TPGOA Technical Insights
Cracking and/or Crazing can be defined as a fracture or surface breakage in the materialof a molded part, usually found in weld line areas, but also on the surface in general. Here are some instances when it may occur and methods with which to address them:
Explanation: Although stress cannot be eliminated, excessive stress can be molded into the parts by using too high an injection pressure, too high a holding pressure, or too fast a filling rate. The molten material is forced into the mold and held there under great pressure until it partially solidifies. When the mold opens and the part is ejected, it is still cooling but is no longer constrained by the mold. Some of the molded-in stresses are allowed to release and a ``splitting'' of the plastic occurs, usually in the weakest area.
Solution: Reduce packing and fill rates by adjusting until the part is properly filled with minimum stresses. Maintain a holding pressure that is no more than 1/2 the primary injection pressure.
Cooling Time Too Short
Explanation: If the cooling time is too short, the part is ejected before the material has formed a skin solid enough to constrain movement of the remaining plastic material while it cools. The surface will split open and form crazing, or cracks will form throughout the part.
Solution: Increase the cooling-time portion of the cycle. This holds the mold closed longer and allows a thicker skin to form on the molded part. The skin will be strong enough to keep crazing or cracks from forming.
Undercuts or Parting Line Blurs
Explanation: For the molded part to eject, there must be no restrictions to a straight push out of the cavity. An undercut, reverse draft, or burr will cause such a restriction. This will try to keep the part in the mold while the ejection system tries to push it out of the mold. The conflict that arises will cause the part to fracture or crack.
Solution: Inspect the sidewalls and edges of the cavity. Make sure there is adequate draft (see the next paragraph) and that there are no burrs or other undercut conditions. If there are, they must be removed by stoning or machining.
Insufficient Draft Allowance
Explanation: A draft angle is simply a tapered side wall that is used to allow easy removal of the molded part from the cavity of the mold that forms it. Without this taper, the vacuum that is created (when plastic displaces air) in the cavity cannot be overcome and the part will simply crack as the ejection system tries to push it out of the mold.
Solution: As a general rule-of-thumb, a draft angle should be at least 1 degree per side to facilitate easy ejection. This does result in a dimensional change in the part and must be considered in the mold design phase. To minimize future problems, the product designer should be made aware of this requirement.
Use of Mold Release
Explanation: Mold release will interfere with the molecular bonding of the plastic. Material enters a cavity in layers and these layers must be allowed to bond together. Mold release interferes with that bonding and will cause crazing to occur on the surface of the part.
Solution: The remedy is to keep the mold as clean as possible and make every effort to eliminate the use of external mold releases.
Improper Ejector Design
Explanation: If ejector pins are too small, or located on thin flat sections of the part, the plastic will be distorted during ejection and cracks will form due to the amount of stress being imparted. Also, if the ejection speed is too great, the plastic will not have time to conform to the normal stress being applied and cracks will form from distortion.
Solution: Any evidence of cracking in ejector pin areas indicates that the pins are too small, or the ejection speed was too great, or there was too much injection pressure used to fill the mold. Injection pressure was covered earlier. Ejector pins should be resized, or relocated. They need to be located such that they are close to side walls or under bosses or other strong areas of the part that can absorb the ejection stresses being produced. They need to be as large as possible in diameter to distribute the ejector forces over a large area.
Explanation: One common cause of cracking is the use of material that has become degraded. This can be the result of overheating in the barrel, but a more common cause is the use of bad regrind. Regrind that has been used over and over can easily become degraded due to the continued exposures to elevated temperature. It melts at lower temperatures than virgin so the regrind can degrade in the barrel, which must be heated high enough to melt the virgin thereby degrading the regrind. Degraded material is weak and does not have a good molecular bonding of molecules. This results in cracking when the part is exposed to any stress, such as that of the ejection system.
Solution: Use only high-grade regrind and use it only once. Mix regrind with virgin at a level of approximately 15% regrind by weight to minimize the tendency to degrade. If this is still a problem, eliminate the use of regrind altogether.
Explanation: Excessive moisture causes cracking or crazing because the water droplets actually turn to steam when heated in the injection unit, and these steam pockets erupt causing voided areas between molecules. This causes those areas to be extremely weak and brittle. The voided areas easily break apart once the mold opens and relieves constraint conditions.
Solution: Although it is commonly understood that non-hygroscopic material do not require drying, do not take chances. Dry all materials. It may be that fillers used in the material are hygroscopic and they will absorb moisture. Every plastic material requires specific drying conditions. And each material should be dried according to the material suppliers’ recommendations. The desired moisture content is between 1/10th of 1 percent and 1/20th of 1 percent by weight. This means that the dry air being used to take moisture from the material should have a dew point of -20 to -40 degrees F.
Explanation: Machine operators who have been told to use mold release sprays sparingly will eventually overuse the spray. The thought seems to be that if a little bit works, a lot will work better. Excessive mold release will interfere with molecular bonding of the plastic and cause weak areas that break apart easily.
Solution: If possible, run the machine on automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is really necessary. And, instruct all employees on the importance of maintaining consistent cycles.
TPG makes every effort to insure that the information contained herein is accurate - however, we accept no liability for the content of this piece, or for the consequences of any actions taken on the basis of the information provided.
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