Vodkaman Posted May 25, 2015 Report Share Posted May 25, 2015 DaBehr - A comprehensive and logical explanation, well written, easy to follow and understand. We have the same thing in car design when modelling bumper and trim panels. We call them dimples or sink holes, the cause of which is exactly as you explained. Why I did not relate the two problems together I will never know. DAve Quote Link to comment Share on other sites More sharing options...
majic man Posted May 25, 2015 Report Share Posted May 25, 2015 In my dictionary, Its called "Poor Design". I do agree, it is "Common" to cut a few minutes off machine time to make more profit. And in hence put out a mold that is sub par... Guess that is exactly what you get when money becomes more important than integrity. I can shoot a 3/4" diameter body that is 4" long with 400 degree plastic without a single dent, So its very well any other mold manufacturer could do exactly the same thing. You "yes men" are just to accustomed to having to read a short story how to make a simple bait turn out. The problem in half these inferior molds is a simple fix. But apparently the big Corporation has not the skills to do so, so they just keep taking your money and smiling.. Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted May 25, 2015 Report Share Posted May 25, 2015 Majic (or should that be Magic) - I cannot help feeling insulted by your aggressive words. Not only accusing the company of poor design and not caring about its customers, but insulting all the members who have contributed to this thread. Dave Quote Link to comment Share on other sites More sharing options...
majic man Posted May 25, 2015 Report Share Posted May 25, 2015 Sorry you feel insulted, 99% of the time the truth is ugly.. Quote Link to comment Share on other sites More sharing options...
DaBehr Posted May 25, 2015 Report Share Posted May 25, 2015 I suspect Majic is more concerned with running his own agenda than helping others....or understanding some some of the basic principles of injection molding. 1 Quote Link to comment Share on other sites More sharing options...
majic man Posted May 25, 2015 Report Share Posted May 25, 2015 I suspect Majic is more concerned with running his own agenda than helping others....or understanding some some of the basic principles of injection molding. Im pretty certain I got my basic principals down just fine. Big ray had a solution in the 2nd post. Once he does it, he will see there is no need to entertain all the other complicated ways to polish a turd. Quote Link to comment Share on other sites More sharing options...
JRammit Posted May 25, 2015 Report Share Posted May 25, 2015 Majic molds sound far superior... Please tell the members where they could purchase these.... Quote Link to comment Share on other sites More sharing options...
DaveMc1 Posted May 25, 2015 Report Share Posted May 25, 2015 (edited) What you are experiencing is called "differential shrinkage" a very common problem with any injection molded plastic part. It is caused by the skin of the bait setting, and when the dense body cools it will pull from the weakest point. Most commonly the closest skin. This is also know as "splay" in a hard plastic part. It could also pull from another part of the part causing it to warp. The common ways to overcome this in plastic molding is to lower the temperature of the plastic you are injecting, texture the location or reduce the thickness of the wall in that area of the mold so that there is no dense, hot core. But we all like those big body baits! There are some additives that can be used to minimize the issue..but we don't use those in plastic bait injection. At a cost of few thousand dollars you could add a heat element to that area of the mold so the skin doesn't set up as quickly. Have you noticed that after you shoot the mold a few times....and the mold is really hot....that the dent seems to go away? That's because the skin is not setting up as fast and becoming the weakest point for the core to suck from. But...it is pulling from somewhere else so you get a dent somewhere else or it will cause the bait a slight warp...which being soft plastic we don't even notice. The most common solution (for both our soft baits and for hard plastic injection) is to hold pressure to help push the hotter core out so the dent is less (probably not gone...just less). And yes...this problem comes up all the time on the Do-it forum....and it's still the same solution. Search Dents on the forum. I would suggest you inject the mold at the lowest temperature that completely fills the mold, and hold pressure for at least a 10 count. If the gate on the mold is designed properly, or in this case opened up, it will not freeze off before the bait does, allowing enough plastic to be pulled in to the cavity as it cools and shrinks. when that gate is too small as in this case, it will cool and solidify not allowing the bait to draw in the plastic it needs. This has been shown by a few that have slightly opened the gates of this particular mold and completely eliminated this issue. Edited May 25, 2015 by DaveMc1 Quote Link to comment Share on other sites More sharing options...
majic man Posted May 25, 2015 Report Share Posted May 25, 2015 Majic molds sound far superior... Please tell the members where they could purchase these.... Sorry, Not the place for that. There is no agenda here to be proven... Quote Link to comment Share on other sites More sharing options...
JRammit Posted May 25, 2015 Report Share Posted May 25, 2015 My wife makes wonderful enchiladas, so wonderful in fact that i belive people are foolish for eating any other kind of enchilada....... But since we dont own a restaurant, i think no less of those people 1 Quote Link to comment Share on other sites More sharing options...
alsworms Posted May 25, 2015 Report Share Posted May 25, 2015 Guys.......this has been a very informative topic. What say we keep it level headed and not ruin it for those who are learning? 4 Quote Link to comment Share on other sites More sharing options...
Basseducer Posted May 25, 2015 Report Share Posted May 25, 2015 YOU GO ALSWORMS!! 1 Quote Link to comment Share on other sites More sharing options...
smallmouthaholic Posted May 25, 2015 Report Share Posted May 25, 2015 (edited) What you are experiencing is called "differential shrinkage" a very common problem with any injection molded plastic part. It is caused by the skin of the bait setting, and when the dense body cools it will pull from the weakest point. Most commonly the closest skin. This is also know as "splay" in a hard plastic part. It could also pull from another part of the part causing it to warp. The common ways to overcome this in plastic molding is to lower the temperature of the plastic you are injecting, texture the location or reduce the thickness of the wall in that area of the mold so that there is no dense, hot core. But we all like those big body baits! I would suggest you inject the mold at the lowest temperature that completely fills the mold, and hold pressure for at least a 10 count. Adding a small mold extender to the top of the molds may reduce the dents as there is more plastic for the cooling plastic to draw from.Sometimes when working w/ multi-cavity molds we can't top off quickly enough to compensate for the draw down. Every mold. especially multi-cavity,custom CNC molds have their own, individual idiosyncrasies and may require different techniques.I.M.O.& experience- Cooler ,injected plastisol produces less dents.I had multi cavity molds that the manufacturer couldn't stop from denting. They were filed open,and adjusted numerous times and ended up in the trash can. One size does not fit all. edited for spelling Edited May 25, 2015 by smallmouthaholic Quote Link to comment Share on other sites More sharing options...
Arkansasman Posted May 26, 2015 Report Share Posted May 26, 2015 I ask could oiling help the op out from debts? Quote Link to comment Share on other sites More sharing options...
Big Ray Posted May 26, 2015 Author Report Share Posted May 26, 2015 First I just want to say I am glad we are cool DF1. Ok now I forgot to mention how long I hold pressure. I know this is going to sound crazy but I hold that sucker for 2 minutes. I tried less and it didn't help at all so I just figured hold pressure until the plastic becomes solid and there will be no chance of it ending up with dents. When you guys first told me to hold pressure I didn't know how long so I just keep trying different times until it worked. Quote Link to comment Share on other sites More sharing options...
mark poulson Posted May 26, 2015 Report Share Posted May 26, 2015 (edited) Ray, I have had to add sprue extenders, in the form of duct taped wrapped around the mold's edge, to keep enough hot plastic available so the baits could suck it down as they cooled. Once I opened up the entry to each cavity, they poured without the extender. I would try preheating the mold with a heat gun before you begin pouring. That should let you inject at 330 and still fill all the mold details. Cooler plastic shrinks less, but it doesn't flow as well, so the heated mold will help it flow. You can also try spraying the mold with PAM, to help the plastic flow at lower temps. I do that, and only have to hold pressure for ten seconds, max, when I inject. Edited May 26, 2015 by mark poulson Quote Link to comment Share on other sites More sharing options...
