Fairprene® - Diaphragm Manufacture

DIAPHRAGM MANUFACTURE

Depending on the type of diaphragm being considered, several manufacturing methods can be used. Flat and non-molded types are simply cut from flat, cured stock as received from the supplier. When an application requires a convoluted or deep-draw diaphragm, tooling and molding conditions are critical for successful to production. The design engineer has to carefully consider tool design. Injection, transfer, and compression molding are three methods commonly used in forming rubber parts. In the manufacture of fabric-supported diaphragms, compression molding is most commonly used. With this method, a set of matched male and female tools is brought together to shape the diaphragm to the desired configuration. The tools, presses and material to be molded have significant bearing on the manufacture of fabric-supported diaphragms.

Cutting Methods for Flat Diaphragms
Steel Rule Dies: The steel rule die is manufactured from high-strength spring steel, sharpened to a fine edge. The steel strip is inserted in a groove cut in a heavy plywood base. This type of die is easily manufactured but is subject to early dimensional loss through continued use. Steel rule dies find wide application in industry where high tolerance values are not specified.
Punch Press: With the punch press method, heavy steel dies are machined and ground from tool stock. Although they are more expensive and difficult to make them steel-rule dies, punch press dies give greater accuracy and service life.

Molds
Several materials are used for the fabrication of molds. These include cast iron, aluminum, phosphor-bronze, low carbon steels, high carbon steels and zinc. The finished mold should have adequate service life along with a good balance of heat transfer properties and corrosion resistance. A low carbon steel is generally used because of its low cost and ease of machining. In some cases, this type of mold is case-hardened or chrome plated to increase its service life or improve its release properties. For molds requiring harder surfaces, steel with high carbon content is used. These molds are more difficult to machine and are more expensive. They do, however, have the advantage of longer life and less distortion under load. For economical, short production runs, aluminum molds can be used. They should be treated carefully, as they are easily damaged.
Design: The design of the mold cavity must be machined to obtain the desired convolution dimensions. Cavities are designed slightly oversize to eliminate stress points during the molding operation. These oversize dimensions also allow for shrinkage which occurs when the part is removed from the mold. Molds should be made in matched pairs and marked so that they are never used interchangeably.
Dowel Pins: Most molds are guided by dowel pins which are made from high carbon steel to provide longer life. To prevent misalignment and subsequent difficulty in opening or closing the mold, dowels must be handled carefully. In some applications dowel pins are not necessary; the mold halves are installed in the press and align themselves as the presses close.
A word of caution: Misalignment will cause serious damage to the mold.
Basic Types: Two basic types of mold openings are used in the industry, parallel and book (or hinged) molds. Parallel molds, which open straight up, should provide adequate space for easy loading and cleaning. This feature is extremely important if the molds are an integral part of the press. Book molds open like the covers of a book. They are easy to clean and load and less subject to damage, but they are more expensive. Molds, whether single or multi-cavity types, should be designed for ease of handling.
Manual Opening: When the molds are operated manually, a means of supplying leverage during the opening stage should be provided. This can be accomplished by machining a slot between the two mold parts. The slot should not interfere with the mating surfaces of the mold.
Stops: Pressure on the part being molded can be controlled by using stops, which are part of the mold. These stops are raised portions on the outer diameter of the tool. They allow it to close only a specified distance. In other molding procedures, no stops are used and pressure is controlled directly by the press.
Pressure Relief: Some means of relieving gas pressure which develops during the molding operation should be provided. Drilling a small hole in the middle of the mold or a rapid opening and closing of the press may accomplish the task. The latter process is called "bumping."

Presses
Various presses are available ranging from simple laboratory to large production models.
Types of Presses: Hydraulic and mechanical presses may be powered by hand, steam pressure or electric motors. The most common type used in production and the laboratory is the hydraulic press.
Platens: Because the molding of rubber diaphragms requires the rubber to be vulcanized, the presses should accommodate heated platens and have the capability to develop temperatures in the 350°F (177°C) range. In addition, they should be designed to insure parallel alignment of the platens under the variable pressure and temperature conditions. Distortion of the platens can cause improper closing of the molds, resulting in rejected production or mold damage.

Materials
Since most materials are supplied to the molder in either an unvulcanized or semi-vulcanized state, space should be available for storage over a period of time to minimize variation in molding properties. Storing the materials in a cool, dry place is recommended and necessary for some materials.
Semi-cured materials can be used to shorten molding time in the manufacture of some types of diaphragms. Whether uncured or semi-cured, the base material should be somewhat thicker than the desired thickness of the finished part.
Mold staining may occur and is a result of the build-up of foreign material on the face of the mold. This deposit may be particles of the surface dusting agent, matter exuded from the rubber, by-products of the vulcanizing process, or release agents. The accumulation can be partially controlled by adjusting molding temperature and changing release agents. Satisfactory temperatures and mold releases may have to be developed through experimentation.
Release from the hot mold requires special attention. If the diaphragm is difficult to remove, the operation becomes costly in terms of time and possible damage to the part and/or tool. Release of diaphragm stock may be controlled by surface dusting agents, compounding techniques, temperature and pressure alignments, and choice of release agent.
A coated fabric for diaphragm molding should be capable of reaching a full state of cure in a relatively short time at 300°F to 340°F (150°C to 171°C). A typical cure cycle is 5 to 8 minutes at 340°F (171°C). This can vary, however, depending upon the material.

Molding Operation
Whether single or multi-cavity tools are involved in the molding operation, precut blanks are usually used. The blanks for single cavity molds are easier to cut than those for multi-cavity molds.
Blanks for single cavity molds are cut from raw stock so that they are slightly larger than the finished part. The over-sizing compensates for fabric "pull-in." Blanks for multi-cavity tools require slits to relieve tension and distortion, as the convolution height increases.
After the blank is cut, it is placed in the tool and heated under pressure to form the part configuration and vulcanize the rubber. Molds with stops are used where there are dangers of distorting or damaging pressures. If accurate pressure control is possible, stops are not needed and the molds are closed using a specified pressure. This pressure can vary widely, ranging from very pounds per square inch to 1000 psi (0.3 to 7 MPa). The correct pressure must be determined by trial. Pressure variability can be the cause of rejected parts.

Diaphragm Inspection
A molded diaphragm may be rejected for many reasons. Some of the more common faults and their causes follow:
Pinholes: Pinholes can be caused by trapped air, moisture, solvent, improper molding temperature and pressure, or mold staining. The effect of moisture can be minimized by storing at low humidity or by preheating the blanks at 160°F (71°C). Trapped solvent is largely the responsibility of the diaphragm-stock manufacturer; it may be removed, however, during the molding stage by exposing the diaphragm to enough heat to evaporate the solvent without damaging the rubber. A temperature of 150°F (65°C) should be adequate. Excessive molding temperatures can cure the diaphragm surface more rapidly than the interior position. This condition prevents the escape of gas from inside the diaphragm. Gases then force their way out through the rubber surface causing pinholes and blisters.
Tear Drop: This is a saucer-like depression associated with air, excess release agent, or other volatile material trapped between the diaphragm and mold cavity. The imperfection can be eliminated by using higher molding temperatures or less release agent, or by mold venting and press bumping.
Cracks: Poor rubber flow can cause cracks in the diaphragm. These flaws may be the result of improper die design, excessive pressure, or partially cured stock material. Too much heat can force the elastomer to cure before it has formed completely.
Foreign Material: Contamination, dirty molds, and improper dispersion of compounding ingredients are hazards in the molding process. Clean molds and good dispersion can eliminate the problem. A source of air should be available to blow dirt from the mold surface between cures.
Shallow Convolution: This is usually a result of poor mold design, improper pressure, poor rubber flow, prevulcanized rubber, or incorrect molding temperature. In most cases, simple adjustments in pressure and curing procedure will correct the problem. Shallow convolution can also result from failure of the press to close rapidly enough.
Flat Convolutions: Compound flowing away from the top of the convoluted area can result in flattened convolutions. One of the most common causes of this problem is an oversized vent hole. A large vent hole acts as a point of stress relief; the rubber migrates to this point, thereby leaving an insufficient amount of compound for the convolution. For this reason vent holes should be kept as small as possible.
Elastomer Tears: Tears usually occur when the finished part has a tendency to stick to the mold. Proper release agents should ease the problem. Adjustment of curing temperatures may also help. In some cases, it is necessary to let the diaphragm cool slightly before removing it from the mold.
Distorted Diaphragms: Here again the problem usually occurs when the finished part has a tendency to stick to the mold. The same corrective measures cited above apply.
Curling: This can be caused by unbalanced coatings or improper molding techniques. Excessive or uneven pressure can lead to curling as rubber is forced from one side of the fabric to the other.