Plastic injection mold design process
plastic injection template
In the past, the production of plastic injection molds was very expensive. These molds were generally used only in mass production and where thousands of samples of products were needed. Molds are usually made of hard steel, prestressed steel, aluminum, and copper-beryllium alloys. The issue of mold selection is purely economic in the first place. The production of steel molds is generally more expensive than other molds, but their longer lifespan, which will result in more production before the mold wears out, balances economic conditions. Pre-hardened steel molds wear out sooner and are generally used in low-volume or large-volume production. The hardness of pre-hardened steel is usually a number between 38 and 45 on the Rockwell scale. In terms of longevity and wear resistance, hard steel molds are undoubtedly the best option available. The hardness of these molds is numerical between 50 and 60 on the Rockwell scale.
Currently, aluminum molds have a much lower production cost than steel molds. While high-grade aluminum, such as QC-7 and QC-10 used in the aerospace industry, is machined and used by modern computer equipment, aluminum can be considered an economical and cost-effective option for molding hundreds of thousands of different pieces. Also, aluminum molds have a shorter manufacturing process due to the rapid heat loss. In addition, for greater resistance to wear, these molds can be coated with a layer of reinforced fiberglass. Copper-beryllium alloys are also used in parts of the mold that should have better heat transfer capacity.
General components of injection mold
The molds consist of two main halves, the injection half (page A) and the filling half (page B). Both halves of the mold are embedded in the plastic injection machine, but only the end half of the mold can move. As a result, the two halves of the mold can be spaced apart or connected to each other. The contact line between the two halves of the mold is called the cutting edge (where the two halves of the mold meet). The two main components of any mold are the core or so-called muscle and the cavity or quilt of the mold. When the mold is closed, the distance between the muscle and the mold quill forms the place where the piece is formed (which is then filled with molten material). Due to this, multi-quilt molds are sometimes used to form multiple pieces simultaneously.
Initially, the plastic resin enters the mold through the spray. The function of the bush sprocket (and the mounting ring) is to hold the mold firmly in front of the injection cylinder nozzle so that molten material can enter the mold from the cylinder. The sprue bush directs the molten material to Quetta through channels created at the surface of plates A and B. Molten material can circulate through these channels and is therefore sometimes referred to as a runner. After circulating through the passage, the molten material passes through a specially designed gate or gates and finally enters the Quetta space to form an injection piece. The amount of resin needed to fill the mold, the aisle and the quilt of the mold is called the shot. Naturally, the molten material that is cooled inside the passages is attached to the main body of the part when the part leaves the mold, and therefore must be separated from it. However, sometimes the gates are heated by a special system and as a result the remaining materials in them are re-melted and separated from the body of the main part. Another type of duct on the mold surfaces is the cooling duct. The standard method of cooling a part is to pass a cooling element (such as water) through holes drilled in the mold plates that form a single passage using a hose. The cooling element circulates in the vicinity of Quetta, absorbs the heat of the mold (which has absorbed the heat of the molten plastic) and keeps it at a suitable temperature so that the solidification of the molten material can be done at an optimal speed.
Ventilation of the mold
Air trapped inside the mold can be vented out through air vents built into the mold cutting border. If this trapped air does not escape, it will be compressed by the pressure exerted by the flowing material in the passage and consequently trapped in the corner of Quetta. As a result, the trapped air will prevent the material from penetrating the mold and will cause defects. In addition, the compression of the trapped air can be such that it causes the molten material to heat up and burn around.
Extract the piece from the mold
As mentioned earlier, the back half of the mold is known as the wing half. The reason for this naming is due to its function in removing the piece from the mold. This half includes the filling system (to which the mold muscle is attached) and the backing plate. The flight system actually consists of a flight screen, box and pins. When the clamping unit of the injection machine separates the two halves of the mold, the filling rod activates the filling system. This rod directs the spinning plate into the sprocket box, which in turn pushes the spinning pins forward toward the molded piece. As a result, the pressure exerted by the spinning pins causes the piece to protrude from the cavity of the mold. Plastic injection molds are generally designed in such a way that the part is securely placed in the half of the filling when opened and the passage as well as the sprue are completely separated from the injection half.
Principles of design of plastic injection parts
In the world of making objects, there are many ways to produce products. If these products are made of plastic, there are several ways we can make them. One of these ways is plastic injection, which requires the design of a plastic injection mold. However, injection molding is not easy. Engaging in such a process requires extensive knowledge of the devices and its process. In this article, we will show you some aspects that we consider in designing plastic parts for plastic injection molds so that we can save our customers time and money in the long run.
What is injection molding?
Injection molding is a method in which molten plastic is injected into a metal mold. The mold is made up of two parts, the "A" and "B" sides of the halves are separated and the plastic piece is allowed to come out of the mold after forming and plastic parts are created.
The image of part A and part B, which form half of the injection mold
What should we consider when designing plastic injection molding?
Draft Angle Draft Angles allow plastic to come out of the mold. Without the suggested angles, this part shows significant resistance due to friction when leaving the mold. Suggested angles should be both inside and outside the piece.
The green image shows the correct predicted angle, and the red image shows the lack of the correct predicted angle
The deeper the piece, the larger the predicted angle. A simple rule of thumb is to have one-degree angles of departure from the mold. Not having a sufficiently projected angle may cause scratches along both sides or a large pin marker.
Having radial edges and corners (inside and out) of the piece is a special advantage. This feature makes it easier for the piece to come out when the plastic is injected - in connection with the pre-designed sides - and for the material to flow more easily. Most importantly, however, this not only prevents excessive costs when forming the mold, but also prevents cracks from being created by concentrating the stress intensity on the part.
An example of radial corners in the design of a plastic part
Keep in mind that radial corners must maintain the same wall thickness, meaning that if the inner thickness is r = ½ then the outer thickness must R = 3 * ½
Undercuts are cases where both halves of the template are removed. These cuts can appear anywhere in the design. These are unacceptable in the design as much as not choosing the right predicted angle. However, some undercuts are necessary or unavoidable. In these cases, the necessary undercuts are made by sliding or moving the piece in the mold. Keep in mind that making undercuts is very expensive when producing a plastic injection mold, and Must be minimal.
تصویر نشان دهنده برش زیرین قابل قبول است به دلیل بخش کشویی قالب
Solid vs. hollow
As the plastic cools in the mold, it shrinks This is a common feature for most materials. Making a hollow (porous) component in front of a solid piece helps reduce the amount of shrinkage or twisting that occurs during the cooling process. It also helps reduce the cost of materials needed to use the part. There are several ways to design a hollow (porous) piece, an example is shown in the diagram below.
Example of designing a solid piece (left) prone to shrinkage and deflection, as opposed to proper design of a hollow piece (right)
Signs of depression
In plastic injection, the signs of depression appear / appear as a spot or part of a plastic surface and / or feel sunken inside the piece.
These symptoms are caused by a number of things:
&bull Uncoordinated wall thickness (may lead to cavities)
&bull Improper radius in the corners
&bull Thickness of retaining beams (discussed later)
&bull Sharp corners
&bull Non-porous parts
The design of the piece shows improperly designed areas prone to depression and cavitation
Retaining beams / joints
Retaining beams and joints are used to further support the product walls. They are mainly used in two ways:
(1) Where the piece has walls that collide at a 90-degree angle
(2) Where a part may be too long or too large and the wall thickness of that part is loose and unsustainable or weak. Retaining beams tend to perform best in the required sections, such as moving along a long piece.
The red image shows the lack of support by the support beams and the green image shows the support beams added to the piece correctly.
The red image shows the incorrect path of the support beams and the green image shows the correct path.
When designing retaining beams, it is important that the predicted angle and the thickness of the base do not exceed 2/3 of the wall thickness attached to it.
Connecting parts to each other
Products generally consist of two or more parts connected together. These parts can be connected with base protrusions, hooks, fasteners, screws, etc. A common way to merge two parts is to use screws through the base ridges. Having edges / grooves where the parts meet and combine is one of the most common ways to combine parts. When planning how to connect the parts, consider the previous points during this presentation, such as prefabricated angles, porous parts, retaining beams, and so on.
The squares above show the edges and grooves, and the bottom two squares show the base ridges (volumes)
Separation lines are lines where the two parts of the mold overlap. These lines create a physical line on the piece that is both visible and tangible. However, these lines can be hidden or minimized if placed on the edges of the piece. When designing a plastic injection piece, always keep in mind its separation lines.
Example of a separation line in a screwdriver
The image shows where the separation lines form in the mold
Location of fly pins
Flying pins allow the piece to come out of the mold. These pins, literally, push the piece out after the material is injected into the mold and formed. However, while the pieces are coming out, these pins leave a mark on the piece These marks are not normally removable, so considering their location is a key point in designing a part.
The arrows point to the location of the fly pins on the plastic holder of a battery
The local image shows that the fly pin mark is in place of these pins (gray area) due to the parts coming out
Position of valves and material flow
Along with the position of the fly pins and separation lines, it is important to know the location of the valves. Valves are where the molten plastic enters the cavity of the piece, in the mold. These valves leave a mark on the position of the valve when the part cools, even when it is intended to be disassembled in a fast process.
The position of the valve is sometimes determined by the following:
Where it is less noticeable
Where it does not interfere with other parts of the piece
How plastic material flows in the piece
Or a combination of all of these.
The location of the valve in this image is shown where it is separated from the plastic part
Material and thickness
A material should be selected depending on (1) how the product works, (2) the environment in which it is located, and (3) the designer's priority. These materials determine how the piece will feel, look and work. Some are flexible, some tough, some strong, and some fragile. The type of material selected often has a significant impact on the design of the part. Some retaining beams may need to be removed or added, and walls may need to be made thicker or thinner, and so on.
Recommended wall thickness based on material type:
All the concepts presented in the previous slides are just some of the correct engineering methods that you should consider when designing parts that are produced by plastic injection molding.
These practices are also known as DFMs (Design for Productivity) and should be used as a continuous checklist throughout the product design and redesign process.
Remember that paying attention to these practices will save on production costs and time for customers in the long run.
Design of plastic injection mold in Catia software
In order to design a plastic injection mold in Katia software, the following steps are followed.
1. Design the desired part in Part design, Generative shape design environments,&hellip
2. Scaling of the cylinder and the matrix according to the percentage of shrinkage of each material
3. Creating a separation line and separation surface around the part, which is mostly done in Generative shape design environment.
4. Filling the holes and empty spaces of the piece in a separate body so that we do not encounter Katia errors in the next steps
5. Create two separate parts and design the final mold block of the piece in both parts
6. Use the Remove command in the assembly environment and subtract the part from the two parts designed in the previous step
7. Use the split command in each part and create the matrix and semicolon using the separation surface
8. Use the Remove lump command to remove the top or bottom of the mold in the mold or matrix
9. Add the body of the holes and spaces removed in step 3 to the matrix or seal according to the mold conditions
10. Create gradients on the surfaces of the mat and the matrix
11. Create a part called from mold and design a block the size of the entire mold (matrix and seal) and remove it from the matrix and seal. This shows the final part of the mold that the mold bugs can be fixed by checking.
12. Design and add other components of plastic injection mold:
- Design of blades and blade plate on the side of the mold
- Designing a box that can be designed individually at each edge.
- Design of gates on both sides of the mat and the matrix
- Design the location of the spiro liner on the matrix side
- Designing a waterway in Sambeh and Matrix