Burn Marks The burn marks are that the gas in the cavity cannot be removed in time, resulting in blackening at the end of the flow. We have a small 97mm diameter octagonal tub with sharp corners, moulded on a twin cavity machine. In the above diagram, the converging flow fronts indicated by red arrows meet. Molding Unbalanced flow of plastic injection defects Solving molding Unbalanced flow problems Definition Unbalanced flow is plastic completely filling some flow paths in the mold before other flow paths have filled. If possible, it is recommended to measure the actual temperature with a resin thermometer or the like. In the above diagram, varying the part thickness and creating flow leaders and deflectors, thinning flow path 1 and thickening flow path 3, is the answer.
The arrow shows the direction of the underflow. The flow freezes off before all of the flow paths have filled. The key result to use to identify high volumetric shrinkage are the shrinkage results. Areas that easily retain the resin 2-2 Insufficient cleaning The fact that the previously used resin remains in the molding machine due to insufficient cleaning is also a cause of black specks. If some of the pellets fed into the hopper are mixed with other resins having a lower decomposition temperature, black spots may be easily caused due to decomposition of the resin. Optimize melt temperature Reduce temperature to avoid material degradation from overheating, or increase it to limit residual stress. Avoid overpacking Select machine with higher clamp force Vent appropriately Use the material supplier recommended venting size.
Black spots tend to stay in the check ring and screw thread in the barrel. Causes Heat decomposition of the material. It may be that fillers used in the material are hygroscopic and they will absorb moisture. The following diagram shows a part with a thick rim. The higher the temperature and the longer the time, the faster the decomposition proceeds. When the divergent flow fronts converge again, hair-like lines appear where they meet. The gate area is an area where the highest fill rates occur.
Each option hence requires consideration of all relevant aspects of the mold design specification. A weld line is a visible dissimilarity between two adjoining areas of a part, caused by differences in flow and cooling time. Causes Excessive ram speed Poor gate position Lack of melt contact with the mold allows jetting to occur. Solution: As a general rule-of-thumb, a draft angle should be at least 1 degree per side to facilitate easy ejection. Polymer is not fully melted. Learn More These are just a few of the different types of degradations that can occur during the injection molding process.
Optimize processing conditions Reduce pressures and shot size to the minimum required. Solution: Inspect the sidewalls and edges of the cavity. Machine defects Including an empty hopper, blocked feed throat, or a worn non-return check valve that causes loss of pressure or volume leakage. Molding jetting of molded plastic defects Solving jetting problems Definition Jetting occurs when polymer melt is pushed at a high velocity through restrictive areas, such as the nozzle, runner, or gate, into open, thicker areas, without forming contact with the mold wall. That is why we have won the praise of customers at home and abroad by providing customers with low price, good quality and timely delivery services. To minimize future problems, the product designer should be made aware of this requirement.
Bozzelli has taught seminars on plastics design and processing for more than 30 years. Remedies Balance flow paths Avoid hesitation and racetrack effects Balance runners Changing the runner system can alter the filling pattern in such a way that the last-to-fill areas are located at the proper venting locations. Therefore, after black specks appear, the barrel must be thoroughly cleaned before taking countermeasures otherwise black specks will never disappear. Machine Molded-In Stresses 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. Causes: 1 The moisture content in the raw materials is too high 2 Air is trapped in the raw material 3 Polymer degradation: material is contaminated, barrel temperature is too high; insufficient injection volume Remedies: Mold design: Check if the exhaust position is sufficientProcess: 1 Choose the right injection molding machine and mold 2 When switching materials, clean the old materials completely from the barrel 3 Improve the exhaust system 4 Reduce melt temperature, injection pressure or injection speed 11.
Move the injection location to a position that will define similar length flow paths. It can however, increase shear heating for better melt flow. To see if hydrolysis is causing molecular-weight degradation, find out whether your polymer is of a type that can undergo hydrolysis, where minute amounts of water react with the polymer in the barrel of the machine. 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. For cracking, crazing, or brittleness, look for process-related causes of stress in the part, such as injection too fast or too slow. This response should rise to the transfer point then drop rapidly to the set second-stage pressure.
Note that the flow paths are not balanced and overpacking will occur in the left of the model. However, by slightly thickening flow path 2 or thinning flow path 1 see flow leaders and deflectors , the plastic could be forced to reach all parts of the thick rim at the same time. Weld line weaknesses Non-optimal crystallinity High residual stress Incompatible materials blended together Too much regrind Improper drying conditions Excessive drying either drives off volatiles in the plastic, making it more sensitive to processing, or degrades the material by reducing the molecular weight. If crack have always appeared i. Ejector pins should be resized, or relocated.