Plastic Injection Molding Site Blog

The Plastic Injection Molding Site Blog is my mini-journal about Injection Molding.


- lets you know when any new postings appear (like brief comments and suggestions on how to improve part quality, productivity and operational efficiencies using current technology and know-how)

- lets you know when any new web pages appear.

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Gas vent

How to improve gas vent for part having mirror face?

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3 Plate Mold Design For Injection Molding

Learn how to design a 3 plate mold and when to use a 2 plate mold design.

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Setting Hold Time and Hold Pressure to Run an Optimized Process

Learn how to make quality parts without spending money

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3D Rapid Prototyping in Injection Molding

Learn how 3D rapid prototyping benefits the injection molding industry

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Injection Capacity Calculator For Injection Molding Machines

Different plastics have different melt densities which alters the injection capacity of a screw and barrel assembly. Use this injection capacity calculator to find the capacity of your machine.

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Production Calculators For Injection Molding

Need to know how long your next production run will take? Or how much material to order? Production calculators used to find the production rate, time & plastic material requirement.

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Stack Molds Improve Productivity And Reduce Your Costs

If you want to double your production per molding machine, then stack molds might be for you.

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Mold Polishing Tips For Injection Molds

Learn about the different types of finishes in mold polishing

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How To Estimate Weight Of An Injection Mold Tool

For those of you who do not have scales here is an easy way to find mold weight.

Injection Mold Weight

Calculation For Metric Units (cm)

Measure overall dimensions of the mold.

Refer to above image of mold for length, width & height.

For example:

If Length=80cm Width=90cm Height=45cm

and Weight(kgs)= LengthxWidthxHeightxDensityOfSteel where density is 7.9 grams/cm^3 for tool steel (P20,H13)

then Weight(kgs)= (80x90x45x7.9)/1000= 2,560 kgs

Calculation For Imperial Units (Inches)

Measure overall dimensions of the mold.

Refer to above image of mold for length, width & height.

For example:

If Length=25 inch Width=35 inch Height=20 inch

and Weight(lbs)= LengthxWidthxHeightxDensityOfSteel where density is 0.28 lbs/inch^3 for tool steel (P20,H13)

then Weight(kgs)= (25x35x20x0.28)= 4,900 lbs

All calculations & technical data are for comparison purposes only and are NOT for design or safe working practices. It is derived from sources that we believe to be accurate, but can NOT guarantee its validity. Your specific application should NOT be undertaken without independent evaluation for accuracy.

Mold Flow Analysis Simulation Benefits

Learn how mold flow analysis software can save you time and money in getting quality parts to market on time.

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Injection Molding Consulting Will Improve Quality and Productivity.

Specializing in Troubleshooting Thin Wall Molding Problems - Injection Molding Consulting

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Life of a wear plate

How long is the maximum life of a wear plate? Answer: The purpose of a wear plate in an injection mould is to act as a sacrificial component in preference

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Gate Type & Mold Steels

Question by Mohamed: How to choose the correct gate type and the correct mold steel? Answer: The type of gate depends upon a number of factors including

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Nox-Rust Paper

Question: What problems other than a visual indicator could be caused by using nox-rust paper on the back plates of injection molds? My answer: I wouldn't

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A Nucleating Agent Can Improve Productivity Immediately

Learn how a nucleating agent can improve part quality and reduce cycle time with this case study

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Plastic Raw Material For Easier Processing

Learn which plastic raw material will improve cycle time and part quality in injection molding

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When To Refurbish An Injection Mold

The quality of the plastic parts produced from an injection mold are a good indicator of the condition of a mold. If the mold has been in production for a while and the parts have the following quality issues then it probably needs refurbishment.

The quality issues are flashing, shorting (short shots), cosmetic defects (such as trapped air), uneven weights between cavities, shifting wall thickness and lower than normal production rates (long cycle times).

Although some of these issues can be due to incorrect processing or poor moulding machine condition, if they are occurring at the same time then it is most likely the mould is at fault and needs refurbishment.

Inspect the most important surfaces such as the part line shutoff faces and the back plates. Disassemble the mold and look for signs of uneven wear, damage, corrosion (see photo) or unusual hard mark patterns. Some simple checks with measuring equipment will confirm the extent of any wear.

The photo below is the surface of a back plate from a single cavity thin wall lid mold (wall thickness 0.55mm). The core was bolted to this plate. The water corrosion is evident especially at the centre, which was 0.1mm deep. This resulted in the wall thickness of the lid to be 0.1mm deeper in one section of the lid. This is too deep for a thin wall part. The back plate had to be surface ground in order to reinstate the flat surface and to maintain even wall thickness (0.55mm) on the lid.

Corrosion on Backplate

Note: when buying used plastic injection molds, they are most likely in need of refurbishment. Click the link below to find out how to source the best quality used molds using this 9-step process.

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Common Ejection Methods

There are 3 main methods of ejecting a plastic part off an injection mould tool. These methods use:

1. Ejector pins

2. Pressurized air

3. Stripper plate

When designing an injection mold, the use of ejector pins should always be considered first since they are cheap and easy to use. They are most suited to parts that are strong and rigid and have an irregular shape.

Pressurized air is often used together with ejector pins to aid in the ejection process. The purpose of the air is to release the vacuum between the part and the core of the mold tool. Without this air, the ejector pins would probably punch holes in the part or distort the part during ejection.

The third method of ejection is done by a stripper plate. This is a very reliable method for parts that are flexible or weak.

To learn more about stripper plates click the link below:

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Medical Device Injection Molding Challenges

Learn how to be successful at medical device injection molding.

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Grinding of Tool Steel and Cracking

When grinding tool steel a lot of heat is generated. Too much heat will cause cracks.

Although cracks are usually not visible to the naked eye, a good indication is the presence of burn marks on the surface of the steel. These cracks can get bigger and lead to failure of the steel during the injection molding process.

Usually the first indication of a crack is a water leak in the injection mold.

To avoid grinding cracks do the following:• Use the correct grade of grinding wheel for the tool steel• Dress the wheel properly so that the wheel actually cuts and doesn’t just generate heat • Use correct grinding parameters such as wheel speed and depth of cut• Use plenty of cooling fluid• When grinding hardened steel only grind steel that has been tempered • Always let the wheel “spark out” before finishing the grinding process.

This will help to minimize the stresses induced by grinding

For more information on this go to

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