Category Archives: Injection Molding

Injection molding process

Cooling the Mold

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This explains the WHY, WHAT, HOW and WHERE of mold temperature control.

Why do we need to control the mold temperature?

The mold temperature has significant impact on molding cycle, dimensional accuracy of molded part, and warping of molded part.

If the mold temperature is too cool, the runner and gate will solidify before the resin could completely fill up the cavity. However, if the mold temperature is too hot, the resin will take more time to solidify affecting the molding cycle time. If some portions of the molded part solidify a lot faster than the other portions, chances are warp or deformed molded part –the dimensional accuracy cannot be satisfied.

What are the methods of controlling the mold temperature?

There are 3 methods to control the temperature of the mold. It depends on the temperature range that the mold will be set for production.

1.    Water:    40~90?

2.    Oil:    80~120?

3.    Heater cartridge: 120? and over.

How and where should we apply the mold temperature control?

The purpose is to control the temperature of the cavity and core so keep the cooling line (water hole) close and evenly placed around the molded part. The image below shows the ideal placement of water hole. But this is only a general rule -mold structure, manufacturability, and ease of maintenance are to be considered.

Molding Problem – Filling Miss

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Filling Miss

Possible Causes in Molding Machine:

  1. Low material temperature
  2. low injection pressure
  3. Material feed shortage
  4. Too small nozzle hole diameter
  5. Cylinder nozzle clogging

Possible Causes in Molding Die:

  1. Small gate or runner
  2. Inadequate gate position
  3. Inadequate position or no air vent
  4. Low mold temperature
  5. Cold slag clogging in runner or gate

Possible Causes in Resin:

  1. Low fluidity
  2. Low lubricity

Rapid Temperature Cycling in the Automotive Sector is a hot issue at AMT 2011

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Mould temperature control has never been more important than today whether in the automotive, packaging or medical sector. Optimising the cycle time and increasing the component quality by means of Rapid Temperature Cycling are among the hot-topic issues for injection moulders today.

This unique variothermal focused event looks at advanced new and specialised processes available to injection moulders today, and how they can be incorporated to:

•  Optimise the process
•  Improve aesthetics and mechanical performance
•  Overcome difficult production challenges
•  Add value to parts and products
•  Achieve new possibilities in design and functionality
•  Cut cycle times
•  Save costs

Visit the website www.advancedmouldingtechnologies.com

Injection Molder and Molding Machines

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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.

Injection molder

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.

Injection Molded Parts Design

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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. A comprehensive reference in part design can be found in The Complete Part Design Handbook: For Injection Molding Of Thermoplastics.

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.

The Complete Part Design Handbook
Industrial Design Books)

Rapid Prototyping

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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.

Injection Machine Tonnage Computation

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How to compute for injection machine tonnage:

  1. Projected area or shadow area including runners and gates: A
  2. Peak pressure: P
  3. Clamping force = A x P
  4. Include factor of safety
  5. 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?

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The cost of injection mold is dependent on the shape or geometry of the molded part, mold steel cost including standard components, and engineering 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:

  1. High Grade (HG) molded parts are parts with all features having important functionality. The product drawings 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.
  2. Medium Grade (MG) molded 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.
  3. 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.

COST ANALYSIS OF PLASTIC INJECTION MOLDS

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

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Injection molding is an economical and very efficient method of producing injection molded 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. A comprehensive reference about injection molds can be found in Injection Mold Design Engineering. There are also Injection Molds 130 Proven Designs for your design reference.

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.

assembled-base

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.