Principles of Centrifugal Rubber Mold Casting : Chapter 3

1. Equipment
2. The Pewter Metal Furnace
3. The Crucible - Seasoning and Maintenance
4. The Vulcanizer and Its Parts
5. The CRMC Machine
6. How to Lay Out a Pewter Casting Shop

3.1: Equipment

The foundation for high quality casting is the equipment on which molds are made and cast. Even the most experienced moldmaker cannot produce fine work if the equipment he works with is neglected, worn or not otherwise up to standard.

A good mold is one that has been properly cured, has smooth, parallel surfaces and has been correctly gated and vented to allow molten metal to flow through the gating system and make satisfactory castings. The equipment on which it is made may look simple but it is often overlooked as a cause of casting problems. Many things can and do go wrong. Vulcanizers that are poorly maintained or in need of repairs can be a major cause of poor molds. Continual use eventually wears the bushings on the platen as it moves up and down on the posts, which in turn, causes lopsided mold curing. Heating elements in the platens may either fail or become inaccurate resulting in improperly cured molds. Timers may malfunction and adversely affect compression or curing cycles. Mold frames eventually wear out causing an excess of rubber flash, mold distortion or model shift. The casting machine may develop drive belt slippage. The pressure gauge can become inaccurate and affect mitering pressure. A malfunctioning heat control unit on the soft metal furnace can be a disaster for alloys sensitive to temperature variations.

The moldmaker and the caster should consider their equipment the tools of a skilled trade-no less important than those used of a toolmaker. Postponing maintenance, no matter how minor, can lead to major problems at a later, more critical time. Weekly inspection and maintenance routines should include oiling. greasing, calibration of all heating and timing devices, and cleaning of all equipment.

3.2: The pewter metal furnace

Also called the ‘pot’ or ‘crucible.’ The soft metal furnace is heated by a gas fired atmospheric burner and requires no additional power or blower to support its combustion. For safety purposes, it must be equipped with some system of pilot flame safeguard to provide an instantaneous closing of both the main and pilot solenoid valves when the pilot flame is not in contact with the ‘flame rod. Whenever any gas fired equipment is being installed, both the equipment supplier and the local gas company should be consulted as to proper installation and safeguards. The standard soft metal furnace used in CRMC has a cast iron crucible with a capacity of 160 pounds of lead (100 pounds of high tin alloy). It may be purchased either mounted on its own metal table (Fig. 3.1) or as a free-standing unit (Fig. 3.2). It should be set up away from drafts to prevent both flameouts and temperature variations. Ventilation sufficient to remove the products of combustion, and to supply ‘make-up’ air for breathing, should be carefully engineered and installed by a properly qualified heating and ventilation company.

Fig 3.1: Soft metal furnace - freestanding and parts

The majority of furnaces in use are gas fired but electric types are available for areas where there are no provisions for gas. They are, however, slower. more costly to operate and may not be capable of reaching or maintaining temperatures high enough for the lead alloys. Furnaces fired by bottled gas would be preferable if available.

All furnaces require a heat control. Since the temperature of any alloy is critical, control of the heat should be paramount. Because the temperature probe is usually made of copper, it is important that it never come into direct contact with the molten alloy in the pot as the alloy will react with and dissolve the copper. A cast iron well (Fig. 3.3.) comes with the furnace and should be used to contain the probe. The level of molten metal in a crucible should never be allowed to fall below the tip of the probe in the well as it could damage both the probe and heat control.

3.3: The crucible-seasoning and maintenance

Before any new crucible is placed into production, it is suggested that it be ‘aged’ (stress annealed) to prevent any possible thermal shock to the cast iron. Even though modern metallurgical processes used in casting are meant to prevent this possibility, aging does seem to help extend the useful life of a crucible. One method of aging used by the Oster Group is simply to fill a new crucible with water and slowly boil the water out. A coating of liquid graphite (‘pot-black’) is then applied to the inside of the crucible to prevent iron and other contaminants from the cast iron dissolving into casting alloys.

Fig 3.2: Soft metal furnace-setup on a stand

Fig 3.3: Cast iron well

The Pottstown Machine Company. Pottstown, Pennsylvania, a manufacturer of crucibles offers the following suggestions in the use of any crucible:

“In cold weather, allow a new crucible to reach room temperature before using it for the first time. Always heat a crucible gradually. Do not simply set the temperature control at 700°F and leave it there. Raise the temperature settings gradually in increments of 100°F. During periods of non-production, keep the crucible under moderate heat, either full or with at least a 2” heel or level of metal, allowing casting alloys to solidify and remelt alternately can cause cracks to develop in the crucible.”

How to clean a crucible: A crucible should always be cleaned whenever an alloy is changed, even if they are ap- proximately the same in composition. First, remove any alloy still remaining in the crucible. Then, while the crucible is still hot, thoroughly scrape its sides with a skimmer. If necessary, the interior should be cleaned with a wire brush after scraping. An OSHA approved face mask must be worn when performing any cleaning operations to prevent the inhalation of toxic dusts and fumes from any lead/tin alloy. After cleaning, apply a fresh coating of graphite to the inside of the crucible. Liquid graphite, besides acting as a contaminant retardant is also an excellent release agent in preventing buildup of solidified metals on the crucible, casting ladles and both the casting machine funnel and pressure plate.

A crucible used for any zinc alloy must never be reused for a tin/lead or lead alloy. No amount of cleaning will permit its reuse for any other alloy except zinc. Zinc alters the chemistry of the cast iron crucible and reacts unfavorably with other alloys.

3.4: The vulcanizer and its parts

A vulcanizer is a positive compression molding machine similar to an hydraulic press. (Fig. 3.4) It cures rubber molds with heat and pressure. All vulcanizers on the market today, operate on the same principle and vary only in design features, such as the manner in which platens are brought together, the number of posts or ways on which they travel and the method of heating the platens.

Because of the weight (700 pounds average) of a vulcanizer, it is extremely important that it be mounted on a strong and secured work table such as that suggested in Fig. 3.5. In Fig. 3.6 in for Vulcanizer. Leverage is important when jacking the mechanical jack operated vulcanizer. The length of the jack handle is a determining factor in the amount of leverage that is being applied to close the platens. It is easy to visualize that the amount of force being exerted by a 200 pound moldmaker leaning on a 4 foot handle can easily tip both the vulcanizer and table if they are not properly secured.

The proper working height of a vulcanizer is one at which the closed platens are approximately at the height of the moldmaker’s elbows. (Fig. 1.10) A platform should be provided for moldmakers who are of shorter stature rather than positioning the vulcanizer so that the taller moldmaker must bend uncomfortably when working.

The important parts of a vulcanizer are:

Platens: The compression plates, consisting of a movable lower platen and a stationary upper platen. The lower platen is raised by a jack (mechanical or hydraulic) to bring the platen and a mold frame into positive contact with the upper platen. In order to create the necessary compression, jacks are rated at a force of 15 to 25 tons. An average pressure of 15,000 psi is usually exerted in the compression stage of curing. This forces the models within the uncured mold set to accept a negative impression of the models, basin former and locators, and, while maintaining this force, to be cured permanently into the rubber by heat. Both platens are heated electrically and must be calibrated to work in unison.

Fig 3.4: Vulcanizer

In order to produce a good mold the lower platen must move upwards perfectly parallel to the upper platen on the posts of the vulcanizer. Too much free play on the bushings will cause it to make a lopsided contact resulting in non-parallel molds. The lower platen should be inscribed with a centering mark to assure that the mold frame is always centered.

Heat controls: Curing temperatures are important, and a variance in temperatures during curing can change both the curing time and the results. A difference in temperature in the two platens can cause half of a mold being cured to enter its ‘free-flow’ stage before the other half. This can cause a mold to cure with poor parting line control since the half of the mold which is more free-flowing will accept a deeper impression of the models. Even when the temperature is properly distributed, if it is the incorrect temperature for the particular job it will cure improperly. Also, a mold that has not entered its free-flow stage at its normal time in the cycle may be incorrectly squeezed causing distortion of models or poor detail in the cavities.

Fig 3.5: Suggested vulcanizer table

Gauge: The newer style vulcanizers are equipped with pressure gauges that give readings in ‘tons of pressure’ being exerted on the platens during the compression process in curing. However, since molds vary greatly, depending upon the rubber, the models and the mold type, a moldmaker would need more information before he could use the gauge as a meaningful reference. Most moldmakers rely on the feel of the jack handle (mechanical jack) rather than from a gauge.

Fig 3.6: Suggested holddown for a vulcanizer

Jack: The jack may be mechanical or hydraulic. Either will work equally as well but the moldmaker must have leverage when using a mechanical jack in order to achieve the force necessary to impress the models into the uncured rubber molds. It is this leverage that makes all moldmakers equal regardless of differences in height, weight, build, strength or sex. Many mold failures, especially those involving lack of detail, can be traced back to the length of the jack handle. Moldmakers should therefore make sure the handle is long enough to suit their own particular need.

Frame: The frame set used for curing molds consists of three parts; top pressure plate, frame ring and bottom plate. They are available in 9”, 10”, 12” and 18” diameters, with rings available for any mold thickness. Special sizes can be custom made. For more on the frame, see Chapter 1.8.

Fig 3.7: Air cast-CRMC machine

3.5: The CRMC machine

The majority of centrifugal rubber mold casting machines in use are air operated and compensate automatically for differences in mold-thickness and non-parallel mold surfaces. (Fig. 3.7). They are also referred to as ‘air casting machines’ (not to be confused with automatic air casting machines which automatically index molds and feed molten metal during the spin cycle). The CRMC machine operates in a range of 300 to 600 rpm’s and molds are mitered with air pressure settings from 19 psi up to 100 psi. They will accommodate 9”, 10” and 12” molds as standard. Machines are available for larger size molds. All CRMC machines require a grounded 110V electric supply and an air compressor which will maintain 100 psi for extended periods of time.

The newer CRMC machines will not start the spin cycle until the air pressure reaches 19 psi and the lid of the machine is closed. In addition they will come to a dead stop the moment the lid is raised. Most of the older machines were not equipped with this important safety device and the machine would continue to spin long after the switch was shut off or the lid raised. The operator would grasp the sprue collar on the still spinning pressure plate with a gloved hand to slow down and stop the machine sort of like putting your hand into a blender.

3.6: How to lay out a pewter casting shop

When the decision has been made to set up a modern casting shop, consideration should first be given as to the best and most efficient layout of ‘man and machine. Electrical requirements, ventilation, security and most importantly, state and federal regulations, especially those set by OSHA (Oссuраtional Safety and Health Act), should be of major consideration in such planning.

Fig 3.8: Suggested setup for a single casting unit

The best location for any casting shop is an isolated room where heat, fumes and dust will not be associated with the rest of the plant. Also, moldmaking should be separated from the casting area. The following points in layout planning should be carefully reviewed, since set-up costs are always lower than the cost of making changes at a later time.

The layout suggested in Fig. 3.8 is considered the most efficient because it requires minimum operator movement resulting in less fatigue. It also provides a segregated but highly visible work- place free from traffic and distractions.

Additional setups would simply be multiples of this single machine layout. (Fig. 3.9)

Lighting: The caster should be located near a source of natural daylight supplemented by artificial lighting. Open windows should be avoided near casting pots because of the effect of drafts on molten metal. (Chapter 4 Alloys)

Floors: Concrete or wooden floors should be easy to clean. Uneven floors cause operator fatigue and should be leveled or provided with ‘duck boards’.

Mold racks: The mold rack suggested in Fig. 3.10, is simple, inexpensive, easy to build and extend. It should be available for easy access for the caster, but away from direct fluorescent lights and electric motors because they create ozone which has a deleterious effect on the life of rubber.

Fig 3.9: Suggested setup for multiple casting units

Scrap containers: Containers for dross, dust and scrap should be made of metal with secure, tight closures because of the toxicity of scrap.

Security: Alloys, because they are expensive, and easily convertible into cash, should always be stored under some form of security. Material sufficient for one day’s production only, should be distributed at the beginning of the work day. Inventory records should show a running in-out and balance on hand at all times before any ‘shrinkage’ can accumulate.

Fire sprinklers: The supplier of the soft metal furnace should be advised where fire sprinklers are located in the casting area. The danger of explosion caused by water reaching hot metal in the furnace is as great as the danger from fire. Sprinkler heads in the area may require higher release temperatures.

Fig 3.10: Suggested construction of a mold rack

Ventilation: The design of any ventilation system for a casting shop is critical and is beyond the expertise of management. Consideration has to be given in its design to the removal of the products of combustion, toxic dusts from both talc and dross, and the fumes from the alloys being used. Sufficient makeup air for breathing and heating must be provided. A properly qualified heating and ventilation company should engineer and install the necessary equipment.

Gas: Whenever gas is used, an emergency gas shutoff valve, clearly visible, properly identified, easily and quickly reached, should be mandatory. Running around the casting room in an emergency looking for a gas shutoff can be hazardous to your health and wellbeing!

Electrical: Local building codes and OSHA regulations must be observed, especially in grounding of all electrical equipment. Switch and fuse boxes should be clearly visible, properly identified, and easily reached in an emergency.

Fire exits: Fire exits, clearly marked and easily reached must be included in any plant layout.

Air compressor: Air compressors must be separate from the casting area be- cause of noise and possible danger. A soundproof and fireproof closet is an excellent place for its installation.

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Chapter 2: Introduction to the caster’s job and basic casting

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Chapter 4: All about alloys