Following are some of the common defects in Injection Molding. The solutions mentioned are possible solutions and must be tried out with safety in mind. Please always follow the material manufacturer and machine manufacturer's guidelines for safety.Paragraph.
Molten Plastic is not reaching the mold cavity section
Molten Plastic is flowing into unwanted sections of the mold cavity
Plastic is shrinking as it cools but additional plastic cannot be for further compensation of the shrinkage
A layer/ streak of a unwanted gaseous byproduct from the melt or moisture in the material comes in between the melt flow and the cavity walls preventing the texture from being picked up and in addition eventually leaving a residue
A differential cooling rate of the melt in two sections of the molded product
When air and gasses get trapped inside the mold cavity during plastic injection, the high pressure results in the dieseling of the plastic resulting in the burning of the plastic.
This can be degraded plastic and/or foreign material that can get mixed with the plastic that needs to be molded.
Usually occur with the parts are thick. The walls solidify and the plastic melt shrinks towards the wall and therefore sucks a vacuum void on the inside of the part
When moisture and/or a gaseous byproduct gets mixed with the melt and is injected in the mold cavity this moisture or gas if embedded inside the melt can show up as bubbles
Shows up at the gate when the material is sheared differently compared to the rest of the part
Usually seen with the part is thick causing the injected plastic to ‘snake fall’ on to the cold mold surface and start freezing immediately. When the fresh plastic come in it does not blend well with the first material causing the jetting marks
The melt flow front is usually cold due to the exposure to the cold cavity. When two flow fronts meet as in a flow around a mold pin they do not fuse uniformly causing a weld line. Sometimes air can also get trapped in to form the defect
There are three phases during the filling of the mold. They are as follows:
In some cases, such as in case of softer materials or larger gate sizes the part weight does not stabilize within a practical time limit of molding cycle times. This is also true in cases of hot runners and valve gated systems. Adding more time than required does nothing else other than pack the gate area when the rest of the part is already below the no flow temperature of the plastic. In such cases the same study mentioned above will yield a graph as shown in Figure 2 where the part weight does not stabilize.
In such cases where gate is not seen the phases of pack and hold must be differentiated from each other. In the pack phase the required amount of plastic must be injected and in the hold phase this plastic must be held in there till the gate freezes off. If the hold phase is terminated before the gate is frozen then the pressurized plastic the cavity will flow back out of the cavity often causing sink and/or dimensional variations and issues. (This is the reason that a molder will notice sink on parts with high pack and hold pressures. When the molder lowers the pressure the sink disappears often baffling the molder since it is opposite to what he expects.)
Following is the procedure that has been used for optimizing the pack and hold times. It is best to illustrate this with an example. Consider the graph shown in Fig 3. This is the same graph in Figure 2. It can be observed that at about 5 seconds there seems to be a change in the slope of the graph. In other words, the % increase of part weight with incremental hold time seems to be lower compared to that before 5 seconds. We therefore can consider that the part has reached the required part weight or in other words the pack phase has been completed. It is similar to visualizing when one is packing his travel bag where initially clothes can be placed in till the bag seems physically full but the remaining clothes can only be put inside after compressing the clothes that were first put in. As the bags are filled more and more, lesser and lesser amounts of clothes can be placed in there. So after the initial quick fill further additions slow down. The pressure used during this initial phase can now be considered as the pack pressure and the time that this pressure is applied for as the pack time.
Going back to the travel bag example, once we have packed the required amount of clothes we must now zip it up in order to hold the clothes in there. If not, the bag top or cover will not be able to keep the clothes in there. Similarly, once the required amount of plastic is now present inside the cavity it must be held in there. This is done by applying another pressure setting that will be lower in value than the pack pressure for a time until the part weight stabilizes or in other words the gate freezes off. The target part weight here will be the same part weight that was obtained at the end of the pack time. The following procedure will better illustrate the steps.
Procedure for determining pressures and times for pack and hold phases.
Note: Optimization of this phase is Step No. 5 in the 6-Step Study for Process Optimization. It is therefore assumed that the previous 4 steps have been completed. Click this link for more info.
Procedure: We will refer to the same info in the graph in Fig 2 starting from the steps to generate the graph.
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