(65) Short cycle molding Injection Molding Process Defect Handling Manual (65 types)

The factors to be considered during short cyclization are as follows:
1. Resin
Fluidity, mold release, curing speed
2. Shape of formed product
Wall thickness (thin wall, uniform wall), release angle
3. Mold construction
Cooling circuit design, splitter/sprue size, gate mode
4. Forming conditions
Mold temperature, resin temperature
5. Molding machine, temperature control machine
Compound action, drying cycle, plasticizing ability, discharge ability
When considering short cyclization, it is necessary to first clarify which of these factors make the molding cycle unable to be shortened
In order to eliminate the tedious heat calculation, our company has developed a mold design system "FACE-2" 4) based on CAE (Computer Aided Engineering), and used this system to calculate the shortest molding cycle
Test and optimize the design of the cooling system to achieve short cycle molding.
When carrying out short cycle molding, it is necessary to consider the various factors mentioned above, but also consider the negative effects caused by the shortening of the molding cycle, and of course, consider the balance relationship with product performance requirements. The basic ideas introduced here are also applicable to general injection molding.
1. The resin is generally selected with a high fluidity grade, which is conducive to short cycle molding. The reasons are as follows:
(1) Generally speaking, molding shrinkage with high fluidity levels has a small anisotropy, and even within a short cooling time, molded products with small deformation or warpage can be obtained.
2) The filling speed to the mold can be accelerated, thereby reducing the filling time.
(3) Even if the resin temperature or mold temperature is lowered, it can be filled, so it can be cured and ejected in a relatively short time.
In terms of shortening the molding cycle, it often seems that little attention is paid to material selection, but when considering the constraints of shortening the molding cycle, it will be known that this is not the case. The constraints on the molding cycle include physical issues such as the drying cycle of the molding machine or the curing time of the resin. However, the actual situation is often that shortening the cycle can lead to deformation and cannot shorten the cycle.
Therefore, it is best to choose materials that are not prone to deformation when selecting materials. The above example is an example where using Duracon M90 cannot shorten the molding cycle and instead uses M270. For the M90, when the cooling time is less than 20 seconds, the surface runout increases, making it impossible to meet the 0.3 mm specification. For the M270, even if the cooling time is only 3 seconds, it can meet the specifications.
2. The shape design of molded products according to the required performance often brings many problems to the molding process due to excessive emphasis on performance. Therefore, when designing shapes, it is necessary to consider both function and formability. Figure 1-2 shows the relationship between the time required for the central solidification of the molded product and the wall thickness of the molded product. The wall thickness of a molded product is one of the most important factors affecting short cycle life. Within the allowable range of functional requirements, it is necessary to strive for thin-walling and homogenization (preferably considering deformation countermeasures in advance).
The constraints on the molding cycle include physical issues such as the drying cycle of the molding machine or the curing time of the resin. However, the actual situation is often that shortening the cycle can lead to deformation and cannot shorten the cycle. An important issue in shortening the molding cycle is to strive to shorten the cycle without deformation during shape design and runner and gate design.
The following two examples show that poor gear roundness (JGMA total meshing error) can lead to periods that cannot be shortened, but can be shortened by improving the shape and gate design.
Due to various factors intertwined in the mold structure, such as the phase transformation of resin and the three-dimensional conduction of heat, it is almost impossible to accurately grasp the heat conduction (resin to mold, mold to coolant) conditions that occur within the mold during injection molding.
However, if you take a more macroscopic view of this problem, you can use the following very simple formula to
Grasp the entire process. This formula uses the average value of mold temperature changes in a cycle to represent how much heat is transferred to the mold within an hour, and how much heat is absorbed through the cooling holes.
It can be seen from this formula that various data required to achieve a certain molding cycle can be roughly grasped, such as the distance from the mold cavity to the cooling hole, the inner wall area, and the flow rate of coolant.
(1) The formula represents the heat transferred from the resin to the mold.
(2) The formula represents the heat transferred through the cooling hole.
The total heat transfer coefficient can be expressed by the following formula:
Q1 and Q2: heat inflow and outflow; Cp: Specific heat of resin; 50: Latent heat of resin solidification; W: The weight of one injection; Tmelt: resin temperature; Teject: extraction temperature; Tmold: mold surface temperature; Tcool: coolant temperature; A: Cooling hole heat transfer area; U: Total heat transfer coefficient; h: The film heat transfer coefficient of the coolant; Hs: pollution coefficient of the coolant; x: The distance between the mold cavity and the cooling hole; λ： Thermal conductivity of the mold; Tc: molding cycle
In other words, to maintain a stable mold temperature, it is necessary to Q1=Q2, while to shorten the cycle, it is necessary to increase Q1. Therefore, it is necessary to try to increase Q2 by the same amount.
The method of adding Q2 can be considered from Equations (2) and (3). As an example of this, Figures 1-3 and 1-4 provide reference data on the film heat transfer coefficient of the coolant 1) and the effect of the distance between the mold cavity and the cooling hole 3).
If the cooling hole is far away from the mold cavity, the temperature of some parts will locally increase (upper left). Most of these parts will deform due to poor demolding, which cannot shorten the molding cycle. It can be seen that the cooling holes should be close to the mold cavity and the number should be more.
4. Molding Conditions One of the most effective methods for short cycle molding is to reduce mold temperature. However, the size and physical properties of the molded product will result in
Due to changes, it is necessary to pay attention when the dimensional accuracy requirements of the product are relatively strict or there is little room for strength.
As an example of this, Figure 1-5 shows the relationship between mold temperature and post shrinkage (the mold temperature referred to here means the surface temperature of the mold cavity during molding).
In addition, when shortening the molding cycle, it is sometimes necessary to pay attention to the molding conditions:
① Grasp and strictly observe the gate closing time
② Shorten the measurement time. ① The so-called "grasping and strictly observing the gate closing time" refers to correctly measuring the gate closing time and setting the pressure maintaining time to meet the requirements of the gate closing time. If the gate closure time is not met, serious problems such as reduced strength and increased dimensional changes can occur.
② In terms of "shortening the metering time", it is recommended to keep the barrel temperature of the second section from the hopper side consistent with the nozzle side, as excessive increase in screw speed can lead to the occurrence of non plasticization. Sometimes even without changing the screw speed setting, the metering time will be shortened.
5. The factor of molding machine or temperature control machine should also be taken into account during short cycle molding by molding machine or temperature control machine.
If plasticizing time is the main reason for shortening the cycle, it would be effective to use a molding machine that can perform a "composite action" (i.e., open the mold while the cooling time is over while plasticizing and metering are not yet complete).
If it is a forming machine that can complete a composite action, it can roughly save measurement time.
Ordinary forming machines cannot perform mold opening and closing and demolding actions within the metering time, while forming machines that can perform composite actions can manipulate the mold within the metering time. It can be seen that as long as the measurement is completed before the mold opening, demolding, closing, and closing forces are applied, the next cycle can proceed.
In order to use complex cooling holes to ensure a high coolant flow rate, it is best to use a thermostat with a large discharge capacity.