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Designing Moulded Precision Gears

09/24/2008 8:25 AM

I need to gather information together in order to design a series of plastic moulded spur gears. These will be single gears (i.e.not compound) in a range from 30 to 48 teeth (30, 40 & 48), have a tooth modulus of 1.0, pressure angle of 20 degrees, and a tooth face width 9mm. I anticipate that the toothform PCDs will need to have a concentricity (total indicated reading) of less than 0.08mm. They will rotate at 500-750 RPM and require a life of about 200 million revolutions (preferably running unlubricated but with an initial application of a small amount of suitable grease).

I've designed a small number of mouldings in the past with a good success rate so I'm comfortable with the basic rules but I would like some help from anyone who has some knowledge of more in-depth tips relating to the design of precision moulded gears or of a web-site that would provide useful information.

Thanks.

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#1

Re: Designing Moulded Precision Gears

09/25/2008 12:05 AM

Can't help you directly with gears, but making truly circular injection moulded parts can be a real challenge.

Suggestion 1: Have space at one side of the gear so that the mould tool can vent/flash that side without compromising the gear perfomance. (ie try to use 8mm width of the 9mm.) (Part line at the edge of the part.)

Suggestion 2: Use multiple radial gates rather than a single gate to fill the cavity. Probably hot runner would be preferred. (Even fill and shrinkage)

Suggestion 3: Use or subcontract the part design to get "mouldflow" or similar analysis done for fill pattern, shrinkage, cooling and stress and CHECK-CHECK-CHECK.

Suggestion 4: Start today with proposed lubricants and raw materials to VERIFY environmental compatability of the materials REGARDLESS of what the suppliers may hint or say. Simplest test is to get multiple sample parts made of the palstic, weigh each as precisely as you can afford, slosh it with heaps of lubricant and seal in a plastic bag. Each month, clean and weigh one more of the samples. ANY weight changes are serious messages that there is risk.

Suggestion 5: Don't have spokes on the gears. Shrinkaged will be more even that way.

Suggestion 6: Mould the gears in a single cavity rather than inserted portions to make the complete circle. (EDM or wire cut) Since any tooling join lines may give irregularity in the gear pitch, alignment and so on.

Suggestion 7: Choose material for stability, self lubrication and shrinkage features ahead of price or ease of moulding.

Suggestion 8: Finally some calculations. Calculate the maximum surface to surface speeds of the gear parts. Some plastics are self lubricating (Like acetal) until the velocity reaches a limit then they spontaneously heat at the interface and that will erode away small portions of the gears. Knowing your gear size, rotation speed and tooth shape, you will be able to determine the maximum surface speeds and choose a material to suit.

By the way, welcome to CR4.

(I work at a place that runs 15 injection moulding machines. We don't make gears, but your tolerance of 0.08mm is typical of our processes for parts up to 100mm in size provided there are no part lines or insert lines involved in the dimension.)

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#2
In reply to #1

Re: Designing Moulded Precision Gears

09/25/2008 1:12 AM

I will cast my vote with our information. Excellent answer. I built a series of molds for gears and other round parts and the customer would not pay for the proper gating in the mold. This was one of those cased where you just do what you are told and consider it a useful lesson. We had a constant battle trying to hold concentricity. Also as to your point #7 I again agree because the higher the shrink factor the the more un-predictable the outcome.

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#3

Re: Designing Moulded Precision Gears

09/25/2008 3:54 AM

Dear Dave,

I can strongly recommend you to contact Alex Kapelevich at AK Gears.

www.akgears.com

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#4

Re: Designing Moulded Precision Gears

09/25/2008 6:35 AM

May be you already know but just in case you could download a nice pdf document of plastic gearing from SABIC.-

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#5

Re: Designing Moulded Precision Gears

09/25/2008 8:03 AM

Hi Dave,

Due to the tolerancing, and the precision that would be involved in the mould, as well as the black magic in the shrinkage at your tolerance level, you may want to look at costing for a real cheap gear mould with large tolerance and seconday processing. Of course this will depend on your volume as well as costs. Generally secondary operations are more expensive, some due to logistical and material handling operation alone.

This may however be a quick way to get the accuracy as well as the cycles you are seeking.

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#6

Re: Designing Moulded Precision Gears

09/25/2008 10:47 AM

In a previous job I purchased many precision gear molds that molded millions of gears per year. The shrinkage of the plastic in a gear mold is different from the shrinkage of, say, a round plaque. (Nearly all plastics shrink as they cool in the mold.) The gear teeth not only shrink in diameter, but also change form as they shrink. This change in form must be compensated for in the mold cavity profile. They also shrink less near the gates, which contributes to runout. At the time (20+ years ago), wire EDM was not advanced enough to cut a good enough gear cavity. There was always a bump at the start-stop of the wire. I know that wire EDM capabilities have increased where this should no longer be a problem. All the gear molds we had built were 3-plate molds, meaning that the runner system was separate from the mold plate that holds the cavity inserts. The gates were a series of "drops" that tunneled straight into the top of the gear. Most of the gear molds had 3 drops. We typically ran 8 cavity molds. The runner system was balanced, so each cavity got the same pressure as it filled. I absolutely agree with Just An Engineer that venting is critical. All the cavity inserts must be vented properly for the material chosen. Too deep of vents will cause the gear teeth to flash. Too little vents will make the gear teeth hard to fill, which ironically, can also cause flash as the mold is "blown open" to fill the part, and cause the teeth to "burn". I haven't had to build a gear mold in a long time, so I am not familiar with web sites and CAD programs for designing and building gears. I don't know if you will be molding the gears yourself or purchasing them from a molder. In either case, you should invest in a machine to measure the gear after molding. I had used a device called a "Vari-Roll" that used a precision cut steel master gear that was placed on one spindle with the gear to be tested on the other spindle. The gears are meshed together and the master gear is rotated. The center distance between the master and test gear is measured and recorded for a full revolution of the test gear. It shows runout, tooth-to-tooth error, total composite error, and pitch diameter of the test gear. It is not an insignificant investment, and you will need a separate master gear for each size of gear molded, but could save a lot of cost down the road. I hope you find some of my rambling useful.

Dennis

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#7

Re: Designing Moulded Precision Gears

09/25/2008 7:31 PM

Hello to all................

I just want to say sorry because of my lack of knowledge in pasting items here. It seems some of the printed word was covered by the picture of gears.

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babybear (1); ferquiza (1); garyceng (1); Just an Engineer (1); mircoconsultant (1); Randolph Toom (1); Zoomer (1)

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