Product diagram of front and rear housings of law enforcement recorder is shown in Figure 1. Maximum size of product is 129.30 mm * 78.90 mm * 12.00 mm, average thickness of plastic part is 2.0 mm, plastic part material is PC, shrinkage rate is 1.006, and weight of plastic part is 29.05 grams and 27.82 grams. Technical requirements for plastic parts are that there must be no defects such as peaks, underfilling of injection molding, flow lines, pores, warpage deformation, silver streaks, cold materials, jet lines, bubbles and so on.

 

It can be seen from Figure 1 that plastic parts are front and rear housings of law enforcement recorder. External dimensions are exactly same. Edges of front and rear housings have buckles, can be butt-fastened together. Technical requirement after assembly is that front and rear housings have same external dimensions and smooth edge feel. There are 10 heat dissipation holes on the top surface of front and rear shells, which need to be rubbed and formed. There are 8 heat dissipation grooves on the top of rear end, which also need to be rubbed and formed. Since front and rear shells need to be fully connected, in addition to 6 buckles and locks, multiple positioning ribs need to be designed on the edge of plastic part to make front and rear shells rigid enough without elastic deformation. Three factors of buckle, full circle stop and multiple positioning ribs ensure that overall structure of plastic part is reliable after assembly, and it will not separate after being dropped to ground to ensure use performance.
There are many gaps on the edge of plastic part, and it is necessary to design pillow position. It should be noted that pillow position should be designed as much as possible instead of rub-through position. Pillow position has a longer mold life than rub-through position. Mold design drawing is shown in Figure 2. When designing parting surface, for rubbing position, if it does not affect appearance of plastic part, try to make inserts in rubbing position to facilitate replacement after wear. In general, one-time service life of S136H does not exceed 200,000 times, only 8407 and other materials have a better mold life after heat treatment. Pillow position also needs to be designed with inserts, mainly for convenience of processing and to reduce EDM clear corner processing.
Mold design layout is 1+1, two plastic parts assembled together are formed on same set of molds, which is called a family mold. Advantage is that plastic parts adopt same materials and molding conditions, which can easily ensure that they have same shrinkage rate and ensure consistent dimensional accuracy. Mold base is standard mold base CI3045, gating system is designed with a latent gate, one point injecting glue, and a fixed mold of latent plastic part. Plastic part is made of PC material, runner needs to be short and thick when designing mold to facilitate flow and filling of molten plastic.

 

Figure 1 Product diagram of front and rear shells of law enforcement recorder

 

 

Figure 2 Diagram of front and rear shell molds of law enforcement recorder
Buckle on the edge of plastic part and two convex buckles are respectively designed with two small sliders to form. All other buckles are designed with lifter forming. Lifter structure is all T-slot lifter seats.
Ejection of plastic parts adopts inclined ejector and thimble ejection. Front and rear molds are designed with cooling and water transportation circuits to ensure normal injection molding.

 

Figure 3 Former mold core

 

Figure 4 Back mold core

In process of plastic melting and plasticization, melt continuously moves to front end of barrel (in metering chamber), more and more, gradually build up a pressure to push screw back. In order to prevent screw from retreating too fast and to ensure uniform compaction of melt, it is necessary to provide screw with a pressure in opposite direction. Pressure in this opposite direction to prevent screw from retreating is called back pressure.
Back pressure is also called plasticizing pressure, and its control is achieved by adjusting return throttle valve of injection cylinder. Back pressure valve is installed at the rear of injection cylinder of pre-plasticized screw injection molding machine, which adjusts oil drain speed of injection cylinder when screw rotates backwards, so that cylinder maintains a certain pressure; screw backward movement speed (resistance) of full motor is controlled by AC servo valve.

1. It can compact melt in barrel, increase density, and improve injection volume, product weight and dimensional stability.
2. Gas in melt can be "squeezed out" to reduce air blooms and internal bubbles on the surface of product, improve uniformity of gloss. Slow down retreat speed of screw to fully plasticize melt in barrel, increase mixing uniformity of toner, masterbatch and melt, avoid color mixing in product.
3. Slow down retreat speed of screw, so that melt in barrel is fully plasticized, mixing uniformity of toner, masterbatch and melt is increased, and product is prevented from color mixing.
4. Properly increasing back pressure can improve shrinkage of product surface and glue running around product.
5. It can increase temperature of melt, improve plasticization quality of melt, and improve fluidity of melt when filling mold. There is no cold glue pattern on the surface of product.

1. When back pressure is too low, screw retreats too fast, density of melt flowing into front end of barrel is small (relatively loose), and there is more air trapped.
2. It will lead to poor plasticization quality, unstable injection volume, large changes in product weight and product size.
3. Undesirable phenomena such as shrinkage, air bloom, cold material lines, uneven gloss, etc. will appear on the surface of product.
4. Bubbles are easy to appear inside product, glue around product and bones is easy to be underfilled.

1. Melt pressure at the front end of barrel is too high, material temperature is high, and viscosity is reduced. Counterflow of melt in screw groove, increase in leakage flow between barrel and screw will reduce plasticization efficiency (amount of plasticized material per unit time).
2. For plastics with poor thermal stability (such as: PVC, POM, etc.) or colorants, temperature of melt increases and heating time in barrel causes thermal decomposition, or degree of discoloration of colorant increases, and surface color of product /gloss becomes worse.
3. Back pressure is too high, screw retreats slowly, and pre-plastic return time is long, which will increase cycle time and reduce production efficiency.
4. Back pressure is high and melt pressure is high. Nozzle is prone to salivation after glue is injected. When glue is injected next time, cold material in nozzle runner will block nozzle or cold material spots will appear in product.
5. During plastic molding process, nozzle leaks due to excessive back pressure, which wastes raw materials and causes heating ring near nozzle to burn out.
6. Mechanical wear of pre-plastic mechanism and screw barrel increases.

Back pressure of injection molding should be adjusted according to properties of raw materials, drying conditions, product structure and quality conditions. Back pressure is generally adjusted to 3-15kg/cm 3. When surface of product has a little air bloom, color mixing, shrinkage and large changes in product size and weight, back pressure can be appropriately increased. When nozzle leaks, salivates, melts overheated and decomposes, product discolors, and material return is too slow, consider appropriately reducing back pressure.  
Back pressure is one of important parameters to control melt quality and product quality in injection molding process. Proper back pressure plays an important role in improving product quality and cannot be ignored!