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How to develop and maintain an efficient deflashing operation.

How to develop and maintain an efficient deflashing operation

The purpose of this article is to show which areas of a cryogenic deflashing operation impact on the efficiency of the operation. The areas covered in this article will be:

* Cryogenic deflashing equipment;

* Deflashing cycle parameters and;

* Flash configuration.

Consumption evaluations comparing the various types of equipment were performed at Minnesota Rubber's Mason City, IA facility. L[N.sub.2] consumption vs. cycle time and temperature evaluation were performed at MG Industries, Valley Forge, Applied Technology Group facility on a Cryojet rotor deflashing unit. L[N.sub.2] consumptions were measured using patented cryogenic subcooler/digital flow monitor systems or an LS 160 liquid storage vessel on a digital scale.

It is important that once the measures described in this article are implemented, they be continuously monitored to sustain a high level of efficiency.

Deflashing equipment

The selection of good cryogenic deflashing equipment is an important part of an efficient cryogenic deflashing operation. Tables 1, 2 and 3 show how the efficiencies vary from one type of unit to another. The economic impact of this efficiency is illustrated in example A.

Example A Company A-B deflashes 200,000 lbs. of parts per month and pays a price of 44 [cents] per 100 scf of nitrogen. 5 cu. ft. belt system 200,000 lbs. of parts/mo. 1.9 lbs. L[N.sub.2]/lb. parts = $276,883.20 annually 5 cu. ft. belt system 200,000 lbs. of parts/mo. 1.2 lbs. L[N.sub.2]/lb. parts = $174,873.60 annually 5 cu. ft. drum system 200,000 lbs. of parts/mo. 0.56 lbs. L[N.sub.2]/lb. parts = $81,607.68 annually

The main factors that impact the efficiency of a deflashing unit are:

* part movement in the deflashing chamber;

* insulation of the cold areas;

* shot flow and separation;

* temperature monitoring and control

Part movement in the deflashing chamber The part movement within the deflashing chamber is very important because it determines how the parts are exposed to both the cold and the blast media. The more intense the movement of the parts in the deflashing chamber the more consistent the deflashing quality and the shorter the cycle time.

In figure 1, the parts are rotated in one circular direction only. The parts build a cylinder which results in overexposing the outside of the cylinder to the cold and the blast media and under exposing the inside. This results in inconsistent deflashing quality and extended cycle time with reduced load quantities.

In figure 2, the parts are moved in the deflashing chamber by means of mixing ribs which transfer the parts from the bottom of the deflashing chamber to the top and from the back of the chamber to the front. This dual rotation generates an intense movement of parts and results in an even exposure of the parts to the cold and blast media. This results in a high and consistent deflashing quality using a minimal cycle time.

Insulation Due to the extreme cold temperatures at which most cryogenic deflashing equipment operate, it is necessary to insulate all areas of the cold deflashing unit exposed to the environment. Any areas which are not insulated well transmit heat from the outside environment to the inside of the deflashing unit requiring additional liquid nitrogen to compensate in order to maintain a deflashing temperature. In addition, insufficiently insulated surfaces exposed to the atmosphere collect ice which when the unit warms up, thaws and results in a water problem. If this water is then allowed to penetrate into the deflashing unit, shot flow and the ability to deflash parts will be severely inhibited. For an efficient deflashing system, insulate well so the cold stays where it is needed (in the deflashing chamber). Common materials used to insulate or seal deflashing units are:

* silicon rubber caulk;

* Armaflex (tubing for shot transfer tubes made for flat areas);

* EPS (expanded polystyrene);

* teflon sheet;

* felt.

The key to the selection of these materials is that they can withstand extreme cold without absorbing moisture. When selecting cryogenic deflashing equipment, evaluate how well the unit is insulated and sealed against moisture penetration. This will minimize problems with shot flow, moisture and high L[N.sub.2] consumption in the future.

Shot handling The shot media are the work horse of a cryogenic deflashing unit and if shot flow is impaired, deflashing of the parts will be limited or inconsistent. There are three main factors which can impair shot flow:

* moisture;

* flash saturation of media;

* shot containment within unit; whereas, one may result in the other or vice versa.

Moisture enters the system through breaks in the shot transfer mechanisms or when the unit is opened for loading or unloading. If the unit is not well insulated, this moisture, usually present as ice, thaws and freezes, causing the media to clump together.

Flash saturation occurs when the unit's separator can no longer perform this function. This is usually caused by moisture or ice collecting on the separator. Once flash or dust saturates the media, it can no longer flow correctly and tends to cake together.

In both cases, moisture levels and shot flow are controlled by sealing off the shot transfer system or separating and transferring the media within a cold dry environment; continuous dehumidification of the system eliminates the buildup of moisture and the resulting freeze/thaw cycle, thereby keeping ice as ice and not allowing it to thaw.

Shot containment problems within the deflashing unit generally result in loss of media out of the system reducing shot amounts available for the deflashing process. Shot containment problems are usually generated by defective seals in doors or openings or breaks in the shot transfer system. These problems are generally resolved by replacing or employing cold temperature gaskets or seals.

Temperature monitoring and control It is essential for efficient and consistent deflashing quality to maintain an accurate deflashing chamber temperature. Precise deflashing chamber temperatures are achieved through:

* precise placement of N2 injection orifices in reference to part and shot distribution within the deflashing chamber;

* insulation of the deflashing chamber against L[N.sub.2] leakage and heat infiltration;

* precise placement of temperature sensing devices in reference to shot and part distribution within the deflashing chamber.

It is important that the temperature sensing device not be exposed to the L[N.sub.2] injection stream directly, but monitor the deflashing chamber temperature. The temperature sensing device should have a quick reaction time. Exposed function sensing devices work very well.

Deflashing cycle parameters

Determination of the current deflashing parameters is essential to the efficient utilization of the deflashing unit as well as consistent deflashing results. One method which is well proven to generate a good deflashing cycle parameter with a minimum of part loss or damage and shortest period of time is as follows, assuming mill belting drum speed at 100%, blast speed at 90% and good shot flow:

* Determine proper deflashing temperature. Select a deflashing temperature which is close to the glass transition temperature of the compound being deflashed. Run a start cycle and look at the results. If parts are being deflashed to a certain extent, you have the proper temperature.

* Once the deflashing temperature is set, the blast cycle needs to be determined. If during the above cycle 10% of the parts were deflashed, then the blast cycle needs to be increased by 90%.

So the next cycle should be run 90% longer. After this cycle, again look at the results. If parts are still not deflashed, increase cycle time or blast speed, but only change one parameter at a time or you will never know what parameter did the job.

* If results are still not acceptable, look at flash configuration or shot flow. Going colder at this point will only waste nitrogen and increase possibility of damage.

These three steps will generally result in acceptable deflashing parameters within three to four cycles. In addition, they will narrow down the variables should the results still not be totally acceptable. What effects the deflashing parameters can have on the efficiency of the deflashing operation are illustrated in table 4.

Flash configuration

Overflows The location of overflows in reference to the part has an impact on the cycle time, deflashing temperature as well as the general deflashability of the part. Overflows should, if they are necessary for the molding process, be moved as far away from the part as possible (x > shot size).

The closer the overflow gets to the part the more difficult it is to remove because the shot cannot penetrate between part and overflows to remove the flash. If the overflow can be removed altogether, the following advantages are usually achieved:

* Shorter deflashing cycle;

* better deflashing quality;

* a more robust mold.

Tear trim design Tear trim design was developed to eliminate the cryogenic deflashing operation. An overflow was placed so close to the part (x = 0) that when this overflow was removed by hand, no flash remained.

This design usually worked well until the mold started to wear and this usually did not take very long because of the knife edge required between the part cavity and the overflow cavity. Trying to cryogenically deflash these parts is very difficult. When the part is cooled down and becomes hard, the overflow becomes part of the part and the shot media cannot penetrate the area. A solution to allow the cryogenic deflashing of this part design is to fill in the overflow cavity leaving only a skin to remove. This improves the deflashability and quality of the part as well as generates a less wear intensive mold.

Parting lines Everyone knows that the parting line and flash base configuration determine the overall deflashing quality. If there is no difference between the size of the flash and the part, the deflashing unit will remove both. No cryogenic deflashing unit will eliminate molding problems (you get out what you put in).

In these cases, if part quality needs to be improved mold rework is necessary.

Ideal flash configuration I believe that everyone who molds rubber parts feels that the ideal flash configuration is no flash configuration. Flashless molding is being developed and will have a future, but with the vast quantities of existing mold inventories cryogenic deflashing will be around for quite a while. So what is the ideal flash configuration? Make the flash as thin as possible with as good a flash base as possible, or in the case where sealing surfaces are involved, try to move the flash away from the critical areas.

Summary

This article has illustrated the important areas which contribute to an efficient cryogenic deflashing operation. The purpose of this was to give an understanding of cryogenic deflashing systems so that a deflasher can evaluate their operation and be able to address problems which they may be having.

Once adjustments have been made to improve the efficiency of the cryogenic deflashing operation, they must continuously be monitored to maintain efficiency. [Tabular Data 1 to 4 Omitted]

PHOTO : Figure 1 - 5 cu. ft. Airmac

PHOTO : Figure 2 - Cryojet rotor
COPYRIGHT 1991 Lippincott & Peto, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Wieland, Rolf
Publication:Rubber World
Date:Nov 1, 1991
Words:1854
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