MECHANICAL,CSS,HTML: Molding Systems

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Friday 1 November 2013

Squeeze Casting

The process is suitable for components with relatively thick wall sections with high mechanical properties, as for example required of safety components in automotive engineering. The cast components can be welded and heattreated, and they can be produced with near net shape. Aluminum alloys can be used which are difficult or impossible to produce by standard die casting.

Buhler selectively utilizes the advantages of the horizontal shot sleeve system.
Advantages using Buhler machines

• The velocity and pressure intensification profile matched to the component geometry can be programmed in very many discrete steps. Real time control maintains these parameters constant.
• Depending on the type of shot unit selected, it is possible to generate high pressure intensification values during the solidification phase.

Your benefits
• Low capital investment, as no special-purpose machines are required.
• Entering of future-oriented market segments using existing SC machines.
• Low maintenance and training requirements thanks to unified machine and die ranges.

Sandy loam soils

Sandy loam soils are dominated by sand particles, but contain enough clay and sediment to provide some structure and fertility. There are four different types of sandy loam soil that are classified based on the size of the sand particles in the soil. You can determine whether your yard has this kind of soil using a simple test.

Classification

Sandy loam soils are broken down into four categories, including coarse sandy loam, fine sandy loam, sandy loam and very fine sandy loam. The size of the sand particles is measured in millimeters and their concentration in the soil is used to determine which category a soil falls under. Sandy loam soils are made of approximately 60 percent sand, 10 percent clay and 30 percent silt particles.

Characteristics

Sandy loam soils have visible particles of sand mixed into the soil. When sandy loams soils are compressed, they hold their shape but break apart easily. Sandy loam soils have a high concentration of sand that gives them a gritty feel. In gardens and lawns, sandy loam soils are capable of quickly draining excess water but can not hold significant amounts of water or nutrients for your plants. Plants grown in this type of soil will require more frequent irrigation and fertilization than soils with a higher concentration of clay and sediment. Sandy loam soils are often deficient in specific micronutrients and may require additional fertilization to support healthy plant growth.

Identification

You can quickly identify sandy loam soil based on its physical characteristics. Pick up a handful of dry soil and slowly dribble water onto it. Work the water into the soil with your hand until it has a smooth consistency similar to putty. Hold the soil in your hand as though you are holding a pipe straight up and down and squeeze it. Sandy loam soils have a very gritty texture. If your soil is a sandy loam, it will form a cohesive ribbon of soil as it squeezes out between your thumb and finger that will fall apart before it reaches one inch in length.

Considerations

Plants that are grown in a sandy loam soil need frequent irrigation and fertilization to maintain healthy growth. The best way to improve a sandy loam soil for gardening is to mix organic matter into the soil. Incorporating a 2- to 4-inch layer of compost or peat moss over the area can significantly improve the ability of your sandy loam soil to hold nutrients and water.

Facing Sand

A sand casting facing composition comprises a dry mixture of about 77% fine sand, 5% binder, 6% green system sand and 12% burned system sand. The fine sand is mulled with 5.8% wt/wt oil per total dry mixture and catalyst at about 0.5% wt/wt. Preferably, fine sand and binder are mulled with oil, catalyst, green system sand and screened burned system. The mulled mixture is rested, mulled again and rested again before use as facing sand for achieving accurate reproduction of the pattern's fine detail. A method of preparing the mold and preserving fine detail comprises riddling the pattern with a thin layer of the facing sand composition, compacting, riddling with dry system sand and riddling with system sand before compacting about the periphery of the pattern. Final layers of system sand are applied and compacted over the entire pattern.

Tuesday 29 October 2013

Green Sand Molding

The most common method used to make metal castings is green sand molding. In this process, granular refractory sand is coated with a mixture of bentonite clay, water and, in some cases, other additives. The additives help to harden and hold the mold shape to withstand the pressures of the molten metal.
The green sand mixture is compacted through mechanical force or by hand around a pattern to create a mold. The mechanical force can be induced by slinging, jolting, squeezing or by impact/impulse.
The following points should be taken into account when considering the green sand molding process:

for many metal applications, green sand processes are the most cost-effective of all metal forming operations;
these processes readily lend themselves to automated systems for high-volume work as well as short runs and prototype work;
in the case of slinging, manual jolt or squeeze molding to form the mold, wood or plastic pattern materials can be used. High-pressure, high-density molding methods almost always require metal pattern equipment;
high-pressure, high-density molding normally produces a well-compacted mold, which yields better surface finishes, casting dimensions and tolerances;
the properties of green sand are adjustable within a wide range, making it possible to use this process with all types of green sand molding equipment and for a majority of alloys poured.

Chemically Bonded Molding Systems

This category of sand casting process is used widely throughout the metalcasting industry because of the economics and improved productivity each offers. Each process uses a unique chemical binder and catalyst to cure and harden the mold and/or core. Some processes require heat to facilitate the curing mechanism, though others do not.
Gas Catalyzed or Coldbox Systems—Coldbox systems utilize a family of binders where the catalyst is not added to the sand mixture. Catalysts in the form of a gas or vapor are added to the sand and resin component so the mixture will not cure until it is brought into contact with a catalyst agent. The sand-resin mixture is blown into a corebox to compact the sand, and a catalytic gas or vapor is permeated through the sand mixture, where the catalyst reacts with the resin component to harden the sand mixture almost instantly. Any sand mixture that has not come into contact with the catalyst is still capable of being cured, so many small cores can be produced from a large batch of mixed sand.
Several coldbox processes exist, including phenolic urethane/amine vapor, furan/SO2, acrylic/SO2 and sodium silicate/CO2. In general, coldbox processes offer:

good dimensional accuracy of the cores because they are cured without the use of heat;
excellent surface finish of the casting;
short production cycles that are optimal for high production runs;
excellent shelf life of the cores and molds.

Shell Process—In this process, sand is pre-coated with a phenolic novalac resin containing a hexamethylenetetramine catalyst. The resin-coated sand is dumped, blown or shot into a metal corebox or over a metal pattern that has been heated to 450-650F (232-343C). Shell molds are made in halves that are glued or clamped together before pouring. Cores, on the other hand, can be made whole, or, in the case of complicated applications, can be made of multiple pieces glued together.

Benefits of the shell process include:

an excellent core or mold surface resulting in good casting finish;
good dimensional accuracy in the casting because of mold rigidity;
storage for indefinite periods of time, which improves just-in-time delivery;
high-volume production;
selection of refractory material other than silica for specialty applications;
a savings in materials usage through the use of hollow cores and thin shell molds.

Nobake or Airset Systems—In order to improve productivity and eliminate the need for heat or gassing to cure mold and core binders, a series of resin systems referred to as nobake or airset binders was developed.
In these systems, sand is mixed with one or two liquid resin components and a liquid catalyst component. As soon as the resin(s) and catalyst combine, a chemical reaction begins to take place that hardens (cures) the binder. The curing time can be lengthened or shortened based on the amount of catalyst used and the temperature of the refractory sand.

The mixed sand is placed against the pattern or into the corebox. Although the sand mixtures have good flowability, some form of compaction (usually vibration) is used to provide densification of the sand in the mold/core. After a period of time, the core/mold has cured sufficiently to allow stripping from the corebox or pattern without distortion. The cores/molds are then allowed to sit and thoroughly cure. After curing, they can accept a refractory wash or coating that provides a better surface finish on the casting and protects the sand in the mold from the heat and erosive action of the molten metal as it enters the mold cavity.
The nobake process provides the following advantages:

wood, and in some cases, plastic patterns and coreboxes can be used;
due to the rigidity of the mold, good casting dimensional tolerances are readily achievable;
casting finishes are very good;
most of the systems allow easy shakeout (the separation of the casting from the mold after solidification is complete);
cores and molds can be stored indefinitely.

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