Bubbles in injection molded parts are a common problem that must be solved.
This article explains three causes of bubble formation and provides solutions.
Unless necessary to achieve desired design, bubbles are not permitted in transparent parts.
Bubbles also weaken mechanical strength of part or reduce customer's specified weight, both of which should be avoided.
Bubbles in injection molded parts can be attributed to three factors: air, moisture, and vacuum.
Air
In barrel
In barrel, air is trapped between plastic particles. When plastic enters barrel from hopper, it is entrained with air. Appropriate backpressure compresses melt ahead of screw, crushing bubbles and preventing them from being injected through nozzle into mold cavity.
Simpler injection molding machine designs do not have backpressure gauges. Backpressure can only be measured by closing of flow control valve. However, backpressure is not linearly related to valve's rotation angle and can only be observed by observing screw's retraction speed.

 

Figure 1: Backpressure Formation
Injection molding machines equipped with backpressure gauges display pressure not melt pressure, but only injection cylinder pressure. There's a roughly tenfold relationship between the two. Some injection molding machines provide a chart of this relationship, attached to injection plate, which can be used to convert backpressure gauge reading to melt pressure.

 

Figure 2: Pressure Conversion
Inside Mold
Whether it's a thick-walled or thin-walled part, there's always some air in mold cavity. When air isn't exhausted from mold, it mixes with injected melt, forming bubbles.
Injection Speed
If injection speed is too high, such as when nitrogen is used to accelerate injection, air in mold cavity may not be able to escape quickly enough, becoming trapped and forming bubbles. For thin-walled injection molding, which requires an extremely high injection speed to fill cavity, venting, low clamping force, and vacuum are the only effective methods.
Venting Grooves
Molds have venting grooves engraved on parting surface, extending from mold cavity to perimeter of mold. Venting grooves have parameters such as width, depth, number of grooves.
Vent groove should be deep enough to allow air to escape, but not allow highly viscous melt to escape (which would otherwise cause burrs). Vent groove should be no deeper than 0.03mm and generally no wider than 6mm. Vent grooves should be spaced 25-50mm apart. Note that depth of vent groove is affected by clamping force.
Operator should set clamping force to a minimum, but sufficient level (not sufficient to prevent burrs), rather than using maximum clamping force. This will not only reduce vent groove compression, but also extend life of mold and injection molding machine's clamping mechanism (including hinge, hinge pin, hinge pin sleeve, tie rod, and platen), and shorten clamping time.

 

Figure 3 Vent Grooves
Breathable Steel
If a glossy finish is not required for product, breathable steel can be used for mold, utilizing micropores within steel for ventilation.
Vacuuming
In some closed areas or on cold runner, a vacuum point connected to a vacuum pump can be created to remove air from mold cavity during injection.
Vacuuming, exhaust troughs, breathable steel are mutually exclusive and cannot be used simultaneously; otherwise, vacuuming will fail.
Moisture
Plastic pellets absorb moisture from air, which must be completely removed to prevent it from being released during high-temperature heating (>1000℃) and entering product.
Drying temperatures and times vary depending on requirements of each plastic. Please refer to table below.

 

Table 1 Drying Temperatures and Times for Different Plastics
Drying hopper draws air from atmosphere, heats it to a drying temperature, and flows upward through plastic in hopper before being discharged back into atmosphere through top.

 

Figure 4 Drying Hopper
Drying conditions in table above are based on an ambient temperature of 200℃ and a relative humidity of 65%, using a high-efficiency turbine to generate airflow. Moisture content of dried plastic is less than 0.02%.
For example, in late spring in South China, where relative humidity exceeds 90%, drying efficiency is poor. Following solutions can be used to address this issue.
Drying time
Extending drying time is an easy-to-understand method. Hot airflow has more time to remove moisture attached to plastic particles, making plastic drier. Increasing hopper capacity can extend drying time.
H = 3.6s*t/c (1)
H = hopper capacity, kg
s = weight of each shot (per beer), including water, g
c = cycle time, seconds
t = drying time, hours
Hopper capacity
Specifications of hopper are indicated by capacity, and there are several types as follows. To simplify calculation, supplier provides following selection guide.

 

Table 2 Supplier's guide to selecting hoppers
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 in order to properly dry plastic. Otherwise, when plastic in hopper is exhausted, plastic near outlet will flow into barrel before it dries for a long time, and moisture cannot be removed.
Hopper capacity calculation example
For injection molding of 20g PET preforms with 32 cavities and a cycle time of 24 seconds, what size drying hopper is required?
Based on Table 1, PET material needs to be dried at 160℃ for 4-5 hours.
From formula (1),
H = 3.6*32*20*5/24 = 480 kg
After looking up hopper capacity in Table 2, a 500 kg drying hopper was selected.
Calculation of Table 2
Assuming that only 80% of injection molding machine's injection volume is used, recommended values in Table 2 represent
t / c = 0.8H / (3.6*s), which is calculated to be between 0.119 and 0.033, that is: drying time, hours = (0.033-0.119) * cycle time, seconds.
Refer to Table 3.

 

Table 3: Ratio of drying time to cycle time.
For the example of preforms, maximum drying time is only 0.119*24 = 2.9 hours, 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 480 kg hopper calculated in previous example.
This indicates that selection in Table 2 is too small in some cases, which may be one of reasons for formation of bubbles. Selection based on formula (1) is more reliable.
Dehumidification dryer
Increasing hopper capacity to increase drying effect is still difficult to ensure dryness of plastic. Reason is that how much increase in atmospheric humidity is compensated by how much increase in drying time? Moreover, atmospheric humidity changes every day, and drying for too long also wastes energy.
Dehumidification dryer can ensure dryness, and it is not related to atmospheric humidity.
A dehumidifying dryer is used in conjunction with a drying hopper. Moist airflow from drying hopper enters dehumidifying dryer. After filtration and cooling, airflow is absorbed by molecular sieve in rotating honeycomb and then returned to air intake of drying hopper. This creates a closed system, unaffected by atmospheric humidity. Molecular sieve in honeycomb is dehydrated and regenerated by an independent, open-air airflow.

 

Figure 5: Dehumidifying Drying Principle
Honeycomb dehumidifying dryer produces a dry air dryness (also known as absolute humidity) reaching a dew point of -40℃, equivalent to a relative humidity of 0.60% or a moisture content of 0.013%, or 128 ppm. Drying capacity of a dehumidifying dryer is measured in kilograms of a specific plastic per hour and serves as a selection criterion.
Two-Stage Drying
Honeycomb dehumidifying dryers are not cheap. Some companies use two-stage drying hoppers to achieve better drying results than a single drying hopper.

 

Figure 6: Two-stage drying
Drying temperature
Plastic suppliers all have recommended drying temperatures. While increasing drying temperature can indeed improve drying results if drying time is constant, excessively high drying temperatures can cause components to become cloudy, affecting color, transparency, and mechanical properties.
Vacuum
Surface sink marks are common when injection molding thick-walled parts. Sink marks are caused by shrinkage as plastic cools from a molten state to a solid state. These can be avoided with proper holding pressure parameters and runner design.
When surface of a thick-walled part has cooled and solidified, but interior is still fluid, shrinkage can only occur within it, creating "bubbles." These "bubbles" contain neither air nor moisture, only vacuum. Method for eliminating these bubbles is same as for sink marks.
If diameter of cold runner is comparable to maximum wall thickness, holding pressure can fill part with plastic through unsolidified runners, eliminating "bubbles."

 

Figure 7: Vacuum "Bubbles"
How to Identify
Three types of bubbles have different causes and require different methods to eliminate. So how do we distinguish which type of bubble is which?
If plastic is transparent or translucent, following methods can be used to identify cause of bubbles.
Number
Air and water bubbles are more numerous, while vacuum bubbles are confined to the thickest area, with fewer or only one.
Location
Location of air and water bubbles is random, varying across several products. Vacuum bubbles are uniformly located in the middle of the thickest area, and their size is nearly uniform across each product.
Expansion by Heating
Air and water bubbles expand upon heating, softening product. Vacuum bubbles, however, do not expand; instead, they shrink or their outer walls sag. This can be determined by placing product under a graduated optical instrument and observing it before and after heating.
Shape
Air and water bubbles are spherical, but vacuum bubbles are not always.