Preparations Before Injection Molding

In this article, we will discuss the preparations before injection molding.  So the following steps are indispensable for the quality of injection molded products.

1. Inspection of raw materials

Check whether the packaging, particle shape and size, appearance color of plastic raw materials are the same as the type and model of the material demand table of the order parts to be produced, to prevent the use of wrong raw materials.

Check if the packaging is damaged and raw materials are contaminated, especially transparent raw materials.

2. Coloring and ingredients of plastic materials

Most plastic materials are primary colors (natural colors), white, milky white, light yellow and transparent. In order to meet the needs for color of the products, we should add the auxiliary materials such as color masterbatch and toner.

In general, the color of the product has been adjusted in the early trial production stage of the mold. The ratio of the toner and masterbatch has been formulated with some color limit samples. In the mass production stage, strictly follow the material demand table and compare the operating instructions to blend the ingredients.

Key points for blending ingredients

before mixing, the inner wall of the hopper of the mixer should be cleaned by an air gun and a soft cloth. Those mixed with toner should be cleaned with mold washing water or kerosene. The preserved original material bags is the best loaded bag. If no original material bags, the replacing material bags should be clean, to ensure that there is no dust and no other raw materials.


3. Drying of raw materials

After the moisture in the raw material exceeds a certain amount, the products will have material flowers (silver streak), bubbles, shrinkage holes and other defects on their surface. And this will cause degradation and affect the appearance and internal quality of the product. Therefore, we must dry  the plastic materials before molding.

Different types of plastic materials have different hygroscopicity. Thus, they can be classified into two categories: hygroscopic and non hygroscopic.

Hygroscopic: ABS, PA, PC, PMMA, etc.

Non hygroscopic: PE, PP, PS, PVC, POM, etc.

There are three factors that affect the drying effect, namely the drying temperature, drying time and the wall thickness of the material layer. The dried raw materials will absorb moisture again after leaving the dryer. If they are not used for a long time, they should be dried again under the same conditions before use.

4. Cleaning of equipment

Different molds, products or orders demand different types or colors of plastic raw materials.

Plastic materials of different types and colors cannot be fully mixed through the plasticizing in the filling tube of the injection molding machine. Even if they can be mixed together, the products may have poor internal quality like easy to break, no elasticity, large overall or partial color deviation, black spots and black lines. There may also be instability in the molding process, and some even can’t be produced (such as stoppers, etc.).

Therefore, in the production of rotary molds, it is necessary to clean up the plastic materials of different colors or types remaining in the machine during the production of the previous mold or product.

5. Preparation for the mold

Last but not least,  these procedures are significant in the preparations before injection molding.

5.1 Mold cleaning

Before injection molding, the anti-rust oil on gaps, the nozzles, the flow channel, etc. Around the surface, cavity and inserts of mold should be cleaned to prevent the oil stains from sticking to the product or blocking the exhaust of the mold. Or else, that will affect the stability of molding.


Mirror products, electroformed shells, and molds with strict post-processing appearance requirements are strictly forbidden to be wiped with cotton wool, rags, and old gloves to prevent damage to the mold surface during operation and lead to scratches on the surface of the product. Generally, it is rinsed with mold washing water and blown with an air gun.

During the operation, it is necessary to prevent the air gun or other objects from touching the mold surface. When disassembling the mold for cleaning, pay special attention to the dismantled inserts and mold shells. Next, put them in special plastic boxes. And wrap them with pearl cotton sheets and soft cloth sheets for storage if necessary. Non-professionals are not allowed to disassemble the mold for cleaning. It is best to clean the mold before starting the machine. The first is to facilitate cleaning, which can better ensure the quality. The second can save the time for mold transfer.

5.2 Water line

Due to the needs of product appearance and production capacity, water line, mold temperature and chiller should be used in the mold during production, so that the temperature of the mold is under a condition that is ideal, relatively constant, and within a small range affected by external influences. After receiving the water on the machine, it will take some time to heat up after the mold temperature machine is started, generally for 15 to 30 minutes.

5.3 Hot runner power supply

For some molds that use hot runners, turn on the hot runner power supply after starting the machine. After preheating for 15 to 30 minutes and the display value of the hot runner electric box reaches the set value, then begin the injection molding.

Now, do you have any other opinions on the preparations before injection molding? If so, you can leave your comment and we can have a further discussion.

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