Injection Mold Making

 injection mold is a device made of metal to produce a plastic product faster, less expensive, and more consistent.

How does injection molding works?

Just heat the resin until it melts then force it into the cool mold. Allow it to solidify. Open the mold then take off the molded part.
That’s it!
As the molten resin is being injected into the mold, it enters the mold opening called the sprue. From the sprue this molten material will then be distributed to the runners then it will be fored into the gate and then into the cavity. The cavity must be filled precisely to avoid short shots but it must not be over packed (over packing is forcing more than enough pressure to the resin and it can damage the mold). The molten resin will stay in the cavity for 30 seconds to 1 minute or more until it cools down and solidify. When the resin solidify a molded part is formed. The mold will open and then the molded part will be ejected. The mold closes and its ready for another shot.

Why is an injection mold expensive?

Injection molds are made up of special, high quality steels that have a good machinability property. Some standard mold components such as springs, bolts, and limit switch are prefabricated and costs lower.

Most standard components such as ejector pins, ejector sleeves, sprue bushings, and leader components are not prefabricated which mean they will only be manufactured when you order them. They are not mass produced thus they are expensive. The cavity and core are mold components which make the mold really expensive. It involves careful design engineering and processing. Most of the tools used in making these components are very expensive.

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Injection Molder and Molding Machine

Injection molder or injection molding machine is a machine for producing plastic molded parts by molding process. It has 2 main parts, an injection unit and a clamping unit.

An injection molder have 2 methods of fastening the mold -horizontal or vertical. Most injection molder are horizontally oriented because molded parts can easily be removed after ejection by leveraging gravity -the molded parts will just fall down into the container below. Vertical injection molders are used for insert molding applications because inserts are inserted into the core side of the mold before molding.
There are also manual injection molder for hobbyists. It is simple in consruction and does require manual force to inject the resin. An example of such machine is from Galomb, Inc. They make Benchtop injection molder.

Types of injection molding machines

Injection machines are classified mainly by the type of system that drives them: Hydraulic, electric, or hybrid.
Hydraulic injection machines uses ram system for its clamping mechanism. This type of machine is used by majority of molding companies.
Electric injection machines are driven by motors. Unlike the hydraulic machines, which continously pump hydraulics to the cylinder throughout the whole molding cycle, an electric machine only makes use of energy during closing and openning of the clamping unit. It uses toggle system for its clamping mechanism in order to build up tonnage. It is also faster, quieter, and have higher accuracy, but it is also known to be more expensive. The first maker of electric injection machine is Nissei Plastic Industrial Co., LTD.

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Injection Molded Parts Design

Posted on December 15, 2010 by Elito

Injection molded parts are made of thermoplastics, a polymer that turns into liquid when heated then turns back to solid when cooled.
Carefull thoughts on the following elements shall be considered for the design of plastic injection molded part.

Wall Thickness

As much as possible use a uniform wall thickness. If you must change the wall thickness, change it gradually to avoid warping.
If the strength is not compromised, design the part with minimal wall thickness to allow faster cooling time and cycle time.

Draft angles

Draft angle is necessary for easy removal of molded part from the mold. Draft angle should be applied in the direction of draw from the cavity and core.
Parts with textured surface require greater draft angle in order to avoid scratches on the textured surface as the part is being removed from the mold. 1 to 3 degrees normally applies to moderately matte or sand-blasted surfaces and 3 to 5 degrees for roucher surface.
But for smaller precision molded parts, it is sometime unnecessary to include draft angles as long as the cavity and core are highly polished.

Corner Radius

Apply radius on corners as much as possible. A proper radius size is not less than the wall thickness of the part.
Radius reduces the stress on the corner thus reducing the warpage compared to corners without radius.

Rib

Apply rib instead of using thicker wall thickness. Uneven cooling of thick wall will lead to uneven shrinkage inside and outside the wall. This could cause warpage and sinkmark due to internal stress. Applying rib will help improve the rigidity and geometric integrity of the molded part. It also improves the cooling time.

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Rapid Prototyping

Rapid prototyping is a technology used to rapidly construct solid objects using three-dimensional computer aided design (CAD) data. The first technique in prototyping, stereolithography, was develop in the late 1980′s to produce prototype parts and models. In todays rapid prototyping techniques, parts can be manufactured in near production-quality in relatively small numbers, sometimes eliminating the use of injection molded parts.
Among the obvious advantages of rapid prototyping are:

1. costly faulty designs can be reduced early in the development stage,
2. effective communications between marketing, engineering, manufacturing, and purchasing,
3. development time can be reduced,
4. minimize engineering changes

These advantages greatly benefits the product development process in terms of cost, quality, and lead-time.
Rapid prototyping process:

1. 3D CAD model is converted to Stereolithography (.STL) format.
2. The Rapid Prototyping machine processes the STL file layer by layer.
3. Manual finishing and cleaning.