|The appearance on injection molded parts of visual defects such as cracking, crazing, grooves, ripples, wave marks and flaky brittleness are aesthetically inacceptable. Also, these defects are regarded as performance defects as most of these common visual defects are an indication of part failure resulting from internal and external stresses exceeding the strength of the part. The most common causes of these stress-induced visual defects range from the environment to the process, polymer, and sometimes the mold or part design. These are some of the more common visual defects:
Appearance of grooves in a part can be blamed on one of three process issues, arising from a pressure or volume problem. Limited first-stage filling pressure or a lack of velocity control can be the source of the problem. Hydraulic pressure during the first stage should be 200 to 400 psi lower than the first-stage set limit. Packing pressure or rate or the melt volume during second-stage pack and hold also may be low.
Incorrect transfer position from the first to second injection stage can cause visual defects. The second-stage pressure should be taken off by reducing the pack or pressure should be held to 300 psi plastic pressure. In case this is not allowed by the machine, the second-stage time should be cut to zero. The result should be a part that is 95% to 99% full. A thin-walled part will fill with only slight under-pack near the gate. Adjust the transfer position to provide appropriate fill volume (95% to 99% full).
Visual defects can also be caused by poor first-to-second-stage switchover response. This response should rise to the transfer point then drop rapidly to the set second-stage pressure. If the pressure drops much below the second-stage setpoint, the flow front may be hesitating and building a high viscosity, which could necessitate repair of the machine.
Melt temperature should be within the supplier's recommended range. Low melt or mold temperatures can cause problems and need to be either increased by 10-15 degree C or cycle time needs to be shortened.
Process-related causes of stress in the part can be responsible for cracking, crazing, or brittleness. Process related causes can include very fast or too slow injection. Fast injection can lead to high molecular orientation, especially in thin-wall parts.
If the part is cracked or crazed right out of the mold, it could be due to ejection. The slowing of ejection will help. Improper undercuts in mold, improper polish in the direction of ejection (draw), too high an ejection velocity, or not enough ejection area could cause this problem.
Over-packing or under-packing the gate can cause brittleness in the part because this can produce parts with high internal stress, especially near the gate. Over-packing at or near the gate causes the polymer chains to be packed too closely together. At room temperature, the polymer chains in an over-packed part may still be free to move a little. But at low temperatures, shrinkage may leave the chains too tightly packed or compressed, so a crack develops. Tightly packed chains lead to retained compressive stress, causing brittleness and part failure. Under-packing at/near the gate causes the polymer molecules to be too far apart during cooling, leading to tensile stresses that also weaken the part around the gate.
A crack occurring at a weld line may signal an improper gate location. It is important to choose a gate location that places the weld line in a least-stressed area. Also, the gate should be placed in a way to maintain some distance of flow after the flow fronts meet to allow good melding and a stronger weld line.
Localized defects can also be related to mold or part design such as a sharp corner with little or no radius. Sharp corners concentrate stresses and act as a notch where the failure can start and then propagate. A radius at a corner spreads the load across a larger area. Some polymers are more notch sensitive than others. PC is more sensitive than ABS. PC/ABS blends reduce the notch sensitivity.
Cracking or brittleness, when found throughout the part, originates in the conditions to which the polymer has been exposed in processing. One of the biggest causes is molecular-weight degradation, due to either excessive processing temperatures or to hydrolysis. Both break the long polymer chains into shorter chains that have lower properties but flow easier. PET, PC, Polyamide & Polyacetals are susceptible to hydrolysis, but styrenics, polyolefins, and acrylics generally are not.
Cracking or brittleness can also occur if regrind material is too degraded or is contaminated. Cracking can also occur if the wrong type or amount of colorant is used. Localized cracking or crazing also can occur from solvents, surfactants, or chemicals.