How to select a Plastic Injection Molding Machine

How to select a Plastic Injection Molding Machine

Purchasing an injection molding machine is not a small investment,buying a too big machine is waste of money,while a machine too small is not suitable. So choosing a machine suitable for products is the most important thing for a buyer,and the supplier suggests the suitable machine and advise how to choose machine is the first lesson the supplier provides to the buyer.

In order to understand the various procedures for selecting injection molding machines in a more systematic way, the following contents and detailed descriptions is for this purpose.

Too choose a machine,only take injection shot into consideration is too simple,and only based on clamping force is also not comprehensive,so we need combine both and make the choice.

Determine the injection weight of IMM.

Injection weight is an important parameter for IMM.It is the most useful and key parameter on choosing a machine. It is expressed by gram or ounze,Although this parameter is simple and easy to understand, it is also easily misunderstood.The reason is very simple. When the plastic material is sure, the weight of the plastic parts can be determined. Therefore, it is easy for them to use the injection weight to choose the machine.In fact, the definition of injection weight refers to the maximum plastic injection weight that can be reached by the injection device when the screw makes a maximum injection stroke under the condition of empty injection (not inject to mold); this parameter reflects to some extent the processing capability of the injection molding machine; it can be measured and can also be from theoretical calculation(the theoretical value and the measured value mayl have a certain degree of difference).The material used for the test is usually a normal PS with a density of 1.05.

Steps to determine injection weight

1, first calculate the weight of the product: W = product weight + gate system weight, in which: W the weight required for injection.In the LOG company's product brochure, all actual injection volume parameters are calculated by using polystyrene (PS) as an example, so in actual situations, it is also considered in two cases.

2. When the injection molding product is polystyrene (PS), injection molding machine should have an injection volume of Wps; Wps = (1.3 ~ 1.1) × W (product weight + total weight of the gate system), when there is a high requirement for product, the coefficient in the above equation should take a large value, and vice versa, it can take a small value.

3.  when the product is other plastic (named X plastic) to imitate the method, first calculate the theoretical injection volume of the plastic should have the Wx; Wx = (1.3 ~ 1.1) × (product weight + gate system total Weight); then based on the density of the plastic (X) is converted into the actual weight of the PS material Wps, the conversion formula is:Wps=Wx × 1.05/Vx

4.  according to the calculation results of Wps and LOG brochure, choose the right injection molding machine.

Remarks: Relationship between injection weight and injection volume

1. The injection weight is not just the injection volume multiplied by the density of the PS. The injection weight is measured and the injection volume is theoretical.Because the plastic material will flow into the gap between the barrel and the screw during injection molding, and the check ring needs to move backwards to reach the closed position, so the injection volume multiplied by the density of the PS is larger than the injection weight.

2.  For products that don’t have high requirements, such as toy figures: the total weight should be 85% of the injection weight; in the injection molding with high requirements; such as crystal products, with 75%.

3. Do not use injection molding machine with too large injection weight

The total weight of plastics and flow path plastics should be between 35% and 85% of the injection weight.The lower limit is due to the following three considerations: the bending of the plate; the residence time of the plastic in the barrel and the power consumption per kg of injection molded parts.Using a small mold for a small injection molded part can cause excessive bending of the platen, causing him to warp (affect product quality) and even cause the platen to crack.Using an oversized injection molding machine to inject small plastic parts, too long dwelling time of the melting material may cause the decomposition of the molten plastic in the barrel.

4, injection speed

For ultra-thin-wall products, using high flow materials and make the molten plastic flows at an ultra-high speed to makes sure the thickness of skin layer created by the cooling-curing of molten material after getting contact with mold wall surface l as thin as possible, which means delay the time for creature of cured skin layer as long as possible.So it involves the issue of injection speed.As far as experience is concerned, the injection speed may be 1 to 1.5 times the flow length ratio.

Second, the injection molding machine clamping force calculation

There are two important factors for calculating the clamping force: 1. The projected area; 2. The cavity pressure.

1. The projected area (S) is the maximum area viewed from the mold opening and closing sight.

2. Cavity Pressure Decision (P)

3. Cavity pressure is affected by the following factors

4. )Number and location of gates

5.  The size of the gate

6. thickness of products

7. the viscosity characteristics of using material

8.  Injection speed

2.1 Grouping of Thermoplastic Flow Characteristics

Group1 PS PE PP

Group 2 PA6 PBT PET

Group3 of PU soft PVC



Group6 PC PEEK hard UPVC

2.2 Viscosity grade

Each of the above groups of plastics has a viscosity (flowability) rating. The relative viscosity grade of each group of plastics is as follows

  Group         multiplication constant (K)

The first group × 1.0

The second group × 1.3 to 1.35

The third group × 1.35 to 1.45

The fourth group ×1.45~1.55

The fifth group ×1.55~1.70

The sixth group ×1.70~1.90

2.3 cavity pressure is determined by wall thickness ,the ratio between flow and wall thickness

Lookup table P0

P=P0•K (multiplication constant)

2.4 Determination of clamping force (F)

F=P•S= P0•K•S


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