Air bubbles in product during injection molding are a common problem to be solved.
This article describes three causes of bubble formation and provides solutions.
Unless it is to achieve design effect, transparent products are not allowed to contain air bubbles.
Air bubbles also reduce mechanical strength of product, or product weight specified by customer, which is to be avoided.
There are three causes of bubbles in injection molded parts: air, moisture and vacuum.

Inside barrel
In barrel, there is air between plastic particles. When plastic is plasticized, it enters barrel from hopper, and air is brought in together. Appropriate back pressure compresses melt in front of screw, bubbles are crushed, and they are not injected into mold cavity through nozzle.
Simpler injection molding machine designs do not have back pressure gauges. Back pressure can only be measured from closing of flow control valve, but back pressure has no linear relationship with valve rotation angle, and can only be observed from screw retreat speed.

 

Figure 1 Formation of back pressure
For an injection molding machine with a back pressure gauge, read pressure is not melting pressure, but pressure of injection cylinder. There is about a 10-fold relationship between two. Some injection molding machines graph this relationship, which is attached to injection baffle, and can be used to convert read back gauge pressure to melt pressure.

 

Figure 2 Pressure conversion
Inside cavity
Whether it is a thick-walled or thin-walled product, there is more or less air in mold cavity, and when it is not discharged out of mold, it is mixed with injected melt to form bubbles.
Injection speed
If injection speed is too high, if nitrogen is used to accelerate injection, air in mold cavity may not be discharged in time, trapped in mold and form air bubbles. If thin-wall injection molding requires a very high rate of fire to fill cavity, it can only be done in exhaust groove, low clamping force and vacuum.
Exhaust slot
Mold is engraved with exhaust grooves on parting surface, extending from mold cavity to periphery of film. Exhaust slot has parameters of width, depth and number of strips.
Depth of vent groove only allows air to escape, and does not allow leakage of melt with high viscosity (otherwise burrs are formed). Depth of exhaust groove is not more than 0.03mm, and width is generally not less than 6mm. Exhaust grooves are opened every 25-50mm. Note that depth of vent groove is affected by clamping force.
Operator should set minimum but sufficient (no burr) clamping force, instead of using full clamping force, so that not only exhaust groove will be flattened less, but also life of mold and clamping mechanism of injection molding machine (including machine hinges, hinges, hinge sets, tie rods and templates) will be prolonged.

 

Figure 3 Exhaust slot
Breathable steel
If appearance of product does not need gloss, breathable steel can be used as mold, and micropores in steel can be used to exhaust.
Vacuum
In some stable closed places or cold runners, open a vacuum point and connect it to a vacuum pump to extract air in mold cavity during injection.
Vacuuming is mutually exclusive with exhaust groove and breathable steel, and two cannot be used at the same time, otherwise vacuum will not be able to be pumped.

Plastic particles absorb water from air, and they must be removed from bottom to prevent them from being released after being heated at high temperature (>1000C) and running into product.
According to requirements of various plastics, drying temperature and time are different. Please refer to table below.

Table 1 Drying temperature and time of different plastics
Drying hopper draws air from atmosphere, heats it to drying temperature, flows through plastic in hopper from bottom to top, and then discharges it back to atmosphere from top.

 

Figure 4 Drying hopper
Drying conditions in the above table are under atmospheric temperature of 200C and relative humidity of 65%, using a high-efficiency turbine windmill to generate airflow, and moisture content of plastic after drying will be less than 0.02%.
For example, in late spring season in South China, when relative humidity exceeds 90%, drying effect is inferior, and following methods can be used to solve it.
Drying time
Extending drying time is an easy-to-understand method. Hot air will have more time to take away water attached to plastic particles, and plastic will be drier. A larger hopper capacity extends drying time.
H = 3.6s*t/c (1)
H = hopper capacity, kg
s = weight per shot (per beer), water inlet, g
c = cycle time, seconds
t = drying time, hours
Hopper capacity
Specifications of hopper are indicated by capacity, and there are following types. To simplify calculations, suppliers have one of following selection guidelines.

Table 2 Guidelines for selecting hoppers for suppliers
It should be noted that hopper must be equipped with a suction machine to continuously replenish used plastic and maintain a constant amount of plastic in hopper, so that plastic can be dried locally. Otherwise, when plastic in hopper is exhausted, it will be added, plastic near outlet will run into barrel before it dries, and moisture will not be eliminated.
Example of hopper capacity calculation
Injection molding of 32-cavity 20-gram PET preforms takes 24 seconds, how much drying hopper is needed?
Look up table 1, PET material needs to be dried at 1600C for 4~5 hours.
From formula (1),
H = 3.6*32*20*5/24 = 480kg
After looking up hopper capacity in Table 2, a drying hopper with a capacity of 500 kg was selected.
Calculation of Table 2
Assuming that only 80% of injection volume of injection molding machine is used for injection molding, recommended representatives in Table 2
t / c = 0.8H / (3.6*s), it is calculated from 0.119 to 0.033, that is:
Drying time, hours = (0.033~0.119)*cycle time, seconds.
Refer to Table 3.

Table 3 Guided drying time to cycle time ratios
Taking preform as an example, drying time is only 0.119*24 = 2.9 hours at most, which is not enough for 4~5 hours required in Table 1.
From another perspective, 32*20 g / 0.8 = 800 g, according to Table 2, a 100 kg drying hopper is selected, which is much different from the 480 kg hopper calculated in previous example.
This points out that selection of Table 2 is too small in individual cases, which may be one of reasons for generation of bubbles. It is more certain to choose with formula (1).
Dehumidifying Dryer
It is still difficult to ensure dryness of plastic by increasing hopper capacity to increase drying effect. Reason is how much atmospheric humidity increases and how much does drying time increase to compensate? Moreover, humidity of atmosphere changes every day, and drying for too long is a waste of energy.
Dehumidifying dryers can ensure dryness independent of atmospheric humidity.
Dehumidifying dryer is used together with drying hopper. Moisture-laden airflow discharged from drying hopper enters dehumidifying dryer. After filtering and cooling, moisture in airflow is absorbed by molecular sieve in rotating honeycomb, then sent back to suction inlet of drying hopper. In this way, airflow is a closed system, unaffected by humidity of atmosphere. Molecular sieves in honeycomb are regenerated by removing water from a separate air stream in contact with atmosphere.

 

Figure 5 Principle of dehumidification and drying
Dry air dryness (also known as absolute humidity) produced by honeycomb type dehumidifying dryer reaches dew point of -400C, which is equivalent to a relative humidity of 0.60% or a moisture content of 0.013% or 128 ppm. Drying capacity of dehumidifying dryer is calculated by how many kg of a certain plastic can be dried per hour, which is standard for selection.
Two-stage drying
Honeycomb dehumidifiers are not cheap. Some manufacturers use two-stage drying hoppers to achieve better drying effect than a single drying hopper.

 

Figure 6 Two-stage drying
Drying temperature
Plastic suppliers have recommended drying temperatures. If drying time is constant, increasing drying temperature can indeed improve drying effect, but too high drying temperature will make ingredients in it muddy, affecting its color, transparency and mechanical properties.

Surface dents are encountered when injection molding thick-walled products. Dent marks are caused by shrinkage of plastic as it cools from a molten state to a solid state. It can be avoided if pressure holding parameters and runners are properly designed.
When surface of thick-walled product has cooled and solidified but inside is still fluid, it can only shrink inside, which is called "bubble". There is no air or moisture in "bubble", only a vacuum. Exclusion method is same as for dents.
If diameter of cold runner is similar to maximum wall thickness, holding pressure can fill product with plastic through runner that has not yet solidified, and eliminate "bubbles".

 

Figure 7 Vacuum "bubbles"
How to tell
Causes of three types of bubbles are different, and methods of elimination are also different. How can we tell which type of bubbles it is?
If plastic is transparent or translucent, following methods can be used to identify cause of bubbles.
Number
Number of air and water vapor bubbles is large, but vacuum bubble only exists in the thickest part, and number is small or only one.
Location
Positions of air and moisture bubbles are random, and in several products, bubbles have different positions. Position of vacuum bubbles is in the middle of the thickest part, which is not biased, and bubble size of each product is almost same.
Heating surge
After air and water bubbles are heated, product softens, bubbles will expand, but vacuum bubbles will not, but shrink, or outer wall sags. Product can be observed before and after heating under a graduated optical instrument.
Shape
Air and water vapor bubbles are spherical, but vacuum bubbles are not necessarily.