Injection Mold Making » Injection Molding

Injection Machine Tonnage Computation

Elito — Tue, 29 Jun 2010 16:00:57 +0000

How to compute for injection machine tonnage:

Projected area or shadow area including runners and gates: A
Peak pressure: P
Clamping force = A x P
Include factor of safety
Choose the machine.

First, calculate the area, A, projected by the part that you will be molding. Include runners and gates and total number of cavities.

Find the pressure, P, that you will use in molding the part. There are multiple pressure in molding process and you have to identify the peak pressure.

Compute for the ideal clamping force, F:

F = P x A

Include factor of safety to prevent flashing on parting lines. This should be more than 1.

Select the machine tonnage that you will be using. The common practice is you have to choose the machine tonnage so that it is greater than the ideal clamping force that you have computed earlier.

For further reference, go to the thread.

How much does a mold cost?

Elito — Fri, 30 Apr 2010 08:57:44 +0000

The cost of mold making is dependent on the shape or geometry of the molded product, mold material cost including standard components, and professional cost (labor cost). It is important to note that labor and material cost greatly varies on geographical location. Labor cost from US, EU, or Japan can be 10 to 20 times more than that of China, India or Philippines but the quality is almost same. Yes, almost same quality. Well, slightly lower but sometimes better.

Mold Cost U.S. versus China

Putting out the geographic factor, the mold can be classified by its precision type and usage factor. Precision types are explained below:

High Grade (HG) plastic parts are parts with all features having important functionality. Plastic parts are completely detailed showing all features being dimensioned with a very tight dimensional tolerances. The general part tolerance is (+/-)0.15mm while specified tolerances are as tight as (+/-)0.02mm. Every part dimensions will be computed individually and must be remodeled into CAD to ensure accuracy and fit. The precision level of every mold part is very high because it involve a lot of fits and shut-offs. A typical example of these parts are wiring harness connectors.
Medium Grade (MG) plastic parts are parts with fewer important functional parts than HG. The general dimensional tolerance are usually (+/-)0.25mm, specified and non-specified. Functional shapes such as clip locks does not go tighter than (+/-)0.05mm. Critical parts does not exceed 50% of the total specified dimensions. A typical example of these are auto fuse box and insulators.
Shape Parts (SP) are part where only a few shapes are functional. 3D CAD file from customer can be used directly with minimal modification for mold making. Examples are caps and covers.

Classification by usage are as follows:

	CLASS-101	CLASS-102	CLASS-103	CLASS-104	CLASS-105
Cycles	1M and up	500K to 1M	100K to 500K	500 to 100K	Lower than 500
Mold base min hardness	28 HRC	28 HRC	8 HRC	mild steel or AL	mild steel or AL
CAV and CORE min hardness	58 HRC	48 HRC	38 HRC	30 HRC	mild steel or AL

Molds are made differently so mold shops should have their own quotation standards. Very busy shops tend to quote higher than shops with very few jobs. The simpler mold, the better and cheaper.

Mold type and classification chart was referenced from EMAMOLD.

What is Injection Molding?

Elito — Wed, 30 Nov 2005 03:37:42 +0000

Injection molding is an economical and very efficient method of producing plastic parts. It can produce millions of parts with exactly the same shape, dimension, and quality. Some examples of injection molded parts are the mobile phones, mouse, keyboard, and many components found inside the automobile.

What is an injection mold?

An 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 plastic 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!

Okey.. as the molten plastic material 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 material and it can damage the mold). The molten material will stay in the cavity for 30 seconds to 1 minute or more until it cools down and solidify.

When the material solidify a plastic part is formed. The mold will open and then the plastic part is being 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 materials 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, and sprue bushings 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.