Have you ever had an injection molded part that didn’t live up to its’ use in the environment for which it was intended? After several attempts to correct the problems did you give up and choose a metal instead?
Would it surprise you to know that many plastic parts don’t make it to market with the part properties originally intended? Did you ever ask yourself what caused this failure?
In the last few years, PMC has picked up 35% of their new customers from part property failures molded by other injection molders. These defects in injection molding parts are not caused by equipment failures but rather by injection molders who not understand what happens to properties during the molding process. The knowledge of what causes the finished part to fail to survive in the intended environment in which it was designed to operate is crucial.
Common Injection Molding Defects
Some of the common defects in cases PMC has resolved include poor mold design, lack of material processing knowledge, and lack of knowledge of design restraints of the resin choice. “Processing guidelines issued by the materials manufacturer are often ignored and it all beings in the first phase of product design.” This often is the beginning of the lack of understanding by the original designer of the part or product, knowing the environment in which it was supposed to survive. Any one of these causes can be traced back to these four processing defects:
- Cavity pressures
- Injection of the plastic being too slow, too fast, or a combination
- Cooling rate of the plastic
- Improper melt temperature
Cavity pressure is directly related to part dimension. The variation of cavity pressure will vary the dimension in a part. The less the variation, the tighter the dimensional control or variation. The higher the cavity pressure value gets the less the part will shrink, all other values remaining the same. One rule of shrink to remember is that plastic shrinks less in the direction of plastic flow and more perpendicular to flow. A common error in this process is when molders experiment with machine settings in order to determine when the gate freezes. At this point, a defect occurs in the parts and then reverses that move until the defect goes away. The problem with this is that the other 3 processes are going to interfere with the finished part properties in ways the molder cannot see. This mistake not only costs manufacturers money but costs customers when they find out their new plastic part does not seem to last very long in its intended use.
Cooling and Injection Rates
The injection rate of plastic into a mold will determine, in combination with cavity pressure, cooling rate, and melt temperature, the amount of stress the plastic is holding up until the finished plastic part begins to show signs of cracking and or warp. This can be a very complex problem to solve. Floyd explains that Experimenting your way through this without having the proper equipment is like trying to learn how to fly an airplane with a blindfold. “You can’t see where you are going, and you can’t find the controls, let alone get off the ground and heaven forbid he or she needs to land,” says Floyd.
Improper Melt Temperature
The final common processing defect is improper melt temperature. When using the processing guides supplied by the materials manufacturer you will see a range of temperatures suggested. This process value is important to making sure that you are not destroying any of the chemistry involved in the making of the final part. The mold temperature is equally as valuable in the development of stress. Before you attempt to mold the first part, make sure the resin purges out the nozzle in an even-smooth-creamy-melt for the entire amount of resin that is destined to be injected into the mold. It’s no good to have a nice creamy melt stream halfway through the injection of the material stream and the last half look like corn-cobbs. This process is already off on a bad start. Smooth is best!
What’s the Bottom Line?
If you own a company and think you have the best molder you could hire and you are not using the technology available, you will be leaving lots of money and customer value on the floor of your molding facility. Most likely, you will be shipping many inferior products which will end up breaking, warping, or cracking in the end-use environment. Defects in molding parts that are far below the expected property values that the customer thought he or she were getting will benefit no one. Customers deserve to know that the material part they chose and paid for made it all the way through to a finished, seamless product.
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