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A Basic Custom Plastic Injection Molding Machine


Plastic Injection Molding Mold Clamping Systems


Mold Clamping Systems

Clamping your molds on the platens for the injection molding process is a fairly basis principle and the methods used to accomplish this task are not all that different from each other.  The most common method is the use of mold clampsmold clampThese clamps are usually made of forged or cast steel and look like the picture shown here.  While this is a fairly common clamp style, you can find a number of variations of this type of clamp.  It consists of the clamp, an adjustment bolt, an attachment bolt, and a spacer or washer for use on the attachment bolt. 


The alignment of these clamps is critical to their function and they must be used properly.  The result of not using these clamps properly, could mean the difference of a mold remaining in your machine and lying on the ground beneath it.  The safety hazard of this is self explanatory, not to mention the damage that could be done to the mold or machine.  It is very important that the clamp adjustment bolt is set the correct height which is level with the clamping surface of the mold.  You can measure the clamp plate thickness and if it’s 1 “, set you adjustment bolt for 1” height.  This provides the most surface contact with the clamp plate and results in the best clamping forces being applied.  A small amount of error can exist without serious issues,  but level is still the desired and safest approach.


Another important factor in proper attachment of a clamp is the depth of the attachment bolt into the platen hole.  The best rule of thumb used to calculate this is “one and a half times the diameter of the bolt being used should be used as a depth into the platen hole”.  In other words, if you are using a ¾” inch attachment bolt, you should use a bolt length that will get you at least one and one eighth of an  inch into the platen.  .75 x 1.5 = 1.125 or 1 1/8”.  The formula applies to all bolt sizes being used.  Make sure you also account for the height of the spacer or washer you are using to determine the bolt length needed.  Let’s assume the spacer is ½” in thickness. We already know we need to get 1 1/8” depth into our platen.  To determine the length of the bolt required you need to make some measurements.  First measure the height of your clamp plate.  Then you need to know the thickness of your clamp and add the spacer thickness to that.  Lets assume our mold clamp plate thickness is 1”.  Our clamp thickness is also 1”.  Our spacer is ½”.  We total these amounts and then add the thread depth needed to determine our bolt length or, 1” + 1” + ½“+ 1 1/8” = 3.625” or 3 5/8” bolt length required for our task. 


This is important for a couple of reasons.  The first is that if we are using too short of bolts, we will likely strip the threads right out of the platen holes when the mold is mounted on the platens.  As the mold is opening, closing, and ejector system running, you could literally strip the mold off the platens and the mold could fall out of the press.  Does this sound like the voice of experience?  That’s because it is and I’ve seen it happen.  The other problem I’ve seen that ends with the same result is using bolts that are too long.  Each platen hole has a finite depth, meaning that you can hit bottom if the bolt is too long.  The result of this if you don’t notice is once again a loose clamp and a dangerous situation, so pay attention these details and you’ll never have a problem.  Keep your platens holes in good condition too, as this makes everyone’s job simpler and safer.


Other methods of clamping a mold to a press is using holes in the clamp plates and bolting directly through them  You will need to apply the same calculation to determine bolt lengths for this method as well.  This method is easier and safer than clamps, but requires precise hole placement in the molds to line up with your press platen holes.  The last common method is hydraulic clamping, which is the fastest, safest, and most expensive method but well worth it if the investment is made as this is a great time saver, saves wear and tear on the platens, and cuts down the risk of mistakes.  These are  the types of  financial decisions your company will have to make for itself.  Another less common and even more expensive clamping method is magnetic platens or clamping plates that are added to your existing platens and use extremely powerful magnets to hold the molds into place.  These are typically only used where time savings and quick mold change are a way of life as their costs are tough to justify in many situations.


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Written by: WM8C, July 28th, 2006.  Not for use without written permission


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