Plunger pot sealing board.

It never ceases to amaze someone who retains a continual experience in the custom molded rubber business, just what could possibly be novel anymore after the many years of process improvement. It is a legitimate question to raise, as anyone can validate the numerous achievements pioneered by more than a handful of very talented individuals. Yet, as I meet with clients on a daily basis, I am often confronted with a situation whereby a client introduces me to what they believe is a very exclusive process which they have personally developed. This, then, leads to the revelation that the particular technique they have in use has also been adhered to by numerous other custom rubber molders for quite a few years.

It is with pride that we in the industry realize the majority of all the equipment manufactured today, from the simplistic to the very expensive and complicated, includes within it somewhere a small rubber part. This equipment will not function until these parts, which are designed and manufactured by the custom molded rubber producers, pass successfully through the molding process. Only then are the expectations of their creators fulfilled and the equipment is successfully passed on to the customers.

This article elects to highlight the technique of transfer molding and how the use of a device called the plunger pot sealing board (PPSB) will augment this type of rubber molding as a cost saving feature (figure 1).

[Figure 1 ILLUSTRATION OMITTED]

When touring a rubber molding factory that utilizes transfer molding, it is not unusual to observe a ribbon of uncured rubber squirting out from the interface between the pot and the plunger of the mold. This occurs frequently as the press reaches full pressure for that individual cure. In fact, it has been my experience to discover that in most instances, the operators, lead people and molding foreman accept this as a normal eventuality.

The existence of this leakage problem is unfortunate, as it most certainly leads to diminished profit; but I assure you, it is evident in most plants and factories. Transfer molders try to counter this with an explanation that they keep the void between the interface of the pot and plunger fitted at all times, and that this phenomenon does not exist in their transfer molding process. If you doubt that this situation is present in your plant, I urge you to take a ten minute walk-through of your transfer molding pressroom and observe the number of times you witness this problem unfolding before your eyes.

Many excellent custom rubber molders endeavor to keep this leakage under control. Only when you have the opportunity to visit, one on one, with these molders will they secretly confide that this leakage and subsequent loss of rubber material is one of their real concerns.

Several methods are commonly implemented in an attempt to solve this problem. When the molds are new and all the alignments are near perfect, this oozing of the rubber between the pot and plunger is not of great concern; but as the cures increase on the mold being run, several steps are taken to correct this dilemma. Each of these remedies increases the cost of the transfer molding operation, which in turn boosts the piece prices of the part, and the intended profit margin goal is generally decreased.

Custom molders order high quality tooling from their mold maker. The plunger/pot interface is matched as best as possible on these molds when they are new. Providing that these molds receive very good care while they are in production and also are run with the same cover or plunger, they will run for a prolonged period without any leakage. The time will come, however, when the seepage starts and the molding operation takes a negative turn towards a profit loss. In this scenario, the mold is removed from the press and sent to the mold shop for repair. This essential repair might require anywhere from one to three days. This depends on how quickly your mold shop or any outside mold shop can break into their scheduled workflow, push other work back and proceed with work on your mold. Generally, the customer has been promised that their parts will be shipped on a given date. Removing the mold from production for this plunger/pot repair will most likely create a delinquent delivery time, leaving your customer with an unfavorable image of your delivery proficiency.

Another frequently used practice is to cover the rubber in the transfer pot with a piece of paper. This could be newspaper, computer printout paper or the precut brown industry paper sectioned so each sheet accurately fits the mold pot. Several cures later there is more leakage at the plunger pot interface, which generally results in an increase in the number of sheets of paper used each cure. This routine proceeds until the transfer plate is deformed completely. The paper and rubber, which are encapsulated in the waste flash pad, are thrown into the waste basket and hauled to the closest landfill, which also adds to the piece price, due to the increased overhead that has now been unnecessarily created. There is, then, the need to add additional personnel to clean up the area around the press, due to these accumulated flash pads.

There is a simple, quick and successful solution to this problem. Plunger/pot sealing board (PPSB) is now available and can correct this leakage problem promptly for a very small investment of money and time. The following are just a few of the improvements that a custom molder who uses this transfer molding technique in their pressroom can also experience:

* Immediate improved seal-off between the plunger/pot interface is experienced. Molders who have evaluated PPSB in their molding process line have taken transfer molds that were leaking so adversely at the plunger/pot interface that the mold was going to be pulled from the molding line for repair. They were convinced to evaluate PPSB before the mold was removed from the press for repair. The net result was that the PPSB stopped all leakage on the plunger/pot sealing surface and the molder continued to run out several hundred cures, completed the order with accountability and shipped the parts on time to their customer.

* A reduction in the rubber compound needed per cure will in most instances be in the range of 10% to 15% when the use of PPSB is introduced to your transfer molding process. Many molders have reported savings up to 20%, and the highest savings per cure reported by my clients were in the 40% range. In some cases, by adding a sheet of muslin, cheesecloth or delta fabric with the PPSB, it is possible to reduce the flash pad to where the only rubber in the flash pad is in the sprue holes. The additional added material that is present will assist in pulling the sprues from the transfer plate. The use of this fabric material to pull out the sprues will save time because the molder does not have to pick out the sprues one at a time with a brass pick.

* Several users of PPSB have realized a reduction in molding rejects after they establish its use in their transfer molding process. The most obvious reason for an increase in rejects, in my view, is that when the ribbon of rubber begins to extrude out between the pot and the plunger, the pressure in the mold is changed and the filling sequence is interrupted, which causes unfills in the parts being molded. The real concern regarding these towlines in the molded parts is that they are very miniscule and difficult to detect. This can become particularly delicate when the customer discovers these rejects or the part fails in application. PPSB will correct this situation.

Plunger/pot sealing board is available for purchase in several thicknesses. The most popular thickness by far is .187". The. 187" thickness is the size that is most always suggested for trial runs when a new client is preparing to evaluate the use of the PPSB on their production transfer molding line.

Plunger pot sealing board is a highly structured, tightly crosslinked polymer. The characteristic that sets it apart from other structured materials that have been evaluated for use in similar applications is its ultra high resistance to deformation, stretching or extrusion at the normal rubber molding temperatures that are utilized today.

The directions for using PPSB are clear and straightforward. They include fitting the plunger/pot sealing board to the transfer mold.

* Step 1. The I.D. dimension of the pot must be measured very accurately. There must be 1/16" overall clearance between the O.D. of the plunger/pot sealing board and the I.D. of the mold pot. The clearance to establish the needed fit between the sealing board and the pot should be 1/32".

* Step 2. Drill a 1/2" hole through the PPSB near the center.

Also included in the directions for use is transferring the molding using the PPSB.

* Step 1. Load and center the accurate weight of rubber needed for a cure/heat into the mold pot.

* Step 2. Place the PPSB on the top of the rubber. Make sure that the PPSB is centered in the mold so it will fit into the mold pot when the press is closed.

* Step 3. Run the press closed in the normal manner and complete the cure cycle with PPSB sealing and closing off the escape of the rubber material.

* Step 4. When the mold opens, the PPSB will normally bow upwards in the middle. If this does not happen, blowing air on the center and top of the PPSB will create the bowing of the PPSB.

* Step 5. Insertion of a brass pick and blowing compressed air into the hole that was drilled through the PPSB will cause the PPSB to release from the mold pot.

* Step 6. Set the PPSB aside for the next cure.

* Step 7. Unload and load the mold in the normal manner and repeat the cycle.