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 Overview No matter how good the design, if the tooling and molding are not up to the task, the transmission will suffer. It is pretty common in this industry to find any molder claiming expertise in molded gearing. They will usually claim tool tolerances to +/-.0001 inches, super-precise molded gear specifications, and qualify it all with an ISO or QS certification. Unfortunately at present there is no industry standard classification of inspection criteria and tolerances specifically for molded gearing. Therefore, such claims are just proof that anyone can say anything in print. Tooling
 Three Plate Tooling Three plate tools allow the part to be gated in the interior region of its body, which means the that actual gear teeth are a significant step away from the turbulent region of injected plastic flow. Also, the melt temperature and pressure profiles are more uniform and circular about the axis of the gear, which results in better molded part dimensional stability. These three plate tools must be made with very high precision, and the gear cavity should be replaceable since part shrinkage can never be completely known before the tool is used to mold the gear.  At Kleiss, we presume that the cavity will be cut twice. The first cavity is designed per our best estimate of final dimension, and the 2nd cavity is then determined from the actual part shrinkage in the 1st try. Additional Precision Characteristics Every precision gear tool requires a few more precision characteristics. Of course the runner and part configuration should be optimized for proper molding. Gate locations and sizes are also critical. Mold temperature control is extremely important. Every piece of steel in the vicinity of molten plastic should be held at a constant determined temperature. The core pin around which the bore is formed is very important. Temperature regulation of that core pin must be achieved and its wear minimized. Usually the mold base should be insulated from the mold machine at its mounting points to prevent uneven cooling from the large steel machine platens. Ejection of the part is quite important. Will a sleeve be needed to eject the gear from the core pin? Can pin ejectors be used without deforming the part? If the gear is helical, will an unscrewing ejector system be required? All of these questions must be addressed in the design of the tool. Molding Once the tool is built and installed in the machine, the last and still critical step is to mold the part. We don't know of anyone in the industry right now that advertises the specific steps they take to assure a precise, repeatable, materially optimal process. That is unfortunate. Let us be the first. The Molding Machine Without the correct molding machine, operating to specification, with precise position and pressure control capabilities, it is impossible to assure consistent dimensions and quality. The basic construction of a molding machine is very straightforward.  Material is fed in through a hopper into a screw and barrel where it is heated to a molten condition. Then the melt is injected under high pressure into the mold while the mold is held closed with the high force of a toggle or hydraulic ram. The goal is to melt the resin in a very uniform and repeatable fashion, inject it into the mold under very precise and repeatable pressure conditions, and hold the steel temperatures in the entire flow system very stable. After the molten plastic is solidified, the mold is opened and the gear is ejected. It then continues to cool in the open air. There are a lot of necessary handling and pre-processing requirements for the materials. All of the handling equipment of course must be quite clean. Engineering polymers quite often must be dried to a specific dew point prior to use. When the part is made, a runner is formed that could be used as regrind and put back in the melt. This quite often raises more quality concerns than the slight savings in material expense may justify. At Kleiss we use only virgin material for molded gears. Injection The method of injecting the plastic into the mold is very important. We believe that in most cases the plastic should be injected under high pressure very quickly in order to maintain the good characteristics of the melt completely during injection. As the mold cavity fills, the injection pressure should be held at the same high value and the speed of injection slowed to maintain that pressure. At the moment when the part is filled, and the mold machine switches over from injection to a pack & hold condition, the plastic melt is held at the same high pressure. This pressure is held for as long as it takes to freeze the gates and achieve the highest weight possible for the molded part. Below is a graphic image of the pressure and fill patterns of a typical plastic molded part.  (Part is filled in 0.23 seconds)
We have not found any publication or explanation from the industry that describes proper molding parameters for precision gearing. We offer this as a first step toward a new openness in this regard. The industry will not advance if everyone in it keeps their own techniques secret. The above approach has worked for us very well to date. We continually experiment with modifications to process and tool design, and will discuss advances in that area as they occur. |
390 Industrial Avenue – Grantsburg, WI 54840
– tel: 715.463.5995
Copyright 2004 – Kleiss Gears Inc.
