Cut downtime and mold wear using plastic.

A K Rubber, Elkhorn, WI, a custom manufacturer of precision rubber products, recognized the limitations of traditional mold cleaning methods and adopted a new method employing the use of plastic media as a blasting agent. This new method has been successful.

The company molds more than five hundred different products; however, its forte lies in molding precision parts that require specific raw materials and compliance to critical tolerances. More than 15 years ago, the company developed and implemented a system to insure 100% traceability of all manufacturing operations. When a defective part is discovered the problem is traced back to the source and the production process is corrected. Mold cleaning was a process that required improvement.

Shortcomings of traditional mold cleaning methods

For AKR, the typical frequency of mold cleaning varies from 100-400 heats. The need for cleaning depends on the compound being molded, the shape of the part, its surface texture and a number of other factors.

Originally they only used alkaline soaking to clean molds. It was effective, but tedious and time consuming. Company president Kirt Fiegel reported, "Using alkaline soaking is a real nuisance. The mold has to be hoisted and carefully lowered into a tank of hot caustic liquid. Someone could be injured if the mold slips and splashes caustic liquid. Rubber gloves, rubber aprons and face shields must be worn. After soaking (usually overnight or longer), the part must be hoisted out and rinsed off in a separate tank, then scrubbed down and dried to prevent rust." Alkaline soaking poses another problem; the long downtime for the molds leaves presses idle until the molds are returned. This problem was most apparent for A K Rubber during long running jobs when molds had to be cleaned repeatedly. The disposal of waste liquid is another concern.

AKR bought a glass bead blast machine and used the finest screen size of glass bead available (to minimize erosion to the mold). They did not recycle glass bead. Once through the blast nozzle the fractured glass bead was disposed of since it was considered too abrasive.

They found that even with their precautions, over many cleanings glass bead caused both processing and aesthetic problems. As Fiegel pointed out, "As the mold began to erode, flash would get thicker. The thicker flash would then aggravate the deflashing process. At this point the mold would have to be replaced or repaired."

Glass bead blasting would also alter the surface of the rubber parts, changing a shiny finish to a satin finish over a very short period of time. In one case, mold wear caused by glass bead prevented the proper functioning of a delicate molded flow control device. A pin in the center of the mold cavity became worn and subsequently, the molded flow controls would not function. Fiegel admitted "This caused a lot of consternation and lost time as we tried to figure out why the part was not performing properly in our testing. We finally concluded it was due to mold wear." The company had to take the part out of production while the mold was refurbished. The total cost of the damage included the repairs to the mold, the mold downtime, wasted effort and the inconvenience to the customer. After that incident, A K Rubber stopped all use of glass bead for cleaning delicate or close tolerance molds.

Investigating an alternative mold cleaning process

Fiegel kept a lookout for better methods of mold cleaning. "Water honing and vapor cleaning methods were considered, but these systems were not seriously pursued because they seemed to be expensive and messy. The only logical choice seemed to be an improvement in the bead blasting process."

Mold cleaning with plastic media

Fiegel was familiar with plastic media deflashing applications. AKR has been using extruded thermoplastic media for cryogenic shot blast deflashing since 1977 and currently operates two 5 cu.ft. cryogenic blast units. Since he first saw the process, Fiegel has been enthused about the benefits of plastic media deflashing vs. steel shot wheelabrating. The benefits include: the easy maintenance, serviceability and compactness of the equipment and the advantages of the media. Plastic media do not pit the parts or leave rust residue, as steel shot does. In 1988, Fiegel read about and began to hear from other rubber molders about a new blasting process for cleaning molds using plastic media.

The plastic media blast (PMB) process, utilizing granulated thermoset media, was first developed in the late '70s by the aviation industry. The PMB process was used to clean paint from propellers, wheels, landing gear and other components prior to structural testing.

In the PMB process, millions of lightweight thermoset plastic particles are propelled by air pressure to remove relatively soft surface layers of paint, coating or contaminants without damaging harder underlying substrates. Plastic media are offered in a range of hardnesses and densities which falls between walnut shell and glass bead. Plastic media's moderate hardness and sharp edged granular shape not only enable it to strip paint and coatings, but also to clean heavy mold residue without damaging mold surfaces.

At one time, AKR experimented with plastic media in its glass bead unit, but the glass bead equipment did not propel the media at a high enough velocity to clean efficiently.

Fiegel tested the plastic media mold cleaning process and was very impressed by the speed and effectiveness of the system. AKR purchased a mold cleaning system consisting of a media propulsion and recycling unit and a blast cabinet. There are two options for pneumatic media propulsion and recycling: suction blast and pressure blast. More sophisticated pressure blast technology is also available.

Some systems include a pressure pot to force plastic media, mixed with pressurized air, through a nozzle at high speed, onto the part. Media are then recovered using a cyclone separator (figure 1). With a higher media-to-air ratio and increased media velocity, a pressure blast system provides significantly quicker cleaning action.

In some cases, a company will have a sand, glass bead or shot blast cabinet, which can be retrofitted to accept plastic media. This typically involves a new blast nozzle and adjustment of air pressure.

The system AKR selected has interior dimensions of approximately 48" wide x 36" deep x 48" high accommodating molds up to 32" without a mold rack. A K Rubber selected an optional 1,000 pound weight capacity cabinet and workcar, as well as an external 6' reinforced track extension to ease mold handling.

An estimate of media consumption for AKR was provided based on air/media flow charts (calculated from blasting pressures, nozzle size, etc.) and actual figures obtained from customers using the same equipment and media: Media -- Multi-Blast, 20/30 mesh (MB-2); blast pressure -- 50-55 psi; blast hrs./wk. -- 15-20 hours. Estimated media consumption: 500 lbs./mo. 500 lb. per month was considered a rough estimate. Media usage rates vary depending on their individual operating patterns.

Installation and operation

The mold cleaning system is easy to operate and maintain. After electricity and air are supplied and the machine is charged with media, an operator can begin cleaning molds.

Proper machine adjustments and operator technique maximizes cleaning efficiency. Machine performance is controlled by varying air pressure and media/air mix, and also by changing the blast nozzle size. The operator can also increase performance by varying the angle of the nozzle and the distance between the nozzle and the part. For consistent results, a small amount of fresh media should be added approximately every half hour during blasting to replace media that have been removed by the cyclone separator after breaking down into unusable dust. Although a typical blast system has a media capacity of approximately 50 lb., small additions at regular intervals maintain an ideal mix of large, medium and small particles providing aggressive and thorough cleaning.

Shortly after installation, AKR noticed a minor difficulty with static electricity build-up. The media are treated for static resistance, however improper grounding will cause media to cling to the mold and the inside of the cabinet. The static problem was eliminated by properly grounding the machine.

In some cases, the mold release agent works somewhat more effectively when applied to a glass bead etched surface. When this type of surface is desired, AKR uses plastic media first to clean the mold and then dusts it quickly with glass bead.

The benefits of using plastic media

It is a common belief among glass bead users, that a shiny metal surface insures a clean mold; however the bright metal indicates that the stained surface has been removed. In a controlled experiment, comparing PMB and glass bead blasting metal removal rates, PMB produced less that 10% of the metal removal of glass bead on K-100 aluminum and approximately 0.1% of the removal on Freemax 45 steel (figure 2).

In addition to reduced mold wear, plastic media will reduce wear to vital parts of the blast cabinet (hoses, gloves, nozzles, etc.) which translates into decreased equipment repair and replacement costs. This could amount to substantial savings over a period of time.

This mold cleaning system offers speed and efficiency, and eliminates hand cleaning. Thermoset plastic media do not melt and can be used to clean hot molds directly from the press. Molds can be cleaned more frequently because they are back in production in a matter of minutes. Fiegel claimed that their molds take from 10-30 minutes to thoroughly clean and return to production (table 1).

Table 1 - mold cleaning times
Mold type   Dimensions      # of        Product    Cleaning time
                            cavities    molded     (min/plate)
Tool steel    24" square    100         SBR            8 - 10
                                        O-rings
Tool steel    24" square    150         Natural/SBR    10 - 12
                                        bushings
Tool steel    30" square    200         Silicone       15 - 20
                                        grommets
Aluminum      48" square     1          EPDM           20 - 25
                                        tires

Both glass bead blasting and alkaline soaking involve more time than plastic media blasting and require training and a great deal of care. With glass bead, there is a significant potential for damage to the mold. If an operator is not careful he could quickly wear a critical edge on a delicate mold.

Plastic media blasting is non-hazardous to employees and the environment. Operators are not exposed to caustic or toxic chemicals or fumes, and waste dust is landfill safe. Although it is not required by OSHA or the system manufacturer, AKR instructs blast operators to wear ear protection when operating the machine.

Plastic blast media are very durable and can be recycled many times. At blast nozzle pressures below 50 psi, plastic media break down at the rate of 1%-5% per cycle (figure 3). Breakdown rates vary due to plastic media type, blast pressure used, nozzle distance from the mold and the physical properties of the mold being cleaned. Not only is plastic media durable, but compared to glass bead, it yields twice the volume of media per pound, making plastic media more cost effective.

Conclusions

There is a strong argument for use of plastic media for mold cleaning. It is particularly useful in working with high cost molds with critical dimensional requirements. While the cost of plastic media is higher than some other types of blast media on a per pound basis, the long term durability, speed, safety and ease of use more than offset the higher unit cost.

[Figures 1-3 ILLUSTRATION OMITTED]