Why monitor the injection molding temperature? Here’s what to know.
Injection molding is a highly relevant and current manufacturing technique used extensively, from manufacturing personal items such as toothbrushes, toys, etc., to industrial components like plastics employed in automobiles, etc. It allows for the manufacture or mass production of hundreds of thousands or millions of identical components of a specific quality and dimension.
However, obtaining the desired or perfect result is almost impossible without controlled temperatures. This article covers injection molding and how important temperature control is to the entire process.
The Injection Molding Process
The injection molding process starts with melting plastic. The molten plastic is then injected into a specific mold and then left to cool in order to produce a hardened product that takes the shape of the mold.
Thermosetting polymers are sometimes used, but thermoplastic polymers – which consist of nylon, acrylic, and polycarbonate, as well as over 80,000 other polymers – are the most common material types used for this particular process.
Thermoplastics are those materials that turn to plastic as soon as heat is applied to them. They usually harden when cooled and can usually be subjected to the heating-molding-cooling cycle recurrently. This is the property that makes thermoplastics recyclable and versatile.
By comparison, thermosets like silicone and epoxy resins are permanently set as soon as they are heated. A chemical reaction that links the polymers into 3-dimensional structures causes this permanent change. This makes thermosets the go-to choice for high-temperature applications.
Thermosets are stronger than their counterparts, i.e., thermoplastics. But the former cannot be recycled and is even more brittle than thermoplastics.
Maintaining Appropriate Conditions during the Injection Molding Process
Up to 3 crucial process conditions must be in perfect order, or else obtaining an aesthetically acceptable and structurally sound component from plastic injection equipment is impossible.
They are:
- Pressure
- Time
- Temperature
Let’s take them one after the other.
- Pressure
Filling molds requires an incredible amount of pressure, the lack of which results in improper filling of molds. However, applying too much pressure during the injection molding process has adverse effects. Excessive pressure can bring about charred plastic, air bubbles, etc.
- Time
In order to achieve the appropriate or desired results for a molded component, there are a few points along the molding cycle that require precisely-set time. One of such areas includes the residence time, i.e., the amount of time molten plastic will remain inside the barrel of the plastic-injection equipment.
It is crucial to correctly set the residence time so that the molten material in the barrel is not heated extensively or for too long or removed too soon from the barrel. The injection mold machine will produce the desired results by setting the residence time correctly.
But insufficient residence time prevents the molten plastic from reaching the desired temperature. This makes the plastic harden prematurely as soon as it enters the mold cavity. It also leaves sinking spots where the plastic didn’t fill the mold cavity.
On the other hand, if the residence time is too long, the plastic gets burnt inside the barrel before it even reaches the mold.
- Temperature
Temperature significantly impacts each step of the entire plastic injection molding process. This is why temperature control is crucial and calls for the need to install a temperature controller on the injection-molding machine. This is one of the surest ways to ensure the injection-molding process is adequately maintained at the right temperature.
As soon as the plastic is introduced into the machine’s barrel, it is heated and mixed thoroughly. It is essential to set up the proper temperature profile as this ensures the thorough melting and mixing of the plastic without allowing it to burn.
The temperature profile is set to increase along the machine’s barrel towards the check valve. The highest temperature should be set immediately before the mold.
This gradient allows the gradual heating of the plastic, which ensures the smooth transition from solid-state to liquid while reducing wear and tear on the reciprocating screw.
The current temperature that the heating bands maintain depend primarily on the shape and size of the component being manufactured and the material being used. A standard range generally lies between 149 degrees Celsius and 426 degrees Celsius.
The maintained temperature within the mold is also as vital as the temperature profile inside the barrel. The mold is designed to maintain a lower temperature compared to the barrel in order to allow the plastic to cool.
This temperature is usually between 65 degrees Celsius and 177 degrees Celsius. It is controlled by running oil or water lines through the mold or by making use of electrical cartridge heaters.
The water cooling lines are employed for lowering temperature-cooling applications, while the heated oil lines are only utilized when the mold must be maintained at optimal temperatures.
On the other hand, electric cartridge heaters are a neat replacement for the typical heated oil lines. However, they have to be carefully positioned in order to prevent them from introducing uneven heat profiles around the injection mold.
The Importance of Temperature Control in Injection Molding
Excessive or inadequate temperature can bring about several flaws in the finished plastic component. It can even severely damage the injection-mold machine itself. The flaws and issues that develop depend primarily on the aspect of the entire process that is maintained with an incorrect temperature.
For instance, when the temperature at the rear zone of the barrel is too high, it can result in bridging between the barrel throat and the hopper. This is where a plastic cap turns solid at the point the material enters the barrel.
Furthermore, keeping the temperature of the barrel at a high level can cause the plastic to burn before it reaches the check valve. The burnt plastic manifests itself as black pieces of the finished component.
When the temperatures are too low during the injection-molding process, it can bring about wavy lines or flow marks that damage the surface of the manufactured component. A too hot mold causes the part to warp or blister the plastic. This leads to raised sections on the surface.
Inappropriate temperature levels can even truncate or compromise the life expectance of the plastic injection-molding machine.
Conclusion
As you can see, the importance of injection molding temperature control cannot be overstated. Inappropriate temperature levels during the molding process can mar the desired outcome in no small measure.
This is why it is crucial to identify and control the temperatures at every point throughout the plastic injection-molding machine.