Clean the machine from any obstructions. Common Molding Defects and How to Avoid Them 1. Lower if the screw turns without engaging, raise if the screw is not turning. Part sticking in tool When the molded part sticks to the mold, it fails to separate from the cavity or is held by a feature of the tool design which can occur for many reasons. Most of the defects are caused due to thermal imbalances in the molten material flow path. Short shot can be caused by unexpected shrinkage in the mold, or by a material that was not viscous enough to totally fill all areas of a mold.
Moldflow Plastics Insight provides detailed shear stress results. Note that the flow paths are not balanced and overpacking will occur in the left of the model. Optimize processing conditions Reduce pressures and shot size to the minimum required. Avoid weld lines in areas which need strength, or which need to appear smooth. Molding sink mark and void of plastic molded defects Solving molding sink mark and void problems Definition Sink marks and voids both result from localized shrinkage of the material at thick sections without sufficient compensation. Molding overpacking of injection plastic molded defects Solving molding overpacking problems Definition Overpacking is when extra material is compressed in one flow path while other flow paths are still filling. These lines occur as the plastic starts to solidify at different speeds, in some cases the molecules are dragged along the surface of the mold causing streaks.
Short shots are observed when the molten plastic turns solid before reaching the end of the flow length. Each option hence requires consideration of all relevant aspects of the mold design specification. Cavities or voids pose a grave danger to the structural integrity of finished parts. The defects can also be observed during production. With a longer cooling time, the part is kept in a controlled environment longer, and is also more likely to keep its shape. What to do To solve problems caused by overpacking, balance the flow paths.
Causes Racetrack effect Hesitation Unbalanced flow paths Flow paths do not need the racetrack effect or hesitation to have unbalanced flow. Cracks may not be obvious until several days or weeks after production. Cracking at the Gate — Cracks that appear near or at the gate location of a molded part are usually caused by molecular orientation. However, this will add to the cost of molding, so alternative actions should be taken first. Warped parts may not be functional or visually acceptable. Description The surface of the molded product collapses somewhat-something that is particularly common in thicker products. Check for the recommended maximum shear stress value for the material recorded in the Materials Database Minimize differential shrinkage Solving one problem can often introduce other problems to the injection molding process.
The issue seems to be only affecting one of the cavities, what could be causing this issue? Remedies Eliminate air traps To prevent burn marks, you should move air traps to places which can be vented, or where ejector pins can be added. In the above diagram, the converging flow fronts indicated by red arrows meet. Non-optimal molding conditions Including, material viscosity, injection rate, and runner system. If you run your injection molding operations using the principles of scientific molding, you will be familiar with our structured, step-by-step approach to solving the problem. Surface cracks that do not appear for a while after the part is molded are problematic. The thicker and longer weld lines make the part structurally weaker. This stress can be caused by a cavity pressure that is either too high or too low.
The temperature plot will show hesitation by a low temperature and a large temperature gradient. Can anyone help me with a solution? Well, we can either change the fill rate and the change could be increase or decrease , or we can reduce the cooling rate of the plastic. Increase the secondary pressure, too. The following animation shows plastic filling a cavity. Be mindful of that fact that, in the case of box-shaped parts, the part will likely stick to the stationary mold if the stationary mold temperature is low.
Molding Air Trap of Injection Molding Defects Solving molding air trap problems Definition Air traps occur when converging flow fronts surround and trap a bubble of air. . Sticking occurs when the part fails to separate from the stationary mold or from the moving mold. This irregular cooling and shrinking of the material around thicker areas is the cause of most sink marks. Causes Adiabatically heated trapped air see Air Traps Air trapped in pockets may compress, heat up and cause burn marks.
Shortening the overall cycle time, reducing residence time, reducing barrel temperatures, or moving the mold to another machine with a smaller barrel can accomplish this. Gradually adjust the nozzle temperature, if necessary, to optimum conditions. The injection molding process can be described as heating plastic pallets to molten state, injecting molten plastic tin the cavities then cooling the plastic to solidify the plastic. Description Streaks of a silvery white color appear in the material flow direction. Q: We are having a problem with one of our parts cracking about 2 days after molding.