But usually mold designer does not have concept of molding itself, don't know how much pressure difference can fill whole set of mold holes; in order to ensure formability, generally adopt a more cautious and conservative way - increase flow area, increase number of gates, etc.; when there is a molding problem, there is another gate to be opened in places where plastic cannot run... This phenomenon is still common today in Taiwan.
To get a good finished product, you need to understand flow behavior inside mold: Plastic melts into a liquid at high temperature, and injection machine gives a pressure. Hot melt will flow from high pressure to low pressure due to pressure difference. Speed on both sides of mold wall is slower than center, which is caused by friction in opposite direction.

 

In fact, in injection filling phase, it can still be subdivided into two parts:
1. Flow: Plastic flows out of nozzle and is injected into cavity through main runner to runner and gate. Moment from start of flow to moment just filling cavity is regarded as flow phase.
2. Holding pressure: When entire mold cavity is completely filled, due to compressibility of plastic melt, when pressure from shooting screw continues to increase, this pressure rise can be filled with an additional 15% more material. After that, due to volume shrinkage caused by cooling, new section of plastic is filled at any time to compensate for volume of shrinkage reduction. Under normal circumstances, an additional volume of about 25% can be filled, which is approximately equal to volume of molten thermoplastic and cold solid plastic.

 

Traditionally, before a plastic product is produced, shape designer first draws an abstract shape, and manually molds clay model. After determining size, mold designer will then submit finished product drawing and drawing of mold, and then finish processing of mold.
Next, maybe three months later for mold trial, trial results are often found to be molding problems, need to repair mold repeatedly. If repair is not good, you need to make up mold. When you try a product, sometimes mold is already riddled with holes; Manpower and time wasted by intermediate process is hard to estimate! And our hope is to get a good product with a tryout.
Why do you need a mold trial? --Because no one can accurately predict shape of plastic inside the mold, and this part of factor determines quality of product. When product obtained is not as expected and defective, it can only be modified to obtain a better product.
What we can control is operating parameters on injection machine, such as temperature, rate of fire, pressure, time, etc.; when finished product is finished, we expect a good product. Surface is beautiful, there are no molding defects, size Stability, no deformation of depression, etc. But what happened in the black box that is not visible inside mold? This part of intermediate process is not known to us. Why are some products of good quality and some have high defect rates?
In order to achieve best design, designer should first understand molding situation inside mold, and take into account relevant factors before mold design to obtain a good finished product; CAE is an effective tool to interpret all information inside mold.

Flow analysis can help us understand molding of plastics inside cavity to find out proper molding conditions and produce good products. Cooling analysis is to design effective cooling pipelines and control cooling conditions to shorten molding cycle. For most products, cooling time is about 70%. If you can reduce this part of time loss, it will greatly increase productivity.
Before you can design an effective cooling, you should first understand concept: Mold can be regarded as a heat exchange system. Heat source is heat brought in by molten thermoplastic, then lost in several ways, finished product is cooled and solidified.
Cooling mechanism is mainly:
1. Metal template heat conduction.
2. Convection and very small amounts of radiation are scattered into atmosphere.

 

Flow pattern of cooling water in the pipeline can be divided into laminar flow and turbulent flow, and turbulent flow can effectively take away heat, so coolant flow rate needs to be controlled within turbulent flow range. Figure below shows temperature gradient between liquid in tube and wall of metal mold during laminar flow and turbulent flow. As can be seen from left figure that mold wall temperature is higher during laminar flow, which is a poor design method.

 

Cooling rate is slow, which affects internal molecular alignment and crystallinity. Uniform cooling, which produces uniform shrinkage, is a ideal design. However, due to geometric asymmetry, different heat concentration zones are easily formed inside and outside product, and zone is cooled at the slowest speed, which also causes a large shrinkage and warpage of finished product.

 

Effective area covered by each cooling waterway should be considered. For example, left side of figure below, effect of two large diameters on same plane will not be evenly distributed than five small tubes on the right side of figure.

 

Similarly, same pipe diameter, but also degree of heat concentration and position adjustment, T-shaped object in figure below, intersection of two sides is heat concentration point, so back of rib is location of more heat, where waterway should be closer; and at inner two corners, it is suitable to add two small tubes to achieve uniform cooling.

 

Arrangement of water flow has two basic types: series and parallel. However since flow in parallel mode is dispersed, efficiency is reduced, series is generally used.

 

For some long and deep areas, where water pipe can be processed, pipe or baffle (bubbler & buffle) is used to increase contact area to improve cooling efficiency.

 

The latest cooling modules take into account detailed mechanisms such as parting surfaces, inserts, and mold base sizes.

 

Before applying cooling analysis, some basic physical terms should be understood first:
Heat: Calorific value required to raise one gram of water by 1℃ is 1 calorie (Cal). Heat required to raise one kilogram of water by 1℃ is 1 kcal. Specific heat: Calorific value of water is set to 1 Cal/g℃, then calorific value of other substances, ratio of other substances to water is called specific heat, which shows that energy required to raise one gram of Celsius by 1℃.
Heat capacity Cp (heat capacity) [Cal / cm ^ 3℃] = specific gravity ρ, [g / cm ^ 3] x specific heat [Cal / g℃], this figure indicates energy contained in 1 unit volume of substance at an elevated temperature of 1℃. Under same volume, substances with high heat capacity have higher energy absorption when same temperature difference is raised. When considering different materials, such as steel, plastic, coolant, etc., because specific gravity is different from heat, heat capacity is different, and temperature is different.
In order to fully consider heat transfer between components, in addition to heat capacity (how much energy is absorbed), heat transfer rate (thermal conductivity) needs to be considered to calculate optimal cooling design and control parameters. Thermal conductivity: [W/M/℃], which indicates ability of a material to conduct heat. A substance with a large thermal conductivity absorbs heat and dissipates heat quickly.
It is really necessary to determine heat transfer characteristics of different materials in the whole set of molds. One is heat capacity, and the other is heat transfer coefficient. Here, another parameter is defined to indicate relationship between these two: heat diffusivity = Thermal conductivity / heat capacity (α = κ / ρCp). A substance having a large thermal diffusivity indicates that heat conductivity is large (heat dissipation is fast) and heat capacity is small (less heat absorption), and heat can be efficiently dissipated. For example, thermal properties of water and plastic, mold steel are:

Ratio of thermal diffusivity, iron and plastic is about 0.517/0.0003=172, which is 172 times different.

Warpage is mainly caused by uneven shrinkage. Finished product with uniform shrinkage is only small in size, and if it is not uniform, finished product is distorted. In a finished product, there may be several types of shrinkage changes:
1. Region to region: Plane position is different.
2. Through thickness: different thickness positions.
3. Directionality: Parallel/Vertical Molecular Direction
Factors that basically affect contraction are:
1. Free Volume Shrinkage: This is data from P-V-T experimental measurement.
2. Crystallinity: Material undergoes phase change during crystallization process, and material with high crystallinity shrinks more.
3. Mold limitation: If mold wall is free from plastic shrinkage, shrinkage will be small.
4. Molecular alignment: Directionality produced during flow process. If not released, will have different shrinkage values parallel to main alignment and vertical direction of molecule.

 

In order to control warpage, we must first understand operating parameters that affect shrinkage rate:

 

Main factors that summarize warping can be divided into three categories:

Cooling effect is affected. Cold surface will shrink first, but it will solidify quickly and shrinkage will be fixed. However, hot surface will shrink slowly, molecules will be rearranged for a long time, and shrinkage will be larger.

 

Varies with thickness or location, depending on pressure holding efficiency.

 

Gate location, flow direction, etc. are main factors. For example, following example is a central feeding disc, and flow direction is mainly radial radiation direction. When radial contraction is larger than vertical direction, product will have a saddle-like warp on the image; if contraction of vertical direction is greater than radial direction, then bell-shaped warp in the figure occurs.

 

Shrinkage analysis - read flow and cooling analysis results, match physical properties of material to calculate finished product:
a. Shrinkage in all directions
b. Molecular alignment (ORIENTATION)
c. Area shrinkage rate
d. Volume shrinkage
e. Perform univariate analysis
Warping - Read results of shrinkage analysis, define boundary conditions, and calculate warp in all directions due to uneven shrinkage. Detailed functions are subdivided into linear, nonlinear, buckling analysis and univariate analysis.
Simulate possible warped shapes
Diagnose to find cause of warpage·
Logical judgment, providing solutions
Reduce warpage of finished product with optimal filling, pressure keeping, cooling and other operating conditions
Reduce warpage by changing thickness of material and reinforcing it
Warping type and calculation method can be further divided into linear and nonlinear; linear relationship is that warpage value is linearly proportional to shrinkage. Nonlinear relationship is due to material buckling and permanent deformation, deformation amount is nonlinearly proportional to shrinkage load, which needs to be calculated by different numerical solutions (large displacement deformation).

 

Calculate warpage value in software, firstly, buckling analysis calculation is needed to obtain eigenvalues, determine deformation shape is linear or nonlinear. Eigenvalue is a proportional value indicating that buckling occurs under contraction load at that time; when eigenvalue is greater than one, it means that product does not buck within whole (100%) load shrinkage range, and calculation of warpage only needs to be described in a linear scale. If characteristic value is less than one, product will buck within contraction load and produce nonlinear deformation.
Bend analysis: should be judged as linear or nonlinear

 

Small displacement analysis: calculating linear deformation

 

Large displacement analysis: calculating nonlinear deformation

 

Constrain: Different limit point selection methods will affect warped shape