Time to read: 6 min
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While the power of a hardware product comes from its internal components, a product is typically recognized by its enclosure, the outer shell that encloses electronic products, making them appealing and user-friendly.
In this post, I&#;m going to walk you through the steps for designing a basic enclosure, using the design of a IoT plant monitor product as an example.
The design is based on this awesome project by Ryan Madson&#;using just a couple of sensors, a WiFi-enabled Photon developer board from Particle, and an online cloud platform called Fathym, he&#;s able to continuously monitor the moisture and temperature of his plant at home.
For the purposes of this example, we&#;re not going to worry about how the enclosure looks, but rather just focus on functionality.
With any design, I like to begin by thinking about requirements, which can help you keep your development in scope and avoid adding cost and complexity where you don&#;t need it.
At this stage, you should ask yourself, what does my enclosure need to do and what are its most basic functions?
Here are the requirements for our plant monitor enclosure:
The above features are necessary for a successful design. Notice how the requirements don&#;t go on to include more specific design decisions such as wall thickness dimensions at this point. In the beginning, keep your requirements as streamlined as possible so you can have flexibility in your design later on.
Pro Tip: Enclosing electronics tends to increase the temperature of the system. You may need to add a fan or some sort of heat rejection method if your components are getting too hot.
Now on to the enclosure. I generally start a design such as our plant monitor example by thinking about how the innards will be held.
Ideally, you have a good idea of what&#;s going inside the enclosure so you can accurately design around it. In our case, we have a Photon Particle board, a temperature sensor, and a soil moisture sensor.
Modeling the larger parts&#;the Photon board and the soil moisture sensor&#;will make the 3D design easier and more relevant. You can often find some sort of dimensional drawing from the manufacturer, if not an actual 3D model.
I was able to find dimensions for both the Photon board and the soil moisture sensor, allowing me to create some simple 3D models.
The placeholder models don&#;t need to reflect every feature of the part. The outer dimensions and any mating features are important to model, but everything else can be left out.
For instance, my models of the soil moisture sensor and Photon board are pretty blocky, but the extents of the parts are accurately represented.
Now that we have models of the electronic parts, we can design our enclosure around them. I start by shelling out a rectangular prism, creating an open box shape.
As we create features, we are striving for uniform wall thicknesses because injection molding, the process we&#;d use for mass manufacturing, requires it.
I&#;m going to use .040&#; wall thicknesses because that will be 3D printable as well as injection moldable.
One of our requirements states that the soil moisture sensor must be inserted at least an inch into the soil. One option is to just run wires from the board to the sensor outside of the enclosure, but I like the idea of a fully packaged product.
I&#;m going to add a slot that will hold the moisture sensor vertically, allowing the probes to pass through the bottom of the enclosure.
We need to leave room for the wires to be soldered on the top of the moisture sensor, so let&#;s remove some material while still maintaining a slot.
I&#;ll also add a cutout for the micro-usb connector. The board will rest with the connector inside this slot, providing some alignment.
The Photon board is currently being held on one side by its micro-USB connector, but we should add supports on which the board can sit.
Luckily, there is nothing mounted on the bottom of the Photon board, so we don&#;t have to worry about hitting anything. A pretty simple way to create supports is to add ribs of our uniform thickness, where the board can rest.
Here&#;s a current view of the assembly so far:
Now we need to think about how the lid will be attached. I&#;m a big fan of the socket head cap screw, so let&#;s add some extra features around the outside of the enclosure to allow a fastener to pass through.
The features you see here are typical in injection molding. Bosses surround the fastener holes and have additional ribs to the outer structure for support. All geometry has our same uniform thickness of .040&#;.
A trick for using metal fasteners in plastic parts is to countersink, or cut, the exact size of the nut on the bottom side of the part, keeping it from rotating while you screw in the fastener.
Finally, we&#;re going to radius the outer corners, which will decrease the stress concentration there and also make the enclosure look a little more friendly.
We are still keeping a uniform thickness, so for the outer corners the outer radius (0.140&#;) will be slightly larger than the inner radius (0.100&#;).
While we&#;re at it, let&#;s radius our internal corners, too. It&#;s important to keep these small to avoid adding too much material and increasing wall thickness.
Here is the completed bottom half of our enclosure:
Now on to the lid! We&#;ll use the same types of features in the lid, shelling a box, adding bosses for the fasteners to pass through, countersinking the fasteners into the top, and radiusing the outer corners to match the bottom.
The bosses for the fasteners look as they do above because we are maintaining our uniform wall thickness, and that&#;s what a countersink looks like from the other side.
I&#;ve also made the bosses slightly shorter than the outer wall height so that there are no interferences.
Just like in Step 9 for the bottom of the enclosure, we will radius the outer corners of the lid to decrease the stress concentration and make the lid match the bottom.
This little boss will mate with the top of the micro-USB connector, securing it in the slot in the bottom of the enclosure.
As per our requirements, holes are created for interacting with the buttons on the board and seeing the LED light.
While the moisture sensor is being pushed into the soil, it will probably come up to contact the lid, which is less than ideal.
To remedy this, I&#;ll add a rib that will hold the moisture sensor down in a more secure position.
The final step is to radius all of those sharp corners that are not only aesthetically unpleasant, but have large stress concentrations. Again, we&#;re going to keep the radii small (.005&#;) to avoid adding too much material.
Now let&#;s add the lid to our full assembly and throw in some hardware.
Be sure to leave space for wires and their bends! It&#;s easy to forget about wire routing while you&#;re designing until you&#;re trying to assemble the product. You can see from the above section view that I&#;ve left plenty of room (nearly half an inch) above the board for wires and the small temperature sensor.
Hopefully this gives you some helpful guidelines for designing and prototyping your own product enclosure. To start 3D printing your enclosure design, hop on over to Fictiv where you can get 3D printed parts delivered in 24 hours.
For more detail on enclosure design features, check out our posts on how to design snap fit components, choosing the best fasteners for 3D printed parts, how to conduct a tolerance analysis for 3D printed parts, and how to design light pipes.
The rapid heating cycle molding (RHCM) injection method has been used to overcome product surface defects generated through conventional injection methods, such as weld marks and flow marks, by contrasting the advanced technology with conventional injection methods and products formed [56]. In this section, three comparisons are made in terms of the working process, mold design manufacture, and part quality.
The polymer melt is thoroughly combined during the injection molding process because RHCM uses high temperature, along with a rapid heating and cooling method. The melt flows much better than in the conventional injection method, with lower viscosity. The surface of the mold cavity is made with a high-gloss surface. As a result, the product surfaces are bright and smooth as a mirror, with no weld marks or flow marks.
In general, the conventional injection molding process consists of five stages, namely, the plasticization stage of the polymer, the injection stage, the filling stage, the cooling stage, and the ejection stage. Once the polymer has been plasticized into a molten state, it is injected at a higher pressure and speed into the mold cavity through a nozzle. Following the packing stage, the polymer melt is cooled to a low temperature using water. After that, the mold is opened, and an ejector pin is used to eject the polymer. At this point, the conventional injection molding process has completed one cycle before beginning the next cycle [56].
In contrast to CIM, RHCM injection technologies can be classified into six phases. The stationary mold is heated prior to injecting the polymer melt into the mold cavity. During this stage, the surface of the mold cavity is heated to the injected polymer&#;s glass transition temperature. The cavity is then filled with the polymer melt. The cavity temperature remains unchanged during the subsequent packing process until the cooling phase starts. Cooling water is utilized as a coolant to preserve the mold and polymer melt inside it to a certain low temperature. The plastic part can now be ejected to complete the RHCM injection cycle. The cavity surface is heated quickly again before the next injection process begins. Then, the next RHCM cycle of injection starts [56].
In RHCM, the initial mold cavity temperature is significantly higher than that in conventional molding, and the temperature is significantly different. However, since the mold temperature rises and falls in a short period of time, the molding cycle is nearly equal to that of a conventional injection molding process [55]. To shorten the molding cycle time, the heating stage for the RHCM mold, which requires heating of both the cavity and the core sides, can begin at the same time as part ejection. If only the cavity side of the mold needs to be heated, the mold can be heated as it opens. During the heating stage, the mold is heated to a preset high temperature, which is usually higher than the polymer&#;s glass transition temperature [87]. shows the RHCM technology principle and the RHCM mold&#;s temperature variation and injection cycles for conventional injection molding. In this figure, a is the heating stage, b is the injection and packing stage, c is the cooling stage, d is the mold opening and part ejection stage, and a&#;, b&#;, and c&#; are the next stages of the injection cycle, while line 1 represents the heat distortion temperature of polymer, line 2 is the mold temperature of RHCM, line 3 is the ejection temperature of the part, and line 4 is the mold temperature in the conventional injection method.
This figure shows the mold temperature changes during RHCM injection cycles, including the polymer&#;s heat distortion temperature and the ejection temperature of part. Although conventional molds are still at the same temperature, the RHCM technology work process is more complex than the conventional injection process. Furthermore, the temperature control of RHCM molds is much more complicated than that in conventional molding, and it is normally controlled by conventional temperature controllers [56].
On the basis of the results from the previous researchers [ 48 , 88 ], it can be said that the application of RHCM in CIM has a big impact on the molded part. Previous studies used the same mold insert for both molding processes. The part molded by RHCM showed greater warpage predicted with crystallinity compared to that predicted without crystallinity. Additionally, the strength of the RHCM molded part was higher than that of the CIM molded part with the same shape and size, while the weld line of the RHCM molded part was smaller compared to that of the CIM molded part. Thus, it can be concluded that using RHCM in CIM yields a significant difference in the molded parts produced, demonstrating that RHCM produces a better result.
Furthermore, Li et al. [ 48 ] investigated the effect of the weld line on the tensile strength of RHCM and CIM components. Tensile testing results showed that, for specimens with a weld line, the strength of the RHCM specimen was significantly higher than that of the CIM specimen due to the smaller dimensions and higher bonding strength of the weld line. Moreover, the RHCM specimen&#;s weld line was smaller than that of the CIM specimen. Furthermore, the two melt fronts that formed the weld line were welded together better due to the higher temperature and pressure in the RHCM process compared to the CIM process, resulting in a higher bonding strength of the weld line. The tensile strength of specimens without a weld line showed no discernible difference between the two molding processes. In addition, a thin surface layer of material containing the V-notch at the weld line was removed from the RHCM and CIM parts to investigate its effect on tensile properties. When compared to specimens with the original full thickness, thinned out specimens with a weld line showed improved tensile strength, while specimens without a weld line showed decreased tensile strength.
Some defects in plastic parts produced by conventional injection molding (CIM) can be solved by RHCM, such as flow marks, silver marks, jetting marks, weld marks, exposed fibers, and short shots [ 43 ]. However, RHCM is not a solution for all injection-molded part flaws. Warpage is one of the defects that cannot be solved using RHCM. Li et al. [ 88 ] investigated a method for predicting the warpage of crystalline parts molded using the RHCM process. To predict warpage, multilayer models with the same thicknesses as the skin&#;core structures in the molded parts were developed. The thicknesses of the layers varied with each molding process. The upper skin layer of each injection-molded part was heated on the stationary side. The prediction results were compared to the experimental results, which revealed that the average errors between predicted warpage and average experimental warpage were respectively 7.0%, 3.5%, and 4.4% in CIM, RHCM under a 60 °C heating mold (RHCM60), and RHCM under a 90 °C heating mold (RHCM90). Apart from that, the microstructure and temperature in CIM were symmetrical along the thickness direction and asymmetrical in RHCM. The predicted warpage was influenced by crystallinity, and the warpage predicted with crystallinity was greater than the warpage predicted without crystallinity, especially in RHCM-molded parts.
Many studies have been performed to enhance the surface quality of plastic products by optimizing the process parameters. Contrary to popular belief, optimized parameters do not eliminate defects, but improve the surface quality of the molded parts. It was recently discovered that raising the mold surface temperature eliminates defects, increases flow length, and improves surface quality [43,87,140,141,142,143,144]. RHCM is an innovative technology that allows for dynamic mold temperature control during the injection molding process. RHCM technology requirements for the temperature of the heat distortion for the injecting polymer should be met before the mold cavity is injected. At the ejected temperature, it must be quickly cooled down. The difference in the mold temperature is significant. Therefore, if the cooling and heating methods are the same as in conventional methods, the production cycle must be extended. The RHCM mold is heated and cooled rapidly to ensure heating and cooling efficiency. Consequently, the structure of the mold differs from a conventional mold, as does the heating method. Companies such as Foreshot Industrial Corporation, Taoyuan City, Taiwan and Letoplast S.R.O., Letovice, Czech Republic have developed and applied RHCM technology, offering the benefits of a perfect product with a glossy surface that does not require painting [145,146]. Letoplast specializes in the production of visual and technical plastic parts made of PC-ABS, ABS, PP, PPE, PC, PA6, and other materials [146]. RHCM technology is used in plastic parts such as network communication equipment computers/communications/consumer electronics, appearance parts, and liquid crystal display televisions (LCD TVs) [145]. This technology allows engineers, technologists, or mold designers to avoid unnecessary premature melt freezing throughout the filling stage, which lowers the melt flow resistance and improves the molten plastic filling capability [38].
Chang and Hwang [147] proposed an infrared heating method for the mold cavity surface. For thermal surface condition assessment, a transient thermal simulation was developed. A change in the mold structure is not necessary with this method. This method has a high construction cost, and it is hard to consistently heat the longitudinal or complex mold cavity. Fischer et al. [148] investigated the effect of a locally different cooling behavior on the injection molding process of semicrystalline thermoplastics. An innovative dynamically tempered mold technology with different temperature zones was investigated, allowing the production of thin-wall components with locally different component properties. The preliminary results showed that, by influencing the inner component properties, significant differences in the optical and mechanical component properties could be achieved. On the other hand, Yao and Kim [142] evaluated the benefits of heating the mold surface through the use of a thin metal coating and thermal insulation. Copper poles were used to control the temperature by heating the thin metal layer. This method was shown to be efficient in terms of energy usage and temperature control. However, the complex coating and tiny parts of the cavity make this process more challenging; in addition, there are safety hazards, such as in the resistor layer&#;s insulation, with the resistor having a limited life time. shows some of the studies conducted on RHCM technology.
Heating stage (110&#;180 °C and 110&#;200 °C)
Cooling stage (180&#;110 °C and 200&#;110 °C)
Mold temperature (50 °C)
Surface marks
Weld line strength
Heating times, 3&#;4 s for mold surface temperature to rise from 110 to 180 °C and 200 °C, along with 21 s for cooling time (return to 110 °C)
Eliminated the surface marks of the weld line and enhanced the strength of the related weld line
Mold temperature (25 °C)
Cooling time (20 s)
Melt temperature (260 °C)
Mold surface temperature
Replication heights of LGP&#;s microstructures
Residual stress in LGP
Induction heating the mold surface to 110 °C could increase the replication rate of the microstructure&#;s height by up to 95%
There was no residual stress in the LGP produced by induction heating
Mold temperature (80 °C)
Cooling time (20 s)
Melt temperature (260 °C)
Mold temperature (110 °C)
Cooling time (20 s)
Melt temperature (260 °C)
Mold temperature (110, 130, and 150 °C)
Cooling time (20 s)
Melt temperature (260 °C)
Injection speed (180&#;200 mm/s)
Packing pressure (1st stage 50&#;70 Mpa, 2nd stage 40 Mpa)
Packing time (4&#;8 s)
Mold temperature (60&#;80 °C)
Cooling time (30&#;40 s)
Mold surface temperature (110&#;150 °C)
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Power rates
Optimum processing parameters
Quality of microfeature heights and angles
Optimum process parameters: injection speed (180 mm/s) packing pressure (70 Mpa), packing time (8 s), mold temperature (70 °C), cooling time (30 s), and mold surface temperature (150 °C).
Replication effect on microfeatures was significantly improved by induction heating
Two gates, cavity temperature = 75 °C, no vapor chamber
Two gates, cavity = 75 °C temperature, with vapor chamber
Two gates, cavity temperature = 110 °C, with vapor chamber
One gate, cavity temperature = 75 °C, no vapor chamber
Tensile strength
The two gate/vapor chamber system&#;s tensile strength increased by 3.2% when preheating temperatures increased from 75 to 110 °C
ABS/PPMA Fiber-reinforced plastic
PP + 20% glass fiber
Heating time (10, 20, 30, 40, 50, and 60 s)
Cooling time (20,30,40, 50, and 60 s)
High- and low-temperature holding time (10 s)
Weld line
Tensile strength
RHCM process could improve the weld line factor for both materials
RHCM process reduced the tensile strength of the part without weld line
Mold heating time (18, 24, 25, and 36 s)
Mold cooling time (25, 32, 38, and 46 s)
Weld line
Moldflow Insight
ANSYS
Weld marks on the part surface could be significantly reduced by increasing the cavity surface&#;s temperature just before filling
Surface gloss of product produced by RHCM was more than 90%.
Mold temperature (between 120 and 150 °C)
Thicknesses of the heated target (10&#;20 mm)
Pitch of the coil turns (10&#;20 mm)
Heating distance (5&#;9 mm)
Position of the induction coil (0&#;12 mm)
Working frequency (30&#;40 kHz)
Waiting time (2&#;6 s)
Heating rate
Temperature difference
Heating rate was increased by 19.5%, from 3.3 °C/s to 4 °C/s
Heating uniformity was increased by 62.9%
Heated mold temperature for RHCM (120 °C)
Mold temperature for CIM (25 °C)
Packing pressure (50 Mpa)
Cooling time (30 s)
Weld line
Tensile strength
Weld line decreased tensile strength, but RHCM reduced the weld line&#;s tensile strength reduction effect.
Silicon insert surface temperature (20, 60, 100, and 140 °C)
Melt temperature (230 °C)
Injection pressure (30 Mpa)
Injection speed (60 mm/s)
Screw back (20 mm)
Sample thickness (0.6 mm)
Weld line
Width of weld lines: 16.4 µm at 20 °C, 11.24 µm at 60 °C, and 5.6 µm at 100 °C
Weld line disappeared completely at 140 °C
Silicon insert surface temperature (20, 40, 80, and 100 °C)
Melt temperature (200 °C)
Injection pressure (30 Mpa)
Injection speed (50 mm/s)
Screw back (15 mm)
Sample thickness (1 mm)
Residual internal stress
Replication fidelity
Residual stress decreased as the surface temperature of silicon insert increased
Coating the silicon insert with carbide-bonded graphene could improve replication fidelity
Melt temperature (240 °C)
Injection pressure (60 Mpa)
Injection velocity (45%)
Packing time (9 s)
Packing pressure (50 Mpa)
Mold heating temperature (60/90/120 °C)
Cooling time (30 s)
Microstructure
Tensile properties
Surface quality
Tensile strength of RHCM parts reached peak at 60 °C mold heating temperature
The sample&#;s surface gloss increased as the mold cavity surface temperature increased, but decreased as the mold heating temperature increased above 90 °C
Chen et al. [39] studied the feasibility, efficiency, and effects of induction heating on the weld line surface appearance for dynamic mold surface temperature control. A simulation and experiment were used to analyze the induction heating with the spiral coil coinciding with the coolant cooling to evaluate its rapid heating/cooling ability and the uniformity of the mold plate surface temperature. Initially, ANSYS 3D thermal analysis was performed on a mold plate with a heating source to verify the analytical capabilities and accuracy. Then, induction heating was performed on the double-gate tensile sample mold to control the surface temperature of the acrylonitrile butadiene styrene (ABS) melt injection mold and determine its influence on the surface marking and weld line strength. From the results, it was indicated that it took 3 s for the center temperature of the plate to rise from 110 °C to 180 °C and 21 s to rise to 110 °C during induction heating and cooling, while, for the second heating/cooling cycle, it took 4 s to reach 200 °C and 21 s to return to 110 °C. The surface temperature of the mold plate could be elevated to around 22.5 °C and cooled to 4.3 °C. The study also proved that the temperature-controlled induction heating of the mold surface also contributed to the elimination of surface markings and to enhancing the strength of the weld line on injection molded ABS tensile bars. However, this study did not yield a positive correlation between the heating time by induction heating and the temperature distribution pattern on mold plates produced by the rapid tooling technique. Therefore, further investigation is required.
Huang and Tai [43] studied the use of induction heating to rapidly heat the mold surface during the injection molding process, thereby improving the microstructure replication effect of light-guided plates (LGP). First, oil was used to heat the mold to 80 °C, and then induction heating was used to heat the surface of the LGP mold above the glass transition temperature. Induction heating was implemented to achieve the high mold temperature essential for the short filling stage throughout the microstructure replication process of LGP, as well as to rapidly lower the mold temperature in the cooling process, allowing the molded part to cool faster. The heating coil was a single copper wire used for small area heating. The geometry of the induction coil is presented in . The molded part of PMMA plastic was a flat LGP of 40 mm × 30 mm, with length and width of 1 mm. Results from these studies showed that the induction heating system only achieved a temperature uniformity of 6.7&#;12.7 °C. In addition, the combination of an oil heater (80 °C) and an induction heating system (up to 110 °C) offered the best replication performance, significantly raising the replication ratio up to 95%, which was 6% higher than when the mold was heated with an oil heater and 7.8% higher than when the mold was left unheated. Thus, these studies prove that induction heating can improve the LGP microstructure filling efficiency. However, in this study, only the performance of the heating technology when heating the mold surface was discussed, without further details on the strength and surface appearance (weld line) of the molded parts produced. Therefore, a further investigation of the correlation between heating technology and the properties of the molded part produced is required.
Huang et al. [44] used the same experimental setup as Huang and Tai [43] to perform rapid induction heating on the surface of a 2 inch LGP mold, but with a different coil design, using three-layered copper wire, to study the influence of high mold temperature in induction heating, as shown in , to enhance the replication rate of LGP microfeatures. The temperature profiles of the Ni heating plate used as the mold insert in 2 inch LGP were investigated in order to identify the optimal power rate setting in the induction heating system; these temperature profiles correlated with the different power rates. Additional testing was carried out to determine the efficiency of the optimal process parameters defined by the Taguchi method. The ANOVA results indicated that the most significant parameters affecting the replication ability of microfeatures were the mold surface temperature and injection speed. As the surface temperature of the mold increased to 150 °C, the replication rate of the average nine-point height of the microfeatures and the average nine-point angle increased to 91.6% and 98.1%, respectively. However, the effects on the strength of the molded parts produced and the temperature distribution surface of the mold insert were not discussed in detail in this study.
Tsai et al. [46] reported the vapor chamber&#;s effect on part tensile strength, which was made using plastic (ABS) material in an injection molding process. This experiment was developed with a copper heat pipe of the flat-plate type. Water was used as the working fluid. Then, to circulate the working fluid, a copper mesh formed using 50 μm wire with 100/200 mesh pore and a solid copper cylinder with a diameter of 2 mm were used to support the vacuum and loading forces. The experiment used the shape of the specimen shown in for tensile testing. The experiment was conducted with two distinct mold designs, one with a single gate and another with two opposite gates. The cavity temperature was used as a parameter in the experiment, while the output was the molded part&#;s tensile strength. As a result of the weld line, the tensile strength of the test part formed by the two gate/no vapor chamber heating system was 11.1% lower than that of the test part formed by the one gate/no vapor chamber heating system ( ). The two gate/vapor chamber system produced a fine weld line, as well as a tensile strength 6.8% higher than that of the two gate/no vapor chamber system. Furthermore, the tensile strength increased by 3.2% when the preheating temperature increased from 75 to 110 °C in the two gate/vapor chamber system. Due to the extended fluid flow, the use of the vapor chamber heating system and the increase in the preheating temperature seemingly improved the tensile strength of parts molded with two opposing gates. However, despite the fact that this study focused on conventional mold steel, additional research on the effect of heating on the mold insert produced by the rapid tooling technique should be considered.
The heating and cooling system designs of electric heating molds are critical for RHCM with electric heating because they directly affect the heating/cooling efficiency and temperature uniformity, which have a decisive influence on the molding cycle and part quality [127]. Electric heating in RHCM can be accomplished through the deployment of electrical heating rods and cooling tunnels in the stationary mold, which can be rapidly heated via electric heating prior to filling and cooled via circulating water after filling [50,88]. In another study, Wang et al. [54] investigated the thermal response of the electric heating with the deployment of a cartridge heater in rapid heat cycle molding, as well as its impact on the surface appearance and tensile strength of the molded part. ABS/PPMA and fiber-reinforced PP were the materials used in the molded part experiment. The mold material used for the mold holder was HT 350, while that for the mold plate was AISI , and that for the cavity plate was AISI H13. ANSYS thermal modules were used to investigate the temperature distribution and thermal response of the cavity surface. The thermal cycling process caused the surface temperature of the cavity to rise gradually. The maximum cavity surface temperature in thermal cycling could reach 185 °C after 30 experiment iterations with 60 s of mold heating time and 20 s of mold cooling time. This implied that the power density of the heater must be high enough to shorten the RHCM molding cycle by reducing the mold heating time. As the power density of the heater increased from 15 to 30 W/cm2, the heating time of the cavity surface to be heated from 30 to 120 °C could be decreased from 58 to 19 s. Experimental results showed that, compared with CIM, the RHCM method reduced the tensile strength of parts without the weld line ( ). In the case of ABS/PMMA plastics, the tensile strength of the weld line part was also significantly lowered due to RHCM. The RHCM method enhanced the weld line factor for ABS/PMMA and 20% glass fiber-reinforced PP. Findings were limited to the effect of RHCM heating technology on the cavity surface and the parts molded using conventional mold inserts. Nonetheless, more research into the effect of RHCM heating technology on mold inserts produced by rapid tooling technique is needed.
Wang et al. [47] studied the design of heating/cooling channels for an automotive interior part, as well as its evaluation in rapid heat cycle molding. Polycarbonate (PC) material was used for the molded part. The general commercial ANSYS was used to simulate the transient thermal response of the mold cavity throughout the heating process to analyze the efficiencies of the original heating/cooling channel design. The designed heating/cooling systems with baffles could significantly improve the cavity surface thermal response efficiency. Steam was used at a temperature of 180 °C and a pressure of 1.0 Mpa. When experimental and simulation results were compared, the reasonable heat transfer coefficient was determined to be 14,000 W/m2·°C. Heat exchange occurred between the cavity plate and the environment via natural air convection. The convective heat transfer coefficient was determined to be 25 W/m2·°C. According to the results, the mold time constant could be lowered from 5 s to 2.5 s, resulting in a 50% decrease in time. Heating efficiency could be improved by 27.3%. The total surface temperature differences could be reduced to 20&#;30 °C. Moreover, by increasing the temperature of the cavity surface just prior to filling, the weld marks on the surface of the part could be significantly reduced ( ). The weld marks could be completely removed as the cavity surface temperature approached a critical level. The critical cavity surface temperature of the plastic material used was approximately 130 °C. Findings from this study proved that the surface gloss of RHCM products was more than 90%. However, further studies on the implementation of RT in RHCM are required.
Nian et al. [45] studied the key parameters and optimized design of single-layer induction coils for external rapid mold surface heating by controlling the process parameters of induction heating (suitable for applications involving mold plates with different thicknesses and coil positions). depicts the spatial dimensions used in the simulation. An experiment was performed to validate the thickness of the heated workpiece (SKD61) and the induction coil (copper) position influencing the heating rate and temperature uniformity. Furthermore, the Taguchi method and principal component analysis were used to identify the best control factor combination to obtain high heating rates with low temperature deviations. According to the simulation results, the position of the induction coil and the thickness of the workpiece were indeed important design parameters. The optimal parameters indicated heating a 10 mm thick workpiece when designing a single-layer induction coil with a 15 mm turn pitch at a distance of 5 mm from the heating target. Additionally, the coil position should not be offset, and the operating frequency and waiting time must be set to 35 kHz and 6 s, correspondingly. Furthermore, the results of the experiments showed that an optimized design of the induction coil outperformed conventional single-layer coils in terms of temperature uniformity. The heating rate increased from 3.3 °C/s to 4 °C/s (an increase of 19.5%), and the heating uniformity increased by 62.9%. In terms of model validity, all MAPE values indicated high prediction accuracy. However, other important tests such as strength tests, as well as an observation of the appearance of the molded part produced by the induction heated mold surface were not performed.
Li et al. [48] investigated the effects of the weld line and its structure on tensile strength, as well as its process dependence, in CIM and RHCM. The specimens were made of isotactic polypropylene (iPP). shows the dimensions of the dumbbell-shaped specimen; the thickness was 2.5 mm. An RHCM mold with a maximum clamping force of kN, a maximum melt volume of cm3, and a maximum injection pressure of 182 Mpa was used to mold the CIM and RHCM parts. Electrical heating rods were used to quickly heat the mold before filling it; after filling, cooling tunnels cooled the mold with circulating water. The only parameter used for the experiment was the mold temperature, and the output was the tensile strength of the weld line on the molded part. According to the results, the RHCM process reduced the dimensions of the weld line, particularly the width of the weld line adjacent to the heated stationary half. The tensile strength of dumbbell-shaped specimens cut from RHCM parts with and without a weld line is shown in . Due to the presence of skin layers or weld lines on both RHCM and CIM specimens, their trends were similar. The RHCM specimen with the weld line had a higher tensile strength than the CIM specimen, but the strength without the weld line was remarkably similar. The weld line reduced the overall tensile strength, while RHCM reduced the decrease in tensile strength. Meanwhile, in the RHCM process, there was no discernible influence on the tensile strength of the specimens not having a weld line. The removal of the surface layer improved the tensile strength of specimens with weld lines but reduced it in specimens without a weld line. Findings from this study prove that RHCM can reduce the effects of weld line on the tensile strength. Nonetheless, further studies on the potential of RT in RHCM are yet to be conducted.
Xie et al. [49] studied the thermal response of the graphene layer, as well as the impact of rapid thermal cooling with carbide bonded-graphene coating on molded product defects such as weld lines, internal stress, and replication fidelity. In this study, the heaters and heating channels implanted in the CIM were detached. They could be rapidly heated to a critical temperature by fine-tuning the voltage applied to the carbide-bonded graphene coating layer, or they could be cooled by a coolant channel. The semicrystalline polymer PP was used for the weld mark experiments, while PS was used in both the residual stress and the replication fidelity experiments. According to the results, with increasing voltage, the rate of heating of the insert surface increased. Once the voltage was set to 240 V, the coating heated up to 145.6 °C in 10 s, indicating that the average and transient heating rates could reach 11.6 °C/s and 16.1 °C/s, respectively. Therefore, the graphene coating could act as a thin-film cavity resistance heater, heating the polymer above Tg in seconds. Additionally, experimental results indicated that improving the surface temperature of the silicon insert could reduce the width of weld lines on molded products. If the temperature was high enough, these lines could also disappear entirely. shows that tensile strength and tensile elongation yields increased with silicon insert surface temperature. The average tensile strength and tensile elongation yields for silicon inserts at room temperature were 27.14 MPa and 128.30% respectively. However, at 140 °C, the average tensile strength and tensile elongation yields increased to 37.39 MPa and 468.77%, correspondingly. Findings from this study concluded that the rapid thermal cooling method could produce sample plates with uniform size thickness (600 µm) and mold products with fewer weld marks, less minimal internal stress, and enhanced replication fidelity. The tensile strength and tensile elongation yield of the products were increased by 37.77% and 265.11%, respectively, with even less energy consumption. Nonetheless, more research on the temperature distribution of the cavity surface of the mold insert is needed.
Liu et al. [50] studied the microstructure, tensile properties, and surface quality of glass fiber-reinforced PP parts molded by RHCM, using PP with 30% short glass fibers. The employed electric heating RHCM apparatus consisted of electric heating rods, a K-type thermocouple sensor, and a mold temperature controller MTS-32II system for measuring, regulating, and displaying the mold surface heating temperature. In the sample thickness direction, Autodesk Moldflow was used to evaluate the glass fiber orientation distribution. The simulation&#;s material properties and boundary conditions were based on the CIM and RHCM experiment parameters. RHCM60, RHCM90, and RHCM120 were the sample mold part designations for the cavities heated to 60, 90, and 120 °C. The tensile tests were conducted according to ASTM D638, and the results showed that, as the cavity surface temperature increased prior to filling, the tensile strength increased first, before decreasing. Tensile strength was the highest in the RHCM60 sample, followed by the RHCM90 and CIM samples, while it was lowest in the RHCM120 sample ( ). These variations were due to the microstructure of GFRPP composites being highly dependent on the cavity&#;s surface temperature. As the cavity surface temperature increased, the thickness of the skin layer with medium fiber orientation along the flow direction decreased. RHCM samples had a thicker shear layer than CIM samples, with RHCM60 having the thickest shear layer. Therefore, the overall crystallinity and fiber orientation of RHCM60 in the cross-section were greater than those of the CIM sample. When an external force applied tensile load to the sample, the increase in fiber orientation caused more fibers to bear the load transmitted by the substrate. The increase in matrix crystallinity indicated an increase in the contact area between the crystal and amorphous regions, which aided in load transfer from the matrix to the fiber, resulting in RHCM60 having a higher tensile strength than the CIM sample. With a weak fiber orientation, the core layer thickness decreased initially, before increasing; the RHCM120 and RHCM60 samples had the thickest and thinnest core layers, respectively. It was also reported that the sample&#;s surface gloss tended to increase as the surface temperature of the mold cavity increased, but the changes in surface gloss were noticeably reduced as the mold heating temperature increased above 90 °C. Furthermore, this study took into account the temperature distribution on the mold cavity surface of various mold types of mold inserts in CIM and RHCM.
According to the above review, RHCM has the potential to eliminate weld lines and improve surface gloss. The main distinction between RHCM and CIM is that the mold is heated and rapidly cooled to the ejection temperature after the filling process. The mold is heated above the molten plastic&#;s glass transition temperature to achieve the optimum surface quality on the molded parts. In the reviewed studies, electric heating, induction heating and steam heating were extensively used as heating methods in RHCM in order to heat the mold in the quickest time. According to the research on parts produced by RHCM, it was found that defects caused by weld lines not only affect the surface appearance, but also affect the strength of the molded parts. A lower strength is obtained due to the stress concentration point between the V-notch and the weak bonding area, and the bonding interface strength at the weld line is not as strong as that in the matrix. In addition, different levels of mold surface temperature were introduced into RHCM research to obtain the optimum molded part quality. However, there is a lack of studies on the temperature distribution of the surface of the mold insert, which will give further information on the effect of heating on the parts produced. Nevertheless, most of the research only focused on using conventional materials as mold inserts, and there is still lack of support for and verification of the application of RT in producing mold inserts that are less expensive than conventional material molds in the RHCM method. The next section in this paper focuses on the effects of RCHM on molded parts. As confirmed in previous research, the RHCM method can increase the strength of the molded parts by reducing weld lines.
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