In the manufacture of plastic parts or PET preform injection molding happens to be an indispensable piece of equipment. This is so because the plastic injection molding machine has the responsibility of converting molten PET resin into the desired plastic parts that are required for the production of various plastic end-products.  

So, to have your desired plastic products from the plastic injection molding machine come as expected, one crucial aspect of the plastic injection molding machine which you should understand very well is the clamping force between the molds. Why this is important cannot be overemphasized since you wouldn’t want to have your raw materials wasted by a plastic injection molding machines whose mold barely have the required force to close up properly which will help it to produce the best piece of plastic materials.

Buy used plastic injection molding machine

Well, as you already know, plastic injection molding machines are a set of equipment that requires enormous capital investment to acquire one. Therefore, with a considerable low budget, you can still enjoy the benefits of this vital equipment if you look toward the direction of the buying a used plastic injection molding machine.

And even before you ask the obvious question about the clamping force of a used injection molding machine which is up for sale, here is the fact and truth. Buying a used plastic injection molding machine from a reliable online intermediary for used equipment such as Exapro is a sure way of investing in a used plastic molding machine that has a clamping force that will do the needed job of producing outstanding plastic parts and other plastic products efficiently and without defects.

Clamping force calculation and effect on plastic parts

In simple terms, clamping force refers to the power which is applied to the mold of a plastic injection molding machine by the clamping unit. This force works basically to ensure that the mold stays closed as at when it is supposed to be closed . To ensure that the clamping force of your used plastic injection molding machine is effective, it must be strong enough to oppose the force which tends to separate the mold when molten plastic is injected into the mold.

Generally, the clamping force of a plastic injection molding machine is usually calculated in tons. This then means that a used plastic injection molding machine with a clamping force of 250 tons can only produce a maximum clamping force equal to 250 tons. Once you can carefully choose a used plastic machine with a capable clamping force for your production need, this will save you from producing plastic parts with defects. Such parts defects may include excessive flesh on products, uneven distribution of molten plastic throughout the formed parts.

Before we go down to calculating the clamping force of a used plastic injection molding machines, let’s consider the structure and functioning of the clamping unit in a plastic injection molding machine.

The clamping unit of plastic injection molding machine 

This is the exact unit where the clamping force that is required to keep the mold closed in opposition to the pressure from molten plastic trying to force the mold open. The clamping unit is made of two main systems;

Toggle system

In a plastic injection molding machine with the clamping force produced by a toggle system, the clamping force is a mechanical power which is produced only when the toggle arm of the plastic injection molding machine is fully extended. So, to get the optimum clamping force from this system, it is essential to adjust the clamping unit to match the thickness of the mold.

Direct pressure system

This system utilizes a hydraulic force to produce the needed clamping force between the molds. There is usually a clamping stroke range which determines the extent of the clamping force on the mold.

While both systems within the clamping unit of a plastic injection molding machine are all meant to produce the much-needed clamping force on the mode, here is a brief comparison between the toggle system and the direct pressure system.

S/N	System Function	Toggle System	Direct Pressure System
1	Mold replacement	Needs an adjustment based on the thickness of the mold	Easy
2	Speed of Mold Opening and Closing	Fast	Slow
3	Mole Opening Stroke	Large	Small
4	Set up of clamping force	Uncertain. Depending on the tightness of the bolt	Easy
5	Mold clamping force	Due to a mechanical force, there is more force than the provided force	Little force from hydraulic pressure

Calculating the clamping force of plastic injection molding machine 

So, having understood what clamping force implies and its importance to the quality of products that your used plastic injection molding machine produces, lets now look at how the calculation is done.

The clamping force of a plastic injection molding machine is directly proportional to the projected area runner, molding, and it must be in opposition with the clamping force. Mathematically, the clamping force formula that acts on an injection molding machine to cause it to close is calculated as;

Force (N) = Injection Pressure (MPA) x The Sample Projection Area (mm2) 

NOTE:

• Sample Projection Area is one line that lies with the mold base parting line.
• Injection Pressure is basically of two types in a plastic injection molding machine. First is the primary pressure (fill pressure) while the second is the secondary pressure (hold pressure). The fill pressure usually has more impact than the hold pressure. In fact, it is this fill pressure that exerts pressure on the molten plastic into the mold cavity to produce the desired shape or product.

Calculating injection pressure of plastic injection molding machine

The formula below is used to calculate the combined pressure from both the primary and secondary pressure.

F = a.p (kgf)

P = apA (kgf/cm2)

Where:

• F = Force created by pressure from the piston
• a = Pressure area of the piston (cm2)
• A: Screw pressure within the inner area (cm²)
• p: Hydraulic pressure (kgf/cm²)
• P: Maximum injection pressure (kgf/cm²)

Here you have it all

With this insightful piece, we trust that you can now determine what quality of products your used plastic injection molding machine is dishing out to you. If the products are not satisfactory, you can employ this simple arithmetic to set your injection molding machine for optimum performance.

You can also contact us for more information, advice as well as the purchase of used plastic injection molding machine of premium quality. We will be delighted to work with you for all your used industrial machines and equipment needs.