In injection molding, bubbles are not a common defect in general small workshops. Bubbles mainly occur in non-large transparent products; small transparent products are not prone to bubbles, while non-transparent products typically do not have specific requirements. Today, we will focus on bubbles in injection molding as we continue our exploration of molding defects.
Bubbles form when gases in the mold cavity are trapped during the flow of the molten plastic. If these gases are not properly vented, they remain trapped, forming bubbles or resulting in voids where the cavity is not fully filled. This affects the subsequent molding effect and quality of the product.
For technicians, addressing bubbles in products often requires more than just reducing speed and melt temperature. Adjusting transparent products is one of the most challenging defects in injection molding.
The most common 2 types of bubbles
There are two causes of bubbles in the plastic injection molding process: one is vacuum voids and other water vapor.
(1) Bubbles due to the presence of moisture
Cause: insufficient drying of plastic material and cause plastic material for hydrolysis.
Solution: Adequate pre-drying pay attention to the hopper insulation management.
(2) Vacuum bubble
Causes of vacuum bubbles:
1. The thick-walled part of the material flow freezes quickly, the shrinkage is prevented, and the mold is not filled enough to produce a vacuum bubble in the injection molding process parameters.
2. The melt temperature setting of the injection mold is not suitable. The barrel temperature is not suitable, and the injection pressure and holding pressure are not enough.
Solutions:
1. Avoid uneven wall thickness structure of plastic product design. Fix the gate position so that the plastic flow material is injected vertically into the thick wall part.
2. Increase the temperature of the injection mold. Lower the barrel temperature. Increase the injection pressure and pressure-holding pressure.
What causes bubbles in injection molded products?
Injection machine:
1. Low injection pressure or hold time
Remedies: Increase the injection pressure and/or the hold time to help force the gases out as the plastic is pushed into the cavity.
2. Insufficient material
Remedies: It is important to establish a feed setting that allows a 1/8'' to 1/4'' cushion of material at the end of the injection stroke. Without this cushion, there is no material against which holding pressure can be applied to force material into the cavity.
3. Improper injection temperature profile
Remedies: Maintain a proper temperature profile. This is readily obtained from the resin supplier, but a rule-of-thumb sets the temperature controls at increments of 50 to 100 degrees F from rear to front, and the nozzle at the same temperature as the front zone. An air shot from the nozzle should produce a bubble-free stream of plastic that has the approximate consistency of warm honey. Remember that the temperature control settings are not the same as the actual temperature of the plastic. They are usually 50 to 100 degrees higher than the actual plastic temperature to accommodate the rapid travel of material through the barrel.
4. Excessive injection speed
Remedies: Start with the supplier's recommendations for injection fill speed. Adjust up or down according to the results. If bubbles appear, slow down the rate. If short shots appear, speed up the rate.
5. Insufficient back pressure
Remedies: Increase the back pressure. Most materials will benefit from a back pressure that is approximately 50 psi. But, some materials require higher settings: in some cases up to 300 psi. However, be cautious, because too high a back pressure will degrade any material. The material supplier is the best source of information regarding proper back pressure settings.
Injection molding Mold:
1. Improper venting
Remedies: Vent the mold even before the first shot is taken by grinding thin (0.0005''-0.002'') pathways on the shutoff area of the cavity blocks. Vents should take up approximately 30% of the perimeter of the molded part. Vent the runner, too. Any air that is trapped in the runner will be pushed into the part. Another rule-of-thumb is to place a vent at every 1-inch dimension around the perimeter of the cavity. You cannot have too many vents.
2. Section thickness too great
Remedies: A good rule-of-thumb is that any wall thickness should not exceed any other wall thickness by more than 25%. There will be little tendency for bubbles at that ratio. Metal inserts can be used to core out sections that do not meet that ratio, or “overflow” wells might be used to move the voided area off the primary part surface. However, the overflow would then need to be removed from the molded part.
3. Improper runners or gates
Remedies: Gates should be of a depth that is equal to at least 50% of the wall they are placed at and should always be located to flow material from the thickest section to the thinnest. Runner diameters should be adequate to avoid a pressure drop as the material fills. Thus, the farther the travel, the larger the initial runner diameter should be. Gates and runners should be machined in the mold to be ``steel safe'' so they can be increased by removing metal. It is a good practice to place gates and runners in individual inserts so they can be easily replaced and/or reworked.
4. Low mold temperature
Remedies: Raise the mold temperature in increments of 10 degrees F until the bubbles disappear. Allow 10 cycles for each 10-degree adjustment (up or down) for the mold temperature to stabilize.
Injection molding process
(1) The material temperature is too high, causing decomposition. The barrel temperature is too high or the heating is out of adjustment, and the barrel temperature should be reduced section by section.
The temperature of the charging section is too high, so that part of the plastic melts prematurely and fills the screw groove, and the air cannot be discharged from the charging port.
(2) Small injection pressure, and short holding time, so that the molten material and the surface of the cavity are not close to each other.
(3) Injection speed is too fast so that the molten plastic is decomposed by large shear, resulting in decomposition trapped gas; injection speed is too slow, can not fill the cavity in time to cause the product surface density is not enough to produce silver lines.
(4) Cooling time is too short (especially for thick-walled products), the solution can be outside the machine with cold water and cold pressing mold for cooling.
(5) The product has the phenomenon of decomposition black line, which is formed due to the high temperature. If the situation is not serious, the temperature can be properly reduced, if the entire barrel decomposes, it must be disassembled and cleaned.
Raw material
(1) Raw materials mixed with foreign plastic or granules mixed with a large amount of powder melt easily in entrapped air, and sometimes silver lines will appear. Raw materials are contaminated or contain harmful crumbs when the raw materials are easily decomposed by heat, poor melt flow pattern is a major cause of bubbles.
(2) Recycled material grain structure is loose, the amount of air stored in the microporous; recycled material regeneration too many times or too high with the proportion of new material (generally should be less than 20%)
(3) Raw materials containing volatile solvents or raw materials in the liquid additives such as dyeing aid white oil, lubricant silicone oil, plasticizer dibutylester and stabilizer, antistatic agent, etc. too much or mixed unevenly, to accumulate state into the cavity, the formation of the silver pattern.
(4) Adding too much or too little material, and the injection pressure is too high, will also produce this phenomenon.
By carefully controlling these factors and implementing appropriate measures, such as ensuring proper venting, optimizing injection parameters, and using high-quality materials, manufacturers can effectively reduce the occurrence of air bubbles in injection molding and produce high-quality, defect-free products.
How to Get Rid of Bubbles in Injection Molding
To avoid air bubbles in injection molding and achieve high-quality, defect-free products, several strategies and techniques can be employed:
1. Some bubbles are mainly caused by material degradation, which can be solved by reducing the speed, increasing the back pressure, and reducing the injection molding temperature and molding speed. Also, do a good job of exhaust.
2. PC transparent product bubbles are generally caused by the exterior cooling faster, the resin will face outward, that is, to the cold direction shrinkage, so that the products form a vacuum, from the appearance of the bubble.
The solution is to raise the mold temperature, lower the material temperature, find the location of the product where the bubble is formed, and shoot the glue in sections.
The speed of injection is adjusted according to the structure of the injection mold, and the appropriate pressure-holding is adjusted to ensure that the products are full and prevent shrinkage.
Auxiliary conditions: the raw material should be well dried, the temperature of the injection nozzle should not be too low, and the injection nozzle should not be too small.
3. Slow injection molding system, the product feels full after the high pressure holding atmospheric pressure pushing for a few seconds, and the speed of holding pressure can be adjusted to about 15. The pressure must be large, the holding time can be longer, and the injection time should not be too long.
By implementing these strategies and closely monitoring the injection molding process, manufacturers can effectively avoid air bubbles and produce high-quality, defect-free injection-molded products.
Summary
Through the above introduction, I believe that you have a certain understanding of the causes of bubbles in injection molding products, there are many reasons for bubbles in injection molding products, when such a situation arises, exclude them one by one.
