How to choose an injection molding machine

A good injection molding machine must manufacture consistent plastic parts, and the same applies to the next reorder. The selection criteria for the machine are usually based on injection weight, mold locking force, and projected area.

1. Injection weight

The injection weight is an important parameter of the injection molding machine device. It is the most commonly used parameter when selecting an injection molding machine, expressed in amperes or grams. Although this parameter is simple and easy to understand, it is also easily misunderstood. The reason is very simple. After selecting the plastic, the injection molding merchant can determine the weight of the plastic part, so they can easily choose the machine based on the injection weight. In fact, the definition of injection weight is the plastic injection weight during empty injection (without injection into the mold), which is measured rather than calculated from theory. The adhesive used is usually a general hard adhesive with a specific gravity of 1.05. When the plastic of the plastic part is different from PS, the injection weight on the specification must be converted as follows before use. The injection weight of non PS plastic is c * b/1.05, (b=the specific gravity of the plastic and c=the injection weight expressed in PS.) For example, the specific gravity of Saigang is 1.42, and it is injected with an injection weight (calculated in PS) of 8 ounces Saigang.

(1) Choose an injection molding machine with sufficient injection weight; Flash; An injection molding machine should not be selected where the injection weight is exactly equal to the weight of the plastic part plus the weight of the runner plastic. In injection molding with low requirements, such as toy portraits, the total weight should be 85% of the injection weight. In high demand injection molding, such as crystal products, 75% is used. The injection weight measured during empty injection is relatively small under injection pressure, while high injection pressure is used for injection molding with high requirements.

For example, when using rigid polyvinyl chloride (UPVC) with a specific gravity of 1.38 to inject a toy portrait, the total weight of the plastic part and the flow channel plastic is 4 ounces. What is the required injection weight for an injection molding machine? The injection weight characterized by PS=4 * 1.05/0.38=3.04 ounces. Using 85% guidance, the required injection weight is 3.04/0.85=3.58 ounces.

(2) Do not use injection molding machines with excessively high injection weight; Flash; The total weight of plastic parts and runner plastics should be between 35% and 85% of the injection weight. The lower limit is due to three considerations: the bending of the template, the residence time of plastic in the material barrel, and the power consumption per kilogram of injection molded parts.

The use of small molds for small injection molded parts can cause excessive bending of the template, causing it to curl (affecting product quality), and even causing the template to crack.

Using an oversized injection molding machine to inject small plastic parts can lead to plastic decomposition if the melting residence time is too long.

Some problems of injecting small plastic parts with large machines can be solved by using the first mock examination with multiple cavities or by increasing the size of the mold. Reducing the temperature of the material barrel also alleviates the decomposition caused by the long residence time.

2. Locking force

The locking force is a very important parameter in the clamping device of an injection molding machine, which is the maximum locking force used to resist injection pressure during injection molding. Insufficient locking force produces a sharp edge. Most injection molding machines are named after the locking force in tons.

Values below the maximum locking force should be used as much as possible. Sufficient locking force is proportional to the projected area of the mold cavity, which is the area projected by the mold cavity on the mold parting surface. Here, tons refer to metric tons (equivalent to 1000 kilograms). There are several methods for estimating the locking force.

The conservative method is to multiply the projected area by a constant that varies depending on the plastic. For example, the thick wall injection constant of general hard rubber is 1 to 2 tons/inch

Thin walled injection molding uses 3 to 4 tons per inch.

For example, to inject a general hard plastic cup with a diameter of 79 millimeters. The thinnest part of this cup is 0.6 millimeters. Find sufficient locking force.

The projected area of the cup (and flow channel) is 3.1416 * 7.92/4=49CM& Sup2 This cup belongs to the category of thin-walled, with a conservative locking force of 0.62 * 49=30.4T

After calculating the required locking force, we can choose a machine of the previous grade according to our company's machine specification table.
3. Projected area

The projection area is an important parameter for selecting the model of injection molded parts. The projection area mentioned here not only refers to the area of the parts, but also includes the area of the garbage bin and water outlet.

The estimation of area is usually obtained by dividing the locking force by a constant: the thick wall injection constant for hard rubber is usually 1 to 2 tons/inch2, while for thin wall injection, it is 3 to 4 tons/inch2.

The calculation of existing beer parts and casting area should be divided into small regular shapes for calculation.
(1) Calculation of s for the area of a circle with diameter d: s= Л D2/4

⑵. Calculation of square area s with side length a: s=a2
⑶. Calculation of the rectangular area s with length a and width b: s=ab

Calculation of trapezoidal area s with upper bottom being a, lower bottom being b, and height h being: s=(a+b) h/2

Note: The above conditions are all necessary but not sufficient. If one of the conditions is not met, a model of the previous level must be selected.
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