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SANKO GOSEI

Injection molding: How to create molding conditions for a box-shaped molded product

How to Set Molding Conditions


In this issue, we will introduce how to create conditions for box-shaped molded products using PS (polystyrene) material.

ポリスチレンで成形された箱形状の成形品

1. Pre-drying of materials


Pre-drying is necessary for some materials.

If highly hygroscopic materials are put into the molding machine without pre-drying, moisture will evaporate and become mixed inside the molded product as voids, causing defects.

Even for materials that do not require pre-drying, it is recommended to pre-dry them if mass production is anticipated, as it is effective in controlling dimensional stability and quality.

The PS used in this study was dried in a box-type drying oven at approximately 80°C for about 2 hours.


2.Purging


Before loading the dried material into the molding machine, the inside of the cylinder and the screw are cleaned with a purging material to prevent the possibility that material used in the previous molding process remains in the cylinder of the molding machine.

*If the color of the material used in the previous molding process differs from the color of the material to be used in the new molding process, purge carefully to avoid uneven coloring.

Also, since there may be material remaining in the hopper from the previous molding, clean it with air and a rag, and always take care to prevent foreign matter from entering the hopper.


3. Molding with primary pressure only


 Cylinder temperature setting


Nozzle: 190℃ Front: 200℃ Middle: 190℃ Rear: 180


 Calculation of molded product volume


 Set the following conditions (when 1 mm of weighing value corresponds to 1 cc of plastic)


Rotation speed: 50% Weighing value: ●● mm Back pressure: 10% Decomp: 1 mm Injection speed: 50% Primary pressure: 30


Mold temperature: 40°C


 Always set the metering value lower than the approximate calculated value (set to short).

 Back pressure refers to the pressure exerted on the molten plastic when the screw of the injection machine is retracted.

 Back pressure refers to the pressure applied to the molten plastic when the screw of the injection machine retracts. This back pressure is set for the purpose of kneading and homogenizing the molten plastic.


 Low back pressure can result in insufficient kneading and loss of material uniformity.

 In addition, air bubbles are more likely to remain, resulting in lower quality molded products.

 High back pressure slows screw retraction and lengthens cycle time.

 Also, excessive shear heat of the material raises the temperature, which may cause decomposition and degradation of the material.


 Therefore, start with a low value.


Check the volume and flow of the molded product that emerges from these settings.

 ➡Increase the metering value little by little so that the mold can be filled with the mold shape.


4. Add secondary pressure (holding pressure)


 (Holding pressure refers to the pressure applied after molten plastic is injected into the mold.

If any of the following symptoms appear in the molded product, consider setting the holding pressure.

 Hiccups due to material shrinkage

 Short-circuiting

 Dimensional defects due to deformation

Set the following conditions


Weighing value: ●● mm Secondary pressure: 20% Switching position: ●● mm


The holding pressure changeover position is usually set when approximately 95% to 99% of the mold cavity is filled. This filling rate is determined based on CAE filling analysis and experience.

The condition settings introduced in this article are conditions determined for one shape, one material, and one molding machine.


If there are any changes, the condition settings need to be changed accordingly.


When changing the conditions, it is necessary to perform about three molding shots because the change may not be obvious after only one shot. Also, it is easier to derive appropriate conditions by leaving only the last shot and writing the conditions on the molded product using a magic marker or the like.


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