This project consisted in the elaboration of a mold for a metric triangle square (45 degrees), which is an indispensable tool for many students, especially in the area of mechanical drawing. 

To achieve this an entire study was developed to determine the most appropriate design for the cavity, considering the dimensions of the feed channels, cooling system, ejection system, material, machining process, and computer assisted analysis 

The selected material was Polystyrene Cristal (PS cristal), which is the product of the polymerization of pure styrene, also known as General Purpose Polystyrene (GPPS). its transparency gives it an indisputable visual appeal, and despite it is hard and fragile it's glass transition temperature is around 100 ° C, which means it is easily processable above that temperature, being able to mold it in almost any way.

Calculations:

1 - Feeding system: the respective calculations were made for the sprue design, feeding channels, inlet, cold wells and vents, considering 4 cavities and an X distribution.

1.a Sprue: considering by convention the diameter of the nozzle (Dn) of 4mm

Dco ≥ tmax + 1.5 mm

Ds ≥ Dn + 1.0 mm

Since the maximum thickness of this piece (tmax) is 2mm, then: 

Dco= 3.5 mm

Ds  = 5.0 mm

However, the diameter Ds exceeds the value of Dco, but this value must be greater to achieve conicity in the sprue, which allows better flow of the polymer in from the nozzle to the mold cavity. 

Knowing that, the value of Dco can be greater. Then Dco = 5.5 mm is taken.
This gives the value of the length of the sprue (L) by the following equation:

Tan  α = (Dco - Ds ) / 2L

Considering α = 1 °, L was cleared and 14.32 mm was obtained.

1.b Feed channels: a cross section of the channels was selected, with the modified trapezoidal shape to obtain both a turbulence reduction during the flow of the polymer, as well as an intermediate machining facility for this type of molds.

For the calculation of D1, D2 and H the following equations were applied:

D2 = tmax + 1.5 mm

D1 = D2/0.7

H= (2/3) * D2

Therefore: 

D2= 3.5 mm (checked value, which is recommended between 3 - 10 mm)

D1= 5.0 mm

H= 2.33 mm

1.c Input: capillary input was selected, because a uniform or continuous flow is required. Making the calculations the thickness of the piece is almost equal to the entrance. So a three-plate mold is recommended,

De = (2/3) * Tmax = 1.3 mm

1.d Cold Wells: the design of this cavity is necessary so that the pellets or particles that have not melted during the plasticizing stage fall in this region, the recommendation is that the diameter of the cold well is greater than the diameter of the channel. so in this case it is placed at the end of the sprue and at the end of each feeding channel.

1.e Vents: they are small fissures in the range of hundredths that are made in the last part that is filled in the cavity, that is, at the end opposite the injection point. In this case, cracks will be made in all corners of the mold of the metric square.

2 - Cooling System: knowing that within the mold plate the temperature difference inside the mold can not be higher than 10 ° C, feeding channels are placed at a distance that will depend on the diameter of the channel (D2)

It is recommended two pass channels for each square and the distance between cooling channels as a function of D, which depends on the maximum thickness of the piece, for this case D = 10mm, then we have that X is 30mm and Y is 15mm corresponding to the distance between the cavity and the feeding channel.

Sistema de Expulsión: se recomienda dos pines de expulsión por cada escuadra, de manera de expulsar con una fuerza distribuida uniforme para evitar deformaciones de la pieza.

3 - Ejection System: two ejection pins are recommended for each square, in order to expel with a uniform distributed force to avoid deformation of the piece.

4 - Filling: In order to achieve the injection of the of assigned cavities (4 cavities), the Injection Capacity and the Closing Force required were considered
In function of Injection capacity:

N= (0.7CI - mazarote + channels weight)/ Piece weight

The injection capacity required by the machine is equal to 80.76ton

On the other hand, depending on the Closing Force, the following formula is applied:

N= [(FC/0.75)-(Ap*(b+c))]/App

The capacity of closing Force was cleared and 229 ton is required. To obtain a machine that met the specifications, the equipment available in the Milacron catalogs was evaluated.

The Milacron Magna T Servo 310 machine was taken, as its closing force capacity covers 85% of the maximum closing force of the machine.

5 - Mold material: It is almost impossible to obtain a steel that meets all the desirable requirements and at a sufficiently high level, therefore the economic factor plays an important criterion in the selection of materials, considering the following aspects:
•    Material
•    Geometry
•    Number of parts to be manufactured
•    Cycle time
Having said that, the ejection pins, the ejector plate, the guide columns and the sprue will be manufactured with AISI 1045 steel (C% 044 max - Si% 0.25 - Mn% 0.70), mainly because the hardness of this steel (70 kg / mm2) makes it ideal to withstand the mechanical stresses and friction wear to which the parts are subjected.

The cavity plates and the ejection box will be made of P-20 steel with chrome treatment (C% 0.32 - 0.38 - Si% 0.3 - Mn% 1.50 - Cr% 1.75 - 2.0 Mo% 0.2 - 0.38). This steel is selected because it is less expensive and gives the product a medium-sized finish, where its polishing facility improves the ejection capacity of the piece.

The cooling system will be manufactured with AISI 420 (WKW4) steel (C% 0.46 - Cr% 13.0 - Si% 0.40 - Mn% 0.40) which, despite being more expensive than the aforementioned steels, has a highly usable stainless property. the cooling channels, through which a cooling liquid flows (usually water). Likewise, this steel presents a good resistance to wear and an ease of machining useful to give the channel a modified trapezoidal shape.

6 - Machining of the plates: Starting from a steel plate with the shape most similar to the final product and exceeding only its dimensions (approximately (0.8m) 3 to avoid the greatest loss of material possible), all the faces on the lathe are faced (around 0.5mm).
Once all the faces are correctly aligned, they are rough-edged to obtain the required dimensions of the mold.
Then the mold is placed in a milling cutter and the 4 cavities with the dimensions of the square will be manufactured, likewise this tool will be used to elaborate the feeding system. (A refining process will be carried out in the cavities to improve the ease of ejection of the piece).
Then proceed to drill the holes through which the guide columns pass, followed by a refining process to reduce the friction felt by the column. In the same way the holes that will serve as cooling system will be drilled.
Finally, the mold will be put back in the grinding machine and the refined or rectified ones will be made where you want to have a superior surface finish. For this design lapping is required so that the piece obtains a smooth surface, since it is required that the mold has a mirror-like surface.

Below are the dimensions of the feed channels, and the cooling channels

Distribution of type X (4 cavities) and Assembly of the model 

Parámetros de entrada al análisis de moldflow:

Machine: Magna300-C65 (47Oz) Cincinnati Milacron

Resin used: Styron 615 APR : Americas Styrenics (GPPS)

Melting temperature: 218 °C

Mold opening time: 5 sec

Cooler: water

Cooler temperature: 25 °C

Reynolds: 10000

Injection time + packing + cooling: 30 sec

Length: 10 sec

Pressure %: 180

For the analysis of this project a convergence study of (6 meshes) was carried out, varying its overall mesh length between 3 mm and 13 mm. It was observed that a decrease in overall length resulted in an increase in the different elements that make up the system and, consequently, in a longer computation time for the analysis of the system. 

As seen in the figure, the location of the injection point generates a flow that divides in both directions from the bottom of the square to the upper section. In this region the two flows collide generating a welding line (the piece tends to fracture in this section).
That being said, the selection of the point of injection in this location was subject to meet the trend of manufacturing companies such as NORMA or Tecnic STAR in view of that regardless of where the entry is placed, always generate a welding line.
Originally the flow lines were not particularly oriented in one direction, but thanks to this graphic representation it was possible to suspect a possible poor flow in the filling.

Once the filling pressure (% Pll) was increased to 180% (originally established by default in 80%), it was observed how the flow lines were oriented in the filling direction of the square, as well as an analysis for a meshing of length equal to 3 mm which allowed to observe in better detail the direction of the flow (As the length of the mesh decreases, more detailed results are obtained, a tendency that will be repeated throughout all the analyzes).

In the following figures it is observed how the percentage of contraction (0.0 - 0.8)% and the percentage of forward closing (0.00 - 0.57)% are in the accepted range for crystal polystyrene (Originally with a filling pressure (% Pll) of 80% had contraction percentages of up to 4%). Again it is seen as a mesh with global length increasingly smaller allows you to obtain results closer to reality (for 13mm was calculated a maximum value of rechupe of 0.3792 while for 3 mm is reported up to 0.5695), in the same way the region more affected by the forward closing is appreciated in more detail in the latter case.

The upper corners of the squares are highly prone to retain air (in the figure of regions of trapped air a mesh of 13 mm was selected to make the presence of air more evident, however, as the overall length of the mesh decreased, a considerable decrease of this defect also appears). To solve this problem, it is enough to make some incisions in the mold to allow the gas to escape.

Although meshing with smaller elements allows for more perfect results, for certain studies the variations of results are so small that it is enough to present the results of a single mesh to define the behavior of said property .