Firstly, the cooling time is the length of time the casting solidifies in the mold cavity until it is mechanically stable enough to remove. We are going to read best tips from popular Plastic molding company in 2022.
Basically, criteria for the necessary stability are the deformation and dimensions of the mold. The practical countdown of the cooling time begins with the completion of the volumetric filling of the mold and ends with the opening of the punch.
Product cooling time estimation
Moreover, the calculation of the physical cooling time determines by the wall thickness of the cast part, the temperature of the mold walls, as well as the thermal conductivity and specific heat of the melt. If the mold wall temperature changes by 10°C, the cooling time changes by 20%. Similarly, a change in melt temperature of 10°C entails a change in cooling time of approximately 3%.
How can we set cooling time?
Secondly, the cooling time set in the injection molding machine is the physical cooling time reduced by the make-up (pressure holding) time. The heat from the polymer melt that injects into the mold cavity must remove before the cooling time has elapsed so that the casting is reliably removed from the mold.
Moreover, it is important that the entire surface of the plastic product has approximately the same temperature, and thus, with a short cooling time, the smallest deformation due to uneven shrinkage achieves.
Formula for calculating the cooling time
Excessively long cooling times in injection molding unnecessarily increase the cycle time. Insufficient cooling time manifests by deformation of cast products, increased subsequent shrinkage and war page when the casting is removed from the mold.
For the successful operation of the injection molding machine and product quality, strict adherence to injection molding technology is necessary. Deviation from the technology will degrade the properties of the product, disrupt operation and accelerate the wear of the injection molding machine or mold. Process optimization bases on three components:
Injection molding machine is working
Plastic molding company in 2022 has a unique molding machines and product quality. As well as, the mold cools down and fixes to the machine with suitable locking force, injection volume, screw diameter and mouthpiece.
The raw material mixes with an acceptable proportion of regenerate and pre-dried.
Adjustment of parameters begins with setting the course of plasticization and decompression; temperature of the material cylinder and the hot runner part of the mold; speed, injection and make-up pressure; jet pressure and the number of revolutions of the screw. The holding time under pressure and cooling are successively set, plus the mold closing force.
Let us explain the universal nuances of adjustment, which uses by a professional.
INJECTION SPEED should be stepwise if possible
First, the melt injects into the mold without much effort. It is enough to set a low speed, thereby avoiding defects in the sprue area. Then, the main segment recommends injecting quickly. Otherwise, the mold cavity will not fill without soldering seams and under filling. At the end of the flow path, it is prudent to reduce the injection speed again.
Allow the gas to escape from the mold in time and prevent the auger from "jumping". This theory is suitable for most Plastic molding company in 2022 products. Too slow forward movement of the auger creates a reverse flow in the material cylinder.
The locking sleeve will not have time to stop the flow of the melt back. As a result: dose fluctuation (under filling, flash).
Boost Injection pressure
It is initially set between 40 and 60 percent of the actual injection pressure. The ability not to push through the residual cushion of the material deserves the highest rating, and to collectively obtain a product without under filling, tightening. Switching to holding under pressure begins when 95 percent of the dosage presses into the mold.
It is expedient to divide the holding time under pressure into parts. In the main period, it is necessary to saturate the casting with pressure that eliminates depressions.
After the residual cushion stops decreasing, it is desirable to reduce the feed pressure in order to prevent over consolidation of the casting and reduce water hammer. The final phase ends with a pressure close to the screw head pressure. Therefore, we recommend hiring best Plastic molding company in 2022.
Having set up injection molding in this way, you will receive consistently high-quality plastic products and long-term operation of the equipment.
Parameters for injection molding
In addition, a high-quality plastic injection molding process can only ensure by the proper operation of all production links. However, some difficult-to-explain defects are only due to injection molding parameters or mold design. Modern research helps to understand the essence of what is happening with the melt along the flow path.
For example, for many years it was erroneously assumed that vacuum cavities (shells) inside thick-walled plastic products are air residues. From now on, the reason for the formation of such shells is explained by the shrinkage stress, which pulls the material from the hot core of the casting to the cooled outer sides.
How to remove the defect near the sprue?
As well as, another hard-to-remove defect is a matte halo near the sprue inlet. On products that inject from the side, the design of the mold should provide a small radius of transition from the sprue to the product. At this point, the melt velocity is very high, and then the annular flow front slows down.
Eventually, a uniform flow rate can be achieved by staging the injection: the speed is slow at first, then faster. Alternatively, it remains to muffle one of the sprue inlets.
A well-known defect called a "gramophone record" eliminates by thermo stating the mold. Too much resistance to polymer flow creates in the cavity.
The surface layer hardens very quickly.
Insurmountable ejector defect
Lastly, one more defect from the "insurmountable" category is the prints from the ejector. If the efforts to extract the casting from the mold are too large, and the ejector end area is small, then the pressure on the surface in this place is too high.
White bumps appear. In addition to reducing the holding pressure, a linear slope and air valves in the rod should provide.
In order for the elimination of defects not to turn into a struggle with additional costs from marriage, the service technician should not ignore the reference information.
