"Plastic Injection Machine"
INTRODUCTION
Motivation
Our motivation to this project is to have a machine that can be ran manually in order to perform objects using a recycled plastic.
Aim of the Project
Standards
We are going to use "SI Unit System" and AGMA specifications to our design.
BACKGROUND
Injection Molding Machine
To understand the meaning of injection molding machine you should firstly understand the meaning of injection molding. As it is a method to obtain molded products by injecting plastic materials molten by heat into a mold, and then cooling and solidifying them. As injection molding produces complex shapes so they can have complex molds design other than other types of molding. Where injection molding machine is divided into 2 units i.e. a clamping unit and an injection unit.
Molds
As the mold plays an important role in the operation of the injection molding machine; there will be a variety kinds of molds such as: the two plate permanent mold, and the three plate permanent mold. Injection machine has a permanent mold because it is high production rate procedure, the mold is continuously used during the running operation.
Clamping units
The functions of the clamping unit are opening and closing a die, and the ejection of products. Thus, there are three main types of the clamping units such as toggle, hydraulic, hydromechanical. Each clamp unit is used in different operation settings. Firstly, the hydraulic clamping unit used in higher tonnage production mass from 1300kN to 8900kN, and also it is very flexible. On the other hand, the hydromechanical is used while having a large tonnage and a higher mass than the hydraulic clamping unit up to 8900kN.
Injection Units
The functions of the injection unit are to melt plastic by heat and then to inject molten plastic into a mold. There is two types of injection motion, you either have the screw perpendicular machine injection unit or the plunger type injection molding machine. The first type has a two stage production stages, and the second has one only. To know more about the material you are using you should know the general characteristics of polymers.
Heat elements
This part will heat up the pipe in order to melt the plastic pellets injected to the machine, and then press it to the mold to perform the operation. These elements connected to a power source and a heater control unit to adjust the heat temperature needed to melt the plastic pellet into its desired state. These heat elements will heat up the pipe into the desired melting temperature of PLA, which is 200 degree Celsius.
Cooling system
There is two types of cooling systems such as air and water. We are considring air to be our cooling system.
polymers
They have a behavior of being a high thermal expansion. By this a shrinkage may occur during the solidification process of the molten polymers which in other words a reduction in linear size will be noticed. There are many factors which leads to this problem:
Factors
(1) Injection pressure, when there is a high injection pressure of the molten plastic to the mold, a shrinkage may occur during this solidification process. (2) Compaction time, if there is a longer time taken while compaction a shrinkage may be reduced. Thus, having a high compaction time is preferable. (3) Molding temperature, is the temperature of the polymer in the cylinder immediately before injection. So having a higher molding temperature will reduce the shrinkage respectively. (4) Part thickness, is the part is thick the shrinkage will occur more to the material, so make sure to have a low to medium thin design to the molded part. (5) Polymer temperature is different than the molding temperature. The polymer temperature is the temperature were the polymer be at it on the molten state. While having a high polymer temperature the shrinkage will be more.
Moreover, the higher the temperature the lower viscosity of the polymer melt. So if the viscosity is high, more material will be packed to the mold.
Problems
Short shots
molding that has solidifies before completing the filling process in the cavity. and the solution is to increase the temperature or the pressure it happens when a larger machine if the short capacity is sufficient.
Flashing
Polymer melt is squeezed into the parting surface between mold plates. And the solution is firstly to reduce size of vents and clearance. Second, reduce pressure. Third, check the melting temperature. And lastly, reduce the shot size. Flashing may occur because the next multiple reasons such as: vents and clearance are too large, injection pressure too high compared with clamping force. Hence, melt temperature may be also too high; and lastly excessive shot size leads to flashing.
Sink marks and voids
sinks and voids may occur when the outer surface solidifies before the inner solidification of the material. The solution is to increase the packing pressure after injection and to have uniform/linear sections of the molds and linear thickness.
Weld lines
Polymer melt flows around a core or other convex detail in the mold cavity, and the solution is to have a venting system. Also, increase the melting temperature and the injection pressure. More is to have an alternative gating location.
Notes: The melting temperature of the plastic is 400 degrees or little bit lower. that's why the solidification rate is fast. The plastic used is considered as a thermoplastic material. We should have a constant injection speed in order to have a proper extrusion process. If we have a two-stage injection the pressure is controlled. And also having higher liquidity of the fluid means higher temperature.
Nozzle
There is two types of nozzle, an open nozzle and a shut off nozzle. We are more interested in having an open nozzle. Nozzle is the front part of the screw and barrel of injection molding machine, it is the necessary and important of injection machine.
DESIGN
Importance of the Design
Why should we do a injection molding machine, if there is already one to purchase? Why it is very important? Injection molding machine especially at FabLab will be beneficial to the society we are working with since 3d printing and other modeling devices have some other limitations that injection molding machine do not. The solidification process and rate can be fast due to the characteristics of the material used and you can notice it through the process of 3D printing. In a larger scale, many industries uses plastic and have yearly tonnage plastic waste produces it is either recycled or dumbed. So why not having products that can be used in our life such as key holders, phone cases, bolts and nuts for the industry machines and etc.
Used Software
The used software in our design is FUSION360, as the cad drawing base software were we can preform any design on. And the other software is MATLAB where we can do our calculations in. And any software are used during the process of the design will be mentioned here.
Design Procedure
The design procedure we are planning to do is, to fabricate the injection molding machine. The fabrication procedure lays into more elementary steps. The first is to design the structure of the molding machine; either purchase or design all the pieces necessary to do it. After constructing the structure of the machine we will be able to calculate some of the fluid and main design calculations, in order to obtain the numbers linked to some of the important part. Thus, doing a whole design analysis to the structure helps designing the mold. The mold will be designed either by CNC or other types, were we are still did not reach to any other types yet. To further design procedure this website will be updated.
Mold Design
The mold is very important component to have in your injection machine. There is two types of molds were they are either permanent molds and impermanent. permanent molds are usually metal but also ceramic molds which can be used repeatedly for casting and are not destroyed after casting. Metal permanent molds are also called dies. The permanent molds should not display a change to the surface quality or the geometrical dimensions in the production process. Thus, permanent molds are exposed to very high temperatures and changes in temperature. On the other hand impermeant molds are the dies where can be used only once or twice a time because of the material they are made with, such as sand.
In our project we are going to use the permanent molds due to its characteristics to withstand the applied conditions, and succeed a very high production rates. The molds dimensions and other specifications will be obtained and found after doing the analysis of the structure, and during.
Structure Design
The structure of our design will be made of different types of materials. The structure contains several parts such as the plumper or the hydraulic press where you can exert the force to do the operation. The main pole where you will have all the parts attached to. Moreover, the structure will contain several other parts such as; bolts, nuts, and washers. These fasteners will help to assemble the parts of the structure together.
Nozzle Design
The nozzle design for now it will be either purchased, or machined. We can do it also using a bolt. The nozzle will be applied under design analysis procedure to test the capability to handle the applied conditions.
Motors
Motor used to run the injection machine are a whole mechanism, where in order to change the movement of the motors from rotational to linear motion. This can be done using belts and polies. Thus, it will smoothen out the motion transition between the systems.
CALCULATIONS AND RESULTS
As part of our fabricated extrusion machine, we managed to do the calculation part in order to understand the physical nontangible parts required to us. Fluid and other materials involved should be studied and well understood. So we have looked firstly for the design of the injection molding nozzle at it can be useful in order to find some of the important parameters, such as viscosity, back pressure, maximum pressure, friction, operation time, speed and other elements.
As our procedure tends to the side of fabricating the components, and then see if the design calculations is the same as experimental or no. It was hard to find the design calculations for the nozzle as we planned to change the injection nozzle to more ordinary one. So the design is some how unique.
The following calculations is the calculations determined to do the design of nozzle, and other designs.
% Drag flow
W = 0.025 ; %m
H = 0.001 ; %m
D = 0.029 ; %m
N = 1.79 ; %rev/s ( Maximum speed for motor)
theta = 30 ; %degrees ( Screw helix angle ) assumption
meu = 0.6 ; %Pa.s ( assumption )
L = 0.38 ; %m
deltaP = 0 ; % Open discharge
density = 1240000 ; % g/m^3
eta = meu ;
D2 = 1.75*10^-3 ; %m
% Drag flow
Qd = (pi/2)*(W)*(H)*(D)*(N)*(cos(theta))
% Pressure flow rate
Qp = (-W*H^3*(deltaP))/(12*meu*L)
% then, volume flow rate
Q = Qd + Qp %m^3/s
% flow rate
Qm = (Q*density)/1000 % Kg/s
% Screw characteristics
% Maximum output
% Q = Qmax
Qmax = (1/2)*(pi)^2*(D^2)*N*(sin(theta))*(H)
% P = Pmax and Q = 0 as determined above
P = (6*(pi)*D2*L*N*(eta))/((H^2)*(tan(theta)))
% Material characteristics
% Deborah Number
% Ndeb = tm/tp
E = 3.5 ; % (Young modulus) GPa
% tm = material relaxation time
% tp = processing time
tm = meu/E % seconds
tm_days = tm/86400 % days
tp = 30 ; %estimation (s)
Ndeb = tm / tp
format long e
And The answers in the MATLAB command window are as followed
Qd =
3.1444e-07
Qp =
0
Q =
3.1444e-07
Qm =
3.8991e-04
Qmax =
-7.3399e-06
P =
-2.1018e+03
tm =
0.1714
tm_days =
1.9841e-06
Ndeb =
0.0057
>>
Discussion
We have calculated the pressure flow rate in the barrel and found its value which was zero. We found out the reason to have a zero value is that we have an open discharge to the fluid flow in the pipe. then Qp = 0, thus, Q = Qd, always which is what we exactly determined in the above code. So, the only parameter I am concerned about is the melted polymer flow rate. In case, flow rate is equal to 3.8991e-04 kg/s. Thus screw characteristics has maximum output, which is 7.3399e-06. For material characteristics, the Deborah Number is equal to 0.0057 which is greater than 1 then the process is not dominantly elastic. Thus we have determined from the material chart which is "Ashby's chart". The young modulus value for PLA as 2 GPA. By this we found the material characteristics values. The material relaxation time is found y the formula viscosity/modulus; the viscosity is equal to 0.6 and the modulus as I said is 2GPam then the relaxation time is equal to 0.1714 seconds. The processing time is estimated to be 30s.And by this we found the Deborah number.
If there is any further modification just copy and paste the above calculation code in MATLAB, and change the constant parameters required to sole them. Some of the calculations are wrong and needs to be modified.
Conclusion
To conclude, our design stopped and did not go into further steps due to us changing our idea into plastic extrusion machine. Some of the calculations and results can be used to do the machine. It is very important to have a molding machine in our manufacturing facilities. But, we stopped doing the machine because we struggled with some limitations and time schedule.
Future Work
In future we can take our research and basic concept into turning the rough idea into a physical idea. And improve it more with the material we already have in our facility.