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A Basic Custom Plastic Injection Molding Machine

 

The Platens & Tie Bars

 

Platens

On most machine there are two platens, referred to as the movable and the stationary platens.  Most commonly you will have two stationary platens, one front and one rear, and one moveable platen. The platens serve a couple of functions.  The first thing that it does is provide a place to attach your mold.  The platens will normally be drilled and tapped with threaded holes, which in turn are used to attach clamps to the mold, holding it in place during the entire molding process.  Some platens in less common cases, are built with slots in them instead of drilled and tapped holes, and this is actually a more versatile way to attach clamps to the mold.  This was more commonly used with European machines then it is on Japanese or American machines.  The placement of these holes are usually determined by an industry standard unless otherwise specified by you for a given platen and machine size.  The sizes of the holes in the platens are typically determined by the platen and machine size as well.  The larger the machine, the larger the bolt sizes required to clamp the molds into them.  I’ve often been asked how many clamps are required for a given mold.  While I’ve not really found a “hard rule” about this, I can tell you from experience that if you have one clamp for every 18 inches of mold length, you will not drop a mold on the floor as long as it is properly attached.  So, on a mold of 36” in length, you would have a total of 12 clamps, 3 per side, and 6 each per mold half.  Now this will vary some in different molding facilities and is meant only to be a guideline.  You should follow whatever rules your company has set forth for this procedure in your injection molding facility.  There are also some “through holes” in the platen in a number of pattern options for ejector rods.  The use of these will be explained in a later section.  These holes are also laid out as per a specific standardized location as with the threaded holes for the clamps.  There are multiple patterns available, which will handle 99% of all standard molds.

 

The second function of the platen is to provide the steel to develop a uniform force on the mold and move the mold halves or cover and ejector, either to the open or closed position.  In the open position, parts can be ejected and removed from the mold by means of an operator or by some other robotic means.  In the closed position, tonnage is applied to the mold halves via the platens and this is when the plastic under high pressure is injected into the mold.  The term “platen deflection” is derived from the amount of movement that is exerted on and results from this injection pressure being applied.  A hydraulic clamp machine usually will resist the platen deflection issue better than toggle clamp press because in a hydraulic clamp press, the tonnage is developed by a large hydraulic ram located in the very center of the movable platen.  On a toggle press, the attachment location of the toggles to the platen are more towards the outside edges and deflection in the center of the platen is more common place.  This can make a difference when you are close to the press tonnage threshold on a given mold, and puts the mold at risk for flash problems along the parting line edges.

 

Tie Bars

The tie bars are the mechanism that actually develops the tonnage of the press.  Tie bars are made of steel.  The size will vary from a few inches in diameter on small machine up to well over 20” in diameter on the very large presses and the length is determined also by the type and size of the press.  The rods will be attached most typically with a very large nut on the outside of both the front and rear stationary platens.  There will be four tie bars in the case of most common injection molding machines.  The only place this won’t really apply is with some of the “C” type clamp mechanisms, which aren’t very common in most standard injection molding machines. The way this tonnage is developed is by moving the movable platen forward until the two mold halves contact each other and then high pressures are applied causing the tie bars to “stretch”.  It is this stretch and the force being applied that will determine the final tonnage capability of an injection

molding machine.  Often, this stretch is measured with a strain gage that is located on the end of one or even all 4 tie bars, and this stretch or “strain” is measured to deliver an output back to injection molding machine controller in order to get a “tonnage reading” from the machine.  This is tie in with safety mechanisms to avoid over stressing the bars which can literally cause them to crack or snap, as well as feed back for the process itself.  This information can also be used by many newer machines that have the option for “auto tonnage setting” of the machine, which takes most of the setup or technician requirement out of the function as it is automatic.

 

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

 

 

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