Mold for Plastic Material
How Plastic Injection Works？
First thermoplastic granules are heated to get rid of the moisture. This can be done in an industrial baker or in the barrel on the injection machine. Then they are melted, compressed in the barrel, shot into a mold and then cool down to form the final product. A typical injection mold is consist with 2 halves, when injection finished, mold open, the final product is then ejected.
- About Rapid Molding
Standard injection molding process aims for massive productions. Normally, when production volume reaches over 5000 pieces, molding start to become more cost-effective than alternatives. However, there is “rapid and simple” molding process designed for small volume production.
- Dual Color
Dual Color injection is done by a injection machine with 2 barrels. The 2 barrels contains same thermoplastic with different color, and both connected to the mold feed nozzle. By adjusting the injection timing of the 2 colors, different color pattern can be formed.
- Hot Runner
The cavities are the empty part in the mold with the shape of the product.
A runner is a channel or channels between then feed nozzle to the cavities with the purpose of transporting molten plastic flow. In a simple cold runner system mold, the plastic in the runner cools down and is ejected when parts are cool down and ejected. A hot runner system is consist with an extra heated manifold and a few heated nozzle, so that when parts cool down and injected, the plastic in runner is still hot and not injected.
Benefit of hot runners are:
Quicker production. runner does not need cool down, less time.
Easier operation. No need to remove the runners plastic. Easier for operation, especially important for automatic grab system.
Material Saving. Plastic in runners now not wasted.
The tradeoff is obviously the mold cost. Hot runner system involves extra heated manifolds and also a heating control system, and hence are more suitable for relative large quantity productions.
2 Types Plastic Material in One Part
Typical applications are plastic handles which consist with a harder plastic core and a softer plastic layer (for example TPU or Silicone). The process to realize that is called overmolding. First the higher melting point plastic (normally is also the harder plastic) is injected, once it is cooled, the lower melting point resin (normally is also the softer one) is then injected on the top of the harder part.
The process does have its limitations. The first injected parts must be able to withstand the temperature up to the second part’s melting point. And the 2 types plastic must not chemically react to each other during the second injection process.
First insert a metal, plastic or other material parts into a injection mold and then inject the melted plastic. This process is called insert molding. The inserted parts is then firmly integrated in the product made after injection. Typical applications are metal thread bushing insert, ceramic isolation core insert etc. One important thing for insert molding is to make sure the inserted parts can withstand the heats and pressures created by injection process without too much deform or crack.
Extrusion Blowing, Injection Blowing and Stretch Blow Molding
- Extrusion Blowing is the easiest and most popular used one among 3. Thermoplastic materials are melted and pushed out to form a hollow tube (the technical jargon for this is “parison”). The tube is surrounded by a metal mold with 2 halves. High pressure air is then blow into the tube, press the plastic to form the shape of the mold cavity. Then a cooling process is introduced to the mold, the 2 halves are open and parts are removed. Compare with the other 2 methods, extrusion blowing mold’s structure is a lot simpler, so is the equipment needed. For smaller volume it is certainly a better choice.
- Injection Blowing Molding involves 2 molds, one for injection and one for blowing. Thermoplastic materials are melted and injected into the injection mold and build a initial shape (preform) around a core rod. The core rod together with the preform are removed from the injection mold and clamped into the blowing mold, high pressure air is then introduced through the core rod and make the final shape. In short, first inject plastic to make a preform and then blow the preform to final part.
- Stretch Blow Molding is basically the same as injection molding. Only difference is before blow the preform, the preform is stretched by the core rod to a longer shape.
Both Injection Blowing and Stretch Blow Molding requires much more complicated tools and equipment. They are more material saving, better in efficiency if massive quantity are involved.
Mold for Metal Parts
A piston pushes melted metal into a mold, after cooling under pressure. Metal forms the shape of the mold cavity. That is die-casting. Most non ferrous metal like zinc alloy, copper alloy, aluminium alloy, magnesium alloy, lead alloy and pewter alloy can be die casted. But die casting metal alloy is not always the same alloy used in forging or extrusion, for example, aluminium casting alloy does not work well with anodizing colors. And die casting mold is more expensive than plastic injection mold as it has to take much higher pressure and temperature. Nonetheless, it is still the most cost-effective way to produce massive amount of metal parts.
Sheets Metal Mold
Although some sheet metal moldings are used for metal sheets over 8 or even 10mm,but usually we call a ”sheet”metal when its thickness is below 6mm. Sheets metal molding is probably the most diversified molding method, it bends, stamps, cuts, rolls, perforates and rivets metal sheets.
There are many different ways to mold sheet metal, each with its own set of equipment, costs and processes. Sheet metal can be formed by bending, curling, rolling, spinning, stamping, laser cutting, ironing, perforating and much more. The process or processes used (in many cases, more than one process is needed) typically depends on the product itself. Aluminum, brass, copper, steel sheets can all be molded into various shapes.
Although in average simple sheet metal mold is cheaper than injection molds but still a good number of production volume is needed to reach a good cost-effectiveness. When production volume reaches a few thousand and more, we start to take sheets metal molding into consideration.
Metal Injection Molding (MIM)
Fine metal powder, wax and binder (normal thermoplastic) are mixed and blended into pellets form. This material is called feedstock, the feedstock is heated to the level that the binder melted and then injected into a mold and form the mid-product with the shape of the cavity. A chemical and a heat process is then applied to remove the binders in the mid-product. Up to this point the mid-product is fragile, it is then sintered under either vacuum or protected environment. After sintering, the products shrink about 15% and reach 98%~99% its natural metal density. The MIM process mostly is applied to small parts, tolerance is about +/-0.05mm or better depends on the size of the parts need to be made. As far as mold concerned, it is basically a standard plastic injection mold. Unit cost is higher than plastic injection parts, as metal powders are much more expensive and extra sintering process is needed.
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