DELAMINATION

 

 Delamination can be defined as a separation of the layers
within a molded part. It can also appear as fish-scaling.

Machine

Injection Speed Too Slow

Explanation: The injection speed determines how fast a molten material is injected into the mold. The material enters the cavity in layers. If the speed is too slow, the layers will solidify before they are packed and fused together. The weak bonding of these layers results in separation as the part cools and shrinks.

Solution: Increase the injection fill speed in small increments (two percent of total) until the delamination tendency disappears. If burning or flashing occurs, the injection speed is too high and delamination is being caused by other factors.

Inadequate Cushion

Explanation: A cushion, or pad, of material is needed at the end of the injection stroke so pressure can be held at all times on the material in the mold as it solidifies. This cushion is created by adding a little more material than is actually needed to fill the mold. Without that cushion, the screw will bottom out at the end of injection and will be unable to hold pressure against the material in the cavity. That material will not be packed out and the layers that were formed during injection will not fuse together. As the part cools and shrinks outside of the mold (after ejection), the layers will pull apart causing delamination.

Solution: Establish a cushion of at least 1/8'' and not more than 1/4''. Too little a cushion causes under-packing while too great a cushion results in uncontrolled packing of the sprue area.

Injection Hold Time Too Short

Explanation: Holding pressure is applied at the end of the injection stroke. The holding pressure is used for maintaining pressure against the molten material to pack it together while the skin solidifies. If the holding pressure is stopped too early, the still-molten material will leak back out of the cavity and pressure will be lost. Without the pressure, the layers of plastic cannot be fused together and they will pull apart as the molded part cools and shrinks.

Solution: Maintain the holding pressure until the gate freezes. Once that happens, the holding pressure will no longer have any effect on the material in the cavity.

Low Barrel Temperature

Explanation: If the material is not heated to high enough temperature in the barrel, the molecules will not bond together properly. When this material is injected into the cavity, it simply pulls apart and delaminates during the cooling and shrinking phase because it is not held together properly to begin with.

Solution: Each material has a range in which it can be molded and the barrel temperature should be adjusted to accommodate the specific material being used and the product and mold design being incorporated. The material supplier is the best source of information for this data. Be sure to use the proper profile and heat the material progressively from the rear to the front of the barrel.

MOLD

Low Mold Temperature

Explanation: If the mold temperature is too low, it will cause the first layers of molten plastic to solidify too soon. The next layers will not fuse properly to the first layers and delamination will occur when the part cools and shrinks.

Solution: Each material has an ideal mold temperature that should be used for proper molding. The material supplier is the best source for this information, but most material should run in molds that are between 120 - 180 degrees F.

Sharp Corners In Gates and Runners

Explanation: Sharp corners, or 90 degree bends that are positioned along the flow path of runner system and gate, will shear the plastic as it goes by and tear the molecules apart. Breaking this bond results in the individual layers cooling at different rates and not bonding back together again in a proper manner. Delamination occurs when the part cools and shrinks.

Solution: It is always a good idea to utilize a radius at every turn in a runner system and at each gate entrance to the cavity. This will help to minimize shear and reduce the chance of resin separation during injection.

Excessive Mold Release

Explanation: Excessive use of mold release will interfere with the normal bonding process that must occur between layers of plastic as they enter the cavity of the mold. If the layers are not properly bonded and fused they will pull apart as the molded product cools and shrinks after ejection from the mold.

Solution: The remedy is to keep the mold as clean as possible and make every effort to eliminate the use of external mold releases.

MATERIAL

Contaminated Raw Material

Explanation: Some common causes of delamination are dirty regrind, improperly cleaned hoppers or granulators, open or uncovered material containers, and poor quality virgin material supplied by the manufacturer. Contaminants are considered non-compatible with the base resin and will interfere with bonding of layers. The non-fused layers pull apart after the part is ejected from the mold.

Solution: This type of contamination can be minimized by dealing with high quality, reputable suppliers and by using good housekeeping practices. Properly trained material handlers will also help reduce contamination.

Foreign Materials or Improper Additives

Explanation: If a pigment is used to color the resin, it is possible that a non-compatible soap is used in its manufacture. If a concentrate is used for coloring, the base resin may not be compatible with the molding compound being used. Accidental mixing of two different grades of the same material may have occurred and they are not necessarily compatible. Even different flow grades of the same material may not be compatible enough to bond properly. These situations all lead to poor bonding of the plastic layers being formed during the injection process. Improper bonding results in the layers pulling apart during the cooling phase.

Solution: Use only compatible additives for all base resins. Use reputable, proven compounders especially when specifying non-standard formulations.

Excessive Moisture

Explanation: Excessive moisture is one cause of delamination. It occurs because the water droplets actually turn to steam when heated in the injection unit, and this steam explodes throughout the plastic, interfering with molecular bonding, causing voided areas between molecules. This causes those areas to be extremely weak and brittle. The voided areas easily pull apart and delaminate when the part cools and shrinks.

Solution: Although it is commonly understood that non-hygroscopic material does 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 the dry air being used to take moisture from the material should have a dew point of -20 to -40 degrees F.

OPERATOR

Poor Housekeeping

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 layers that pull 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. 

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