Injection molding is a manufacturing process that is widely used to produce plastic parts and products at scale. One key consideration when injection molding plastics is maintaining the right temperature, as excessive heat can cause the plastic resin to become brittle. As a professional plastic injection molding manufacturer, I will share some essential tips and best practices to ensure optimal temperature control during the injection molding process.
Understanding plastic resin brittleness
Plastic resin brittleness refers to the tendency of a plastic part to break or crack when subjected to stress, due to degradation of the polymer chains that compromises strength and flexibility. This can occur when the plastic resin becomes too hot during injection molding.
Overheating plastics causes the long polymer chains to break down through a process called thermal decomposition. As these molecular chains shorten, the plastic loses strength and becomes more rigid and glass-like. Even small stresses can then cause the brittle plastic to crack or shatter.
Brittleness temperature threshold
Every plastic resin has a “ductile-to-brittle transition temperature” (DBTT) at which it starts to become brittle and lose impact resistance with increasing temperature.
The DBTT can vary significantly between different plastic materials:
- Polypropylene (PP) – Below 32°F/0°C
- Polystyrene (PS) – Below 50°F/10°C
- Polyethylene (PE) – Below -4°F/-20°C
- Polyamide (PA/Nylon) – Below 50°F/10°C
- Polycarbonate (PC) – Below -22°F/-30°C
Exceeding the DBTT leads to compromised strength and brittleness. The margin between DBTT and melt temperature is referred to as the “processing window”. Staying within this temperature window is key for injection molding quality plastics.
What causes plastic resin to become too hot?
There are several potential ways that plastic resin can become too hot during the injection molding process:
Barrel and nozzle temperatures too high
The key processing temperatures – barrel, nozzle and mold – need to be precisely set for each plastic resin type and grade. If barrel and nozzle temperatures exceed recommendations, the plastic can overheat causing material degradation.
Excessive residence time
As plastic resin sits in the hot barrel, it is exposed to heat over time. Using overly large machines or low injection speeds leads to prolonged heating, potentially taking the resin past its limits.
High shear heating
The intense pressures and friction plastic resin experiences as it is pushed along the screw and into the mold can cause localized overheating. Issues like under-sized gates, poorly designed runners or excess injection pressure exacerbate shear effects.
Repeated regrind passes
Reusing sprues and runners as regrind material can degrade plastic over successive passes through the barrel, lowering its melting point and causing brittleness faster.
Contamination
Contaminants introduced along the production line – whether paper, dust, chemicals or incompatible resins – may negatively impact material properties causing premature brittleness when heated.
Signs of plastic resin becoming too hot
Identifying the signs of overheating is key to addressing problems before quality issues emerge:
- Discoloration – Darker shades, speckles or burnt residues indicating plastic decomposition
- Residues – Flakes, streaks or plates that have broken from main resin mass
- Gas bubbles – Trapped expanding gases from vaporizing components
- Smell – Distinctive odors from overheated plastic
- Changes in melt flow rate – Resin flows faster when molecular chains shorten due to heat damage
- Variations in part dimensions – Clearance issues arise as hotter plastic resin expands more than expected
Solutions for plastic resin overheating
There are several processing adjustments that can counter material degradation from excessive heat:
Adjust barrel temperatures
Reset barrel and nozzle temperatures closer to the lower end of recommendations for that grade resin. This provides a safety buffer, allowing for some variability without crossing limits.
Modify injection speed
Slower injection speeds allow material to flow into the mold with less shear heating. This must be balanced with ensuring the cavity fills completely.
Lower back pressure
Reducing back pressure lessens the heat and shear forces exerted on the plastic. This can improve mixing without degrading polymers.
Shorten cycle times
Keeping cycle times as short as possible minimizes the duration plastic spends in the hot barrel. This relies on the mold cooling adequately within the timeframe.
Switch resins
Select an alternate grade or brand of the base resin with slightly different properties that make it less prone to heat damage. LMW resins with some chain branching are typically more heat stable.
Improve dryer efficiency
Adjust dryer settings or repair issues to ensure plastic resin is properly pre-dried before injection molding. Residual moisture creates steam when heated, accelerating material breakdown.
The risks of running too cold
Lower temperatures reduce risks of material degradation but cannot be lowered indefinitely without consequence. Very cold plastic becomes too viscous to flow correctly into thin sections or across long flow paths. Incomplete mold filling produces parts with defects and dimensional inaccuracies. Finding the sweet spot for temperatures is key.
Conclusion
Plastic resin that overheats during injection molding can pass its ductile-brittle transition point and become rigid and crack-prone when cooled. There are various processing factors – such as temperatures, pressures and cycle times – that influence the level of heat exposure. Identifying indicators of excess heat early on allows adjustments to be made before loss of material properties and part quality. With plastics having a narrow optimal temperature band, vigilance and fine tuning is essential to avoid making injection molded components overly hot or cold.
